The good, the bad and the grizzly : assessing human impact on grizzly bears (Ursus arctos horribilis) at army cutworm moth (Euxoa auxiliaris) aggregation sites in the Shoshone National Forest, Wyoming

Abstract

Successful management has saved the grizzly bear (Ursus arctos horribilis) from extirpation in the contiguous USA. As the population has grown, it has expanded its distribution. Grizzly bears now congregate from July to September in large numbers on alpine talus slopes in the eastern part of the Greater Yellowstone Ecosystem, Wyoming, to feed on army cutworm moths (Euxoa auxiliaris). This ecological phenomenon receives much attention from international film crews, permitted outfitters, and bear enthusiasts. Although the general consequences of human activities on bears has been investigated thoroughly, the impact of human use on grizzly bears at moth sites is poorly understood. In this thesis I describe the extent of human use on Franc`s Peak in the Shoshone National Forest, and its influence on grizzly bears foraging at moth aggregation sites using a resource selection function. I have also explored the spatial habitat use by grizzly bears on Carter Mountain, also using a resource selection function. We observed 72 people in 26 groups on Franc`s Peak between 13 July and 17 September 2017; 88 % of which approached the summit from the northern side. We documented 17 human-grizzly bear encounters in July and August, and the grizzly bears were displaced in 94 % of the them. We did however, not find any evidence of a permanent displacement of grizzly bears by humans. We recorded 121 grizzly bear-locations on Franc`s Peak and 22 grizzly bear-locations on Carter Mountain. Grizzly bears on both sites selected for higher elevation and less steep slopes. In addition, grizzly bears avoided herbaceous habitat on Franc`s Peak and selected for south and east aspects on Carter Mountain. These findings indicate that grizzly bears visited Franc`s Peak and Carter Mountain primarily to consume army cutworm moths. The current level of human use on Franc`s Peak appears to be too low to have a permanent impact on grizzly bears` habitat use. However, the relative high rate of encounters between grizzly bears and humans, and the bears` responses, suggests that the disturbance potential is high if the number of humans were to increase.