Food resources for grizzly bears at army cutworm moth aggregation sites in the Greater Yellowstone Ecosystem

Abstract

Army cutworm moths (Euxoa auxiliaris) (ACM) migrate annually to peaks on the eastern edge of the Greater Yellowstone Ecosystem (GYE). Grizzly bears (Ursus arctos horribilis) feed on these moths from mid-to late summer. The Shoshone Forest is preparing a management plan to address the conservation of these sites and foraging bears. Increased human use and GYE-wide changes in grizzly bear food availability and related foraging patterns are concerns prompting plan preparation. This study addresses grizzly bear diet and vegetation foraging locations on a prominent moth site ('South Site'). A 1991 study identified 4 forb genera utilized by bears at ACM sites. A 2017-2018 study identified 5 more and postulated that biscuitroot (Lomatium spp.), found in high elevation meadows, was an important resource for grizzly bears. During 2020-2021 we clarified these findings using scat collection and descriptions of available vegetation. We determined the frequency and volume of food items in 298 scats. We quantified vegetation at peak meadows (elevation: 3,078 - 3,657-m) and in cirque basins (elevation: 3,658 - 3,931-m) to record the percent cover of nine forb genera. We also described the density of biscuitroot and craters where bears excavated roots to determine if biscuitroot influences foraging site choices for grizzly bears. We confirmed use of 7 of the 9 previously identified forb genera. The most frequently consumed foods by grizzly bears were ACM (23% volume) and roots and tubers (38% volume). Similarly, the 2017-2018 study found 20% ACM by volume and 45% roots and tubers by volume. There was a positive, linear relationship between the density of flowering biscuitroot and craters from grizzlies digging roots in several peak meadows (p < 0.001). Rather than foraging solely on ACMs, grizzly bears on this moth site relied highly on vegetation in their diet, specifically roots and tubers from biscuitroot and clover. Our findings suggest grizzly bears have a diverse diet at this moth site that may allow them to adjust to variations in ACM abundance. They focused foraging on roots and tubers at 5 peak meadows near talus where moth foraging occurs; information that can potentially help mitigate human-grizzly bear interactions involving climbers.