Biological Corridors: Form, Function, and Efficacy

Biological Corridors: Form, Function, and Efficacy Linear conservation areas may function as biological corridors, but they may not mitigate against additional habitat loss

Daniel K. Rosenberg, Barry R. Noon, and E. Charles Meslow

abitat loss and fragmenta- local biological diversity. However, tion are among the most The ability of biological the creation of linear patches in- H pervasive threats to the con- tended to function as corridors as a servation of biological diversity corridors to ameliorate tool to allow for further habitat re- (Wilcove et al. 1986, Wilcox and moval may ultimately cause the local Murphy 1985). Habitat fragmenta- high local extinction extirpation of species, and thus erode tion often leads to the isolation of biological diversity. Because of these small populations, which have higher rates remains concerns, it is important to evaluate extinction rates (e.g., Pimmetal. 1988). critically both the effectiveness of Ultimately, the processes of isolation controversial because biological corridors and the trade- and population extinction lead to a off with diminished habitat area that reduction in biological diversity. Con- the evidence often accompanies habitat conserva- cern for this loss has motivated con- tion plans. servation biologists to discuss the ac- is inadequate Biological corridors may include tions that are needed to increase the linear patches, such as streamside effective size of local populations. Pre- dividuals among populations may riparian areas, shelter belts, forest dominant among these possible strat- increase local and regional popula- remnants remaining from tree har- egies has been the recommendation tion persistence, particularly for vest, and, in agricultural areas, that corridors be included in conser- small, isolated populations (Fahrig fencerows. Although all linear vation plans (Figure 1) to increase the and Merriam 1994, Sjiigren 1991). patches share certain common struc- connectivity of otherwise isolated The importance of such interchange tural attributes (length is much patches (Meffe and Carroll 1994). in both reducing extinction rates and greater than width), they may func- The inclusion of corridors in re- increasing colonization rates has tion in different ways. The literature serve designs has become an impor- become a paradigm in conservation on corridors is contradictory because tant conservation tactic for protect- biology (Doak and Mills 1994, of the ambiguous use of the term ing biological diversity. This strategy Simberloff 1988). This understand- “corridor,” which is often used to was motivated by theoretical and ing, together with the public interest describe landscape components with empirical observations demonstrat- in “greenways” (e.g., Little 1990) divergent functions. Moreover, the ing that increased interchange of in- has led conservation biologists and ability of biological corridors to ame- land planners to advocate corridors liorate high local extinction rates Daniel K. Rosenberg (e-mail: rosenbed@ as essential components of reserve remains controversial because the ccmail.orst.edu) is a research associate design. evidence is inadequate. In this ar- with the Oregon Cooperative Wildlife Although connectivity can be ticle, we define and distinguish the Research Unit, Oregon State University, achieved in many ways, including two primary functions of linear Corvallis, OR 97333. Barry R. Noon is movement through low-quality habi- patches relevant to vertebrate popu- a research scientist with the US Forest tats surrounding reserves, corridors lations-as wildlife habitat and as Service, Redwood Sciences Laboratory, have been advocated as the primary biological corridors. We argue that 95521. Arcata, CA E. Charles Meslow is means to connect isolated popula- both functions of linear patches are the northwestern representative of the Wildlife Management Institute, Corval- tions (Meffe and Carroll 1994, Noss potentially critical to conservation of lis, OR 97330. Their research focuses on 1983, Saunders and Hobbs 1991). biological diversity as the landscape the population ecology of vertebrates in Protecting naturally existing corri- becomes increasingly fragmented into managed ecosystems. 0 1997 American dors likely promotes ecological pro- smaller, more isolated patches (Harris Institute of Biological Sciences. cesses and may benefit regional and 1984, Wilcove et al. 1986).

November 1997 677 tion of these two landscape elements for species that occupy them needs to be clarified. We define these two landscape elements operationally as follows (although we note that a single landscape element may be defined in either way depending on the species involved):

l A corridor is a linear landscape element that provides for movement between habitat patches, but not necessarily reproduction. Thus, not all life history requirements of a species may be met in a corridor. l A habitat is a patch (or collection of patches) that provides resources needed for survivorship, reproduction, and movement. If average survivorship and rerpoduction rates allow a stable or growing population that produces emigrants, a habitat patch is a source patch; if it is depen- dent on immigrants to sustain its populations, it is a sink patch (Pul- liam 1988).

