Responses of Invasive Species to A Changing Climate And ...€¦ · climate. Greenhouse gas...

Fifty Years of Invasion Ecology: The Legacy of Charles Elton, 1st edition. Edited by David M. Richardson© 2011 by Blackwell Publishing Ltd

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Chapter 26

Responses of Invasive Species to A Changing Climate And Atmosphere Jeffrey S. Dukes Department of Forestry and Natural Resources and Department of Biological Sciences, Purdue University, West Lafayette, IN 47907 - 2061, USA

346 New directions and technologies, new challenges

The changing atmosphere directly affects species in many ways. Plants, which depend on CO 2 for growth, respond most directly to the change. Marine organisms face new challenges as atmospheric CO 2 dissolves in the ocean and causes it to become more acidic. And, because the additional CO 2 changes the climate by trapping more infrared radiation in Earth ’ s atmos-phere, it indirectly affects the environment for all species. None of these issues would have concerned Elton as he wrote his book, but they concern virtually all ecologists today.

Already, researchers have documented shifts in fl owering dates for plants, developmental milestones and migration dates for animals, and the distributions of many taxa in terrestrial and aquatic environments (Parmesan 2006 ). The directions of these shifts are heavily biased towards the directions expected under recently observed regional changes in climate (Parmesan 2007 ; Thomas 2010 ). Some taxa have shifted phenology faster than others, but across taxa, spring phenology has advanced by approximately 2.8 days per decade in the Northern hemisphere (Parmesan 2007 ), consistent with trends expected in a warming climate.

Greenhouse gas releases have been accelerating, and by the end of the century, we expect Earth ’ s envi-ronment to be quite different than it is now. How dif-ferent will depend heavily on societal decisions, but most future scenarios anticipate CO 2 to surpass 600

26.1 INTRODUCTION: A RAPIDLY CHANGING ENVIRONMENT

In 1958, when Charles Elton published The Ecology of Invasions by Animals and Plants , an atmospheric chemist named Charles Keeling was busy establishing a research outpost atop Mauna Loa, on the island of Hawaii. In March of that year, he began the fi rst highly accurate series of measurements of atmospheric carbon dioxide, a series that continues to this day. At the time, little was known about how CO 2 concentra-tions might change from month to month, let alone from year to year. The idea that humans could dra-matically infl uence the composition of the atmosphere seemed far - fetched, and was rarely mentioned in the scientifi c community, let alone among the general public.

Now, more than 50 years later, the data collected by Keeling ’ s outpost have produced a compelling illustra-tion of our society ’ s power to change the environment. By 2009, CO 2 concentrations at Mauna Loa were 23% above those of Keeling ’ s fi rst measurement, and 39% above those of pre - industrial times (Fig. 26.1 ), with concentrations increasing more rapidly every year. Researchers around the world have warned that this trend will increasingly affect the climate, and govern-ments around the world are wrestling over how best to reduce CO 2 emissions from fossil - fuel burning and deforestation.

Fig. 26.1 Atmospheric CO 2 concentrations as measured by the station established by Charles Keeling in 1958 atop Mauna Loa. Based on data from Keeling et al. (2009) .

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Invasive species and rapid climate change 347

interactions. There remains little basis for predicting the net outcome of changes in these indirect responses.

Over the past two decades, invasion biology and climate science have reached the point where condi-tional predictions can be made that help inform man-agement decisions. Managers, landowners and policy makers must already make planning decisions in the face of an uncertain future, whether the uncertainty is due to economic, political or other factors. Imperfect predictions based on available knowledge, taken with caveats, can help inform decisions with long - term implications and focus attention on potential new problems or opportunities. Indeed, in the face of uncer-tain climate change impacts, resource managers are already acting, for instance to remove barriers to dis-persal in stream habitat in Oregon, USA, allowing native fi sh access to a wider variety of microsites and stream temperatures (Lawler et al. 2010 ).

