Evaluation of Ranking Procedures for Invasive Plants · PDF fileEvaluation of Ranking...

Evaluation of Ranking Procedures

for Invasive Plants:

Application to British Columbia

Brian Wikeem, P. Ag. Solterra Resource Inc.

Prepared for:

Val Miller Range Branch

B.C. Ministry or Forests and Range

March 2007

Evaluation of Ranking Procedures for Invasive Plants ______________________________________________________________________________

_____________________________________________________________________ BC Ministry of Forests and Range

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Contents

EXECUTIVE SUMMARY .............................................................................................. IV INTRODUCTION .............................................................................................................. 1 WEED RANKING OBJECTIVES AND CRITERIA........................................................ 2 REVIEW AND ANALYSIS OF CURRENT SYSTEMS.................................................. 3

Weed Ranking Systems .................................................................................................. 3 Exotic Plant Ranking System...................................................................................... 6 Northern Prairie Alien Plants Ranking System .......................................................... 6 Virginia Ranking Invasive Exotic Plant Species ........................................................ 8 Australian Weed Assessment Guide........................................................................... 9 NatureServe Criteria for Ranking Invasive Plants.................................................... 10 Alaska Weed Ranking Project .................................................................................. 11

Weed Risk Assessments ............................................................................................... 12 Application of Weed Risk Assessments ................................................................... 14 United Kingdom Non-native Risk Assessment Scheme........................................... 14 Invasive Alien Plant Program (IAPP) Species Prioritization Algorithm.................. 16 Alberta Invasive Species Risk Assessment Tool ...................................................... 17

EVALUATING WEED RISK ASSESSMENT SYSTEMS ............................................ 18 Variable Objectives....................................................................................................... 19 Questions and Content .................................................................................................. 19 Scientific Knowledge.................................................................................................... 19 Biological Characteristics ............................................................................................. 20 Ecological Impact ......................................................................................................... 21 Subjectivity ................................................................................................................... 22 Scoring Procedures ....................................................................................................... 22 Abiotic and Autecological Features.............................................................................. 23 Strengths of Weed Risk Assessments ........................................................................... 25 Summary and Conclusions ........................................................................................... 26

POTENTIAL RANKING PROCEDURES ...................................................................... 28 Unique Features Affecting Weed Populations in BC ................................................... 28 Review Objectives ........................................................................................................ 29 Potential Weed Rating System...................................................................................... 30

Environmental Tolerance.......................................................................................... 32 Abundance and Distribution ..................................................................................... 33 Management Feasibility............................................................................................ 34 Rationale for Provincial and Regional Ranking ....................................................... 35

CONCLUSIONS AND RECOMMENDATIONS ........................................................... 36 LITERATURE CITED ..................................................................................................... 38



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APPENDIX Appendix 1. Glossary of terms. .................................................................................... 43 Appendix 2. Exotic plant ranking system questions..................................................... 46 Appendix 3. Northern Prairie alien plant ranking system questions. ........................... 47 Appendix 4. Virginia ranking invasive exotic plant species questions and scoring..... 48 Appendix 5. Australian weed assessment guide. .......................................................... 49 Appendix 6. NatureServe questions and scoring. ......................................................... 50 Appendix 7. Alaska weed ranking project questions and scoring. ............................... 51 Appendix 8. IAPP species prioritization algorithm. ..................................................... 52 Appendix 9. Alberta invasive species risk assessment tool questions and scoring. ..... 55

Tables

Table 1. Number of species listed by provincial government legislation and other sources in British Columbia...................................................................................1

Table 2. Criteria and characteristics of highly ranked species.............................................2 Table 3. Selected primary and secondary criteria used to rank invasive plants. .................4 Table 4. General characteristics of invasive plants..............................................................5 Table 5. Comparison among protocols of criteria and total number of questions

asked. .....................................................................................................................7 Table 6. Desirable information for conducting weed risk assessments. ............................15 Table 7. Examples of questions and responses..................................................................20 Table 8. Potential criteria for ranking invasive plants in British Columbia. .....................31 Table 9. General guidelines for selecting weed ranking criteria. ......................................32

Figures

Figure 1. Potential priority matrix to rank invasive plants in British Columbia............... 35



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EXECUTIVE SUMMARY

This report reviewed and evaluated selected invasive plant ranking (weed ranking systems) and weed risk assessment procedures from North America, Australia, and the United Kingdom to assess their relative application in British Columbia at both a provincial and regional scale. Eight procedures were specifically evaluated, but other literature and protocols were also reviewed to put the process of evaluating invasive plants into an historical and current context. The review revealed that numerous overlapping protocols are presently available and many are currently under development. The main findings and recommendations from the report are summarized below.

• Weed Ranking Systems were developed in the mid-1980s to screen innocuous invasive plants from those that could threaten national park resource values in the United States. Over the next decade, the original protocols were modified and applied in many similar settings throughout the United States.

• Weed ranking systems were initially designed to evaluate small areas. More recently, some protocols are applied to states (provinces) and countries.

• Most weed ranking systems were developed for conservations lands, and exclude agricultural lands and native plant communities outside conservation areas.

• Weed ranking methods combine professional opinion with “scientific” documentation to predict relative invasiveness among non-native plants based on a set of predetermined criteria.

• Generally, people with good taxonomic and ecological skills can reduce the number of weed species of potential management concern when the criteria for weed ranking are rigorously applied. Training will be required to ensure consistent application of these tools.

• Weed Risk Assessments were introduced in late-1990s, and blend weed ranking objectives with other legal, political, and policy issues involved with the prevention, introduction, spread, and control of exotic organisms into countries.

• Weed risk assessments are strictly technical evaluations that take no account of the potential benefits from new organisms being introduce into novel environments, although the process has been moving in that direction.

• Numerous guidelines have been developed that provide general direction on the content and methods for conducting weed risk assessments, but specific protocols vary considerably in scope and application.

• Weed risk assessments have become more comprehensive over time, and international risk analysis standards have been established to evaluate the effects of weeds and plant pests on the environment and biological diversity.



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• The transition to more comprehensive analyses has not necessarily improved the weed risk assessment process, and in some cases has made relatively simple procedures cumbersome, confusing, and appearing imprecise.

• Weed risk assessments have been criticized as procedures that are narrowly focused, subjective, often arbitrary and unquantifiable, and subject to political interference. There is also widespread concern that they are severely limited in their ability to identify potential of invasive plants in new geographic environments because they lack of broad scientific principles or reliable procedures.

• Most weed risk assessments rely heavily on biological traits to determine “invasiveness” and to project impacts. The inability of “invasive” traits to predict actual invasions, however, has created skepticism, and some weed managers doubt they will ever be capable of predicting which species are invasive and where.

• Ecological impacts of weeds on plant community composition, structure, and ecological processes are the most pervasive criteria used in weed ranking and risk assessments.

• Although ordering invasive species is important for prioritizing management efforts, little is known about which species cause the greatest ecological impacts, or the relative effects of the same species from one ecological setting to another. Consequently, questions regarding impacts are of limited value for objective analysis until more information is available on the actual effects of weeds on ecological structure and processes.

• Risk assessments have used several approaches to scoring weed threats and impacts. Measures of uncertainty, and the option for assessors to record that answers are “unknown,” are common procedures used to compensate for the subjectivity introduced in these procedures.

• Until recently, most weed risk assessments have concentrated on native plant communities on conservation lands where “disturbances” may be minimized compared to other lands.

• Climate and habitat factors have not been used extensively in weed risk assessments, although “climate-matching” is gaining interest and application in some of these procedures. Similarly, environmental factors have only begun to infiltrate weed risk protocols recently.

• Weed risk assessments that are carefully documented and explain the logic of their rating systems are more likely to result in repeatable conclusions by different assessors, and allow public scrutiny and independent evaluation.



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• Weed risk assessments can provide a consistent and logical decision-making process for ranking exotic plants. Such an analytical framework encourages biologist and managers to consider the full range of factors and consequences of their decisions.

• Although all of the methods reviewed provide valuable information for conducting weed ranking elsewhere, the existing protocols ranged from highly specific to very comprehensive in geographical scope, organisms addressed, and the sophistication of questions and summary analyses. None of the applications reviewed appear satisfactory in their present format for application in British Columbia.

• The following conclusions and recommendations are submitted: o No weed ranking system presently exists in BC that is widely endorsed

and systematically used.

o A pragmatic and scientifically-based process for ranking invasive plants, and evaluating the relative susceptibility of habitats, would be a valuable management tool for the province.

o None of the weed assessment tools reviewed is adequate in its present format to meet the needs of British Columbia.

o A weed ranking system for British Columbia should account for the unique features of the province including its geographic size, and its diversity in climate, soils, and vegetation, and the requirements of invasive plants relative to ecological conditions in the province.

o BC needs to be divided into “management zones” based on geographic separation, such as Ministry Forests and Range districts, or regional district electoral areas. Divisions should be based on anticipated management authority so that geographic areas and management responsibilities are linked.

o Criteria for land uses with different management priorities, legal responsibilities, and government policies need to be considered in setting priorities.

o Weed ranking needs to be linked to inventory and mapping so that weed lists can be updated at regular intervals based on actual data collected in BC.

o Generally, invasive plant lists should be regarded as guidelines rather than a definitive list of ranked species. Weed lists should be considered “living lists” with the expectation that they will modified as more knowledge is gained, and as provincial, regional, and local priorities change over time.



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INTRODUCTION

Although a host of non-native1 plants have been introduced world-wide, most species have not become problems in their new setting (Hiebert and Stubbendieck 1993; Nelson 1994; Williamson 1996; Radosevich 2005). Those that do become pests, however, can be highly competitive, and affect the integrity and productivity of natural and agricultural ecosystems. Management of these species can be difficult, expensive, and often requires a long-term commitment. Therefore, land managers must direct financial and human resources to those species that are most likely to interfere with management goals and objectives. In 1994, Douglas et al. (1994) listed 553 non-native vascular plants as part of the British Columbia flora, and numerous introductions have occurred since (Rankin and Associates 2004). Presently, 82 of these species are covered in overlapping lists under three provincial statutes (Table 1), while management of the remaining species is at the discretion of those agencies or people responsible for Crown and private lands.

Table 1. Number of species listed by provincial government legislation and other sources in British Columbia.

Invasive Plant Designation Source Number Listed

Weed Control Act 49

Forest and Range Practices Act 42

Community Charters Act 50

Ministry of Forests and Range Southern Interior Forest Region Invasive Plant Categories 43

Ministry of Forests and Range Coast Forest Region Invasive Plant Categories 49

Ministry of Forests and Range North West BC Invasive Plant Categories 68

Ministry of Forests and Range North East BC Invasive Plant Categories 68

Ministry of Forests and Range Invasive Alien Plant Code Index >120

1 See Appendix 1 for a glossary of technical terms.



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Many other invasive plant lists have also emerged in the province over the last decade that complement, or supplement, those covered under provincial legislation (Table 1), but no clear and consistent rationale for these lists has been documented. Each of these lists pertains to large geographic areas and a wide range of ecological conditions. While they are intended to provide general direction for classifying the possible risks and ecological threats of invasive species, there exists some debate on their veracity even under local conditions. In addition, no comprehensive system is presently available for ranking invasive plants at a provincial and regional level that is broadly accepted. Many protocols have been developed over the last two decades to assist in separating innocuous introduced species and those that may become harmful or damaging (Hiebert and Klick 1988; Hiebert and Stubbendieck 1993; APCC 2002; Randall et al. 2003; Morse et al. 2004; DEFRA 2005; Pheloung 2007). The objective of this report is to review selected invasive plant ranking and risk assessment procedures used in North America, Australia, and the United Kingdom (UK) to evaluate their applicability for use in British Columbia at both a provincial and regional scale.

WEED RANKING OBJECTIVES AND CRITERIA

Initially, weed ranking procedures were developed to provide resource managers with a pragmatic tool to classify the relative threat of non-native species to indigenous natural resources. These models sought to separate harmless exotic species from those deemed “disruptive” according to their “innate ability to become a pest” and the “level of impact” they might cause (Table 2). In addition, the process aimed to collect consistent and accurate information for establishing priorities and making management decisions (Heibert and Stubbendieck 1993).