The corridor function The primary ecological rationale for Figure 1. Representation of a linear patch of trees, a potential corridor connecting two corridors in wildlife conservation is forest patches. The surrounding area of homes, farms, and fields represents the matrix to increase population persistence environment. Linear conservation areas connecting otherwise separated habitat patches by allowing continued exchange of have been suggested to facilitate movement of animals across degraded matrix environ- individuals among a previously con- ments and thus to function as biological corridors. Drawing: John Megahan. nected population. Movement of individuals among subpopulations may Defining linear patches as (i.e., enhancing movement) role for reduce regional extinction rates by a corridor or as habitat biological corridors, the different number of mechanisms: by decreas- definitions suggest that the existence ing variability in birth and death The use of the term corridor in di- of either of these roles is sufficient to rates (Beier 1993, Den Boer 1981), verse contexts has contributed to its define a linear patch as a corridor. by increasing (re)colonization rates vague and often contradictory defi- One definition of a corridor thus of unoccupied patches (Hanski and nitions (Rosenberg et al. 1995). Land- emphasizes a movement function Gilpin 1991), by decreasing inbreed- scape ecologists Richard Forman and (i.e., passageways from one location ing depression (i.e., by increasing gene Michel Godron (1986) characterized to another), whereas the other flow; Shonewald-Cox et. al. 1983), corridors only in terms of their shape stresses form-that is, shape, com- and by increasing potentially adaptive and spatial- context, defining corri- position, and context (i.e., narrow genetic variance for maintaining dors as “ ...narrow strips of land and contrasting with the environ- population fitness (Lande 1995). which differ from the matrix [the ment on its edges). Consequently, Few studies have tested the first environment in which habitat and the criteria to determine if a linear mechanism; however, the empirical linear patches are embedded] on ei- patch serves as a corridor have been evidence that does exist supports the ther side. Corridors may be isolated ambiguous. When issues such as the notion that reduction in demographic strips, but are usually attached to a significance of corridors to the main- variability will increase persistence patch of somewhat similar vegeta- tenance of biological diversity are (Den Boer 1981, Forney and Gilpin tions” (p. 123). Biological corridors debated (e.g., Noss 1987, Saunders 1989). Empirical evidence also sup- have also been defined as continu- and Hobbs 1991, Simberloff and Cox ports the second mechanism (Sjiigren ous, narrow patches of vegetation 1987), these different interpretations 1991) as important in maintaining that facilita te movement among habi- of the corridor concept may result in regional persistence. The relative tat patches , thereby preventing iso- controversy (Rosenberg et al. 1995). importance of the genetic mecha- lation of populations (Merriam To determine whether a particu- nisms compared with demographic 1984). Although the structural defi- lar linear patch functions as habitat factors remains controversial (Lande nition does not preclude a functional or as a biological corridor, the func- 1988, Mills and Smouse 1994). How-

678 BioScience Vol. 47 No. 10 ever, in a large metapopulation (spa- the second to maintain populations, sumed corridors. However, these tially semi-isolated populations that in fragmented landscapes. Thus, cor- studies, also not replicated, fre- are linked by dispersal of individu- ridors are potentially meaningful quently did not include individuals als) the contribution of only a few components of conservation reserve that moved through the matrix, and immigrant individuals to local gene design, whether viewed at the indi- outside of the corridor. For example, pools per generation is sufficient to vidual or population level. Dmowski and Kozakiewicz (1990) minimize deleterious inbreeding ef- investigated the movement of birds fects and to sustain genetic diversity Linear patches as corridors: evidence between two pairs of patches; one (Lande and Barrowclough 1987, from observational studies. Although pair was presumed to be connected Slatkin 1985). many researchers have asserted that by a corridor, the other to be uncon- In addition to connecting local corridors are vita1 elements of connected. Because a higher proportion populations, corridors may facilitate servation plans (e.g., Noss and of birds from one habitat patch were movement of an individuaI within its Cooperrider 1994, Saunders and subsequently found in the second home range. Although the latterfunc- Hobbs 1991), few studies have dem- patch in the connected pair than in tion has received less attention than onstrated that corridors actually in- the unconnected pair, and because the former, it may be important to crease the rate of successful move- movements were observed along the the survival of individuals, and thus ment of animals between patches. edge of the presumed corridor, the populations, in fragmented environ- Rather, most studies have documented researchers concluded that corridors ments. Facilitation of movement may animal presence in linear habitat increased connectivity. However, be particularly relevant for species patches lying between larger patches because the authors did not directly whose home range area requirements and have inferred that the linear estimate movements through matrix exceed the average patch size or for patches were acting as corridors. habitat outside of the corridor, it is species or populations whose sur- An early study that is often cited unknown what effect the corridor vival requires that individuals move as evidence of the value of corridors actually had on movement rates. between discrete patches (landscape encompassed two woodlands: one that Studies on small mammals, which complementation; Dunning et al. was connected to a larger forest and are likely to be more selective of 1992). For example, some species, one that was isolated (MacClintock et dispersal habitat than birds, are such as migratory amphibians (e.g., al. 1977). The connected woodland widely cited as evidence that linear red-spotted newts, Notophthalmus contained more forest interior bird patches function as corridors (e.g., viridescens; Gill 1978) may require species than the unconnected tract, Bennett 1990, Henderson et al. 1985, links among seasonally used habi- and the connecting “corridor” con- Merriam and Lanoue 1990, Wegner tats. The lack of such connectivity tained several of these species as well. and Merriam 1979). Again, many of may cause local extirpation; it is Although the differences in species these studies lacked replication and therefore one possible mechanism numbers between the connected and direct measurement of movement, for loss of biological diversity result- unconnected tracts may have been instead inferring movements of indi- ing from habitat fragmentation the result of a corridor effect, the viduals by counting the numbers of (Wilcove et al. 1986). study was not a valid test because it individuals that were captured in an How well do corridors facilitate lacked replication and did not di- array of traps dispersed throughout movement? Demonstrating the effi- rectly measure the movement of in- the study area. Several studies did, cacy of corridors in promoting move- dividuals. Nevertheless, this study has however, evaluate the efficacy of ment among populations and within been the basis for many of the early corridors based on differences in home ranges requires knowledge of arguments in favor of corridors, as movement rates. For example, the probability of entering habitat Simberloff et al. (1992) noted. Similar Wegner and Merriam (1979) com- patches that vary in configuration studies with parallel interpretations pared the number of transfers of and landscape context. The effect of dominate the literature: Authors have individuals among a forest patch, corridors on rates of immigration, a frequently concluded that their re- fencerows, and fields; they found population-level process, can ulti- sults document the importance of much greater use of fencerows than mately be explained at the level of corridors in connecting habitat fields in movements to and from the the individual animal by asking how patches even though they have not forest patch. How the animals trav- actually shown that individuals used eled between patches was unknown, the corridors to move between but because few individuals were cap- patches. Although their conclusions tured in fields, the authors concluded were not fully supported by the data, that movement across fields was un- What are the ultimate effects on the these studies have greatly influenced likely. To what extent movement animal’s fitness? Given this under- both the popular and scientific views rates may have been reduced with- standing, it is possible to scale up of the biological value of corridors out the fencerows is unknown. These from an individual’s pattern of move- (Rosenberg et al. 1995, Simberloff et findings were later substantiated by ment within a patchy home range to al. 1992). the observation (Merriam and the collective patterns of movement Several recent studies have at- Lanoue 1990) that displaced white- of individuals traveling across a tempted to test more directly for footed mice (Peromyscus leucopus) patchy landscape. The first process increased movement of individuals traveled most frequently along necessary to mai ntain individuals,is between patches connected by pre- fencerows rather than across fields.

