Abstract 251 – Evidence for density-dependent effects on body composition of grizzly bears in a changing Greater Yellowstone Ecosystem

Andrea Corradini, Mark A. Haroldson, Francesca Cagnacci, Cecily M. Costello,
Daniel D. Bjornlie, Daniel J. Thompson, Jeremy M. Nicholson, Kerry A. Gunther, Katharine R.
Wilmot, Frank T. van Manen

The Greater Yellowstone Ecosystem, although recognized as one of the world's least impacted
temperate ecosystems, has undergone environmental alterations over the last decades. During
this period, concerted management efforts have allowed the grizzly bear population to increase
significantly. As a result, the range and density of the bear population have also increased,
despite a decline of some high-calorie foods. This study investigated the intraspecific processes
driving bear population demographics in the face of climatic and human impacts affecting the
availability of some key food resources. We examined lean body mass and percent body fat
from >400 grizzly bears over two decades and in relation to a temporally and spatially explicit
index of grizzly bear density, individual traits, and geographic areas. Specifically, we
hypothesized that individual lean body mass declined as population density increased, and that
density had an age-dependent effect. Further, we hypothesized that the ability to gain body fat
during the active season was independent of population density and environmental changes, as
omnivory helped buffer energy intake from fluctuation in high-calorie food sources. We found
that lean body mass was negatively related with grizzly bear population density, particularly in
young females. Although higher bear densities had a more negative impact on female bears,
they still reached their typical total body mass as they matured (>7 years of age), which may be
due to delaying reproduction or dispersal to areas with fewer bears. In contrast, we found that
the seasonal rate of body fat gain remained constant as grizzly bear population density
increased, despite recent environmental changes, possibly by shifting feeding tactics. Our study
shows that individual performance is influenced more by intraspecific competition than
landscape-level food changes, highlighting a notable resilience of Yellowstone grizzly bears to
environmental alterations.