Table 2. Criteria and characteristics of highly ranked species.

Primary Criteria Selected Characteristics for High Priority Species

Innate Ability as a Pest Highly fecundity, specialized dispersal abilities, ability to germinate in a wide range of environmental conditions.

Significance of Impact Occur in high quality natural areas, have large populations that invade and replace natural communities.

Feasibility of Control Not widely distributed, small seed banks, and easily controlled with cultural, mechanical or chemical methods.



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Since their introduction, exotic plant ranking systems have undergone numerous modifications to adapt the process for use in other geographic regions and ecological conditions. Additionally, interest in listing and ranking invasive plants has gained international interest, and these protocols have been incorporated into other risk assessments with broader objectives (Heffernan et al. 1999; Hiebert et al. 1999; Morse et al. 2004; ANHO 2005). For example, the United States (US), Australia, New Zealand, and the United Kingdom have developed risk assessment procedures that are primarily directed towards controlling the importation, introduction, and spread of non-indigenous species into their countries (NRC 2002; DEFRA 2005; Pheloung et al. 1999; NWRAS Review Group. 2006). Weed risk components of these systems have been adapted to address the legal, political and regulatory issues surrounding transportation and importation of exotic species (Pheloung et al 1999; DEFRA 2005; NWRAS Review Group. 2006). Consequently, both the objectives for making these assessments, and the criteria for evaluation, have undergone significant changes in recent years (Table 3).

REVIEW AND ANALYSIS OF CURRENT SYSTEMS

Weed Ranking Systems

Weed ranking systems appear to have evolved from early work by Hiebert and Klick (1988) who constructed a method to classify invasive plants based on a model developed at Point Reyes National Seashore (Self 1986). Between 1989 and 1991 this procedure was applied in numerous national parks across the United States, and revised based on the experiences and recommendations of users and expert reviewers (Hiebert and Stubbendieck 1993). Most weed ranking methods combine professional opinion with “scientific” documentation to predict relative invasiveness among species based on a set of predetermined criteria (Hiebert and Klick 1988; Dunster 1990; Hiebert and Stubbendieck 1993; Hiebert 2001). Early experience indicated that applying these systems could greatly reduce the number of weed species of potential management concern when the criteria were diligently applied by a person with good taxonomic and ecological skills (Hiebert and Stubbendieck 1993). Plant characteristics related to “weediness” (Table 4) were considered the principal attributes that enabled weeds to invade habitats and form persistent populations,

Evaluation of Ranking Procedures for Invasive Plants _____________________________________________________________________________________________________________________

_______________________________________________________________________________________________________ BC Ministry of Forests and Range 4

Table 3. Selected primary and secondary criteria used to rank invasive plants.

Primary Criteria Secondary Criteria

Innate Ability (Invasive Traits)

Seed production, viable seed, average seed mass, interval between seed crops, long flowering and fruiting period, evidence of reproductive failure in native habitat. Asexual reproduction, small genome size, biennial or annual. Relative growth rate; produces spines, thorns, burrs, parasitic, pathogenic. Allelopathic, unpalatable, causes allergies, toxic to animals. Geophytic, hybridization, self-fertilization, minimum generative time. Tolerates mutilation, cultivation, fire. Other species in genus occur in area of interest. Tolerates range of conditions or stressful conditions.

Impact (Environment, Economic, Social)Affects ecosystem processes, community structure, and community composition. Competes with native plants and animals. Genetic impacts, creates fire hazard. Grows on other vegetation; forms dense thickets. Impact agriculture infrastructure, affect agriculture operations, potential risk to forestry operations. Obstructs vision or movement, diminishes aesthetic appeal.

Feasibility of Management and Control Well controlled by herbicides; biocontrol agents available; cultural/mechanical control possible.

Abiotic Factors Climatic suitability, soil requirements, light requirements.

Distribution and Abundance Current global distribution. Current local distribution, proximity distribution in adjacent jurisdictions, trend in distribution, naturalized beyond native range, garden disturbance weed, weed of agriculture or forestry, environmental weed.

Disturbance Extent and kind of disturbance. Requires anthropogenic disturbance to establish.

Ecological Types Aquatic, riparian, grassland, shrub land, forest land, other.

External Introduction and Spread Capable of surviving in transit, likelihood of detecting at entry point, likelihood of introduction into country, re-introduction, or dispersal from human pathways. Species listed elsewhere (province/state, country).

Introduction and Spread Propagules dispersed intentionally, dispersed unintentionally, persistent seed bank. Capable of long-distant dispersal, wind-dispersed, buoyant and water-dispersed, animal-dispersed. No known predators (native or non-native); capable of surviving in transit.

Land Use Practices Conservation areas, native ecosystems outside conservation areas, agricultural, forestry, parkland, pasture, rangeland (non-arable grazing), roads and trails, utility corridors, other.


_____________________________________________________________________ BC Ministry of Forests and Range 5

Table 4. General characteristics of invasive plants.1

• Capable of germinating in many environments.

• Continuous seed production throughout the growing season.

• High seed production and long viability of seeds in the soil seed bank.

• Ability to reproduce both sexually and vegetatively.

• Ability to self-fertilize.

• Ability to cross-pollinate easily by a variety of vectors such as insects and wind.

• Adaptations for short- and long-distance dispersal.

• Rapid growth from the vegetative phase to flowering.

• If a perennial, vigorous vegetative reproduction or regeneration from fragments.

• If a perennial, resistance to being broken from ground easily.

• Ability to compete vigorously with other plant species by special means such as rosettes, rapid growth, or chemical properties (allelochemicals).

1 Modified from Baker (1974). but potential impacts, and feasibility to control populations, were also regarded as important factors for making management decisions. Those species considered to be the greatest threat, and most easily controlled, were given the highest priority for management; while benign species that were virtually impossible to control were regarded as a low priority. This information was used to formulate control and management decisions for the areas surveyed (Hiebert and Klick 1988; Dunster (1990, cited in White et al. 1993); Hiebert and Stubbendieck 1993; Hiebert 2001). Initially, weed ranking systems were designed for making evaluations on relative small areas such as specific sites, or individual national parks. More recently, the scope of these procedures has broadened significantly, and some applications are now used to survey entire states (provinces) and countries (Hiebert and Stubbendieck 1993; Morse et al. 2004).



Exotic Plant Ranking System

This system was among the first published and widely circulated weed ranking protocols developed (Hiebert and Stubbendieck 1993). Designed primarily for US National Parks and small areas dominated by native vegetation within parks, the system has been adopted and revised for use primarily in conservation areas throughout the United States. The system was developed as an analytical approach for decision-making, and with the hope of reducing subjective decisions for weed control, which were often based on incomplete knowledge (Hiebert and Stubbendieck 1993). The procedure contains 24 questions for each weed species organized into two main sections: 1) Significance of Impact, and 2) Feasibility of Control or Management. The significance of impact section is divided further into the “Current Level of Impact” and “Innate Ability of Species to Become a Pest” (Table 5; Appendix 2). Each question has a variable number of multiple choice answers which are independently scored on a relative scale ranging from 0 to 15 points. Each of the two main sections is based on a scale of 100 points. Species that score more than 50 points for Significance of Impact are regarded as “seriously disruptive” and require management intervention, while species that score high in the “Feasibility of Control” section will be easier to manage than those with low scores. A final question relating to urgency of action ranks plants as high, medium or low.

Northern Prairie Alien Plants Ranking System

The Alien Plants Ranking (APR) system was cooperatively developed by the US National Parks Service, University of Minnesota, and the US Geological Survey. Based on earlier prototypes prepared by the National Parks Services (Hiebert and Klick 1988; Hiebert and Stubbendieck 1993), this computer-based system was modified for application in grassland parks in the central United States (Hiebert et al. 1999). Like the previous protocol, it is designed to help identify invasive species that are currently affecting grassland sites, and other invasive species that may be a high risk in the future. The system also evaluates the feasibility of control of each species, which helps managers compare the cost of control against potential impact. The APR system contains 23 questions for each weed species known to occur at, or near, the area of interest (Table 5). The questions are organized into three sections: 1) Current

Evaluation of Ranking Procedures for Invasive Plants _____________________________________________________________________________________________________________________

_________________________________________________________________________________________________________ BC Ministry of Forests and Range 7

Table 5. Comparison among protocols of criteria and total number of questions asked.

Criteria

US

Nat

iona

l Pa

rk S

ervi

ce

1993

Nor

ther

n Pr

airi

e 19

99

Vir

gini

a 20

01

Aus

tral

ia 2

002

Nat

ureS

erve

20

04

Ala

ska

2005

Uni

ted

Kin

gdom

200

5

BC

IAPP

200

6

Alb

erta

200

7

Invasive Attributes (Biological Traits) 9 10 3 5 9 20 18 13

Impact (Environment, Economic, Social) 6 7 5 6 5 4 16 6 12

Feasibility of Management and Control 9 6 1 4 3 7 2

Abiotic Factors (climate, soil, light) 2 2

Current Distribution and Abundance 3 1 4 5

Disturbance and Land Use Yes n/a n/a Yes n/a n/a n/a n/a n/a

Introduction and Spread 35 6

Trend in Distribution and Abundance 7

Total Questions 24 23 14 12 20 21 > 601 31 33 Uncertainty No No No No No No No No Yes

1 Exact number of questions difficult to determine because of the number of modules in the scheme and sub-questions.



level of Impacts, 2) Potential of the Species to be Invasive, and 3) Feasibility of Control Including Costs (Appendix 3). Multiple-choice answers are provided for each question with the associated scores variably ranging from 0 to 10 points depending on the weighting given to the question. “Unknown” is provided as a response for all questions and does not enter into the final scoring for a species. Scores are tallied within the computer program, and an on-screen summary report can be produced or a hard copy printed. The program provides electronic fact sheets for each weed species. In 2000, this system was expanded to 25 questions and the web-based system was linked to GIS and quantitative sampling systems (Hiebert 2001).

Virginia Ranking Invasive Exotic Plant Species

Beginning in 1991, the Virginia Department of Conservation and Recreation (VDCR) began compiling a list of invasive plants species to create awareness among land managers and the public (Heffernan et al. 2001). By 2003, the list had undergone several revisions and invasive species were classified based on three main criteria: 1) Invasive Rank, 2) Physiographic Rank, and 3) Habitat Requirements (VDCR 2003). Invasiveness Rank classified species as highly invasive, moderately invasive, and occasionally invasive based on cumulative impact on natural areas, effects on other species, potential to disperse and invade native habitats, distribution and abundance, and difficulty to manage. Physiographic Rank was determined by dividing the state into three physiographic regions and assigning each plant to regions where they occurred. Habitat Requirements focused on light and moisture requirements to provide a broad indication of their adaptation to different environments. Adaptation to light was partitioned into full sun, partial sun or shade; and moisture categories included hydric, mesic and xeric. Although not regulatory, the list provides managers with a relative ranking of invasive species, a regional assignment of where they may be problematic, and basic habitat information regarding species adaptation to abiotic habitat features (Heffernan et al. 2001; VDCR 2003). This appears to be the first time habitat conditions were introduced into an invasive plant ranking procedure. In 1999, the VDCR List of Invasive Species was combined with a modification of the Association for Biodiversity Information (ABI) ranking criteria (Randall et al. 2001) to study 11 non-native species in Virginia (Heffernan et al. 2001). Four components were used to rank species invasiveness representing aspects of the species biology, ecology, and potential for management: 1) Impacts on Native Species, Habitats, and Ecosystems,



2) Biological Characteristics and Dispersal Ability, and 3) Distribution and Abundance in Virginia and the U.S., and 4) Management Potential (Appendix 4). Although the protocol contains a series of 14 multiple-choice questions among the four components (Table 3), some questions are comprised of an additional series of queries. For example, the first question (Biological Characteristics) under Section 2 of the protocol (Appendix 4) contains a checklist of 14 biological traits that cumulatively contribute to the species’ overall “aggressiveness.” Species with three or more of these traits are classified as “Extremely Aggressive.” In general, scoring for this procedure is convoluted and complicated. Each question is answered as insignificant, low, medium, or high; and each category is further assigned a weighted score. The answers to each question are converted to weighted scores to tally sub-ranks for each area, and these in turn are adjusted to new weighted scores to provide an overall rank for the species. A greater weighting is placed on those criteria that most strongly demonstrate impact on native plant and animal habitat, and biological characteristics; whereas less emphasis is focused on distribution, abundance, and difficulty of control. Species invasiveness is ranked as insignificant, low, medium, or high. Comments and supporting references are provided for each question. References include published research articles, internet sources, databases, and personal communications from field biologists or land managers.

