CTSP Newsletter (Web Version) PAGE 8
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CTSP Grantee Stories, Successes and Struggles

Craighead Environmental Research Institute (CTSP 1996 Grantee)

(Photo: Lance Craighead explains his reserve design ideas at the 1998 ESRI Conservation Program Showcase)

We used our CTSP grant to delineate landscape routes offering the best chance of success for wildlife moving among the three large core protected areas in the Northern Rockies -- the Salmon-Selway, Northern Continental Divide, and Greater Yellowstone Ecosystems. Using ARC/GRID and Montana Gap Analysis data, we derived habitat suitability models for three umbrella species, which we combined with road density information to create kilometer-scale cost surfaces of movement. For each of the three species -- grizzly bear, elk, and cougar -- we performed a least-cost-path analysis to locate broad potential corridor routes. From this first approximation we identified probable movement routes and as well as critical barriers, bottlenecks, and filters where corridor routes intersected with high-risk habitat. This analysis is being used to identify priority areas for wildlife management to improve the connectivity between the core protected ecosystems in the Northern Rockies.

Habitat reduction and fragmentation at a variety of spatial scales has been widely acknowledged as a primary cause of the decline of many species worldwide (Ehrlich 1986, Lovejoy et al. 1986, Harris 1984). Habitat fragmentation generally leads to smaller and more isolated animal populations. Smaller populations are then more vulnerable to local extinction, due to stochastic events (Shaffer 1978, Gilpin and Soule 1986), and they are more susceptible to the negative effects of inbreeding depression. To reduce the isolation of habitat fragments, many conservation biologists (e.g. Noss 1983, 1987, Noss and Harris 1986, Craighead et al. 1997, Craighead and Vyse 1995, Paetkau et. al. 1997) have recommended maintaining landscape "connectivity" -- preserving habitat for movement of species between remaining fragments.

While a significant body of research has demonstrated the deleterious effects of habitat fragmentation, work examining critical thresholds of habitat connectivity has lagged behind. For several species, Population Viability Analysis (PVA) has been employed to estimate minimum viable populations as well as the amount of habitat necessary to support a minimum viable population (MVP) (Shaffer 1978, 1981, 1987, Gilpin and Soule 1986). Yet few similar analyses have been done explicitly to demonstrate minimum thresholds of connectivity for adequate protection of species. The scientific debate over the efficacy or even necessity of species corridors is a reflection of this problem (Simberloff and Abele 1976, Simberloff et al. 1992). However, since habitat fragmentation has been shown to contribute to population declines, by inference some degree of "lack of fragmentation", or habitat connectivity, should help ensure long-term species survival (Noss 1987, 1992, Beier 1993, Noss et al. 1996).

At regional scales, connecting large core areas of wildlife habitat requires corridors -- land managed for its function as routes for wildlife movement and dispersal (Saunders and Hobbs 1991). The notion of connective habitat corridors implies a system of corridors and the core areas of habitat which they serve to link. Conceptual models of core areas, movement corridors and buffer zones have been proposed by several workers (Soule, Noss 1992, Noss and Harris 1996) as frameworks for long-term regional scale conservation of wildlife. The Ninth U.S. Circuit Court Of Appeals (1990) provided a reasonable definition of a corridor:

"...avenues along which wide-ranging animals can travel, plants can propagate, genetic interchange can occur, populations can move in response to environmental changes and natural disasters, and threatened species can be replenished from other areas."

Presently, corridor or connectivity analysis has been mandated in the Upper Columbia River Basin Integrated Scientific Assessment (Quigley et al. 1996), in the U.S. Forest Service conservation strategy for Western forest carnivores (Ruggiero et al. 1994), and in the Interagency Grizzly Bear Recovery Plan (USFWS 1993). Corridor analysis is a logical tool for compliance with the letter and spirit of the National Forest Management Act of 1976, the Endangered Species Act of 1973, and the National Environmental Policy Act of 1972. Wildlife movement corridors have been mentioned or broadly outlined in Environmental Assessments such as Montana Department of Fish, Wildlife and Parks conservation easements for the Sieben Ranch (MDFWP 1996) and Lewis and Clark National Forest Oil and Gas Leasing documents (USDA 1996). Despite this recent acknowledgment of the importance of linkage zones, the only guidelines so far provided by government agencies is a list of 8 types of information layers to be used in a GIS for linkage zone assessment, described in the grizzly bear recovery plan. A 5-year process of regional-scale linkage assessment for grizzly bear populations in the lower 48 states is currently underway (USFWS 1993).

In order to contribute to defining a scientific framework for linkage zone assessment (or corridor analysis) we report an independent effort to delineate wildlife corridors according to the conservation biology model (cores, corridors, buffers) at regional scale in a specific geographic area. Using Geographic Information System (GIS) software, the best available spatial data on habitats, and considering the habitat preferences of 3 select umbrella species, we have modeled potential regional-scale wildlife corridors between core protected areas in the Northern Rockies of the United States. Our approach offers a biologically defensible assessment of probable corridor routes and suggests one means (least-cost) of estimating the relative "connectivity" of alternate routes. A flowchart of the process we followed appears above.



All text © by the respective organizations, November 15, 1999

Compilation & web design: Charles Convis, ESRI Conservation Program, November 15, 1999