26.3 INVASIVE SPECIES ’ RESPONSES TO CLIMATE CHANGE

The fi rst discussions of how invasive species might respond to environmental changes (see, for example, Cronk 1995 ; Huenneke 1997 ; Dukes & Mooney 1999 ) had relatively few studies on which to build conclu-sions. Since then, the topic has received increasing attention, particularly in the form of studies examining potential future distributions of species. A variety of recent publications review potential responses of inva-sive species to climate change in particular (see, for example, Thuiller et al. 2007 ; Hellmann et al. 2008 ; Rahel & Olden 2008 ; Walther et al. 2009; Bradley et al. 2010 ; Ziska & Dukes 2011 ). In 2009, Australia ’ s Invasive Species Council began publishing an eBulletin called Double Trouble to focus specifi cally on potential interactions between climate change and invasive species.

Hellmann et al. (2008) highlighted fi ve ways in which invasive species will respond to climate change. These are as follows. 1 Species will be introduced through new pathways. Pathways of introduction will change as humans try to mitigate or adapt to the changing climate, whether that involves planting new species for bioenergy use (Raghu et al. 2006 ; Barney & DiTomaso 2008 ), taking advantage of newly thawed Arctic waterways for transporting goods (Pyke et al. 2008 ), importing new species for ornamental use or intentionally moving

parts per million by the end of the century, and envi-sion Earth ’ s average surface temperature rising by 1.1 to 6.4 ° C by 2100 (Meehl et al. 2007 ). The Earth system models that produce these numbers also foresee greater warming over land than over the oceans, and much greater warming as one approaches the higher latitudes in the Northern hemisphere (see Plate 8). Precipitation patterns are also likely to change, with some of the wetter regions of the world receiving more winter precipitation, some of the drier regions of the world becoming drier still, and droughts and deluges becoming more frequent everywhere (see Plate 8). Fossil fuel burning and intensive agriculture also lead to increases in nitrogen deposition in many ecosys-tems, and increases in low - level ozone.

This cabal of environmental changes will contribute to shifts in species distributions and competitive inter-actions in ecosystems throughout the world. The results of these shifts, though, will not be simple to predict.

26.2 CLOUDS IN THE CRYSTAL BALL: WHAT CAN BE PREDICTED?

Many types of uncertainty make it challenging to predict responses of ecosystems and communities to environmental change: uncertainty about the rate and nature of the environmental changes (which stem from smaller uncertainties in the science and larger uncertainties about society ’ s future path), uncertainty about how a specifi c ecosystem functions and how individual species respond to those changes, and uncertainty about the relationships among species within communities, just to name a few (Dukes et al. 2009 ). Although this uncertainty should make a reader sceptical of the precision of any single prediction for the future, an understanding of the scope of likely changes is valuable for predicting ecological impacts. The scientifi c community has become increasingly confi dent in the direction of most environmental changes, even if the rates of some are still uncertain. At the same time, a growing body of experimental research has illuminated the more basic responses of organisms to environmental changes in many simpli-fi ed ecosystems. Direct responses of many species to climate change can be inferred from elements of their distribution across the landscape. However, species will also respond indirectly to climate change, for instance as climate affects the strength of interspecifi c


survive at and carry avian malaria to higher altitudes, causing problems for native birds whose last remain-ing habitats are on the mountain - tops (Benning et al. 2002 ). Warming may also lead to the retreat of ranges along the warmer edges, through direct physiological responses and/or changes in interactions among species. Changes in precipitation patterns will similarly affect distributions of some terrestrial species. Species ’ responses can be estimated through climate envelope - type (niche) models, and are further discussed below, in section 26.4 . 4 Impacts of existing invasives will change. Impacts of invasive species are likely to change for a variety of reasons, although predicting the nature of these changes will not always be straightforward. Impacts are generally thought of as the product of the range of the invasive species, the per - capita effect of the species on the process or property of interest, and the abundance of the species within its range (Parker et al. 1999 ). With environmental change, this impact will also depend on any direct response of the process or property to that change. For instance, in South Africa, invasions of woody Acacia and Pinus species into upland watersheds have decreased streamfl ow levels (Le Maitre et al. 2000 ; Dye & Jarmain 2004 ). Because water is scarce in this region, loss of any water from the streams is a major concern. If less pre-cipitation fell in this region as a consequence of climate change, the relative impact of these invasive species would increase, because each drop of water would be more valuable (Fig. 26.2 ). Abundances and range sizes of many invasive (and native) species are likely to change in the future (see below), but evidence for changes in species ’ per - capita impacts is less clear. In