Australian Weed Assessment Guide

This guide was developed as a tool to help make standard, informed decisions on weed control priorities in South Australia. The protocol consists of 12 questions to compare the relative importance of various weeds under different land use conditions in recognition that the same weed species may respond differently to various practices (Table 3; Appendix 5). The questions are divided into three categories: 1) Invasiveness, 2) Impacts, and 3) Potential Distribution (Appendix 5). Invasiveness focuses on the relative rate of spread among species, with weeds that disperse faster having a higher priority for control. Impacts are related to the economic, environmental and social effects resulting from weed invasion, and Potential Distribution predicts the total area the weed could eventually cover. A sequence of multiple choice responses is presented for each of the 12 questions. Scores are attached to each of the alternatives, which are ranked from high to low. A fifth category, “Don’t Know,” is also included for those questions where a subjective



judgment cannot be made. A score, ranging from 0 to 10, is assigned to each of the criteria, and the product of the three scores determines the “Weed Importance Score” out of 1000. Weeds can be assessed separately for various land uses, so that the most important weeds can be identified in different management situations. These include aquatic, crop/pasture rotation, forestry, irrigated crops and pastures, native vegetation, non-arable grazing, perennial horticulture, and urban.

NatureServe Criteria for Ranking Invasive Plants

Two drafts of this procedure were reviewed (Randall et al. 2003; Morse et al. 2004). This protocol was developed for biologists to evaluate the effects of individual invasive plants on native plant communities within a specified geographical region (country, state (province), ecological region, and site). The second draft states that it is “designed to make the process of assessing and listing invasive plants objective and systematic, and to incorporate scientific documentation of the information used to determine each species’ rank” (Morse et al. 2004). The method focuses on native plant communities in conservation areas. It is not intended for evaluating weed species that grow in cultivated areas, or on disturbed sites outside cultivation. Two questions are asked before a survey is conducted to determine whether these criteria are met. If an invasive plant grows outside cultivation, but not in native plant communities, it is ranked as insignificant (Appendix 6). This protocol integrates previous weed ranking criteria developed by the US National Park Service (Hiebert and Klick 1988; Hiebert and Stubbendieck 1993; Hiebert 1997) with a synthetic approach to explaining relationships among invasive plants and ecological responses based on models (Parker et al. 1999). The procedure consists of 20 multiple-choice questions organized into four categories: 1) Ecological Impact on Ecosystem Processes, Communities, and Native Species; 2) Current Distribution and Abundance; 3) Trend In Distribution and Abundance; and 4) Management Difficulty (Appendix 6). Five answers are possible for each of the 20 questions; four pertaining to the physical, biological, ecological, or economic response to the questions, and the fifth is “Unknown” (Table 3). “Unknown” is recorded when the evaluator is unable to respond to the question because of “lack of information.” Evaluators are asked to record comments and the sources



of information used to arrive at conclusions for each answer in a citation format. This may include publications, internet sources, expert opinion, and other sources. The ranking for a species is determined in a three-step process. First, scores are assigned to each question within sub-rank categories and then tallied to produce a sub-rank score within each of the four categories. Each of the four sub-ranks, however, is weighted differently depending on the perceived importance of the category. For example, the highest weight is credited to Ecological Impact and the lowest weighting is given to Management Difficulty. The final score is produced by summing the weighted sub-rank scores to provide the overall invasive rank (I-Rank) for the species. Each species is then placed in one of the four I-Rank categories: high, medium, low, or insignificant. Although the protocol was originally designed to assess invasive plants in large, contiguous, and ecologically diverse regions, NatureServe states it can be adapted to discrete regions (such as states, provinces, or smaller geographic areas). NatureServe is now using this protocol to assess the effects on biodiversity of nearly 3,500 exotic plant species established outside cultivation in the United States (Morse et al. 2004). In 2004, the NatureServe method was tested in British Columbia by ranking 36 non-native plants inhabiting coastal communities (Rankin and Associates 2004). Based on this review, the authors concluded that the method “forced careful and relatively objective” consideration of a wide range of factors involved in assessing invasive plants” but further refinement of the protocol would likely be necessary to apply the process on a regional basis. The following observations and conclusions were drawn that are pertinent to this report:

• The scope of the process is comprehensive with respect to the organisms involved; international, national and regional scope; and the potential policy and political issues addressed.

• The ranking process is subjective. • A sound “risk assessment procedure” with clear priorities is a necessary first step

in managing invasive plants. • Actions should be based on a clear direction, and the best scientific and socio-

economic information available. • Changes would be necessary before the process could be adopted by BC.

Alaska Weed Ranking Project

This protocol is a cooperative project among the Alaska Natural Heritage Program, several US federal agencies including the Forest Service, National Park Service,



Agricultural Research Service, Cooperative Extension Service, and the Geological Survey. The protocol is a web-based system that appears to have elements of both the National Park Weed Ranking System and the NatureServe model although no references are provided. The procedure is divided into two sections. The first section (Climatic Comparison) correlates the potential climatic suitability of each species to one of three eco-geographic regions in the state (Appendix 7). The second section (Invasiveness Ranking) contains 21 questions assembled into the same (but reworded) four sections as the NatureServe protocol: 1) Ecological Impact, 2) Current Distribution and Abundance, 3) Distribution, and 4) Feasibility of Control (Table 3). Two to four scaled answers follow each question, and “Unknown” is allowed when the assessor cannot select an answer among the choices provided. No score (including “0”) is applied for Unknown. The total score for each of the four sections varies from 40 points for Ecological Impact to 10 points for Feasibility to Control (Appendix 7). The total score for a species is the sum of the total points for all four sections out of 100 possible points. This value is converted to a decimal fraction by dividing the score attained by 100. Information for all species is displayed in a tabular format listing the score for each species and the climatic similarity of the species relative to each of the three eco-geographic regions listed for Alaska. Climate similarity for each species is expressed only as Yes or No for the eco-geographic region.

Weed Risk Assessments

Weed risk assessments were introduced into the invasive plant forum in the mid- to late-1990s, and probably owe their beginning to the multi-stage process developed in the US to assess human health (NRC 1983). Risk assessment was defined as a general process for linking science to decision-making (NRC 1983). In 1992, the US Environmental Protection Agency (EPA) described a basic framework for evaluating scientific information relating to undesirable effects of “stressors” on the environment. The term “stressor” was defined as “any physical, chemical, or biological entity that can induce an adverse effect.” “Adverse ecological effects” included a variety of disturbances ranging from mortality of an individual organism to a loss in ecosystem function (EPA 1992). Weed risk assessment has amassed considerable momentum over the past decade to the extent that the process has been characterized as a “new and developing discipline”



(Groves et al. 2001; Rahman et al. 2003). The first international workshop for weed risk assessments was held in Adelaide, South Australia in 1999 with presentations delivered from at least five countries including Australia, Ecuador (Galapagos Islands), New Zealand, United Kingdom, and the United States. Since then, weed risk assessment procedures have been introduced or applied in New Zealand (Pheloung et al. 1999); Australia (Pheloung et al. 1999; Pheloung 2007), American states (Thomas 1999; Westbrooks and Madsen 2006), United Kingdom (DEFRA 2005) and Canada (Alberta) (EMEC 2007a). Although weed risk assessments can be conducted at any stage of a species’ entry or spread into a country or specific geographic area, the current focus appears directed towards preventing the ingress and dispersal of species that will likely become invasive in a country (IPPC 2004). These procedures are concerned primarily with assessing the risk of a plant species becoming of sufficient importance to be classified as a pest. Generally, weed risk assessments are strictly technical evaluations that ignore the potential benefits that may accrue from the species being introduced or from not controlling it. Moreover, no mechanism presently exists to balance the risks and the potential benefits from political processes even though advocates have been trying to move in that direction (IPPC 2004). Growing international interest in quarantine legislation concerning weeds (and other non-indigenous organisms) added further impetus to develop weed risk assessment systems to help prevent the introduction of weed species, and to prioritize existing species for control (IPPC 2004). Much of the current experience originates in Australia and New Zealand, where quarantine protocols for preventing the introduction and spread of unwanted plant species are in place. The success of weed-risk assessment systems in these countries results from a regulatory environment that enables plant protection organizations to restrict the movement of plants across the borders and within the country. Without such legislation, weed-risk assessment by itself cannot prevent the entry and spread of weeds (IPPC 2004). More recently, risk assessments have become even more comprehensive and address some of the legal and policy issues related to international trade (Powell 2002). In addition, international risk analysis standards have been established to evaluate the effects of vascular plants and plant pests to the environment and biological diversity, including those risks affecting uncultivated/unmanaged plants, wild flora, habitats and ecosystems (IPPC 2004).



Application of Weed Risk Assessments

Numerous guidelines have been developed over the last decade (IPPC 1996; APHIS 2002; Williams 2003; Lehtonen 2004; IPPC 2004), which provide general direction on the content and methods for conducting weed risk assessments (Table 6). Although these assessments often aim to be quantitative, they are mostly qualitative procedures (Powell 2004). In the generic risk assessment process (EPA 1992), “experts” first estimate probabilities (as low, medium, or high) that a species will disperse, establish, and spread; and then they forecast the environmental and economic consequences of the species’ introduction (Simberloff and Alexander 1998). The total risk of establishment is “quantified” as the product of the independent probabilities of dispersal, survival, initial establishment, and spread. The “Total Effect of Establishment” is defined as the product of the perceived economic and environmental consequences, which are arbitrarily, but consistently, scored according predetermined tabular values (Simberloff and Alexander 1998). Categories of uncertainty often accompany the estimated probabilities for each component of the assessment. These are usually expressed as “very certain, reasonably certain, or moderately certain,” or as “reasonably or very uncertain.” These estimates do not contribute to the calculation for “total organism risk potential” but they can serve as a guide for decision makers in evaluating the strength of the assessment (Simberloff 2005).

United Kingdom Non-native Risk Assessment Scheme

Impetus for this protocol emerged from the Review of Non-native Species Policy: Report of the Working Group analysis conducted in 2004, which recommended that “a scheme for assessing the risks posed by any non-native organisms to species, habitats or ecosystems in all or part of the UK” be undertaken (DEFRA 2005). The project was conducted by a consortium from six universities and institutes in the UK using a draft pest risk assessment scheme developed by the European and Mediterranean Plant Protection Organization, and the procedures developed for plants invading Western Australia (Pheloung et al. 1999). The procedure is divided into two parts. The first section contains 14 questions that determine whether a detailed risk assessment is warranted. The second part includes 51 detailed questions in three categories: 1) Entry and Establishment; 2) Capacity for Spread; and 3) the Extent to Which Significant Economic, Environmental or Social Impacts may Occur (Table 3). Each question is accompanied by five levels of responses



Table 6. Desirable information for conducting weed risk assessments.1

Indicator Characteristics

Taxonomic Information Taxonomic description and valid scientific name including synonyms of accepted scientific name and common name(s).

Pathway or Source of Entry Possible pathways include: • Plant purposefully imported for urban horticulture, commercial

horticulture, or forestry. • Contaminants of imported commodities such as seeds for sowing

or as a food source for humans or livestock. • Accidental introduction of plants not included in either of the

above pathways e.g., attached to clothing. Species History and Closely Related Taxa

• Has the species become a weed elsewhere? • Does the species have weedy relatives in the same genus or

family in other countries or the receiving country? • What effects has the species had where it has become a weed?

