Chapter 3: Corridors - An Overview

Natural Resources Conservation Service (NRCS)

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Figure 3-1: The three elements of landscape structure - patch, corridor, and matrix - are clearly evident in this photograph.

INTRODUCTIONLandscape ecologists Forman and Godron suggestthat a landscape is a heterogeneous land areaconsisting of three fundamental elements: patches,corridors, and a matrix (Figure 3-1). They define eachelement as follows:

Patches, corridors, and the matrix interact inecologically significant ways. Consequently, thisconceptual model is very useful in the study offunction, structure, change, and the conservationpotential of corridors in the landscape.

Patch: Generally a plant and animal communitythat is surrounded by areas with differentcommunity structure; however, a patch may bedevoid of life.

Corridor: A linear patch that differs from itssurroundings.

Matrix: The background within which patchesand corridors exist (the matrix defines the flow ofenergy, matter, and organisms).

TYPES OF CORRIDORS

Corridors can be natural (a tree lined stream channel)or the result of human disturbance to the backgroundmatrix (a strip of native prairie left unplowed betweentwo fields). Corridor structure may be very narrow(line) such as a hedgerow, wider than a line (strip)such as a multi-row windbreak, or streamsidevegetation (riparian). Corridors may be convex, tallerthan the surrounding matrix like a shelterbelt betweenwheat fields; or concave, lower than the surroundingvegetation, such as a grass strip between twowoodlots. Line or strip structure may be found inmany different kinds of corridors. Five commonlyused categories of corridor origin are:

· Environmental corridors

· Remnant corridors

· Introduced corridors

· Disturbance corridors

· Regenerated corridors

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In recent years, engineered corridors such asoverpasses and underpasses have been designedspecifically to accommodate wildlife movement.

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EnvironmentalCorridorsEnvironmental corridors arethe result of vegetationresponse to an en-vironmental resource suchas a stream, soil type, orgeologic formation. Theyare typically winding

(curvilinear) in configuration with widths that are highlyvariable. Sinuous strands of riparian vegetationparalleling stream courses are prominent examplesin all regions of the country (Figure 3-2).Environmental corridors are frequently the mostimportant habitats in the watershed.

Remnant CorridorsRemnant corridors are themost obvious products ofdisturbance to the adjacentmatrix (Figure 3-3). Stripsof vegetation on sites toosteep, rocky, or wet to putinto production are left asremnants after land iscleared for agriculture or

other uses. Some remnants are line corridors left toidentify property boundaries. The width andconfiguration of most remnant corridors varyconsiderably. Remnant corridors often contain thelast assemblages of native flora and fauna in awatershed.

Introduced CorridorsIntroduced (planted)corridors date back to circa5000 BC. More corridorsmay have been plantedbetween the 14th and 19thcenturies in England than atany other time or place inhistory. Under the Statuteof Merton, 1236, landlords

were granted the right to enclose portions ofwoodland and pasture. Over the next 500 years,thousands of miles of hedgerows were planted.Some of these hedgerows persist to this day andare valued as national landscape treasures. In theUnited States the Shelterbelt Project of the 1930swas the largest conservation project of theDepression Era; over 200 million seedlings wereplanted into shelterbelts and many were maintainedby Civilian Conservation Corps (CCC) work crews(Figure 3-4). In agriculturally dominated landscapes,introduced corridors have become critical habitat formany wildlife species.

DisturbanceCorridorsDisturbance corridors areproduced by land manage-ment activities that disturbvegetation in a line or strip;a mowed roadside or brush-hogged powerline right-of-way are examples (Figure 3-

5). Continued disturbance of the strip is oftenrequired to maintain vegetation in the desiredsuccessional stage. The widths of disturbancecorridors vary, but they tend to be more strip-like.Configuration is typically straight line. They may besufficiently wide to constitute a barrier for somewildlife species, splitting a population into twometapopulations. Disturbance corridors are oftenimportant habitats for native species that require earlysuccessional habitat.

RegeneratedCorridorsRegenerated corridorsresult when regrowth occursin a disturbed line or strip(Figure 3-6). Regrowth maybe the product of naturalsuccession or revegetationvia planting. Regrowth in

abandoned roadways, trails, and railroad right-of-ways are examples. Corridor width and configurationare dependent upon the nature of the previousdisturbance. Regenerated corridor vegetation is oftendominated by aggressive weedy species during theearly stages of succession. East of the MississippiRiver, regenerated corridors occur as hedgerowsalong fence lines and roadside ditches. They areless common in the West. In highly fragmentedlandscapes, regenerated corridors are oftenimportant habitats for small mammals and songbirds.