species of concern to new locations (Richardson et al. 2009 ). 2 Newly introduced species will experience different biotic and abiotic conditions, affecting the probability that they will become established in the new region. Climate change could alter the probability that a newly arrived species could establish a self - sustaining popula-tion through two primary mechanisms. First, it could alter the climatic suitability for the introduced species. Second, it could affect the physiology of resident species directly, potentially altering their competitive abilities or fecundity. Unless native communities rapidly reor-ganize, and species evolve in response to climate change, existing communities may become more sus-ceptible to invasion by species that arrive pre - adapted to the new conditions. Dukes & Mooney (1999) pre-dicted that ecosystems dominated by long - lived organ-isms with long juvenile periods and little long - distance seed dispersal will experience the greatest increases in invasibility, because populations of those species will likely be slowest to adapt to the new conditions. 3 Distributions of existing invasives will shift. As envi-ronmental conditions change, species distributions will respond. A warming of 3 ° C, well within the end - of - century projections from the IPCC (2007) , corre-sponds to isothermal shifts of approximately 250 km in latitude and 500 m in elevation (Gates 1990 ). Depending on the topography, mean temperatures are expected to migrate horizontally across Earth ’ s land surface at rates of 0.11 to 1.46 km per year (Loarie et al. 2009 ). Warming will allow many species to extend their ranges on the poleward or uphill sides, where they are currently cold - limited. For instance, in Hawaii, warming may allow invasive mosquitoes to

Fig. 26.2 Mechanisms through which climate change could affect the impact of invasive species. Figure adapted from Dukes (2011) .

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Europe (see, for example, Beerling et al. 1995 ), North America (see, for example, Zavaleta & Royval 2002 ; Peterson et al. 2003, 2008 ; Roura - Pascual et al. 2004 ; Bradley et al. 2009 ; Jarnevich & Stohlgren 2009 ) and Africa (see, for example, Richardson et al. 2000 ; Parker - Allie et al. 2009 ). Most of these studies base predictions purely on climatic tolerances, and predictions range from dramatic range increases to range decreases, depending on the species and the study. Richardson et al. (2000) found that including soil type in their analysis restricted the potential change in distribution for all fi ve of the invasive plant species they examined, and climate change was pre-dicted to reduce the total area of suitable habitat for all of these.

Similar studies have also examined invasive animals, such as cane toads ( Bufo marinus ) in Australia (Sutherst et al. 1995 ) and gypsy moth ( Lymantria dispar ) in New Zealand (Pitt et al. 2007 ). These climate envelope - type studies currently provide the best mech-anism for estimating future species ranges, but they have several limitations. For instance, current ranges may not have climatic limits that are relevant for use in predictions. The native ranges of species often occupy more restricted climatic zones than those occu-pied in the introduced region (Sutherst et al. 1995 ; Beaumont et al. 2009 ), and populations in the intro-duced region may not have had the opportunity to spread to the limits of their climatic tolerances. In addi-tion, models that have a coarse spatial resolution omit topographic variation, and thus underestimate cli-matic and environmental heterogeneity. Grid cells classifi ed as climatically unsuitable for a species may actually have patches of suitable climate within them (Luoto & Heikkinen 2008 ; Randin et al. 2009 ). These factors could lead to an underestimation of the future range of the species. Other factors could lead to over-estimation of ranges; limits imposed by competition with other species, by a lack of mutualists or by a dependence on non - climatic environmental condi-tions such as a specifi c soil type could restrict ranges to areas much smaller than the zones that models identify as cli matically suitable. Beyond this, other environmen tal changes such as increasing CO 2 con-centrations may alter the climatic tolerances of a species in ways that are currently impossible to predict without experimentation.