Environmental Tolerances • Characteristics of the climate and soils in both its home range and its extended weedy range.

• Do similar climates and soils exist in the present country? Note however, that many species tolerate a wider range of environmental condition when released from the constraints present in their home range.

Biological Characteristics Undesirable biological traits that defined it as weed elsewhere or characteristics that may contribute to it becoming invasive in the new location.

Introduction and Spread Biological traits that promote the spread and persistence of the species in native or agricultural ecosystems.

Potential Impacts Possible adverse effects if the species is permitted entry and not controlled including: • Damage to human health. • Loss of production in agricultural/horticultural systems. • Reduction in population levels of valued native plants and

animals. • Disruption to natural ecosystems resulting in reduced ecosystem

services e.g., lower water supply, reduced tourist attraction. Ability to Control Cost, effort, efficacy and possible outcomes of control options. 1 Source: IPPC 2004.



(very low, low, medium, high, very high) and three levels of uncertainty (low, medium, high). Written comments are required for each question justifying the response, and a literature reference (or personal communication) is recommended where possible. The manual provides guidance on the protocol, and for actions that should be taken when information is lacking or uncertain (DEFRA 2005). Six additional modules provide methods for identifying invasive attributes, evaluating pathways of introduction, determining the vulnerability of receptors, quantifying economic impacts, summarizing risks and uncertainties, and selecting risk management options. Three of the modules are designed to determine whether the species has invasive attributes (Module 1), to quantify economic effects (Module 4) and to summarize risk and uncertainty (Module 5). Two other modules allow assessment of the relative importance of introduction pathways (Module 2) and the vulnerability of receptors (Module 3) to invasion. Module 6 provides a decision-making framework for selecting risk management options should the assessor believe they are warranted. Draft Excel templates are available to conduct analyses, but these are considered rudimentary at this time and require further development (DEFRA 2005).

Invasive Alien Plant Program (IAPP) Species Prioritization Algorithm

This draft protocol was developed to provide a means for ranking invasive plants in coastal British Columbia. The procedure is an Excel-based system that has integrated components of existing ranking procedures into a product applicable to temperate, maritime conditions. The procedure contains 30 questions distributed among four sections: 1) Biological Criteria; 2) Impact Criteria; 3) Controllability Criteria, and 4) Containability Criteria (Table 3; Appendix 8). In the first section, the assessor answers each of 17 questions independently and if the answer is “Yes” for the question, the appropriate points (ranging from 2 to 5 points) are entered into the “Score” column of the spread sheet. If the answer is no, the question scores zero. No “Unknown” choice is available for any question. Sections 2 to 4 contain five, four and three alternatives to select among, respectively. In these sections, the assessor chooses the most applicable impact, or scenario, and then records the point value for that question in the “Score” column beside the choice and at the bottom of the section in the “Subtotal” cell (Appendix 8). Sub-total scores for each of the sections are automatically calculated in the Excel version or hand-calculated on paper. These values are transferred to a summary table and



weighted with a “prorated factor” as follows: Biological (1.3), Impact (2.0), Controllability (1.5) and Containability (1.5). The total score for each species is calculated by summing the four prorated scores from each section and the dividing by three to produce a final score with a maximum value of 10 (Appendix 8). Although many of the questions found in this protocol are similar to those found in weed risk assessments, the objectives of this procedure are not stated, and weed ranking may be the intent of the procedure.

Alberta Invasive Species Risk Assessment Tool

This risk assessment tool is a computer software application that allows a “predictive, quantitative assessment of the likelihood of adverse impacts from potential invasive alien species in Alberta” (EMEC 2007a). The tool is intended to predict invasive species impacts by providing information for management decisions, prioritizing management strategies, and by assisting in enhancing management communications in the province. Similar to the UK scheme, this protocol has adopted the “classic risk assessment paradigm” developed in the US (NRC 1983; EPA 1999), and is broader is scope than invasive vascular plants alone. Hence, the protocol addresses all potentially invasive organisms from pathogens and parasites to vascular plants and animals. It also accommodates organisms from a variety of environments including aquatic, terrestrial, agricultural, natural areas; and native, disturbed habitats. It does not directly deal with issues concerning national or international policy. The procedure contains 32 questions distributed among six “Indicators:” 1) Biological Characteristics, 2) Introduction, 3) Environmental Impact, 4) Economic Impact 5), Social Impact, and 6) Control Management (Table 3; Appendix 9). All but one question is accompanied by four levels of responses (ranked 0, 1, 2, 3, 4) based on an ordinal scale from “least” to “most.” Zero in the scale means “low” or “none.” The procedure also provides the assessor with the opportunity to answer “unknown” for each question. When this occurs, or when no answer is provided, the question does not contribute to the final score. The protocol also accommodates uncertainty when the assessor is not sure of the strength of a subjective answer. These are ranked as low, medium or high and averaged over all six indicators and 32 questions. The risk assessment for a species is “quantified” by averaging the response scores for each of the six “Indicators” and converting the value to a percentage. Each indicator is adjusted with an “indicator-specific weight” and these values are then summed to produce an overall score “reflecting the adverse impacts from the species.” No rationale



is provided for the weighting, but in the “default” mode, each indicator is assigned equal emphasis. Modifications to the weighting scheme provide a mechanism to “represent scenarios in which certain aspects of the impacts or potential “invasiveness” might be emphasized.” An “Overall Risk Score” and “Overall Confidence Score” is presented in the final summary table for each species (EMEC 2007a). A manual accompanies the protocol providing rationale and guidance for each question, and for actions that should be taken when information is lacking or uncertain (EMEC 2007b).

EVALUATING WEED RISK ASSESSMENT SYSTEMS

The ability to predict with confidence that non-indigenous plants will become invasive in a new environment could yield enormous ecological and environmental benefits. Such predictions, however assume that information about the biology of new species, their capacity to survive and spread; and their actual impacts on organism, ecosystems, and economies are known, or can be reasonably estimated. These assumptions have plagued weed risk assessment protocols since their inception; and as the process escalates in biological complexity, technological innovation, and theoretical and statistical development, there is little evidence that verifiable predictions are at hand. For example, Simberloff (2005) offers “risk assessment procedures are narrowly focused, subjective, often arbitrary and unquantifiable, and subject to political interference.” In addition, there is widespread conviction that the “lack of broad scientific principles or reliable procedures” severely limits their ability to recognize the potential of invasive plants in new geographic environments (NRC 2002; Powell 2004; Simberloff 2005). Powell (2004) speculates, however, that the demand for these systems to contribute to international legal, policy, and political processes will require greater scientific certainty in the future. This will require a “balance between the demand for accuracy and precision,” and “the constraints of limited information, time and other resources” (Powell 2004). The National Research Council report, Predicting Invasions of Non-indigenous Plants and Plant Pests (NRC 2002), concluded that a scientifically based system to predict invasiveness must satisfy three general criteria:

• It must be transparent, be open to review, and have been evaluated by peers.

• It must have a logical framework that includes independent factors that are identified through critical observation, experimentation, or both; and that are important in the invasion process.



• Use of the framework must be repeatable and lead to the same outcome, regardless of who makes the predictions.

Some of the specific limitations of weed risk assessments are summarized below.

Variable Objectives

Beginning as a simple process to provide objective lists of weeds to assist managers in making informed decisions (Hiebert and Stubbendieck 1993), risk assessments have evolved over the last decade into a much larger process biologically, geographically, legally, and politically (NRC 2002; Powell 2004; Simberloff 2005). Legal, political and policy issues are now challenging the weed risk assessment process to extend beyond its traditional scope of identifying hazards, pathways, and the biological consequences of plant invasions (Rahman et al. 2003; Powell 2004; Simberloff 2005). The transition to more comprehensive analyses has not necessarily improved the process, and in some cases has made relatively simple procedures cumbersome, confusing, and appearing imprecise.

Questions and Content

The number, and the content, of questions contained in weed risk assessments continue to escalate. The modest number of questions (ranging from 20-24) found in early protocols has now surpassed 60 (Table 3; Table 5). While some commonality remains, there is considerable dissonance among protocols as counties, states (provinces), and regions attempt to modify existing procedures to include new objectives and address local conditions. The construction and clarity of questions varies considerably among protocols. Some protocols present clear, concise questions containing a single concept. Others submit questions that are more complex, contain multiple concepts and unnecessary “ecological” or “biological” jargon, and the linkage between the question and the prospective responses is not clear (Table 7). Such lack of clarity can lead to faulty conclusions, and reduce the accuracy and precision of the protocol.

Scientific Knowledge

In an evaluation of the risk assessment process in the public health sector, the US National Research Council stated “the basic problem with risk assessment is the sparseness and uncertainty of the scientific knowledge of health hazards addressed.



Table 7. Examples of questions and responses.

Question Possible Responses Comment

Number of seeds per plant. 1) Few (0-10); 2) Moderate (11-1000); 3) Many-seeded (> 1000).

Question and responses clear and concise. Response easily connected to question.

Is the production of offspring prolific and consistent? Guidance: Does the organism produce viable offspring? Does the organism have higher fecundity than other organisms its size?

1) Very few or none; 2) Few; 3) Moderate; 4) Many.

Question vague, guidance introduced additional concepts into the question (e.g. “viability” and “other organisms its size.” Not clear which part of the question and guidance the responses refers.

Re-organization of the risk assessment function will not create the data and underlying knowledge that assessors need to make risk assessments more precise” (NRC 1983). While these conclusions were directed to health, they apply equally well to risk assessment in the biological sciences. For example, Parker et al. (1999) concluded “Despite the considerable attention invasive plant species receive, our lamentable paucity of data on impacts leaves us largely ignorant about the ecological changes they have brought about.” Five years later, Powell (2004) reported “there are currently no known broad scientific principles or reliable procedures for identifying the invasive potential of plants… in new geographic ranges.”

Biological Characteristics

The recurrence of some invasive species in widely separated parts of the world elevates speculation that weeds possess traits that enhance their ability to flourish in new environments. Although the importance of traits as a main factor determining invasive potential holds considerable appeal in weed risk assessments, there are few data available that convincingly demonstrate their predictive capability (Reichard and Hamilton 1997; Parker et al. 1999). While Baker (1974) listed 11 botanical characteristics of the “ideal weed” (Table 3), no individual plant species has acquired all these traits (Radosevich and Holt 1984). Moreover, many species that possess these traits are not invasive (NRC 2002), while other non-native species, which are invasive, have few or none of these characteristics (Schierenbeck et al. 1994; Rejmanek and Richardson 1996).



Nonetheless, biological characteristics have been extensively used in weed risk assessments, and extrapolated to a wide range of habitats even though species with the same genetic makeup often demonstrate different phenotypic responses in new environments (Marsden-Jones and Turrill 1945; Radosevich 2005). Consequently, it is difficulty to predict the response of an introduced species based on biological characteristics without considering the biotic and abiotic conditions in a new environment. Unfortunately, environmental factors have only begun to make their way into weed risk protocols recently (Westbrooks and Madsen 2006). In many protocols, biological characteristics are evaluated as if the effects of perceived negative characteristics are cumulative, and progressively define one plant as more invasive than another. Williams (2003), however, suggests there may be no “suit of endogenous plant characteristics …that reliably predicts potential weediness.” Accordingly, the inability of “invasive” traits to predict actual invasions has created skepticism, and some doubt it will ever be possible to predict successfully which species will become invasive (Crawley 1987; Williamson and Fitter 1996; NRC 2002).

Ecological Impact

Ecological impact is the most pervasive criterion motivating weed ranking and weed risk assessments, and the need to manage invasive plants. Some typical criteria used to assess weed impacts among many (Table 3) include:

• Plant Community composition and structure;

• Ecosystem Processes including fire occurrence, erosion, sedimentation rates, hydrological regimes, nutrient cycling, among others;

• Higher Trophic Levels such as extirpation or endangerment of native plants and animals; and

• Genetic Integrity including production of fertile or sterile hybrids that can out compete or lower the reproductive potential of native species.