CORRIDOR FUNCTION

Corridors perform important ecological functionsincluding:

· Habitat

· Conduit

· Filter/barrier

· Sink

· Source

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These five functions operate simultaneously, fluctuatewith changes in seasons and weather and changeover time. Their interactions are often complex andin many cases are not well understood.

HabitatA corridor may function ashabitat or a component ofhabitat, particularly for thosespecies with small home

ranges and limited mobility, ruffed grouse (Bonasaumbellus) for example. For some species, largemammals for instance, a corridor may serve astransitional habitat during seasonal migrationsbetween patches. The habitat function of corridorsis discussed in greater detail in Chapter Four.

ConduitA corridor functions as aconduit when it conveys

energy, water, nutrients, genes, seeds, organisms,and other elements. Biologist Michael Soule hasidentified three general categories of animal needfor the conduit function of corridors:

· Periodic migration to breeding or birthingsites; elk migration from wintering habitat tocalving grounds, for example.

· Movement between patches within theanimals home range to access food, cover, orother resources.

· Some populations must receiveimmigrants if they are to persist in isolatedpatches; for example, male cougars migratingfrom one metapopulation to another to breed.

Filter/BarrierA corridor functions as a filteror barrier when it interceptswind, wind blown part-icles,surface/subsurface

water, nutrients, genes, and animals. Corridors mayfilter out sediments and agricultural chemicals fromrunoff that originates in the adjacent matrix. Theymay also act as barriers that reduce wind velocityand decrease erosion. Some artificial corridors likehighways and canals are barriers to wildlifemovement and may genetically isolate populations.

SinkA corridor functions as a sinkwhen it receives and retains(at least temporarily) objectsand substances that originate

in the matrix; soil, water, agricultural chemicals,seeds, and animals for example. Corridors canbecome sinks for wildlife, when the rate of mortalityin the corridor from predation and other causescreates a net loss in the population of either corridorresidents or migrant species.

SourceA corridor functions as asource when it releasesobjects and substances intothe adjacent matrix.Corridors may be sources of

weeds and �pest� species of wildlife. They may alsobe sources of predatory insects and insect eatingbirds that keep crop pests in check. High qualitycorridors are often a source of wildlife; reproductionin the corridor exceeds mortality and individuals areadded to the population.

CORRIDOR STRUCTURE

The physical and biological characteristics ofcorridors such as width, connectivity, plantcommunity, structure (architecture), edge to interiorratio, length, and configuration determine howcorridors function (Figure 3-7). Corridor width,connectivity, and plant community architecture areboth ecologically and visually the most important ofthese characteristics.

Low High Complex Simple High Low

EDGE TO INTERIOR RATIO PLANT COMMUNITYSTRUCTURE

CONNECTIVITY

Figure 3-7

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All five corridor functions are enhanced by increasedwidth and connectivity. Corridors with the fewestnumber of gaps have the highest levels ofconnectivity. As gap width increases, the number ofwildlife species for which the corridor functions as aconduit decreases. Biologist Michael Souleemphasizes the importance of connectivity formaintaining wildlife population viability in highlydeveloped landscapes. Ecologist Richard Formansuggests that there is value in maintaining severalparallel connecting corridors or patch �steppingstones� between large patches. Some ecologistscaution that corridors can also be conduits fordiseases, predators, exotic species, and fire whichcan threaten populations. However, corridors remainamong the best options for maintaining biodiversityin agricultural landscapes.

The vertical and horizontal structural characteristicsof vegetation within a corridor, its architecture, alsoinfluence ecological function. The vegetativestructure of corridors may vary from a single layer ina grassed waterway to four or more layers in aremnant woodlot or riparian corridor. Verticalstructure is a particularly important habitatcharacteristic for some species of birds. Horizontalstructure within corridors also varies. Patchiness (the

Figure 3-8: The overstory, middlestory, and understory vegetationin this woodlot, its plant community architecture, provide a varietyof niches for wildlife.

density of patches of all types) is most common inremnant and riparian corridors. Researchers reporta direct correlation between an increase in plantspacing heterogeneity and an increase in bird speciesdiversity. In general, the greater the structuraldiversity within a corridor, the greater the habitat valuefor an array of species (Figure 3-8).