November 1997 679 Figure 2. Aerial view of transit, individuals moved rapidly one of the first experi- across poor-quality habitats (Garret ments (La Polla and and Franklin 1988, Wigget and Boag Barrett 1993) on the 1989). These observations, together effects of biological with those discussed above, suggest corridors on the popu- that corridors, when available, may lation dynamics of ani- be selected for movement between mals. Population size patches. However, these studies do of meadow voles was not indicate how movements would assessed in the old-field habitat patches, which be affected if corridors were not avail- were created by mow- able. The observational studies con- ing the surrounding veg- ducted on movements of animals etation. The research- occupying landscapes with linear ers created potential habitat patches lead to some inter- corridors of different esting hypotheses, but they provide widths by maintaining only weak evidence of how corridors strips of vegetation be- affect animal movements in dis- tween patches. Three turbed landscapes. study plots with different corridor treatments were arranged Linear patches as corridors-evi- by experimental units dence from experimental studies. (blocks). The left plot Field experimentation offers a rigor- of the lower block in- ous means to test how corridors af- cluded a wide corri- fect movement patterns of individu- dor (5 m), the middle als and dynamics of populations. plot no corridor, and However, the large spatial and tem- the plot on the right a poral scales that are appropriate to narrow (1 m) corridor. real-world conservation plans make Photo: Gary Barrett; re- printed from La Polla relevant experimental studies diffi- and Barrett (1993) with cult. We discuss studies from two permission of Kluwer classes of field experiments: first, Academic Publishers. manipulative and replicated experiments, and second, quasi-experimental studies, whose study design in- Although there is cluded neither randomization of only weak evidence manipulations nor adequate replica- that corridors in- tion. Nevertheless, we included a study crease immigration of this latter type because it is one of rates, it is well known the few conducted at a spatial scale that animals will that is relevant to conservation plan- repeatedly use spe- ning and reserve design. cific routes during One of the first field experiments transit. Observations published on biological corridors (La of ground squirrels Polla and Barrett 1993) tested the moving between co- effects of the presence and width of teries (Garret and linear patches of vegetation on the Franklin 1988, Wig- population dynamics of meadow get and Boag 1989) voles (Microtus pennsylvanicus). The show that animals study design consisted of nine ex- follow linear patches perimental plots that were arranged 1 Figure 3. Oregon Ensatina. Animals of habitat while dispersing. For ex- by blocks in a three-treatment x three- occupying relatively small habitat ample, black-tailed prairie dogs replicate design. Each plot consisted patches, such as Ensatina eschscholtzii, (Cynomys ludovicianus) often dis- of two habitat patches (20 x 20 m) have been the subjects of most of the persed from one colony to another that were either connected by a lin- experimental work on biological cor- through vegetated ravines (Garret ear patch (1 x 10 m or 5 x 10 m) of ridors. The microlandscapes-in which and Franklin 1988), and Columbian suitable habitat or separated by a th ground squirrels (Spermophilus continuous belt (10 x 20 m) of mowed ar ca columbianus) were found to use pre- (i.e., unsuitable) habitat, designed to to existing pathways made by other be a barrier to movement (Figure 2). allowing a more rigorous approach to colony members (Wigget and Boag The unsuitable areas separating evaluating the effects of landscape at- 1989). However, when pathways plots failed to confine voles; animals tributes on animal populations. were not available or not used for dispersed across the habitat that had

680 BioScience Vol. 47 No. 10 Figure 4. (below) View of a section of a study plot from a study of biological corridors that investigated movements of the Oregon Ensatina salamander (Rosenberg et al. in press). Experimen- tal treatments included pathways with narrow linear patches of vegetation re- tained as potential corridors, as shown here, and pathways with all surface organic matter removed. Vegetation along the perimeter of the fence was removed prior to each experiment. Photo taken at the Starker Forest site.

