Assessing the Population Consequences of Disturbance and Climate Change for the Pacific Walrus

Abstract

Climate change and anthropogenic disturbance are increasingly affecting wildlife at a global scale. Predicting how varying types and degrees of disturbance may interact to influence population dynamics is a key management challenge. Population Consequences of Disturbance (PCoD) models provide a framework to link effects of anthropogenic disturbance on an individuals behavior and physiology to population-level changes. Bioenergetic models often constitute the basis of these frameworks, wherein an individuals daily energy balance is simulated over the course of its lifetime, allowing many individuals to be subjected to different environmental conditions and ultimately simulate population-level vital rates under varying degrees of disturbance. In the present study, we develop a Pacific walrus (Odobenus rosmarus divergens) PCoD model to encompass the population-level effects of both anthropogenic disturbance and climate change. Pacific walruses are an Arctic/subarctic ice-associated pinniped. As the Arctic has become increasingly ice-free with climate change, walruses spend more time on land-based (rather than ice-based) haulouts from which they must expend more energy to reach foraging areas, and where they have a greater risk of predation and disturbance-based mortalities. Concurrently, sea ice loss is increasing the anthropogenic footprint in Arctic regions (e.g., fisheries, shipping, energy exploration) which creates additional disturbance. We developed a bioenergetic Dynamic Energy Budget (DEB) model for the Pacific walrus and applied it to four scenarios (ranging from optimistic-pessimistic) which incorporate different global sea ice model projections along with varying degrees of anthropogenic disturbance. All scenarios indicated a decline in Pacific walrus carrying capacity and population growth rate (and thus overall abundance) to the end of the 21st century, but demonstrated that the intensity of that decline could be mitigated by global efforts to reduce carbon emissions (i.e., lessening the rate of sea ice loss) and local management and conservation efforts to protect sensitive habitat areas. In summary, we introduced a flexible PCoD modelling framework in a novel context which will prove useful to researchers studying walruses and other species similarly threatened by rapid environmental change.