CHANGE

Plant communities change over time. Corridorstypically have fewer plant species than larger patchesbut species diversity appears to increase with corridorage. Disturbance and consequent succession arethe principal agents of change in corridor vegetation.Disturbance may be natural, wildfire for example, orinduced by land management activities in or adjacentto the corridor such as mowing or grazing. Becausemost corridors have a high edge to interior ratio theyare particularly prone to the effects of disturbance inthe adjoining matrix. Human-induced disturbancehas the potential to push corridor vegetation beyondthe point where it can recover through naturalprocesses. This may lead to degradation of thecorridor ecosystem and a successional path thatdiffers significantly from the norm.

Changes in plant community function and structureas a result of plant succession have significant effectson wildlife. Both species composition and densitymay be altered. However, mature corridors, with theexception of riparian corridors, seldom achieve thewildlife species diversity of large patches.

Wildlife biologists have advocated managingsuccessional change in corridors to meet a varietyof outcomes. Sensitivity to biodiversity is growing,however, even in situations driven by single speciesmanagement.

Changes in plant community structure caused bydisturbance or succession also affect other corridorfunctions. For example, windbreak efficienciesdecline dramatically when the shrub layer is removed,a common occurrence when livestock are allowedto graze unmanaged in windbreaks.

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EXPANDING PERSPECTIVE

NRCS project-scale conservation practicescapitalized on the function and structure of corridors.Windbreaks, grassed waterways, field borders andother conservation practices, functioning as filters,barriers, and sinks, have reduced soil erosion,improved water quality and increased crop andlivestock production. Both native and introducedplants and wildlife have been the indirect beneficiariesof the habitats created by these practices.

Conservation corridors planned specifically for wildlifehave tremendous potential to preserve and enhancebiodiversity at a landscape scale. Land managersnow realize that by emphasizing wildlife planning atthese larger scales they can:

Maintain within the landscape or watersheddiverse self-sustaining wildlife populations of bothnative and introduced species at population levelsin harmony with the resource base and local socialand economic values.

WHAT IS THE CURRENT STATUS OF

CORRIDORS?The limited information on the quantity and quality ofthe nation�s corridors suggests:

· A decline in the number, length, and areaof some types of corridors.

· A significant degradation of the functionand structure of many types of corridors,especially stream/riparian corridors.

· A general reduction in the value of corridorsfor human use and environmental services.

In 1992, the National Research Council completedan extensive study of aquatic ecosystems includingstream corridors. They concluded that the functionand structure of many stream/riparian corridors havebeen substantially altered and their ecological integritycompromised. Agricultural chemicals, feedloteffluent, urban runoff, and municipal sewagedischarge were noted as major causes of waterquality degradation. Increased sediment loading fromurbanization, agriculture, grazing, and forestry andthe construction of dams, channelization and waterdiversions have further compounded the problem.

In addition, the separation of many floodplains fromtheir stream channels by levees, filling and channelentrenchment have disrupted natural cycles of plantsuccession (Figure 3-9). These stresses havereduced the value of many corridors for wildlife habitatand for recreation and other human activities. Theyhave also eliminated or greatly curtailed theenvironmental services normally associated withriparian corridors; particularly flood management,pollution abatement, groundwater recharge, andfloodwater dispersal.

Figure 3-9: This entrenched stream will no longer support theriparian vegetation (wildlife habitat) that lines its upper banks.

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There are an estimated 3.2 million miles of rivers inthe United States, yet only 2% of these meet therigorous criteria for designation as a Wild and ScenicRiver. An estimated 75% of the nation�s streamsare degraded to levels where they can only supporta low level fishery; only 5% of the streams support afishery of high quality. A 1995 National BiologicalSurvey report stated that 85 to 95% of southwesternriparian forests have disappeared since theSpaniards first settled the area (Figure 3-10a). Thelost scenic values and recreation opportunities arestriking. However, these habitats can respond wellto proper land management (Figure 3-10b).

Researchers conducting the NRCS Natural ResourceInventory (NRI) estimated there were approximately160,000 miles of windbreaks in 1982. By 1992, thefigure had decreased to roughly 150,000 miles, areduction of over 6%. During that same 10 yearperiod, the area in windbreaks was also reduced byan estimated 6%. Of equal concern is the decline inwindbreak quality, the result of old age, neglect, andpoor management practices. Grazing, herbicidedamage, and excessive competition from introducedgrasses in shelterbelts can contribute to degradation.Degraded shelterbelts are less efficient as filters,barriers, sediment traps, nutrient sinks, and as habitatfor wildlife.

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Figure 3-10a: This riparian corridor is in poor condition due toimproper grazing management.