For plants in particular, in situ experimental manip-ulations can provide important indications of how

a fi eld experiment in Montana (USA) grassland, Maron and Marler (2008) found little evidence for changes in competitive effects of invasive plant species on natives under increased precipitation. On the other hand, many studies have shown that the outcome of com-petition can depend on environmental conditions (see, for example, Patterson 1995 ). For instance, in the Rocky Mountains of North America, native bull trout ( Salvelinus confl uentus ) outcompete non - native brook trout ( Salvelinus fontinalis ) in cooler streams, but warmer water would favour the brook trout (Rieman et al. 1997 ). 5 The effectiveness of strategies for managing and controlling invasive species will change. Environmental changes will alter the effectiveness of management and control strategies by affecting the physiology of inva-sive species and biocontrol species. Physiological responses of weed species, for instance, can alter their response to herbicides. In an agricultural fi eld, Ziska et al. (2004) found that Canada thistle ( Cirsium arvense ) rebounded from herbicide treatment more rapidly under elevated CO 2 than under ambient CO 2 , likely because it had allocated more resources to below - ground structures that were then less completely killed by the glyphosate. The effectiveness of biocontrol tech-niques also depends on environmental conditions. If distributions of biocontrol species respond differently to climate change than those of their target species, the area in which the biocontrol organism achieves its intended effect will shift.

Much of the research on these issues so far has com-prised niche modelling studies and experimental manipulations. I explore results from these studies in more depth below, in sections 26.4 – 26.6 . The subse-quent sections concern feedbacks of invasive species to climate change, consequences of societal responses to climate change, future advances and challenges in the fi eld.

26.4 THE ART OF PREDICTION: HOW WILL CLIMATE CHANGE AFFECT INVASIVE SPECIES ’ DISTRIBUTIONS?

Many studies have tried to predict future distributions of invasive species based on current species ranges, taking a variety of approaches. Niche modelling has been used to estimate future ranges of invasive plants in Australia (see, for example, Kriticos et al. 2003 ),


ability of resident communities to exploit available resources, then the invasibility of those communities would increase. It seems likely that invasibility of some plant communities, such as those dominated by a few longer - lived species with similar geographic distribu-tions, might increase substantially as the dominants must increasingly tolerate conditions in which they did not evolve. In contrast, invasibility of communities dominated by a large mixture of species with a range of lifespans and largely different distributions might change very little, as shifts in dominance among the resident species could prevent any reduction in resource use by the community. There is some indica-tion that plants entering new ranges experience less impact from pathogens and herbivores than do the resident species, suggesting a potential mechanism for accelerated establishment of new species, and transi-tions from one vegetation type to another (Engelkes et al. 2008 ). Climate change seems unlikely to decrease invasibility, except perhaps in extreme environments where conditions become even more extreme, such as deserts that become increasingly water limited. There, the chances of better - adapted species arriving seem slim. Effects of climate change on invasibility of animal communities will similarly depend on the capacity of the extant organisms to tolerate or adapt to new conditions.

Although there is little experimental evidence to describe how climate change may affect the invasibil-ity of communities, there is increasing evidence that, at least for plants, invasive species have a variety of traits that will provide advantages in a changing climate. For instance, within a given community, invasive species are likely to tolerate a wider range of climates than natives, and thus may experience less of a disadvantage. Studies have shown that invasive plant species tolerate a wider range of climates than their non - invasive relatives (see, for example, Rejm á nek & Richardson 1996 ; Qian & Ricklefs 2006 ) or other, non - invasive resident species (Goodwin et al. 1999 ). Also, invasive plants in at least one community have adjusted their phenologies to ‘ keep up ’ with climate change much more rapidly than competing native plants (Willis et al. 2010 ). Invasive species have also demonstrated the potential to spread rapidly through the current landscape, either aided or unhin-dered by roads and human land use. Not all native species will disperse so easily across the patches of fragmented habitat that exist in many parts of the world.