Figure 5. Aerial view of a landscape-scale experiment conducted with butterflies. Nick Haddad and Robert Cheney at the Savannah River Site in South Carolina designed experimental plots in which the US Forest Service manipulated the existence and length of pathwa ys of clear-cut s intended to function as biological corridors connecting clear- cut habita t patches (1.6 ha). The clear-cuts provided habitat for many of the butterfly species studied by Haddad (1997). Photo courtesy of the US Forest Service.

and eastern chipmunks (Tamius from between the corridors and the striatus), may reside entirely within fence. In addition to testing the ef- narrow linear patches (Henderson et fects of corridor widths on move- al. 1985, Lorenz and Barrett 1990). ment patterns, Andreassen and co- been assumed to be unsuitable. Male If this residency occurred in the vole workers tested the effects of predators voles moved more readily between study, then similar results would have and conspecifics on movement be- study plots over mowed areas than been obtained without a corridor if havior. The presence of predators between habitat patches within a one of the patches was increased in and conspecifics was simulated by plot, suggesting that the linear size by the area of the linear patch. placing fox scat and voles, respec- patches intended to function as cor- The mechanism that regulated vole tively, in enclosures placed inside the ridors probably did not stimulate or densities at the scale of the experi- corridors. During each day of the direct movement at the spatial scale mental plots of La Polla and Barrett study, two male voles were released of the experiments. Nevertheless, at (1993) may have been related to the into the habitat patches in the morn- the end of their study the authors social structure of the vole popula- ing and tracked until the evening. found higher mean densities in con- tions (Collins and Barrett 1997). There were no spatial replicates; in- nected than in unconnected patches, Further experiments with similar de- stead, the different trials were con- and they concluded that corridors signs would need to be conducted to ducted on the single enclosure, which effectively increased dispersal among discriminate between the corridor hy- was modified as needed for each trial. patches. pothesis and patch-size hypothesis. Although the presence of preda- The mechanisms that promoted In a recent study of root voles tors (as simulated by fox scat) or higher average densities in connected (Microtus oeconomus), Andreassen conspecifics (as simulated by indi- patches were not identified; what et al. (1996) tested behavioral mecha- vidually caged voles) did not affect such mechanisms might be is unclear nisms that may be responsible for movement patterns, corridor width because individuals frequently moved increasing immigration among had a strong effect. Immigration rates across habitat that had originally patches that are connected by corri- to the habitat patches were highest been considered unsuitable. One dors. A fenced pathway of length in the mid-width (1 m) and lowest in plausible explanation for the results 310 m and width 7.5 m was created, the widest (3 m) corridors because of is that the corridor increased the within which a narrower strip of differences in the resistance to and effective habitat area of the patch grass (“corridor”) of width 3 m, 1 m, the rates of movement along the cor- and thus allowed a greater popula- or 0.4 m connected habitat patches ridors. Resistance to movement was tion size. Indeed, small mammals, (5 x 5 m) placed at both ends of the greatest in the narrowest corridor such as house mice (Mus musculus) pathway. Vegetation was removed (0.4 m), as demonstrated by the high

