Applications of stable isotope analysis to advancing the understanding of brown bear dietary ecology

Abstract

Dietary ecology is one of the most important drivers of brown bear fitness at the individual and population levels. However, researchers do not have an in-depth understanding of the trophic niche breadth, diet composition, and seasonal diet variation for most Alaskan populations. I set out to better understand multiple facets of brown bear dietary ecology using stable isotope analysis (¹³C & ¹⁵N) as the primary tool to infer brown bear diet and gain insights into their trophic niche, dietary seasonality, dietary generalism and specialism, and isotopic trophic discrimination factors. I determined that using sectioned hair samples is the best practice for determining the isotopic trophic niche of brown bears. Additionally, I determined amino acid trophic discrimination factors for brown bears and explored the ability to separate salmon species in bear diets. I also used stable isotope mixing models with sectioned hair samples to infer seasonal dietary patterns of individual bears in five distinct Alaskan ecosystems. Approximately one-quarter of bears relied solely on vegetation over multiple years despite access to other sources of nutrition; these bears could be considered specialists. Other bears, approximately half, switched diets seasonally but had the same pattern of resource use year over year, a foraging class that I termed persistent seasonal generalism. Approximately one-quarter of bears did not have a persistent dietary pattern across years and could be considered true generalists. Most bears appear to have preferred dietary patterns that are persistent through time, which may be indicative of foraging inertia; maintaining foraging patterns even when faced with changing resource availability due to natural fluctuations, disturbance, or climate change. The sum of this work advances our understanding of brown bear dietary ecology from the individual seasonal level to population level degrees of generalism and specialism, and the methods developed can be applied to many species for which dietary ecology information is difficult to obtain.