invasive species may perform in future environments, and could provide useful tests of niche model projec-tions. However, these experiments are expensive, and therefore rare. So far, experimental manipulations have investigated the effects of CO 2 , climate change (both warming and precipitation changes) and nutri-ent deposition on the success of invasive species. These experiments have tested how the abundance of a species may change within its range, as opposed to examining how the range itself may shift. Manipulations that increase resource availability, for instance by adding precipitation (or nutrients; see below), often increase the success of invasive plant species (Dukes & Mooney 1999 ; Scherer - Lorenzen et al. 2007 ; Blumenthal et al. 2008 ). Manipulations of CO 2 and temperature are still too rare to make con-fi dent generalizations, but many CO 2 experiments have found positive responses of invasive plant species, at least in some years (see, for example, Smith et al. 2000 ; and see below). Climatic variability studies are also rare; White et al. (2001) investigated the effect of extreme heating and rainfall events on community invasibility, and found that C 4 plant species may invade C 3 grasslands more successfully as climatic variability increases and climatic extremes become more frequent.

Several observational studies and experiments have examined how the abundance or growth of invasive animal species changes in response to variation in tem-perature or moisture availability (see, for example, Stachowicz et al. 2002 ; Chown et al. 2007 ; Saunders & Metaxas 2007 ; Heller et al. 2008 ; Weitere et al. 2009 ). As with plants, results are largely species spe-cifi c, although Stachowicz et al. (2002) found increas-ing recruitment of the three most abundant non - native sea squirts in years with warmer winter waters off the eastern coast of North America. Again, though, few (if any) experiments have examined how climate changes will affect invasive species ’ distributions.

For the most part, our understanding of how inter-actions with resident species constrain (or expand) the large - scale distribution of invasive species is poor, and so it is diffi cult to estimate how climate change might alter these constraints. Few studies have examined how climate change alters community invasibility. Results from several experiments suggest that commu-nities suppress establishment success and growth of some invading species (native and non - native) through resource pre - emption (see, for example, Dukes 2001 ; Hooper & Dukes 2010 ). If climate change reduces the


Field experiments with invasive plant species grown in competitive environments provide the most realis-tic simulations of future environments. Those con-ducted so far have found that some invasive species respond quite strongly to elevated CO 2 , particularly when other constraints on the species ’ growth are relaxed (see, for example, Smith et al. 2000 ; Belote et al. 2003 ; H ä ttenschwiler & K ö rner 2003 ; Williams et al. 2007 ). For instance, the annual grass Bromus madritensis ssp. rubens responded much more posi-tively to elevated CO 2 than native annual species in a very wet year in North America ’ s Mojave Desert (Smith et al. 2000 ). However, in drier years, Bromus failed to germinate or failed to survive and was unaf-fected by CO 2 . Although very few species have been studied in realistic environments, it appears that several invasive species are likely to preferentially benefi t from the increase in CO 2 , at least in some years or some conditions (Table 26.1 ).

Although the main byproduct of fossil fuel burning is CO 2 , reactive nitrogen molecules are also released during combustion. This nitrogen, along with nitrogen released from agricultural fi elds, gets deposited on land and in the sea, fertilizing many natural systems. Although this fertilization may have little effect on the abundance of invasive plant species in regions where the native fl ora already contains many responsive plant species, there are many regions of the world in which the native fl ora evolved under nutrient - poor conditions. Several lines of evidence suggest that nitro-gen deposition increases the prevalence of invasive plants in these regions. Scherer - Lorenzen et al. (2007) recently reviewed this evidence in detail.