November 1997 681 percentage (73%) of trials in which than those that selected a pathway the contrast between patch and ma- voles did not leave the habitat patch without a corridor. Pathways with- trix environments. in which they were released. Move- out corridors were selected less often On the surface, this study seems ment rates were lowest in the wide than corridor pathways, and move- to offer strong support for a func- corridor, which the researchers at- ment through noncorridor pathways tional role for corridors. However, tributed to frequent cross-directional was faster. The contrast between interpretation of the results is con- use of the corridor. pathway types, however, was great- founded by landscape differences that Andreassen et al. (1996) con- est following several weeks without were not amenable to experimental cluded that wider corridors are not rain. Under these conditions, ani- manipulation. For example, treat- necessarily more efficient conduits mals in pathways without corridors ments (i.e., corridor or no corridor) for movement than narrower corri- experienced weight loss and in- were not located randomly-the for- dors, so long as the corridors are not creased mortality, but increased the ested linear patches were adjacent to so narrow that they are avoided as rates of movement (Rosenberg et al. a lake, whereas the clear-cuts were movement pathways. As Andreassen in press). Only under such “drought” inland. Thus, the increased move- et al. noted, these conclusions as- conditions did greater numbers of E. ment could have reflected factors sume that predation rates are the eschscholtzii reach target patches con- other than the corridors. Moreover, same, regardless of corridor width; nected by corridor pathways than tar- different methods were used to esti- if, however, survival rates vary get patches connected by pathways mate movement in the two habitat among corridor types, then immi- without corridors. Under nondrought types. Nevertheless, this study is gration rates (a measure of corridor conditions, by contrast, even though noteworthy because it is one of the efficiency) will be a function of both E. eschscholtzii preferred corridor few that was conducted at a scale time spent in each corridor and cor- pathways, those that selected path- appropriate to land management. ridor-specific survival rates. As the ways without corridors moved faster, Preliminary results from another authors also noted, this study cannot and as a result, the immigration rates large-spatial scale experiment (Fig- be considered a “proper” experiment resulting from movement along both ure 5) that investigated the effects of because there were no spatial repli- corridor and noncorridor pathways landscape pattern on movement be- cates. Nevertheless, this study iden- were similar. havior of butterflies further suggests tified behavioral mechanisms affect- This compensatory behavior be- the potential of corridors to increase ing movement patterns, such as tween survival costs and movement interpatch movement and popula- resistance to entering linear patches rates contributed to the lack of con- tion density (Haddad 1997). Nick and rate of movement once entered, sistent differences in immigration Haddad and Robert Cheney used an that should be considered in deter- rates. The influence on immigration innovative approach in which the US mining if a linear patch of habitat rates of enclosing pathways by barri- Forest Service created clear-cut for- is likely to function as a biological ers was unknown but may have con- est patches (1.6 ha) in a predomi- ’ corridor. tributed to similar immigration rates nately pine forest matrix at the Sa- We have used a similar approach under nondrought conditions. Al- vannah River Site in South Carolina. (Rosenberg et al. in press) to study though our study did not reveal a The study design included variation movements of the Oregon Ensatina consistent positive effect of corri- in the distance between patches and salamander (Ensatina eschscholtzii; dors on immigration rates, it did in whether or not the patches were Figure 3). In our experiment, each show that behavioral mechanisms connected by linear patches (32 m study plot contained pathways with that are important to successful dis- wide) of similar habitat. and without linear patches (“corri- persal to target patches may be af- For butterfly species requiring dors”) of habitat (Figure 4). This fected by compensatory behavior open habitats (e.g., buckeye, Junonia design allowed us to investigate involving corridor selection, move- coenia), the pine forest seemed to movement patterns of displaced in- ment rates, and corridor and matrix create a partial barrier to movement dividuals in a setting in which they quality. because a higher proportion of indi- were allowed to select among path- A recent landscape-scale study has viduals moved between patches con- ways. Each study plot consisted of found increased movements of birds nected by corridors than between four target patches (1 x 3 m) of between forest patches connected by unconnected patches. The propor- unmodified habitat that were con- corridors (Machtans et al. 1996). tion of transfers of all species studied nected to a central source patch (3 x Although few birds were observed decreased as distance between 3 m) by pathways (3 x 40 m) that dispersing, the authors concluded patches increased, regardless of the were enclosed by fences. These path- that more juvenile birds dispersed presence of a corridor. For some ways were of either high quality (i.e., through corridors than through ad- species, no matter what the length of they contained a 1 m wide strip of jacent clearcuts. Interestingly, the au- the corridor (which ranged from 64 natural vegetation surrounded by bare thors observed a greater proportion to 384 m), transfer rates between mineral soil) or low quality (i.e., all of birds within the clear-cut areas patches connected by a corridor were surface organic matter was removed). that contained residual large trees greater than those between uncon- We hypothesized that animals that than clear-cut areas with no trees nected patches. At all distances, these selected a corridor pathway from the remaining. Thus, similar to the E. butterflies moved from one patch to source to a target patch would be eschscholtzii study, the likelihood of another in the absence of corridors. more likely to reach the target patch using a corridor may be a function of Thus, these preliminary findings sug-

