Landscape and Urban Planning 55 =2001) 195±208 Communicating clearly about conservation corridors George R. Hessa,*, Richard A. Fischerb,1 aForestry Department, North Carolina State University, Raleigh, NC 27695-8002, USA bUS Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199, USA Received 22 August 2000; received in revised form 20 February 2001; accepted 26 February 2001 Abstract Con¯icting de®nitions lead to confusion when people communicate about `corridors', particularly when they come from different disciplinary backgrounds. Usage of `corridor' in game management, island biogeography, and metapopulation literature focused on function, namely, the movement of ¯ora and fauna from one area to another. A structural usage of the term arose in the ®eld oflandscape ecology as it developed in North America with the matrix±patch±corridor paradigm of landscape structure. `Corridor' is now used to describe both the structural and functional aspects of linear landscape features, often implicitly, in a wide range of disciplinary literature. Lack of a clear and consistent terminology leads to confusion about the goals or implied functions of corridors. Consequently, the manner in which corridors should be designed, managed, and evaluated is also unclear. Proper design and management ofa corridor depends critically on a clear and explicit statement ofits intended functions.Ifcorridors are not designed to perform well-de®ned functions, the outcome may be disappointing, or even deleterious. The roles corridors play derive from six ecological functions: habitat, conduit, ®lter, barrier, source, and sink. These ecological functions have been recognized widely and adopted by a number ofdisciplines, including conservation biology, wildlifemanagement, landscape ecology, and landscape planning. We review brie¯y the history ofthe term `corridor' in the context ofconservation, catalog some ofits de®nitions and uses, review the functions of corridors, and differentiate between the structural and functional aspects of corridors. We reject the notion ofde®ning `corridor' succinctly, because ofthe complex and multiple functionsa corridor may serve. Instead, we suggest that conservationists and planners consider and document explicitly all ofthe possible functionsofcorridors when designing them. Addressing explicitly these functions when designing a corridor should eliminate much of the confusion surrounding their roles, and focus attention on establishing design criteria for corridors that function as intended. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Conservation corridors; Habitat fragmentation; Metapopulation literature ``Landscape linkages, land bridges, wildlife cor- ridors, greenways, shelterbelts, turkey trots Ð call them what you will.'' Chadwick =1991: xxi): ``Perhaps the generation ofoverarching princi- ples for the evaluation and design of corridors * Corresponding author. Tel.: 1-919-515-7437; could be assisted by the development ofmore fax: 1-919-515-8149. rigorous definitions that reflect the range of E-mail addresses: [email protected] =G.R. Hess), [email protected] =R.A. Fischer). potential values ofretained areas fornature con- 1 Tel.: 1-601-634-3983. servation.'' Lindenmayer and Nix =1993: 629) 0169-2046/01/$20.00 # 2001 Elsevier Science B.V. All rights reserved. PII: S 0169-2046=01)00155-4 196 G.R. Hess, R.A. Fischer / Landscape and Urban Planning 55 52001) 195±208 1. Introduction from many disciplines. Instead, we propose that con- servationists and planners consider and document Habitat loss and fragmentation have been cited explicitly all ofthe possible functions ofcorridors widely as the major contemporary threats to biologi- when designing them. cal diversity as people transform the landscape to accommodate their needs =e.g. Harris, 1984; Wilson, 1988; Saunders et al., 1991; Alverson et al., 1994; 2. Brief history and de®nitions McCullough, 1996; Pickett et al., 1997; Fielder and Kareiva, 1998). Corridors have become popular tools Corridors for faunal movement have a relatively in efforts to mitigate fragmentation and conserve long history in game management =Harris, 1984, biodiversity, but the term is used in ways that can 1988; Harris and Scheck, 1991). They have been used be contradictory and confusing. as a conservation technique since early in the 20th Numerous researchers working with corridors have century, particularly for birds =see extensive list of noted that lack ofa clear and consistent terminology examples in Harris and Scheck, 1991). Indeed, a sys- leads to confusion about the goals of corridors =Saun- tem of stepping-stone wildlife refuges is central to the ders and Hobbs, 1991a; Loney and Hobbs, 1991; migratory corridor planning critical to the restoration