26.5 LESSONS FROM EXPERIMENTAL MANIPULATIONS: INVASIVE SPECIES ’ RESPONSES TO A CHANGING ATMOSPHERE

It has long been recognized that the increasing atmos-pheric CO 2 concentration could alter the balance of competition among plants (with implications for the effects of weed species on crop yield, for instance (Patterson 1995 ), or for competition between C 3 and C 4 plants). Many experiments have examined responses of single plants or single species growing in pots to elevated CO 2 . Dukes (2000) suggested that such exper-iments, collectively, did not indicate that increasing CO 2 favours invasive species over non - invasive species. However, some individual studies of this type have found that invasive species could preferentially benefi t. For instance, Song et al. (2009) grew three plant species that are invasive in China along with three similar, but native, co - occurring species in growth chambers at two CO 2 concentrations, and reported stronger biomass responses in the invasives. Ziska (2003) found strong responses of six of North America ’ s more pernicious weed species to both current (relative to pre - industrial) and future CO 2 concentrations.

More realistic tests of this question have featured competition between the invasive species and a native community. Two pot experiments (Bradford et al. 2007 ; Dukes 2002 ) collectively show little effect of CO 2 on the overall impact of invasive plants. However, competition experiments in pots still fall short of pro-viding a realistic environment from which to draw strong conclusions.

Table 26.1 Results from fi eld experiments testing the response of invasive species to elevated CO 2 in a community context.

Species Community Response Reference

Bromus madritensis Mojave Desert + /0 Smith et al. 2000 Lonicera japonica Tennessee forest understorey + /0 Belote et al. 2003 Microstegium vimineum Tennessee forest understorey − /0 Belote et al. 2003 Prunus laurocerasus Swiss forest understorey + Hattenschwiler & Korner 2003 Hypochaeris radicata Tasmanian grassland 0 * * Williams et al. 2007 Leontodon taraxacoides Tasmanian grassland 0 Williams et al. 2007 Centaurea solstitialis California valley grassland + J.S. Dukes et al., unpublished data

+ , Signifi cantly positive effect of CO 2 on biomass production in some years of study; 0, no signifi cant effect of CO 2 on production or population growth rate in some years of study; − , marginally signifi cant negative effect of CO 2 on produc-tion in one year of study; * * , a marginally signifi cant CO 2 - driven increase in population growth rate under ambient temperatures was offset by warming.


ceptible to invasion by more productive plant species that gain footholds in disturbed microsites. For taxa other than plants, other measures of departures from historical conditions might be more appropriate.

As discussed above, several studies have used niche models to project changes in invasive species ’ distribu-tions under climate change. However, these models do not incorporate other environmental changes, such as increases in CO 2 and nitrogen deposition. Mechanistic models that simulate a variety of species - level and environmental processes could be used to explore the impacts of a variety of environmental changes, but these models are more diffi cult to parameterize. Incorporating effects of CO 2 , nitrogen and other factors into niche models would not be straightforward, but is worth attempting, considering the evidence that these factors can play important roles in species ’ success. Such modifi ed niche models could then be used to esti-mate, at least roughly, the importance of various inter-actions among environmental change factors for invasive species ’ success.

26.7 FEEDBACKS FROM INVASIVE SPECIES TO CLIMATE CHANGE

Some plant and animal species, including some inva-sives, strongly infl uence disturbance regimes and the cycling of elements in ways that tangibly affect the composition of the atmosphere or the energy balance of the landscape (Dukes & Mooney 2004 ). Thus some invasive species that respond to an environmental change can create weak to moderate feedbacks to the rate of that environmental change. Species most likely to create perceptible feedbacks are those that dramati-cally alter the landscape over large regions. Examples include cheatgrass ( Bromus tectorum ) and some insect pests.