682 BioScience Vol. 47 No. 10 gest that linear patches did increase Figure 6. The effects on 1 Low quality matrix I movement rates- for some butterfly corridor use of habitat I species but that the lack of corridors quality in the corridors did not isolate patches (Haddad 1997). and in the surrounding environment (matrix). The solid circles in the Linear patches as corridors-com- source patches repre- mon patterns. The research to date sent dispersing animals; shows that it is difficult to devise the size of the circle in- High quality convincing tests of the efficacy of dicates the probability Moderate quality corridor corridor linear patches as biological corri- that an animal in that dors and as a means to increase popu- location will find and lation persistence. This difficulty select the corridor, rela- holds even for species that are opera- tive to the probability that it will disperse High quality matrix 1 tive at limited spatial and temporal I through the matrix. In i---- p---J scales. Because organisms respond in I the system surrounded species- and landscape-specific ways by a low-quality matrix, to potential corridors, no single study animals will be more is likely to provide a definitive answer likely to select the high- to the question of the ecological func- quality corridor than tion of linear patches. Therefore, indi- they would when the vidual case studies will continue to be corridor is surrounded important in evaluating the benefits by a high-quality ma- Moderate quality High quality of various landscape designs as a trix; however, if they corridor corridor means to increase connectivity. do select the low-qual- Several general principles arising ity corridor, they will move through it more from the diverse array of published quickly. Animals adjacent to the corridor are more likely to select either corridor than studies may assist in evaluating the to disperse through’a low-quality matrix. If the matrix is composed of high-quality efficacy of biological corridors. First, habitat, the chance that an animal will select a corridor instead of simply moving given a choice between habitat and through the matrix decreases. Whatever the quality of the matrix, dispersing animals nonhabitat, individual animals are that enter corridors travel through them at a speed (indicated by arrow length) that is more likely to select movement path- affected by the quality of the corridor and the matrix. ways that include components of their habitat. This behavior is par- ability of successfully traversing the The second critical factor is se- ticularly pronounced for individuals corridor (Figure 6). lecting a dispersal pathway. During moving within the bounds of their The likelihood of finding a corri- dispersal, the difference in survival home range. When movement among dor is related to the distance of the costs between the corridor and the local populations is considered, how- animal from the corridor: As the surrounding environment (matrix) ever, most studies found that indi- distance increases, the probability of is likely to affect pathway selection; viduals were not adverse to moving locating the corridor decreases. This as the survival costs within a matrix through areas that did not contain relationship is likely to have a large increasingly exceed those within a suitable habitat-a typical matrix en- effect for species that are relatively corridor, the probability of selecting vironment. Second, the relative use sedentary but a small effect for spe- the corridor increases (Figure 6). The of the matrix as movement habitat cies that can physically assess the hypothesis that survival costs affect depended on the degree to which it availability of more distant path- selectivity of a pathway is intuitively contrasted with patch habitat. Third, ways. Empirical evidence supports appealing and has empirical support. animals may show compensatory be- the hypothesis that the probability For example, our findings on sala- havior in less favorable environ- of locating a landscape element is manders provide evidence that the ments. For example, they may move inversely related to distance. For ex- quality of the matrix surrounding more rapidly in low- than in high- ample, movement of bark beetles (Ips otherwise isolated patches affects the quality habitat. typographus) to a pheromone trap efficacy of corridors that may con- These general principles suggest a declined with increasing distance nect them (Rosenberg et al. in press). simple conceptual model for evalu- (Helland et al. 1984), and patch se- If the contrast between patch and ating the likelihood of a linear patch lection by flea beetles (Phyllotreta matrix, in terms of resistance and functioning as a biological corridor. spp.; Kareiva 1982) and cabbage costs to movement, is small, then Assuming that once an individual butterflies (Pieris rapae; Fahrig and animals may be largely indifferent to reaches the target patch it remains Paloheimo 1988) decreased as dis- the presence of corridors (Figure 6). there, then the probability of suc- tance increased. Variation in distance If animals do not select a corridor, cessful dispersal through a corridor sensitivity may be achieved by spe- they may simply compensate for this can be represented by three succes- cies-specific levels of exploratory “mistake” by moving more rapidly sive events: the probability of find- movements along the periphery of a through matrix habitat. However, ing the corridor, the probability of home range, as reported for wolves when the contrast between patch and selecting the corridor, and the prob- (Canis lupus; Fritts and Mech 1981). matrix is large, the costs of failing to

