Biogeochemical tracers of change in Pacific walruses past and present

Abstract

Reduced sea ice and projected food web shifts associated with warming of the Arctic have raised concerns about the future of Arctic species. Pacific walruses (Odobenus rosmarus divergens) use sea ice as a platform for molting, giving birth, and resting between foraging bouts. Exactly how sea ice loss will affect walruses is difficult to predict, due to a lack of information about regional ecosystems and their responses to climate change. The objectives of the research in this dissertation were to 1) examine how walrus diet changed in response to shifting sea ice conditions over the last 4,000 years, with the goal of generating predictions about how current and future ice loss may affect the walrus population; 2) make it easier to directly compare the results of retrospective and contemporary stable isotope studies of walruses; and 3) generate new tools to assist wildlife managers in monitoring the walrus population in an uncertain future. Stable carbon and nitrogen isotope ratios of walrus bone collagen indicated that diet was similar during previous intervals of high and low sea ice; however, diet variability among individual walruses was greater when sea ice cover was low, suggesting decreased abundance of preferred mollusk prey. Modern walrus diet was different from both previous high and low ice intervals, meaning that food webs in the Arctic are still in a state of flux, or that recent changes are novel within the last 4,000 years. Tissue-specific stable isotope discrimination factors were generated for walrus muscle, liver, skin, and bone collagen to improve comparisons between retrospective and contemporary studies of walrus diet. Additionally, lipid normalization models were parameterized for walrus skin and muscle, thereby making future walrus stable isotope research more feasible by reducing analytical costs and allowing the use of non-lethal sample collection. Finally, a novel technique for estimating the age at onset of reproductive maturity using concentrations of zinc and lead in the teeth of female walruses was established. This new approach has the potential to become a powerful tool for monitoring the walrus population and may be applicable to other species. Use of this technique on archived specimens may make it possible to examine changes in wildlife population dynamics across thousands of years.