Cheatgrass has replaced native shrubland systems in a large region of western North America, altering carbon storage and energy balance in this area. This annual grass, whose tinder allows fi res to spread widely, now dominates at least 20,000 km 2 of the Great Basin (Bradley & Mustard 2005 ), and has the potential to spread across 150,000 km 2 (Suring et al. 2005 ; Wisdom et al. 2005 ). Fire - aided conversion of sagebrush shrubland to cheatgrass grassland has transformed parts of western North America from a carbon sink to a carbon source (Bradley & Mustard

26.6 INTERACTING ENVIRONMENTAL CHANGES

The future environment will differ from today ’ s in pat-terns of human land use, temperature and precipita-tion, CO 2 , nitrogen deposition and runoff, ozone pollution, ocean acidity and many other respects. We might expect some aspects of these environmental changes to counteract each other in some locations. For instance, plants growing under elevated CO 2 often close their stomata partly, leading to water savings. The additional water savings could at least partly counteract increases in water stress due to warming or drought. With so many different environmental changes happening at once, the potential for such interactions is great, and predicting the most conse-quential interactions presents a challenge.

So far, relatively few studies have examined invasive species ’ responses to more than one environmental change at a time. Data from Zavaleta et al. (2003) , Williams et al. (2007) and Blumenthal et al. (2008) suggest interactions may occasionally be observed, but the frequency with which they will be important is unclear. In North American mixed grass prairie, Blumenthal et al. (2008) found that the success of three invasive forbs depended on addition of supple-mental snow. When snow was present, one of the species also responded to nitrogen addition. Results such as this suggest that interactions among environ-mental factors may be less important than the ‘ sum ’ of environmental changes acting on a site. Although the increase in CO 2 will be relatively uniform worldwide, sites that experience dramatic changes in climate and other factors such as nitrogen deposition will move farther from the conditions in which local species evolved than sites in which climatic changes are more subdued and changes in other factors are minimal. I hypothesize that the sites in which environmental con-ditions depart the farthest from their historical mean will experience the greatest increases in invasibility, as the native species will no longer be well adapted to local conditions. It is unclear how best to measure departures of conditions from a historical mean across multiple factors. An integrating and quantifi able measure in terrestrial plant communities might be aboveground biomass production. Sites in which biomass declines sharply might become more suscepti-ble to invasion of stress - tolerant plant species, whereas those in which biomass increases sharply may be sus-


or adapt to environmental changes will alter the success of invasive species. Examples may include direct introductions, such as those of potentially inva-sive biofuel species such as Arundo donax and Miscanthus × giganteus in the USA (Raghu et al. 2006 ), or of ornamental species that might not have been invasive in a previous climate. We may also have new opportunities to purge invasive species from portions of their ranges that become less hospitable (Bradley et al. 2009 ). In other cases, threats from invasive species and climate change may lead environmental managers to implement ‘ managed relocation ’ of some native species (Richardson et al. 2009 ). Will any of these species, now transported by humans to a new location, become invasive pests? Similarly, climate change will allow some native species to spread unaided from their historical range into contiguous regions with newly suitable climates. Will these species be considered ‘ alien ’ in these new locations (Walther et al. 2009 )? Questions such as these, which were rarely discussed before this century, are rapidly becom-ing more common.

26.9 LOOKING FORWARD

Governments and managers already wage a continu-ous, demanding battle to intercept, detect and control invasive species. Unfortunately, other global environ-mental changes complicate this effort. To this point, relatively little guidance has been provided to the inva-sive species community as to how to prepare for or deal with new challenges arising from other environmental changes. It seems unlikely that shifting environmental conditions will require dramatic changes in invasive species detection or management practices. However, longer - term planning, improved databases, and new predictive tools, such as improved and user - friendly niche models could help prepare managers for likely challenges to come. Resources dedicated to such efforts and tools are likely to provide benefi ts in any future scenario.