November 1997 683 select a corridor when one is present animals use linear patches as habi- patches of trees, and other types of may be high. Thus, the value of cor- tat, as reflected by established home vegetation that are left after land- ridors as landscape connectors may ranges that are sometimes entirely altering activities provide habitat for be dynamic, reflecting variation in within such patches (e.g., Henderson many vertebrate species (Martin the degree of contrast between patch et al. 1985, Lorenz and Barrett 1990). 1980, Wegner and Merriam 1979), and matrix environments. Therefore, Although there is evidence that lin- and even small strips of uncultivated given a choice of a pathway through ear patches may facilitate movements land between crops provide important a corridor or through the matrix, of individuals between patches, it is habitat for invertebrates (Maelfait and optimal behavior is a conditional, difficult to separate this function DeKeer 1990). Many areas created as not an obligate, response. from that of simply adding habitat. potential corridors may, in fact, re- The matrix environment further The difficulty of distinguishing these alize their greatest function as rem- influences movement through the po- functions has added to the contro- nant habitats. tential corridor by affecting the prob- versial nature of corridors. If viewed Linear patches of vegetation in ability of directional movement once as habitat, linear patches may pro- urban environments, often called the animal is in the linear patch. If vide an important component in a greenways (Little 1990), may like- selectivity is low, animals may “wan- specific reserve design, whereas this wise serve as habitat for species that der” and experience high mortality may not be true if the linear patch is do not require extensive wild areas. rates outside of the linear patch; such intended to function as a corridor Greenways play an increasing role in a phenomenon may have occurred (i.e., increase movement). Such con- nature education, recreation, and sce- with dispersing cougars (Felis controversy is unfortunate because it nic quality of landscapes (Saunders et color) in southern California (Beier detracts from the potentially more al. 1987). Because greenways are in- 1995). Other factors that may affect important ecological importance of creasingly popular as a means of selectivity include conspecific attrac- linear habitats. Furthermore, the improving urban and suburban liv- tion (Smith and Peacock 1990) and public values linear habitats not only ing environments (Little 1990) and avoidance of conspecifics and preda- for their environmental quality but may themselves serve as important tors (Fritts and Mech 1981, Joule also for recreation and for protec- habitat, biologists should work with and Cameron 1975); the research con- tion of scenery; many conservation urban planners and community groups ducted on root voles by Andreasson et efforts have focused on their restora- to design greenways that contribute to al. (1996) provided an initial investi- tion and protection (e.g., Little 1990, urban wildlife conservation and edu- gation into these factors. Noss 1987). Linear habitats include cation, rather than arguing whether The third critical factor influenc- streamside riparian zones, remnant greenways function as corridors. ing the efficacy of a particular corri- habitat patches, urban greenways, Much of the argument against in- dor is the likelihood that the animal and many others. cluding linear patches in reserve de- can move successfully through a path- Streamside riparian areas support signs is based on the possible ill ef- way. The probability of successfully a wealth of biological diversity (e.g., fects of attracting wildlife to areas in reaching a habitat patch by moving Naiman et al. 1993) and are ecologi- which a high proportion of linear though a corridor is a function of cally important regardless of their patch is exposed to the surrounding survival costs (mortality rates) and role as corridors. Areas preserved environment (e.g., Simberloff and transit time. The rate of movement is along streams include a diversity of Cox 1987, Simberloff et al. 1992), a affected by an animal’s physical abil- habitats and maintain the integrity landscape component that is often ity and by the environment. Average of aquatic ecosystems by providing referred to as “edge habitat.” Corri- movement rate through a corridor is shade, nutrients, and structure while dors in which the matrix environ- likely to be related to the survival cost reducing sedimentation and pollu- ment affects the environment of the in the corridor and to the width of the tion (Gregory et al. 1991). Conser- corridor’s interior are considered corridor. Based on evidence of the vation and restoration of these habi- edge habitat. The amount of edge movements of salamanders (Rosenberg tats are, therefore, important to habitat is increasing in many hu- et al. in press), fruit flies (Dobzhansky maintaining the biological diversity man-modified environments, often et al. 1979), field voles (Microtus of ecosystems that include riparian at the expense of interior habitat agrestis; Stenseth and Lidicker 1992), habitats. Thus, the importance of (e.g., Robbins et al. 1989). This trend and flea beetles (Kareiva 1982), move- streamside riparian areas as wildlife is a matter of concern because edge ment rates are likely to increase with habitat is unrelated to their potential environments may not provide habi- increasing survival cost. Thus, dis- to function as corridors. tat for species that are most in need persal success is a function not sim- The importance of remnant habi- of protection; many edge species have ply of distance, but also of velocity, a tat patches in wildlife conservation high birth and survival rates in dis- factor that is almost entirely ignored has recently been emphasized (e.g., turbed areas, and providing habitat in discussions of the potential of Saunders et al. 1987, Shafer 1995). for these species will not maintain linear patches to serve as corridors. Linear patches may often be consid- regional diversity (Robbins et al. 1989). ered to be remnants; in many land- Land that is allocated to corridors The habitat function scapes, they provide the only remain- in conservation plans may require ing habitat, thus contributing to the the size of core habitat patches to be Not surprisingly, most studies of maintenance of local and regional smaller to account for the area of the corridors demonstrate that individual biological diversity. Fencerows, small corridors, thus increasing the pro-

