Juvenile predation overwhelms nutritional effects on female ungulate fat reserves in a high-predation system

Ungulate body fat reserves reflect the nutritional environment, often serving as a useful indicator of bottom-up resource availability. However, body fat reserves also integrate energetic costs associated with avoiding predation risk and reproductive effort, and it is unknown how bottom-up and top-down factors integrate to affect body fat reserves. We used generalized mixed-effects models to evaluate how bottom-up, top-down and intrinsic factors explained variation in winter ingesta-free body fat (IFBF) of female elk (Cervus canadensis) near Banff National Park, Alberta, Canada from 2015 to 2021. We found two top models were competitive, with calf survival during the first 3–4 months of life and female age included in both models. There was more support for bottom-up forage effects than indirect predation risk effects. Based on model predictions, females with a calf surviving through the previous summer had 3.6 percentage points (95% CI: 2.4%, 4.9%) lower IFBF the subsequent winter than females without a surviving calf. Because calf survival during the first 3–4 months of life is largely driven by predation in this system, we suggest top-down effects of predation through calf mortality influenced body fat around nine times more than bottom-up factors in our study. Under high predation, variation in body fat levels of female ungulates across a population may not reflect only bottom-up influences on body condition, but also differences in predation. Thus, both bottom-up and top-down factors must be considered when assessing the nutritional environment. Read the free Plain Language Summary for this article on the Journal blog.