Although ordering invasive species is usually the primary objective of virtually all procedures, surprisingly little is known theoretically or operationally about which invasive species cause the greatest ecological impacts. Moreover, less is known about the relative impacts of an individual species from one ecological setting to another (Parker et al. 1999).



Such fundamental knowledge gaps make assessments of complex processes at best highly spurious, and at worst virtually impossible. Consequently, questions regarding impacts are of limited value in an objective analysis until more information is available on the actual effects of weeds on ecological structure and processes.

Subjectivity

Subjectivity is perhaps the most consistent criticism levied against protocols for weed ranking and weed risk assessments (Simberloff and Alexander 1998; NRC 2002; Powell 2004). These systems rely principally on observational data, or data that originate in different geographic locations and climates compared to the present distribution of the weed. Currently, there is no known way to collect, or introduce, the actual data that are required to conduct unbiased weed assessments using existing protocols (NRC 2002). “Measures of uncertainty,” and the option for the assessor to record that the answer is “unknown,” are common procedures used to offset the subjectivity introduced in these protocols (Hiebert et al. 1999; NRC.2002; Morse et al. 2004; DEFRA 2005). Unfortunately, these measures of uncertainty do not compensate for inadequate or incorrect information. “Expert” opinion is usually the other main source of information used to determine the invasive characteristics of weed species, and their anticipated environmental and economic impact in new environments (Hiebert and Stubbendieck 1993; NRC 2002; Powell 2004). Opinions of a weed’s performance in a new environment, however, often vary among professionals even when they have the same information (Simberloff and Alexander 1998; Parker et al. 1999). Although disagreement among experts is not unexpected (Parker et al. 1999), the prospect that different results and interpretations can be produced using the same protocol detracts from the aspiration of veracity and repeatability generally sought in assessment protocols (NRC 2002).

Scoring Procedures

Risk assessments have used several approaches to scoring weed threats and impacts. One technique simply sums the scores for each of the “risk elements” evaluated in the assessment that reflect impact. Relative weed risk potential is assigned in descending order of the highest score (Hiebert and Stubbendieck 1993; Hiebert et al. 1999; ANHP 2005). A second approach allocates weights to individual questions, or to each of the risk elements, to reflect their relative contribution to potential risk. The specific procedures



for weighting and for calculating a final score vary among protocols (Heffernan 2001; Morse et al. 2004). Other protocols use more complicated algorithms that involve summing scores, weighting risk elements, converting scores to probabilities, and combinations of these mathematical procedures (APCC 2002; DEFRA 2005; EMEC 2007a). In addition, uncertainty may be involved in some of the calculations but not others (DEFRA 2005; EMEC 2007a). Many risk assessments use slightly different characteristics and methods for determining probabilities or likelihoods, depending on the specific conditions where they are applied (DEFRA 2005; EMEC 2007a). These changes can be valid, though, if the ecological characteristics of the plant, the environmental features of the location, or both, justify modifying the procedure to accurately account for local conditions. Although the simple enumeration of scores for each question has been considered a weakness in some protocols, no analysis has been conducted to validate any algorithms used to classify invasive plants and their impacts (NRC 2002). Despite this, weed risk assessments are likely to continue in their present form until a less subjective and more quantitative system can be developed. In the interim, weed risk assessments must have a clear rationale for scoring if protocols are to gain credibility (NRC 2002). Risk assessments that are carefully documented and explain the logic of their rating systems are more likely to result in repeatable conclusions by different assessors, and allow public scrutiny and independent evaluation (NRC 2002).

Abiotic and Autecological Features

All plants, including weeds, are simultaneously affected by the amount of heat, light, moisture, and nutrients available to them (Daubenmire 1974). Consequently, their relative distribution on the landscape reflects the degree to which a particular environment can provide an optimum mix of these factors for individual plants to complete their life cycle, and for populations to grow and spread. From a predictive standpoint, the specific components driving weed populations cannot be evaluated individually. Consequently, indices that integrate these factors are sought to help predict where suitable habitats for weeds exist, and where other habitats limit invasive weed growth. So far, four inter-related components have been considered in risk assessment procedures: climate, habitat type, light, and disturbance. Climate. Although some non-native plants can adapt well to a variety of habitats, they usually are most successful within the climatic variation of their native range first



(Panetta and Mitchell 1991; Radosevich 2005). In the past, climate has not entered into weed risk assessments except in the most rudimentary manner (Heffernan et al. 2001; APCC 2002; ANHP 2005; EMEC 2007; Pheloung 2007). More recently, however, “climate matching” appears to be gaining interest and application in some of these assessments (Busby 1986; Panetta and Dodd 1987; Panetta and Mitchell 1991; Cramer and Solomon 1993; Lehtonen 2004; Randall and Stuart 2000; Williams 2003; Powell 2004; Westbrooks and Madsen 2006). Ideally, climate matching should compare the climatic conditions from the species’ native and extended weedy range to the climate in the present area where it grows or could infest. One of the limitations of climate matching, however, is the assumption that climate alone determines the distribution of invasive plants in new environments, even though other abiotic and biotic factors (including random dispersal) may be important (Sutherst and Maywald 1985; Mack 1995; Davis et al. 1998). Habitat Type. Several analyses have concluded that the most useful criterion for predicting whether an invasive plant will thrive in a new environment is the history of where it has succeeded elsewhere (Reichard and Hamilton 1997; Daehler and Carino 2000; Daehler et al. 2004). Such comparisons are usually expressed in terms of “ecosystem types” or “habitat types,” which are described in relatively board terms such as aquatic, riparian, grassland, open forest, closed forest, etc. Even though habitat matching was addressed in some of the procedures reviewed (Heffernan et al. 2001; NatureServe 2004; DEFRA 2005), it has generally received a low weighting for scoring. While climate and habitat matching may have limitations and be considered a first evaluation for screening new species (Kriticos and Randall 2001), there is growing agreement that both deserve greater emphasis in profiling invasive plants than most current weed risk assessment protocols assign now (NRC 2002). Recent protocols have given habitat matching a higher profile (Warner et al. 2003; Duncan 2005), and the Nature Conservancy suggests that all species proposed for introduction into the US should be screened based on invasiveness outside their native range (Randal and Gordon 2007). Requirement for Light. Shade can either promote or restrict the potential for an invasive plant to establish and flourish in new environments. Knowledge of the light requirements of weed species can be a powerful tool for predicting weed distribution in novel environments. So far, shade tolerance has not been given much consideration in most weed risk assessment protocols (Heffernan et al. 2001; DEFRA 2005; IAPP 2006; Pheloung 2007).



Disturbance. The importance of natural and anthropogenic disturbance in promoting the establishment and spread of weeds is generally well accepted. For example, Radosevich (2005) considers disturbance as a primary factor favoring plant invasions, while Prieur-Richand and Lavorel (2000), state that disturbance is “the only clear factor lending to a community’s invasibility.” Most weed risk protocols have concentrated on native plant communities in conservation lands and largely ignored disturbance as a factor promoting weed populations (Hiebert and Stubbendieck 1993; Heffernan, et al. 2001; Morse et al. 2004; ANHP 2005). Even on conservation lands, natural events such as fire, floods, and storms disturb soils, modify plant communities, and create temporary openings in forested canopies. These disturbances allow entry of invasive plants when light, moisture, heat, and competition are modified. Outside conservations lands (protected lands), forest harvesting, road construction, livestock and wildlife grazing, and recreation also create conditions that foster invasive plant dispersal into new areas (APPC 2002; Radosevich 2005).

Strengths of Weed Risk Assessments

Currently, the limitations of weed ranking and risk assessments appear to exceed the strengths and benefits. Most limiting is the extent to which they both depend on subjective decisions to make “scientific” predictions (Hiebert and Stubbendieck 1993; NRC 2002; Morse et al. 2004; Simberloff 2005). Nevertheless, weed risk assessments have value, provided that conclusions drawn from them are supported with relevant data, scientific literature, and they are peer-reviewed (NRC 2002). One of the most important benefits of these systems is that they provide a consistent and logical decision-making process for ranking exotic plants. Such an analytical framework encourages biologist and managers to consider the full range of factors and consequences of their decisions. If the procedure documents the methods used, rationale for decisions, and the assumptions made in conducting an assessment, then assessors can conduct evaluations that should be consistent and defendable within the bounds of the knowledge available and the assumptions made (Hiebert and Stubbendieck 1993; NRC 2002). Consistently using an analytical tool helps ensure that ecological knowledge contributes to the process, and can reduce the subjectivity often associated with decisions based on personal judgment and precedent alone. Consequently, the credibility of management decisions is enhanced when they are based on acceptable biological information (Hiebert



and Stubbendieck 1993). Additionally, the process of conducting risk assessments may be as valuable as the specific results they produce because they provide a medium for gathering and analyzing relevant information and knowledge about invasive plants, and their management (NRC 2002). Experience with weed ranking and weed risk assessments suggests that the number of weed species of potential concern is often reduced when assessments are conducted by experienced ecologists who have a sound understanding of weed ecology (Hiebert and Stubbendieck 1993). However, the results of these analyses should be considered as guidelines and not interpreted as definitive.

Summary and Conclusions

Weed Ranking Systems and Weed Risk Assessments are terms that are now commonplace in the invasive plant and weed management vernacular. Although these tools have similar origins and overlap extensively in objectives, methods, and scope of application, there are differences. The following summary identifies the most pertinent features of both tools (hereafter collectively called “weed assessment tools”) based on the survey of procedures listed above and additional readings.

• Weed Ranking Systems were developed in the mid-1980s as tools to discriminate among innocuous non-native plants and those that could threaten natural resource values primarily in US national parks.

• Weed Risk Assessments emerged in late-1990s, and blended weed ranking objectives with other legal, political, and policy issues involved with the prevention, introduction, spread, and control of exotic organisms at a national and international level.

• Numerous guidelines provide general direction on the content and methods for conducting weed risk assessments, but specific protocols vary considerably in scope and application.

• The focus of most weed assessment tools has concentrated on native plant communities and especially on conservation lands.

• Almost all weed assessment tools exclude agricultural lands and native plant communities outside conservation areas, where disturbance usually plays a significant role in promoting the introduction, establishment, and spread of invasive plants.

• Weed assessment tools combine professional (expert) opinion with “scientific” documentation to predict relative invasiveness among non-native plants based on a set of predetermined criteria.



• Weed assessment tools have been criticized as narrowly focused, subjective, often arbitrary and unquantifiable, and subject to political interference.

• There is concern that the ability of weed assessment tools to identify potential invasive plants in new environments is severely limited because they lack broad scientific principles and reliable procedures to make such determinations.

• Most weed assessment tools rely heavily on biological traits to determine “invasiveness” and to project “impacts.” The inability of biological traits to predict actual invasions has created skepticism, and some doubt whether they will ever successfully predict which species are invasive and where.

• Ecological impacts of weeds on plant community composition, structure, and ecological processes are the most common criteria used in weed ranking and risk assessments.

• Although ordering invasive species is essential for prioritizing management efforts, little is known about which species cause the greatest ecological impacts or the relative effects of the same species from one ecological setting to another. Consequently, questions regarding impacts are of limited value for objective analysis until more information is available on the actual effects of weeds on ecological structure and processes.

• Measures of uncertainty, and the option for assessors to record that answers are “unknown,” are common procedures used to compensate for the subjectivity introduced in these assessments.

• Climate and habitat factors have not been used extensively as predictors in weed assessments, although interest in “climate matching” is growing and some procedures contain questions regarding climate. Similarly, other habitat factors, such as shade tolerance and disturbance, have only begun to infiltrate weed risk protocols recently.

• Generally, people with good taxonomic and ecological skills can reduce the number of weed species of potential management concern when the criteria for weed ranking are rigorously applied. Training will be required to ensure consistent application of these tools.