Simberloff et al., 1992; Lindenmayer and Nix, 1993; ofwaterfowl populations in North America. Other Rosenberg et al., 1995, 1997; Hobbs and Wilson, game species, including tree squirrels and ungulates, 1998; Bennett, 1999). Consequently, the manner in also have been the subject ofcorridor management which corridors should be designed, managed, and =Harris, 1988; Harris and Scheck, 1991). evaluated is also unclear. Ifcorridors are not designed More recently, corridors have been associated with to perform a well-de®ned function, the outcome may the theories ofisland biogeography =MacArthur and be disappointing, or even deleterious. The situation is Wilson, 1967) and metapopulations =e.g. Levins, further confounded when one considers de®nitions 1969; McCullough, 1996; Hanksi and Gilpin, 1997). across the several disciplines that use the term: con- These two theories form the basis for many current servation biology, landscape ecology, landscape and approaches to conservation biology, including the use urban planning, and landscape architecture. ofcorridors to enhance the movement of¯ora and Some conservation biologists have attempted to fauna. In island biogeography, the number of species de®ne the term `corridor' succinctly and precisely, on an island is a function of island size and the rates at often by focusing on a speci®c function =e.g. Loney which species immigrate to and become extinct on the and Hobbs, 1991; Beier and Loe, 1992; Simberloff island =MacArthur and Wilson, 1967). This concept et al., 1992; Rosenberg et al., 1995, 1997). However, has been extended by analogy to `islands' ofhabitat because corridors serve a variety offunctions at many embedded in a hostile landscape. By enhancing the scales, the concepts and terms associated with them ability ofspecies to move among these islands, corri- defy simple de®nition. Many de®nitions have excep- dors increase the rate ofspecies immigration and, tions and caveats, and corridors designed for one thereby, the number ofspecies occupying the island speci®c function will serve other functions, perhaps =Willis, 1974; Wilson and Willis, 1975). unintentionally. Further, confusion arises because In metapopulation theory, individuals ofa single terms included in the de®nitions ofcorridor are often species are distributed among multiple patches of themselves loosely de®ned or the subject ofmisun- habitat Ð perhaps as a result ofhabitat fragmentation. derstanding and debate =e.g. dispersal, habitat). The dynamics ofa metapopulation are a function of In this paper, we review brie¯y the history ofthe within-population dynamics and among-population term `corridor' in the context ofconservation, catalog movement. Corridors enhance the ability ofindivi- some ofits de®nitions and uses, review the functions duals to move among habitat patches, lowering the ofcorridors, and differentiatebetween the structural expected time to recolonization ofa patch that has and functional aspects of corridors. We reject the suffered extinction. Emigrants from well-populated notion ofde®ning the term `corridor' succinctly, patches may also `rescue' patches with depleted because ofits widespread and varied use by people populations =Brown and Kodric-Brown, 1977). In G.R. Hess, R.A. Fischer / Landscape and Urban Planning 55 52001) 195±208 197 combination, these phenomenon are believed to and Forman =1995) identi®ed six societal goals corri- reduce the probability ofspecies extinction. dors contribute to corridors =Table 3). Use of`corridor' in game management, island The roles corridors play in addressing societal goals biogeography, and metapopulation literature focused derive from a number of ecological functions: habitat, on function, namely, the movement of¯ora and fauna. conduit, ®lter, barrier, source, and sink =Fig. 1; For- A structural use ofthe term arose in the ®eld of man and Godron, 1986; Forman, 1995; Smith and landscape ecology as it developed in North America. Hellmund, 1993; Dramstad et al., 1996). Forman and Forman and Godron =1981, 1986) introduced the Godron =1981, 1986) introduced discussion ofthese matrix±patch±corridor paradigm oflandscape struc- six ecological functions, along with their structural ture, in part because ofits utility in describing and de®nition ofcorridors. These functional roles are analyzing the structure oflandscapes as seen in aerial presented in a wide range ofconservation literature, photographs and satellite images =Forman, 1991). In from wildlife conservation to greenways and hiking this terminology, a matrix is the most extensive and trails. They have been widely adopted, including use most connected landscape type, a patch is a non-linear in a recent corridor planning manual produced by the surface area differing in