In addition to riparian buffers and windbreaks, theNRCS and others have long advocated the use ofother types of conservation corridors including:contour buffers, filter strips, field borders, and grassedwaterways. No national database is kept on thesecorridor types. However, based on a survey of NRCSState and field biologists in each region, a roughestimate of conditions and trends was made.

Questionnaires were sent to NRCS State and fieldbiologists in each of the 50 states. Thirty usablequestionnaires were returned; a return rate of 60%.At least three questionnaires were returned from eachof the six NRCS regions. The results presentedbelow estimate the general status of the nation�scorridors.

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Figure 3-10b: This photo depicts the same view of the ripariancorridor after 10 years of proper grazing management.

Type Increased Same Decreased NA N

Riparian/stream corridors on 1st & 2nd order streams 4 9 16 0 29

Riparian/stream corridors on 3rd and higher order streams 4 13 13 0 30

Wetland, lake, and reservoir buffers 6 9 13 0 28

Field borders 7 3 18 2 30

Field buffers (in field) 11 10 7 2 30

Filter strips 21 4 5 0 30

Grassed waterways 18 11 1 0 30

Vegetated ditches 4 13 11 2 30

Grassed terraces and diversions 9 10 5 3 27

Windbreaks/shelterbelts 7 9 5 8 29

Hedgerows 1 8 16 3 29

Other (Please specify)

Type Increased Same Decreased NA N

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Table 1: Estimated change in various conservation corridor types from 1988 - 1998. Data indicate the numbers of states responding.

NA - Not ApplicableN - Total Number of States Responding

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0 5 10 15 20 25 30

Number of states responding

Riparian/Stream corridors on 1st and 2nd order streams

Riparian/Stream corridors on 3rd and higher order streams

Wetland, lake, and reservoir buffers

Field borders

Field buffers (in field)

Filter strips

Grassed waterways

Vegetated ditches

Grassed terraces and diversions

Windbreaks/shelterbelts

Hedgerows

Table 4: Ranking of the overall importance of various corridor types for conservation of soil, water, air, plants, and wildlife.

RELATIVE IMPORTANCE

Type Excellent Good Fair Poor NA N

Riparian/stream corridors on 1st & 2nd order streams 2 10 11 6 0 29

Riparian/stream corridors on 3rd and higher order streams 2 8 13 7 0 30

Wetland, lake, and reservoir buffers 2 10 12 6 0 30

Field borders 0 5 12 13 0 30

Field buffers (in field) 0 2 9 14 5 30

Filter strips 0 7 10 12 0 29

Grassed waterways 0 2 10 14 4 30

Vegetated ditches 0 4 11 11 2 28

Grassed terraces and diversions 0 3 8 15 4 30

Windbreaks/shelterbelts 2 11 4 5 8 30

Hedgerows 2 8 9 4 10 29

Other (Please specify)

Type Excellent Good Fair Poor NA N

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Type VeryImportant Important Somewhat

ImportantNot

ImportantDon’tKnow N

Roadside 4 11 10 3 1 29

Powerline ROW’s 4 6 12 4 2 28

Railroad ROW’s 1 10 15 2 1 29

Pipeline ROW’s 4 2 12 7 4 29

Table 3: Estimated importance of four non-NRCS corridor types as habitat for wildlife. Data indicate the number of states responding.

NA - Not ApplicableN - Total Number of States Responding

Type VeryImportant

Important SomewhatImportant

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Table 2: Estimated habitat value of various conservation corridor types. Data indicate the number of states responding.

NA - Not ApplicableN - Total Number of States Responding

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The literally millions of miles of roadside corridors inthe United States represent a potentially rich habitatresource. Many roadsides are dominated by a single(often exotic) grass species that is of limited habitatvalue. Only 10% of the roadsides in Cache County,Utah were rated high quality habitat for pheasantsand ground nesting songbirds in a recent study.Roadside management practices further reducehabitat value. Roadside mowing during the nestingseason is a common practice that destroys nests,kills adult birds and small mammals and degradesroadside habitat. Roadsides that are disturbedfrequently harbor numerous large patches of noxiousweeds.

Some states have initiated integrated vegetationmanagement or roadside wildflower programs thatemphasize native plants and ecologically basedmanagement practices. However, the habitat andaesthetic benefits roadside corridors could providegenerally go unrealized. The status of powerline,pipeline, canal, and railroad corridors is unknown.The quality of these corridor types may be similar tothose of roadsides.