Many researchers are currently working to develop expanded and improved invasive species databases, and to develop useful predictions of invasive species ’ distributions. A major obstacle to the improvement of predictions is the insularity of data within region - , discipline - or taxon - specifi c databases, databases that only consider invasive (or native) species, and

2006 ; Prater et al. 2006 ), releasing about 8 Tg of carbon so far (Bradley et al. 2006 ). The invasion also alters albedo (J.S. Dukes, unpublished data) and eva-potranspiration (Prater & DeLucia 2006 ; Prater et al. 2006 ) across the landscape, with implications for regional climate. Bradley et al. (2009) suggest that the area suitable for cheatgrass may decline under some projected temperature and precipitation regimes. On the other hand, growth chamber studies suggest cheatgrass may have responded positively to increas-ing atmospheric CO 2 (Ziska et al. 2005 ), and will con-tinue to respond in the future (Smith et al. 1987 ; Ziska et al. 2005 ). Further acceleration of cheatgrass growth would lead to more rapid and greater fi ne fuel accumu-lation in the region, likely facilitating more frequent and hotter fi res, and greater cheatgrass dominance.

Pathogens and insect pests can also produce feed-backs. Probably the most important feedback mecha-nism occurs when environmental changes facilitate larger outbreaks of these organisms in forests, leading to widespread tree death. Cold winter temperatures are thought to restrict the range of hemlock woolly adelgid ( Adelgis tsugae ), a non - native pest responsible for the decline of eastern hemlock ( Tsuga canadensis ) in eastern North America. The insect ’ s range could expand rapidly northwards into remaining hemlock stands as annual minimum temperatures rise (Dukes et al. 2009 ). A similar phenomenon appears to have occurred with the mountain pine beetle ( Dendroctonus ponderosae ), a native insect pest in pine forests of western North America (Kurz et al. 2008 ). Recent warming has led to more severe outbreaks of mountain pine beetle, and tree deaths from the latest outbreak will release an estimated 270 Mt of carbon over a 20 - year period, accelerating climate change. Mountain pine beetle has the potential to create even stronger feedbacks if its range expands to new regions of North America, as would be expected under continued warming.

26.8 THE HUMAN ELEMENT: SOCIETAL RESPONSES TO GLOBAL ENVIRONMENTAL CHANGE, AND CONSEQUENCES FOR INVASIVE SPECIES

We, as a species, will respond to environmental changes ourselves. Some of our attempts to mitigate


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databases that incorporate climatic or environmental but not biological information. To provide the greatest benefi ts, data in consolidated, global biological and environmental information systems should be inte-grated with improved niche models and climate projec-tions in a user - friendly, preferably web - based tool (Lee et al. 2008 ). Such a major integration project would face several challenging obstacles, and would likely take decades to develop. However, such a system would provide valuable information and predictions for managers around the world.

In The Ecology of Invasions by Animals and Plants , Elton (1958) wrote ‘ … provided the native species have their place, I see no reason why the reconstitu-tion of communities to make them rich and interesting and stable should not include a careful selection of exotic forms, especially as many of these are in any case going to arrive in due course and occupy some niche. ’ Elton clearly recognized that the challenge posed by invasive species was too great to expect to maintain pristine communities. Before we embark as a society on major campaigns to protect ecosystems from new threats from invasive species, we have the responsibility to ask, is it worth it? Are the benefi ts of any current or future reduction in invasive species ’ prevalence worth the cost of combating them? Which invasives should we battle, and what is the right amount of money to spend on keeping them in check? At the heart, these are political and philosophical questions, but they are questions that can only be addressed with the help of a strong scientifi c base of knowledge. Our understanding of biological invasions has grown rapidly in the past half century, and par-ticularly in the past three decades, at about the same time that our awareness and understanding of global environmental changes has blossomed. Although we will never fully describe the ecological, evolutionary or economic consequences of invasive species, many of their consequences are sobering. Other environ-mental changes have similarly profound conse-quences. Recent research suggests that in some cases the changes will amplify each other (e.g. through feed-backs), or amplify each other ’ s consequences (e.g. when climate change and an invasive species both decrease availability of the same goods or ecosystem services). Further research will clarify how frequently this is likely to happen. In the meantime, to the extent possible, it will be useful to identify policy and manage-ment options that can deal with these problems simul-taneously (Pyke et al. 2008 ).


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