POTENTIAL RANKING PROCEDURES

No weed ranking system presently exists in British Columbia that has widespread endorsement. Invasive plant management in BC, however, could benefit from a simple, objective, and scientifically-based process that evaluates the potential threat of individual weed species, and the susceptibility of ecologically diverse habitats. Such a tool should be provincial and regional in scope, but flexible enough to address local needs. A comprehensive approach to listing invasive species should recognize that weeds have variable potential in different environments, but effective management requires agreement among stakeholders in assembling provincial, regional, local and management mandated lists. Accord among participants fosters communication, cooperation and joint understanding of invasive plant issues and solutions.

Unique Features Affecting Weed Populations in BC

A weed ranking system for British Columbia must take into account the unique features of the province. British Columbia covers nearly 1 million square kilometers, extends over nearly 110 of latitude, and spans an elevational range from sea level to 5950 m on Mount Logan. This encompasses a region nearly equivalent to the combined area of Washington, Idaho, Oregon, and California; and a latitudinal variation similar to that from San Francisco to the BC border. As a result, the province has a diverse climate ranging from dry desert-like conditions in the southern interior, to Mediterranean and temperate forests on the coast, and to alpine and arctic climates at the highest elevations and far north. Plant communities reflect the diversity in climate, soils, and topography in British Columbia. Fourteen biogeoclimatic zones and over 75 subzones (Meidinger and Pojar 1991) have been classified in the province. Each of these can be further divided into smaller units and contain many seral vegetation types. This range in climate and vegetation types provides enormous opportunity to both accommodate invasive species in some biogeoclimatic units, and to restrict them in others. It should be recognized that ecological conditions in British Columbia will likely meet or exceed the ecological limits of many species of Mediterranean or Central Asia origin somewhere in the province. Although numerous biological factors contribute to the introduction and spread of invasive plants globally, forest canopy is one of the most pervasive influences controlling their distribution throughout most of British Columbia. The presence of climax forest stands, or seral communities dominated by deciduous trees and shrubs, restricts weed



introduction and growth for most species that are already in BC, or those that immigrate north from grassland communities in the United States. Conversely, the removal of canopies at any seral stage can provide favorable conditions for invasion and further spread, especially when soils have been disturbed when the canopy is removed. About 3% of the land base in British Columbia is agricultural lands (MOAL 2006), while 14% resides in provincial parks and protected areas (MOE 2006). Regional parks and conservations areas also contain other “natural areas.” The larger part of the provincial land base (>80%), however, remains forest land, rangeland, alpine, and urban areas; which fall under federal, provincial, corporate, and private jurisdictions. These areas are primarily managed as natural ecosystems used for a wide range of purposes such as forestry, mining, livestock and wildlife grazing, recreation, and other activities. Jurisdictions and land use practices often overlap, and are not equally distributed throughout the province. Therefore, each jurisdiction and land use represents a different opportunity or threat for weed invasion and control. To be effective, a weed ranking system must address species within all jurisdictions and disturbance regimes throughout the province. Although the potential for weed invasion may vary significantly between highly disturbed agricultural lands and minimally disturbed natural areas, a weed ranking system must take into account effects on all provincial lands including agricultural, natural areas, and integrated-use lands.

Review Objectives

In addition to the weed ranking and risk assessment systems reviewed, many more protocols are available world-wide that could be drawn upon in planning a weed ranking procedure for British Columbia (Thomas 1999; Tye 1999; Duncan 2005; Westbrooks and Madsen 2006; Pheloung 2007). It may be tempting to modify an existing system for immediate application, however, none of the protocols reviewed appear satisfactory for use in the province in their present format. Before adopting a particular approach, the objectives for a weed ranking system in BC requires considered contemplation. Such critical review is imperative given the wide range of products and approaches currently available, and the large variation in objectives, requirements, and criteria used in these tools. In particular, the objectives for a weed ranking system in BC should consider the following questions:

• What will the tool be used for? Is the tool intended for ranking weeds within an individual Ministry, for provincial scope weed ranking through the Invasive Plant Council, or is it intended as part of a risk assessment process for all invasive



organisms as suggested in the Invasive Alien Species Framework for BC (Rankin et al. 2004)?

• Is there a need for a comprehensive tool that embraces the ecological, physiographic and geographic variability in the province? Will it focus on protected and conservation lands or consider the broader perspective of all land susceptible to weed infestation in the province?

• Will the tool apply to the wide range of jurisdictions that must by considered in developing a comprehensive system that will achieve broad acceptance?

Potential Weed Rating System

The selection of criteria for weed ranking and risk assessments has gone through considerable vacillation over the last two decades as researchers and managers attempt to determine the best mix of biological, physical, and management factors to use in these assessments. Most previous protocols have emphasized criteria related to “invasiveness” and “impact,” which are usually unknown or highly speculative. In contrast, relatively less importance has been assigned to climatic and habitat factors, which can be more objectively defined, although these factors are becoming more prominent in new protocols. Three primary factors are suggested for a weed ranking system in BC: 1) Environmental Tolerance, 2) Abundance and Distribution, and 3) Management Feasibility (Table 8). Each primary criterion is broad in scope and implies a “potential effect” that is not measurable. Secondary criteria are indicators more directly related to weed biology, ecological adaptation, current and potential distribution, and likelihood of control. The veracity of any weed ranking system depends upon the quality of information available to assess the primary and secondary factors for each species. Therefore, the secondary criteria proposed have emphasized questions where actual data can be acquired or reasonable estimates can be made. Some of these criteria may require more precise definition and amplification if they are selected, and other criteria can be considered depending on the specific objectives and scope of the weed ranking system. General guidelines for selecting primary and secondary criteria for weed ranking are found in Table 9.



Table 8. Potential criteria for ranking invasive plants in British Columbia.

Environmental Tolerance 1 Adaptation of species to BC climates.

Range in climatic conditions the plant species is adapted to in its native range compared to local climates in British Columbia.

2 Adaptation of introduced species in novel environments to BC climates. Range in climatic regimes within the introduced range of non-native species outside BC compared to climatic profiles for actual or potential habitats occupied by the species within the province.

3 Proportion of province, region, local area with similar climate. The geographic area (hectares) of the province with a similar climate as the native or exotic range of the plant defines its potential distribution provincially, regionally or a locally.

4 Habitat suitable for supporting populations. This analysis entails matching ecological types in British Columbia to analogous types outside BC, and throughout regions of the province. Ecological matching is primarily based on biogeoclimatic or ecoregion classification. This analysis will require comparisons with other vegetation classification systems to make “relative” comparisons. Ecological types include forest, grassland, riparian, aquatic, alpine, etc.

Abundance and Distribution 5 Biological characteristics that promote introduction and establishment.

The most conspicuous trait that will limit weed infestation in BC on most forest range is shade tolerance. Other factors, however, that promotes dispersal and establishment to sites where light conditions are adequate may also be relevant.

6 Does the species occupy similar habitats in adjoining US states, Alberta or Yukon. The relative proximity of species already in the province, or species that inhabit similar habitats to those in BC outside the province, provide an estimate of potential for dispersal.

7 Current estimated area covered provincially, regionally. This question attempts to quantify in general terms the area presently covered by individual invasive plants (high, medium, low). Documentation of the area cover combined with the habitats used provides an index of plant success and relative threat to successfully establish in similar habitats in the province.

8 Recent trend in total area infested within the province. Estimated rate of spread provincially, regionally, locally based on observations (increasing, stable, declining).

9 Disturbance regime matching. Requirement for natural or anthropogenic disturbance for the species to establish and form persistent populations. Generally these relate to land use practices that result in various kinds, degrees, timing and frequency of disturbance.

Management Feasibility 10 Likelihood of control.

Relative ability to control invasive plants using cultural/mechanical, chemical or biological means within the context of integrated pest management principles. The effects of management on non-target organisms are also important, especially when weeds occur in natural environments.



Table 9. General guidelines for selecting weed ranking criteria.

1. All criteria should relate to clearly defined objectives for ranking weeds in British Columbia.

2. Criteria must be relevant to British Columbia conditions and management priorities (ecologically, economically, and socially).

3. Each criterion can be evaluated in an objective and unbiased manner (accuracy)? 4. Each criterion should be easy to measure or estimate; or factual information can be

acquired to provide an answer. 5. Repeated assessments (same person, other people) will produce the same result

within reasonable limits (precision). 6. Criteria should relate to legal requirements and government policy where appropriate

and necessary. 7. Each criterion should contribute (additive) to making management decisions. 8. All statements for each criterion should be clearly stated in simple, but accurate, text

with minimal extraneous jargon. 9. Where possible, each criterion should be presented unaccompanied by other criteria

that could result in contradictory or ambiguous answers for the same question. 10. The criteria should be flexible so they can be adapted to meet local or regional needs

as required.

Environmental Tolerance

Environmental tolerance endeavors to predict the response of new non-native plants into British Columbia by matching existing climates and habitats in the province with habitats where the species has succeeded in its native or introduced range elsewhere (Table 8). Such analyses can provide meaningful information on the potential range and distribution of invasive species within the province although they do not imply measures of relative “invasiveness” or “aggressiveness.” Climate matching could be achieved using computerized tools such as CLIMEX (Vickery 1974) or BIOCLIM (Bioclim 2007), or by using climate atlases such as Hardiness Zone Maps (USDA 1990; AAFC 2000). Other products may be available such as climate maps and long-term climate data from Environment Canada. Habitat suitability is the interaction between the abiotic components of the habitat and the biological characteristics of the plant. Habitat-matching can be accomplished by surveying literature to determine the native distribution of individual species (range in



latitude, longitude, and elevation), their non-native global distribution outside North America, and their current North American distribution. This information can be compared to the species’ current distribution in British Columbia or projected by comparing analogous habitats where the plant is found elsewhere with biogeoclimatic units in BC. These comparisons can provide a “suitability index” that indicates a “likelihood of invasiveness.” Actual impacts on habitats, native plants and animals; and rare and endangered species are unknown. These can only evaluated only by unquantifiable inference.

Abundance and Distribution

Abundance and distribution depicts the diversity of conditions under which a species can subsist (Table 8). Abundance generally refers to the size of a population and relates to ecological factors such as cover, density, and frequency. Distribution may be geographically oriented, or it may refer to the extent of infestation within various ecological types. Many biological characteristics are used in weed risk assessments to predict invasive potential. Shade intolerance , however, is likely the most important biological trait affecting the introduction, establish and spread of invasive plants in BC, where forest canopies play such an important ecological role in plant communities. Most invasive species presently in BC are best adapted to open conditions and are mostly shade intolerant. Therefore, those biogeoclimatic units with no canopy, or with an open forest canopy, are at greater risk of invasion than those that will attain a closed canopy. Consequently, weed adaptation to shade (shade tolerance) is an important biological factor that determines the ultimate range of a species. Disturbances such as logging, fire, and beetle kill can create artificial opening in biogeoclimatic units that would otherwise be forested. Under these conditions, the distribution and abundance of species, which were previously restricted by shade, can change dramatically as previously marginal habitats become suitable. Local experience provides ample information to predict shade tolerance of many non-native species that presently occur in the province. Other information can be obtained through literature and experience elsewhere can help predict response of new invaders. Confirmation of the presence of an invasive species and its estimated area provincially, and regionally, are the first steps in establishing its persistence and distribution in the province. As more inventory data become available, the distribution and the frequency of infestations can be determined on a local, regional and provincial basis. With GIS



analyses, these data can be superimposed on biogeoclimatic maps and climate maps (if they exist) to provide information on the geographic distribution of species, and the habitat and climatic regimes where the species occurs. Results from these analyses can help develop projections of possible infestations elsewhere. Disturbance is a primary factor favoring plant invasions in virtually all ecological systems (Radosevich and Holt 1984; Prieur-Richand and Lavorel 2000). Land use practices such as forest harvesting, road construction, livestock and wildlife grazing, agriculture, urban, and recreation remove forest or herbaceous cover and disturb soils, which permits invasive plants to disperse, establish, and form persistent populations in new areas. Land use and disturbance can be inferred from forest cover maps and possibly overlaid onto biogeoclimatic maps to determine cover and distribution of various land use types by biogeoclimatic unit. Plant response to disturbance and abundance under various land uses can potentially be mapped from inventory data and specific responses of plants determined from literature review. Not every weed will necessarily need to be cross referenced against each land use, but important land use activities that promote weed invasion can be identified for the most important species that are presently known.

