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

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