USING DIFFUSION-MODELS TO SIMULATE THE EFFECTS OF LAND-USE ON GRIZZLYBEAR DISPERSAL IN THE ROCKY-MOUNTAINS

Timber harvests proposed for Trail Creek Watershed, in southwestern Mo ntana, U.S.A., have been opposed because grizzly bear (Ursus arctos ho rribilis) dispersal from northern Montana wildernesses into the Greate r Yellowstone Ecosystem may be less likely. We used an individual-base d model to simulate grizzly bear responses to: 1) region-level managem ent practices represented by ownership patterns, and 2) watershed-leve l changes in habitat availability due to proposed harvests and road bu ilding. We assigned permeabilities (i.e., values that represent how ea sily a bear can move through a patch) to ownership blocks (region-leve l) and habitat patches (watershed-level) based upon a literature revie w, and used a correlated random-walk diffusion model to simulate movem ents. Simulated bears were placed into rasterized landscapes in a stra tified random manner. At the regional level, bears moved less than or equal to 1500 times (i.e., approximate to 1530 km), and their destinat ions were tallied. At the watershed level, the number of moves require d for bears to leave the watershed were tallied. Sensitivity analyses were used to determine the variability of the results with respect to changes in some parameters of interest (i.e,, permeabilities of privat e lands, harvest permeabilities, and disturbance indices). With the pe rmeability of private land set at 50 (range: 0 to 99), simulated grizz lies did not disperse from the Scapegoat and Bob Marshall Wildernesses into Yellowstone National Park (0 of 10000 simulated individuals), Un der the assumptions of this model, a linkage between the wildernesses in northern Montana and Yellowstone does not appear to exist. However, a significant number of simulated grizzlies (41%) dispersed from Anac onda Pintler Wilderness, which is near Trail Creek Watershed, into the wilderness ES in eastern Idaho. A linkage may exist between these sit es. At the watershed-level, removal of forest habitat under proposed H arvest I (1.77% of the watershed cut) led to simulated grizzlies using slightly more moves (i.e., less than or equal to 5.6%, P = 0.042) to exit the watershed than under existing conditions. Harvests of 3.5% of the watershed (planned Harvest Il) did not alter the number of moves required to exit the watershed (P = 0.068). When disturbances associat ed with roads and harvests were also examined, large increases in numb er of movements required to exit the watershed occurred (less than or equal to 151%, P = 0.002). These analyses suggest that grizzly bears w ould be disturbed while timber harvests were ongoing, but that long-te rm changes in movement would not occur if roads were closed following harvests. The analyses demonstrate the utility of applying individual- based diffusion models to landscape-level movements of animals, and id entifies the need for telemetry studies to determine movement rates th rough specific habitats.