Management Feasibility

Management feasibility evaluates the ability to prevent the introduction, establishment, and spread of invasive plants (Table 8). It also assesses the secondary impacts of treatments on non-target species and the physical environment, as well as the financial costs of associated with management of the species. Species that are difficult to manage are expected to have a greater potential to adversely affect other species, environments, and economies than those that can be easily controlled. Predictions on expected management feasibility can be determined from current experience locally, or for the species in similar habitats elsewhere. Literature already exists in British Columbia to provide as background on numerous species (OLA 2002), but information on other species can be gathered through literature review. The implications of management practices on non-target organisms such as native plants and red- and blue-listed species are also important to consider. Predictions and costs of the expected effects of treatment applications can be gathered from local experience, professional advice from neighboring states or provinces, or literature searches where relevant information is available.



Rationale for Provincial and Regional Ranking

A classification of invasive plants as of provincial, regional or local significance should follow a hierarchical order based primarily on climate, biogeoclimatic distribution, geographic distribution, and land use. Although the three categories are described separately, they are linked and overlap. Figure 1 provides a simple priority matrix to assist decision-making. Provincial Priority. A plant of provincial priority is one that is adapted to a broad range of climate and habitat conditions, and has the potential to establish and limit land use management objectives within the majority of the regional units designated. Provincial and regional invasive plant lists should be prepared that are consistent with provincial level government policies and responsibilities in various ministries.

High

Medium Priority Adapted to BC climates and wide range

of biogeoclimatic units. Presence confirmed and dispersal potential high.

Containment unlikely; biocontrol possible.

High Priority Adapted to BC climates and wide range

of biogeoclimatic units. Presence confirmed and dispersal potential high. Eradication and containment possible;

biocontrol likely.

Inva

sive

Pot

entia

l

Low Priority Poorly adapted to BC climates and few

biogeoclimatic units. Presence confirmed and dispersal potential low. Containment and biocontrol low or not

possible.

Medium Priority Marginally adapted to BC climates and

few biogeoclimatic units. Presence confirmed and dispersal potential low to moderate. Containment/biocontrol likely.

Low Manageability High

Figure 1. Potential priority matrix to rank invasive plants in British Columbia.1, 2

1. Invasive Potential. Ecological suitability to local climates and suitability to habitat conditions

in British Columbia. Habitat conditions are best described as biogeoclimatic units, likely at the subzone level.

2. Manageability. Expected responses to management based on local experience in British Columbia or experience from abroad in similar habitats and climates.



Regional Priority. A plant of regional priority is one that is adapted to a moderate range of climate and habitat conditions, and has the potential to establish and limit land use management objectives within most of the local sub-units designated within the region. These may be based on geographic designations (MOFR Districts, Electoral Areas, etc.), biogeoclimatic units within the region, or both. Local Priorities. A plant of local priority is one that is adapted to a narrow range of climate and habitat conditions that has the potential to establish and limit land use management priorities within particular biogeoclimatic units, plant community types (aquatic, riparian, grasslands), land use practices (range use, forestry, utility corridors, parks and protected land), and/or sensitive areas (sites for red- and blue-listed species). Lists at this level they should focus more on specific jurisdictional concerns and mandates within local ecological areas.

CONCLUSIONS AND RECOMMENDATIONS

No weed ranking system presently exists in British Columbia that is widely endorsed and systematically used. A simple, objective, and scientifically-based process for ranking invasive plants, and evaluating the relative susceptibility of ecologically diverse habitats, would be a valuable tool for management. Such a tool should be provincial and regional in scope, but flexible enough to address local needs. None of the weed ranking or risk assessment procedures reviewed, however, is adequate in its present format to meet the needs of British Columbia. The following recommendations are made for consideration in developing a weed ranking system for the province.

• A weed ranking system for British Columbia should account for the unique features of the province including its geographic size, and its diversity in climate, soils, and vegetation.

• A weed ranking system for BC should be cognizant that many species of Mediterranean or Central Asia origin will be at their northern limits of ecological tolerance somewhere in the province. Therefore, focus should be directed to discerning the relative susceptibility of habitats (biogeoclimatic units) to the introduction and spread of invasive species. Attention to biological traits should center on those characteristics that clearly promote or limit spread of weed species, such as shade tolerance.

• BC needs to be divided into “management zones” based on geographic separation. Ministry Forests and Range districts, or regional district electoral areas, are two possibilities of existing, recognized boundary designations in the province. These divisions provide a hierarchy of geographic areas of diminishing size from province, to regions and local areas. Divisions should be based on anticipated management authority so that geographic areas and management responsibilities



are linked. Whatever choices are made, there should be consistent application across the province.

• Criteria for land uses with different management priorities, legal responsibilities, and government policies need to be considered in setting priorities. Local concerns, and the mandated responsibilities of particular agencies, may over-ride geographically based priorities occasionally, and the flexibility to accommodate such possibilities needs to be provided for.

• Weed ranking needs to be linked to inventory and mapping so that weed lists can be updated at regular intervals based on actual data collected in BC.

• Generally, invasive plant lists should be regarded as guidelines rather than a definitive list of ranked species. Weed lists should be considered “living lists” with the expectation that they will modified as more knowledge is gained, and as provincial, regional, and local priorities change over time.



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Appendix 1. Glossary of terms.

Annual (plant). A plant species that lives for only one year or growing season. Biodiversity (Biological diversity). The diversity of plants, animals, and other living

organisms in all their forms and levels of organization, including genes, species, ecosystems, and the evolutionary and functional processes that link them.

Biogeoclimatic zone. A geographic area having similar patterns of energy flow, vegetation, and soil as a result of a broad, regional climate.

Biological control. The use of living organisms, such as predators, parasitoids, and pathogens, to control invasive plants.

Chemical control. The application of herbicides to control or eradicate plant species. Climate. The average weather conditions of a place over many years.

Community. Any group of organisms interacting among themselves.

Containment. An invasive plant practice that aims to geographically isolate infestations and prevent them from increasing beyond the edge of their current infestations.

Control. An invasive plant practice that aims to prevent seed production and recruitment of new plants within the target patch, and eventually reduce the area and density of the target plant over time. Control measures acknowledge that a low level of the invasive plant will likely persist after treatment.

Crown land. Land that is owned by the government of Canada or British Columbia. Cultural control. An invasive plant management practice that manipulates plant

populations by cultivation, pulling, cutting, or other hand-applied techniques. Dispersal. The scattering of seeds or spores of a plant to a new habitat.

Ecosystem. Organisms together with their physical environment, forming an interacting system, inhabiting an identifiable space.

Endangered species: Any indigenous species, or sub-species, threatened with imminent extinction throughout all, or most of its range.

Environment. The sum of all external conditions that affect an organism or community and influence its development or existence.

Eradication. Elimination of every individual plant of an invasive plant population, including all viable seeds, and vegetative propagules.

Establishment. The process of a plant species entering and reproducing in a new habitat at a sufficient level to ensure continued survival of subsequent generations without immigration of new plants from outside the habitat.

Habitat. The natural dwelling of a plant or animal, including all biotic, climatic, and edaphic factors affecting life.

Herbicide. A chemical that kills or regulates growth of plant species or groups of species.



Hybrid. A plant (or animal) that is the product of a cross between two genetically different plants.

Integrated pest management. An approach to invasive plant management that uses more than one control option including prevention, cultural, mechanical, chemical, and biological controls in an integrated program.

Invasion. The arrival of an organism in an area where it was not formerly represented. Invasive plant. A plant that is non-native to the ecosystem under consideration and

whose introduction causes, or is likely to cause, economic or environmental harm or harm to human health.

Landscape. The fundamental characteristics of a specific geographic area, including its biological composition and physical environment.

Mechanical control. Control of invasive plants by physical and mechanical means such as mowing, cultivation, chain sawing, and weed-whacking.

Native plant. A plant species that are part of the original flora of an area. Non-native. A species that is not native to the region in which it is found.

Non-target. Any plant that a management practice is not aimed at, but may accidentally be injured by the application.

Noxious weed. Any plant species so designated by the Weed Control Act of British Columbia.

Perennial. A plant species that lives for more than two years.

Plant community. An association of plant species growing together in different areas with similar site characteristics.

Prevention. All activities that interrupt the dispersal of new invasive plant species into a geographic area or specific location where they were not previously found.

Propagule. A plant part, such as a bud, tuber, root, or shoot, that can be detached and is able to grow in a new environment.

Range. The geographical area in which a plant or animal species normally lives or grows.

Risk. In species risk assessment, the probability that an adverse effect (injury, disease, or death) will occur under exposure to a specific agent.

Risk Assessment. A process that includes the identification, assessment, management and communication of the risks associated with a particular organism or function.

Species at risk. A species that is extirpated, endangered, threatened, or of special concern. Target species. Invasive plant(s) that are the subject of eradication, control or

containment. Weed Ranking System. A process to evaluate the relative threat of non-native plant

species on plants, animals and other resource values, and order species relative to their ability to be managed.



Weed Risk Assessment. A process for identifying and describing the ecological and economic risks associated with the introduction or transfers of non-indigenous species on native species, habitat, or economic values in new environments.

Weed. 1) A plant growing where it is not wanted, 2) A plant that interferes with management objectives for a given area of land at a given point in time.



Appendix 2. Exotic plant ranking system questions.

I. Significance of Impact A. Current Level of Impact

1. Distribution relative to disturbance regime (10 points) 2. Abundance

a. Number of populations (stands) (5 points) b. Arial extent of populations (5 points)

3. Effect on natural processes and character (15 points) 4. Significance of threat to park resources (10 points) 5. Level of visual impact to an ecologist (5 points)

B. Innate Ability of Species to Become a Pest 1. Ability to complete reproductive cycle in area of concern (5 points) 2. Mode of reproduction (5 points) 3. Vegetative reproduction (5 points) 4. Frequency of sexual reproduction for mature plant (5 points) 5. Number of seeds per plant (5 points) 6. Dispersal ability (5 points) 7. Germination requirements (5 points) 8. Competitive ability (5 points) 9. Known level of impact in natural areas (10 points)

II. Feasibility of Control or Management A. Abundance Within Park

1. Number of populations (stands) (5 points) 2. Aerial extent of populations (5 points)

B. Ease of Control 1. Seed banks (15 points) 2. Vegetative regeneration (10 points) 3. Level of effort required (15 points) 4. Abundance and proximity of propagules near park (15 points)

C. Side Effects of Chemical/Mechanical Control Measures (15 points) D. Effectiveness of Community Management (10 points) E. Biological Control (10 points) Urgency (High, Medium, Low)



Appendix 3. Northern Prairie alien plant ranking system questions.

Significance of Threat Impact (Site Characteristics)

1. Distribution relative to disturbance regime. ( 10 points) 2. Arial extent of populations (answer in per cent or hectares). (15 points) 3. Numerical dominance of species within a community. (15 points) 4. Association with native community. (10 points) 5. Hybridization with native species. (5 points) 6. Degree of threat and impact. (10 points) 7. Effects on management goals. (10 points)

Innate Ability to be a Pest (Species Characteristics) 8. Mode of reproduction. (4 points) 9. Vegetative reproduction. (4 points) 10. Frequency of sexual reproduction for mature plant. (5 points) 11. Number of seeds per plant. (5 points) 12. Dispersal ability. (5 points) 13. Germination requirements. (4 points) 14. Seed banks. (10 points) 15. Competitive ability. (4 points) 16. Ecological effects (select all that apply). (4 points) 17. Known level of impact in natural areas. (10 points)

Difficulty of Control 18. Likelihood of successful control. (10 points) 19. Saturation of surrounding region. (5 points) 20. Effectiveness of community management. (10 points) 21. Vegetative regeneration. (10 points) 22. Biological control. (10 points) 23. Side effects of control measures. (5 points)

Total Points (175)



Appendix 4. Virginia ranking invasive exotic plant species questions and scoring.