SUMMARYThe nation�s corridors are clearly in decline. Yet theneed for conservation corridors as part of anintegrated approach to conserving biodiversity hasnever been greater. Why the apparent indifferenceto the loss of some types of corridors? Biologist AllenCooperrider argues that the underlying causes ofindifference toward environmental decline in generalare perceptual and attitudinal. He suggests that wemust begin to see, think, and act more holisticallyand reestablish an attachment to the land as anecological system, of which we are an integral part,if we are to become good stewards.

Landscapes managed on cultural concepts of naturethat embrace neatness and productivity can be quitedifferent than those managed on scientific conceptsof ecological function and structure.

�The farmer identifies with the agriculturallandscape, and this landscape represents thefarmer. A farmer�s work is constantly on view,and the farmer�s care of the land can be readilyjudged by his peers. Consequently, theagricultural landscape becomes a display of thefarmer�s knowledge, values, and work ethic.�(Nassauer and Westmacott 1987: pg 199).

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Yesterday a thousand mile wind stilled here. Waxwings fleeing winter�s wrath stopped briefly. Hunters stalk quail in the frosty edges.The farmer�s soul warmed by fall�s flaming foilage. Gifts of an autumn windbreak. Poem by Craig Johnson Drawing by Kyle Johnson

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NATURAL CONNECTIVITY SHOULD BE

MAINTAINED OR RESTORED.

Case Study:

LOUISIANA BLACK BEAR USE OFCORRIDORS

CONNECTED RESERVES / PATCHES

ARE BETTER THAN SEPARATED

RESERVES / PATCHES.

Corridor Planning Principles discussed in Chapter 5 that are exhibited by this casestudy include:

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Case Study: Louisiana Black Bear Use of Corridors

This case study illustrates the importance ofconservation corridors in maintaining viablepopulations of large mammals in fragmentedlandscapes.

The Louisiana black bear (Ursus americanusluteolus) was once abundant in east Texas,southern Mississippi and all of Louisiana. Habitatloss and fragmentation have diminished the rangeof the black bear by 90 to 95%. In January 1992,the U.S. Fish and Wildlife Service designated theLouisiana black bear as threatened under authorityof the Endangered Species Act.

In 1994, wildlife biologists at the University ofTennessee initiated a study of corridor use andfeeding ecology of black bears in the Tensas RiverBasin in northern Louisiana. The 350 km2 privatelyowned study area contained four major isolatedhardwood patches, some linked by woodedcorridors. The patches were surrounded byagricultural fields of corn, soybeans, cotton, wheat,and other small grains.

Corridors in the study area are rivers, bayous, andditches bordered by wooded strips 5 to 75 m wide.The corridors are typically linked to wooded tracts.Four major corridors in the study area ranged from50 to 73 m in width. The height and density ofvegetation in most corridors was sufficient toconceal bear movements.

Radio collars were placed on 19 Louisianablack bears, 6 males and 13 females and theirmovement was tracked over 18 months.Analysis of the telemetry data indicates thatthe bears were located in forested patchesand corridors more than expected inproportion to their occurrence in thelandscape. All 6 male bears in the studymoved to a wooded patch other than the patchthey were originally captured in; only 3 femalesmoved to another patch. Fifty-two percent ofthe male bear patch-to-patch movement and100% of all female bear movement werebetween patches connected by corridors.Adult male bears used the corridors mostintensively in June and July, the breedingseason. Sub-adult bears used the corridorsfor dispersal from their natal home range.Bears also used the corridors to access foodresources outside wooded patches.

Figure 1: This cub will use corridors to access food resourcesoutside of the wooded patches.

Figure 2: Wooded corridors become important conduits for bearmovement between wooded patches, particularly during the matingseason.

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Tensas River Basin, Louisiana

· Bears preferred corridors to agriculturalfields when outside of a forest tract.

· Corridors allowed bears to move fartheraway from forested tracts.

· Bear movement between woodedpatches connected by corridors was morefrequent than between patches that werenot connected.

This study demonstrates that wooded corridorsbetween forested tracts were used by both maleand female bears. Long-term management shouldinclude maintenance and enhancement of woodedcorridors that link substantial forested patches andconstruction of new corridors.

Numerous research projects report black bearsrequire large unbroken tracts of suitable habitat tosustain a population. This study suggests thatcorridors may be vital to the survival of Louisianablack bear in highly fragmented landscapes.

The material for this case study was abstracted with permissionfrom Anderson, D.R. 1997, Corridor use, feeding ecology, andhabitat relationships of black bears in a fragmented landscapein Louisiana, Masters thesis, University of Tennessee,Knoxville.

Researchers concluded that:

Figure 3: The importance of wooded corridors in linkingwooded patches in Louisiana is clearly illustrated in thisdiagram.

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