Potential paths for male‐mediated gene flow to and from an isolated grizzly bear population

Abstract

For several decades, grizzly bear populations in the Greater Yellowstone Ecosystem (GYE) and the Northern Continental Divide Ecosystem (NCDE) have increased in numbers and range extent. The GYE population remains isolated and although effective population size has increased since the early 1980s, genetic connectivity between these populations remains a long-term management goal. With onlỹ 110 km distance separating current estimates of occupied range for these populations, the potential for gene flow is likely greater now than it has been for many decades. We sought to delineate potential paths that would provide the opportunity for male-mediated gene flow between the two populations. We first developed step-selection functions to generate conductance layers using ecological, physical, and anthropogenic landscape features associated with non-stationary GPS locations of 124 male grizzly bears (199 bear-years). We then used a randomized shortest path (RSP) algorithm to estimate the average number of net passages for all grid cells in the study region, when moving from an origin to a destination node. Given habitat characteristics that were the basis for the conductance layer, movements follow certain grid cell sequences more than others and the resulting RSP values thus provide a measure of movement potential. Repeating this process for 100 pairs of random origin and destination nodes, we identified paths for three levels of random deviation (h) from the least-cost path. We observed broad-scale concordance between model predictions for paths originating in the NCDE and those originating in the GYE for all three levels of movement exploration. Model predictions indicated that male grizzly bear movement between the ecosystems could involve a variety of routes, and verified observations of grizzly bears outside occupied range supported this finding. Where landscape features concentrated paths into corridors (e.g., because of anthropogenic influence), they typically followed neighboring mountain ranges, of which several could serve as pivotal stepping stones. The RSP layers provide detailed, spatially explicit information for land managers and organizations working with land owners to identify and prioritize conservation measures that maintain or enhance the integrity of potential areas conducive to male grizzly bear dispersal.

Incorporating wildlife connectivity into forest plan revision under the United States Forest Service's 2012 planning rule

“…For instance, we assumed that roads were an impediment to animal movement, but some species have been shown to use roads as travel corridors (Forman et al, 2003). Indeed, our results agree with several species-specific analyses conducted in portions of the study area (e.g., Cushman, McKelvey, & Schwartz, 2009;Peck et al, 2017). We did not account for effects of topography on resistance to movement because these effects are highly variable among species, but topography may be an important determinant of resistance to movement for individual species (e.g., bighorn sheep [Ovis canadensis], which prefer to move through steep terrain to avoid predators; Geist, 1971).…”

“…Our approach improves upon coarse-filter habitat-based approaches by explicitly considering habitat requirements, sensitivity to human modification, and dispersal distance, thereby bridging the gap between coarse-filter approaches and dataintensive fine filter approaches. Indeed, our results agree with several species-specific analyses conducted in portions of the study area (e.g., Cushman, McKelvey, & Schwartz, 2009;Peck et al, 2017). We were able to identify important locations for facilitating movements for a variety of different species across a large landscape containing multiple private, state, and federal jurisdictions.…”

“…We modeled connectivity within and around CGNF using randomized shortest paths (RSP; Saerens, Achbany, Fouss, & Yen, 2009), which incorporates elements of both least-cost paths and circuit theory and has been used to model movements of grizzly bears (Ursus arctos horribilis) and elk in the study extent (Brennan et al, 2018;Peck et al, 2017). Randomized shortest paths are least-cost paths modified by introducing random deviations that simulate exploratory movements away from the optimal route, which may more accurately reflect movement behavior of many species in real landscapes.…”

Conserving transboundary wildlife migrations: recent insights from the Greater Yellowstone Ecosystem

“…As an example in the GYE, ungulate migration data could be coupled with spatially explicit assessments of habitat needs for grizzly bear dispersal north of YNP (Peck et al . ) to identify multispecies priorities.…”

“…Finally, future assessments could integrate the corridor needs of multiple species and even larger taxonomic groups. As an example in the GYE, ungulate migration data could be coupled with spatially explicit assessments of habitat needs for grizzly bear dispersal north of YNP (Peck et al 2017) to identify multispecies priorities.…”