I. Impact On Native Species, Habitats and Ecosystem 1. Ability to Invade Natural Systems

2. Impact on Ecosystem Processes

3. Impact on Natural Community Structure

4. Impact on Natural Community Composition

5. Conservation Significance of the Natural Area(s) and Native Species Threatened

II. Biological Characteristics and Dispersal Ability 6. Biological Characteristics 7. Other Regions Invaded

8. Speed of Spread

9. Current Trend in Total Range Within United States

10. Potential to be Spread by Human Activity

III. Distribution and Abundance In Virginia and the United States 11. Approximate Number of Distinct Natural Areas or Other Wildlands Infested

12. Extent of the Species US Range Where it has been Identified as a Problem

13. Potential Cover of the Species in Strata Where it Occurs

IV. Management Potential 14. Difficulty to Control Species



Appendix 5. Australian weed assessment guide.

I. Invasiveness 1. What is the Weed’s Ability to Establish Amongst Existing Plants (3 points)

2. What is the Weed’s Tolerance to Average Weed Management Practices in the Land use (3 points)

3. What is the Reproductive Ability of the Weed in the Land use (3 points)

4. How Likely is Long-distance Dispersal (>100m) by Natural Means (3 points)

5. How Likely Long-distance Dispersal (>100m) by Human Means (3 points)

II. Impacts 6. Does the Weed Reduce the Establishment of Desired Plants (3 points) 7. Does the Weed Reduce the Yield or Amount of Desired Vegetation (4 Points)

8. Does the Weed Reduce the Quality of Products or Services Obtained from the Land use? (3 points)

9. Does the Weed Restrict the Physical Movement of People, Animals, Vehicles, Machinery and/or Water? (3 points)

10. Does the Weed Affect the health of Animals and/or People? (3 points)

11. Does the Weed Have Major, Positive or Negative Effects on Environmental Health? (3 points)

III. Potential Distribution 12. What Area of the Land use is Suitable for the Weed? (10 points)



Appendix 6. NatureServe questions and scoring.

Screening Questions 1. Is this a non-native species in the region of interest and is it currently established outside cultivation? If yes, proceed to question 2; if no Stop. 2. Does this species occur in conservation areas or other native species habitats or is it highly probable that it will do so? If yes, proceed to criteria below; if no Stop, rank as Insignificant.

I. Ecological Impact (5 questions; 50% of I-Rank Score) 1. Impact on Ecosystem Processes and System-Wide Parameters (33 points)

2. Impact on Ecological Community Structure (18 points)

3. Impact on Ecological Community Composition (18 points)

4. Impact on Individual Native Plant or Animal Species (9 points)

5. Conservation Significance of the Communities and Native Species Threatened (24 points)

II. Current Distribution and Abundance (4 questions; 25% of I-Rank Score) 6. Current Range Size in Region (15 points)

7. Proportion of Current Range Where Species Is Negatively Impacting Biodiversity (15 points)

8. Proportion of Region’s Biogeographic Units Invaded (3 points)

9. Diversity of Habitats or Ecological Systems Invaded in Region (3 points)

III. Trend in Distribution and Abundance (7 questions; 15% of I-Rank Score) 10. Current Trend in Total Range Within Region (18 points)

11. Proportion of Potential Range Currently Occupied (3 points)

12. Long-Distance Dispersal Potential Within Region (9 points)

13. Local Range Expansion or Change in Abundance (18 points)

14. Inherent Ability to Invade Conservation Areas and Other Native Species Habitats (6 points)

15. Similar Habitats Invaded Elsewhere (9 points)

16. Reproductive Characteristics (9 points)

IV. Management Difficulty (4 questions; 10% of I-Rank Score) 17. General Management Difficulty (18 points)

18. Minimum Time Commitment (15 points)

19. Impacts of Management on Native Species (15 points)

20. Accessibility of Invaded Areas (3 points)



Appendix 7. Alaska weed ranking project questions and scoring.

A. Climatic Comparison 1. This species is present, or may potentially establish, in the following eco-geographic regions:

1) South Coastal; 2) Interior-Boreal; 3) Arctic-Alpine. 2. This species is unlikely to establish in any region in Alaska. B. Invasiveness Ranking 1. Ecological Impact (40%) 1. Impact on Ecosystem Processes (10 points) 2. Impact on Natural Community Structure (10 points) 3. Impact on Natural Community Composition (10 points) 4. Impact on Higher Trophic Levels (cumulative impact on animals, fungi, microbes, and other

organisms in the community (10 points)

2. Biological Characteristics and Dispersal Ability (25%) 5. Mode of Reproduction (3 points) 6. Innate Potential for Long-Distance Dispersal (3 points) 7. Potential to be Spread by Human Activities (3 points) 8. Allelopathic (2 points) 9. Competitive Ability (3 points) 10. Forms Dense Thickets, Climbing or Smothering Growth Habit, or Otherwise Taller Than the

Surrounding Vegetation (2 points) 11. Germination Requirements (3 points) 12. Other Species in the Genus Invasive in Alaska or Elsewhere (3 points) 13. Aquatic, Wetland, or Riparian Species (3 points)

3. Ecological Amplitude and Distribution (25%) 14. Is the Species Highly Domesticated or a Weed of Agriculture? (4 points) 15. Known Level of Impact in Natural Areas (6 points) 16. Role of Anthropogenic and Natural Disturbance in Establishing (5 points) 17. Current Global Distribution (5 points) 18. Extent of the Species in US Range and/or Occurrence of State or Provincial Listing (5 points)

4. Feasibility of Control (10%) 19. Seed Banks (3 points) 20. Vegetative Regeneration (3 points) 21. Level of Effort Required (4 points)



Appendix 8. IAPP species prioritization algorithm.

Note to User: Only input information into yellow fields to maintain integrity of embedded formulas. This ensures the auto-population of an Overall Score.

Criteria Subtotal Score Prorate Factor Prorated Score

Biological 3.8 x 1.3 5.0 Impact 5.0 x 2.0 10.0 Controllability 5.0 x 1.5 7.5 Containability 5.0 x 1.5 7.5 Grand Subtotal 30.0 Overall Score 10.0

Common Name:

Scientific Name:

Biological Criteria

Precede down this list answering each Yes/No question. If the answer is Yes to any question, then move the appropriate points for that question over to the right and into the Score column. The Subtotal will be calculated automatically.

Points Score

Is this species: New to the area (e.g. Within the last 25 years) and suited to thrive in the local climate? 5 5

Adapted to thrive in an aquatic, riparian or sensitive ecosystem? 5 5 Tolerant a wide range of soil conditions? 3 3 Able to live in excess of 5 years? 4 4 An annual or biennial with high reproductive success? 4 4 Able to quickly dominate a site without disturbance and form a dense monoculture? Or 5 5

Capable of slow domination of a site, but in patches without disturbance? Shade tolerant at some stage of its life cycle? 3 3 Geophytic? (it has underground storage organs - bulbs, corms, or tubers) 3 3 A producer of seed whose viability exceeds 5 years? (if unknown, then score 2) 5 5 Capable of hybridizing naturally? 5 5 Apomictic or self-fertilizing? 5 5



Able to reproduce by vegetatively, by stolons, rhizomes, bulbils or other asexual means? 5 5

Seed, spore or cuttings adapted to dispersal by birds/animals, water or wind? 4 4 A prolific seed producer @ 5,000 or more seeds per plant? Or 5 5 A moderate producer of seed @ 1,000 to 5,000 seeds per plant? Capable of setting seed after its first year? 4 4 Stimulated by mutilation, cultivation, or fire? 3 3 A plant woody (including stems or roots)? 5 5

Subtotal: 3.8

Impact Criteria

Select the single most applicable impact and place the associated points into the bottom Subtotal cell. Points Score

Does this species negatively affect: Human health and/or safety? 5 5

- Lethal or toxic to ingest - Causes pain or discomfort - Obstructs visibility along transportation routes - Degrades or poses risk to public infrastructure e.g. Fire hazard

Animal health (domestic or wild)? 4 - Lethal or toxic to ingest - Causes pain or discomfort

Natural or agricultural environments? 4 - Taints forage crop or is unpalatable - Reduces crop yields - Increased erosion or restricted water flow in riparian areas

Native plant communities? 3 - Alleopathic (alters soil chemistry preventing other plants to establish) - Smothers or climbs on native plants - Reduces biodiversity

Recreation, and/or human or animal migration? 3 - Impenetrable thickets - Turf grass invasion

Landscape aesthetics? 2 - Visual blight

Subtotal: 5.0



Controllability Criteria

Select the single most relevant scenario and place the associated points into the bottom Subtotal cell. Points Score

Is this species: Extremely difficult to control? 5

- Few if any treatment options and extremely expensive 5.0 - Multiple re-treatments are necessary

Difficult to control? 4 - Options are available but expensive - Re-treatments are necessary

Moderately difficult to control? 3 - Multiple options are available but less expensive - Re-treatment(s) may not be necessary

Easy to kill but re-treatments are necessary? 2 - Garden escapees, annuals

Subtotal: 5.0

Containability Criteria

Select the single most relevant scenario and place the associated points into the bottom Subtotal cell. Points Score

Is this species found in: Isolated areas? 5

- Good chance of containment given that species is confined to only a few areas with jurisdiction 5.0

- A newly arrived species Moderate distribution? 4

- Moderate change of containment given that species is moderately distributed throughout jurisdiction

And throughout the subject area (e.g. Is ubiquitous)? 2 - Poor chance of containment given that species is found throughout jurisdiction

- A species with a long history in the area Subtotal: 5.0



Appendix 9. Alberta invasive species risk assessment tool questions and scoring.

BIOLOGICAL CHARACTERISTICS Dispersal Ability

1. Does the organism possess biological attributes that contribute to long-distance dispersal? 2. What is the rate of dispersal once the species is released or disperses into a new area?

Reproduction Characteristics 3. Is asexual reproduction (e.g. vegetative reproduction or self-fertilization) an important aspect of

this organism’s reproduction? 4. What is the frequency of sexual reproduction? 5. Is there rapid growth to reproductive maturity? 6. Is the production of offspring prolific and consistent? 7. Are the organism’s specific requirements for reproduction available in Alberta? 8. Does the opportunity to hybridize naturally with native species in Alberta exist? 9. Are there known natural predators or natural control agents in Alberta?

Habitat Suitability 10. How many natural sub-regions or watersheds have a climate suitable for the organism? 11. How many natural sub-regions have a physical habitat suitable for the organism’s

establishment? 12. Are the biological requirements of the organism available in Alberta?

INTRODUCTION Introduction Potential

13. Is this species established in Alberta or present in areas adjacent to Alberta? 14. Do anthropogenic mechanisms for the introduction, re-introduction, or dispersal of this

organism exist? 15. How great is the potential for introduction, re-introduction, or dispersal from human

pathways? 16. How likely is the organism to survive in transit? 17. What is the probability of detecting the organism at the entry point? 18. If introduced, how many individuals are likely to be released?

ENVIRONMENTAL IMPACT Effect On Environment

19. Is the invasive species known to compete with native species or species at risk for resources? 20. Is the organism a parasite or predator of native species, species at risk, or species of

management concern? 21. Is the species a host or vector for known diseases, parasites, or pests? 22. What is the potential impact on ecosystem processes?

ECONOMIC IMPACT General Effects

23. What is the potential impact to infrastructure of economic importance? Effect On Agriculture

24. What is the potential risk to agricultural operations? Effect On Forestry



25. What is the potential risk to forestry operations? Effect On Animal Health

26. What is the potential risk to the health of animal species of economic importance? SOCIAL IMPACT Effect On Human Health And Well-Being

27. What is the likelihood of severe health risks after association with this species? 28. What is the impact on aesthetic values and social convenience?

Effect On Culture, Including Community And Traditional Knowledge 29. What is the potential of an invasive species to reduce local biodiversity (i.e. native plants)

valued by local communities or aboriginal communities? 30. What is the potential risk to culturally valued spaces and features?

CONTROL MANAGEMENT 31. Are there potential side effects of control methods to other species? 32. What level of effort is required for the control of this organism?

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