684 BioScience Vol. 47 No. 10 portion of edge habitat in a land- cause the local extirpation of spe- scape. This tradeoff is apparent in cies, and thus erode biological diver- many forest planning alternatives sity. Corridors are often advocated (e.g., USDA 1996). If large reserves as a means to lessen the effects of are compromised in size to allow for habitat loss. If, however, population linear patches, regardless of whether growth rates are influenced more they function as corridors or simply as negatively by habitat loss than posi- habitat, then their establishment may tively by the creation or maintenance represent a net loss to wildlife protec- of corridors, the population will de- tion. Recent observational (Schmitz cline. Because of the lack of strong 1997) and theoretical (Fahrig in press) empirical data supporting the bio- work on the importance of habitat logical function of corridors, and quantity and composition (Dunning because even if linear patches do et al. 1992), rather than its configu- have this function, they do not nec- ration in the landscape, supports this essarily compensate for additional hypothesis. habitat loss and fragmentation, cau- Figure 7. Comparison of tradeoffs in tion is necessary when incorporating Compromise in connectivity reserve design. Although a corridor de- corridors into reserve designs. and patch size sign (a), in which the linear patch has similar vegetation as the habitat patches, Conclusions The tradeoffs in the type and size of is often advocated as the best means for patches in reserve designs are impor- ensuring connectivity, there are poten- Linear patches have often been tant considerations for conservation tial tradeoffs with such a design when equated with biological corridors, strategies (Figure 7). Larger, high- the quality of the matrix or patch size is but in fact, linear patches serve two quality habitat patches will reduce compromised because of land allocated different functions: as movement to corridors. An alternative to creat- the level of connectivity needed for ing a potential corridor with high- pathways and as habitats for resi- population persistence. Indeed, when quality habitat that is otherwise sur- dent animals. Distinguishing be- habitat patches are large, a low level rounded by unsuitable habitat is a tween the two requires knowledge of connectivity-just enough to al- matrix design (b), which increases con- of the life history stages that the low a few individuals per generation nectivity by embedding moderate-qual- species in question fulfills within the to reach other habitat patches and ity patches throughout the matrix. The linear patch. Evaluation of the corri- reproduce successfully-may be suf- matrix design shown here has discrete dor function of linear patches should ficient for the population to persist. patches, but a more homogenous dis- include the three stages of successful Increasing the size of the reserve or tribution of moderate-quality habitat dispersal to a target patch through a the permeability of the matrix may within the matrix is another alterna- corridor-finding, selecting, and tive to corridors. Creating potential increase viability of the population corridors may also result in a tradeoff moving successfully through it (Fig- more than adding corridors as dis- with reserve size. The patch-size de- ure 6). In each stage, environmental tinct linear elements, especially if the sign (c) demonstrates that patch size influences, such as landscape pat- matrix allows some movement of can be increased by including the area terns, and species-specific behavioral individuals among populations (Fig- that was formerly allocated to a corri- components must be considered ure 7). Most of the work on corridor. With increases in patch size and, when attempting to determine whe- dors suggest that movement through presumably, in population size, there ther a linear patch could indeed func- the matrix occurs, especially if the is less need for high levels of connec- tion as a biological corridor. To matrix retains elements of the tivity, and edge effects are minimized. evaluate the role of a linear patch as patches, such as remnant live or dead habitat, survival and reproduction trees, as Machtans et al. (1996) dem- nectivity with corridors may be the rates within the patch need to be onstrated with songbirds in recently most effective way to increase popu- considered. Again, we emphasize that harvested forests. Increasing reserve lation viability (cf. Beier 1993, 1995). determining whether a particular lin- size and enhancing the matrix for The importance of a particular linear patch serves as a biological corri- dispersal was recommended to pro- ear patch as habitat for maintaining dor or as a habitat patch will indeed be tect Northern spotted owls (Strix or restoring local or regional bio- species and landscape specific. occidentalis caurina; Thomas et al. logical diversity should also be evalu- Although much of the literature 1990). ated. Most important, however, ele- on corridors has discussed the occur- Although quantitative analyses to ments in reserve designs must be rence of animals within linear ascertain the relative benefits of al- viewed as tradeoffs, especially when patches, differentiating between the ternative reserve designs that include the issue of creating corridors, rather corridor and habitat functions of lin- different types of connectivity would than of simply retaining existing linear patches requires knowing whe- be difficult, it is important to con- ear patches (e.g., streamside riparian ther the patch increased animal move- sider options other than corridors. areas), arises. In some cases, the cre- ment to connected patches. However, In some cases, of course, few alterna- ation of corridors as a tool to miti- even demonstrating that a linear tives are available (e.g., highly ur- gate against habitat loss due to de- patch has increased immigration rates banized areas), and incre asing con- velopment activities may ultimately over what would otherwise have been

November 1997 685 achieved does not necessarily trans- insightful discussions. Our work on Forney KA, Gilpin ME. 1989. Spatial struc- late to increased viability for the corridors has been funded by the US ture and population extinction: a study or biological Forest Service, Redwood Sciences with Drosophila flies. Conservation Biol- population, species, ogy 3:45-51. community; it simply shows that the Laboratory, Arcata, CA; the US Fish Fritts SH, Mech LD. 1981. Dynamics, move- linear patch functioned as a corri- and Wildlife Service; and the Oregon ments, and feeding ecology of a newly dor. The level of connectivity needed Cooperative Wildlife Research Unit, protected wolf population in northwest- to maintain a particular population with the cooperation of the Wildlife ern Minnesota. Wildlife Monographs 80: l-79. 11 vary tremendously with the dy- Management Institute, Oregon De- Garret MC, Franklin WL. 1988. Behavioral namic properties of the demography partment of Fish and Wildlife, Or- ecology of dispersal in the black-tailed of the population, including popula- egon State University and the US prairie dog. Journal of Mammalogy 69: tion size, survival and birth rates, Fish and Wildlife Service. We are Gill DE. 1978. The metapopulation ecology and genetic factors, such as the level indebted to Starker Forests, Inc., and of the red-spotted newt, Notophthalmus of inbreeding and geneti variance. the US Forest Service Siuslaw Na- viridescens. Ecological Monographs 48: The movement and habitat functions tional Forest for facilitating our field 145-166. of linear patches are clearly not mu- work on biological corridors. Publi- Gregory SV, Swanson FJ, McKee WA, tually exclusive, but we believe that cation costs were supported by Cummins KW. 1991. An ecosystem per- spective of riparian zones: focus on links it is important to determine which is Starker Forests, Inc., and by the Tho- between land and water. Bioscience 41: primary. mas G. Scott Achievement Fund. 540-551. We suggest that if corridors are Haddad NM. 1997. Do corridors influence effective in facilitating movement References cited butterfly dispersal and density?: A land- among patches of habitat, then their scape experiment. Ph.D. dissertation. Andreassen HP, Halle S, Tms RA. 1996. Op- University of Georgia, Athens, GA. effectiveness may be due to one of timal width of movement corridors for Hanski I, Gilpin M. 1991. Metapopulation two mechanisms: increasing the prob- root voles: not too narrow, not too wide. dynamics: brief history and conceptual ability of successful movements with- Journal of Applied Ecology 33:63-70. domain. Biological Journal of the Lin- in the home range of an individual Beier P. 1993. 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