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

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 of changes, concerted management efforts have allowed the grizzly bear population to increase significantly. As a result, the range and density of the bear population has also increased, despite a decline of some high-calorie foods. This study investigated the intraspecific processes driving bear population demography 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 the two decades and in relation to a temporally and spatially explicit index of grizzly bear density, individual traits, and landscape zonation. Specifically, we hypothesized that individual lean body mass declined as population density increases, and that density had an age-dependent effect. Further, we hypothesized that individual body fat levels were independent from 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 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 moving to areas with fewer bears. In contrast, we found that percent body fat remained constant as grizzly bear population density increases, despite recent environmental changes, possibly by shifting feeding tactics. Our study shows that individual Yellowstone grizzly bear performance is influenced more by intraspecific competition than landscape-level food changes, highlighting a notable resilience of grizzly bear to environmental alterations.