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 individual's behavior and physiology to population-level changes. Bioenergetic models often constitute the basis of these frameworks, wherein an individual's 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.

Assessing the Sustainability of Pacific Walrus Harvest in a Changing Environment

“…Although it is difficult to fully predict how walrus population size will respond to these factors, our scenarios provide a useful framework to assess harvest sustainability and quasi-extinction risk in a changing environment. The four climate/disturbance scenarios considered (Table 1) were designed to simulate a broad range of outcomes to the end of the 21 st century (Johnson et al, 2023). Generally, these scenarios indicated the walrus population will decline over time even in the absence of harvest (e.g., Fig.…”

“…The theta-logistic modeling approach we apply in this study is a useful framework to conceptualize the sustainability of harvest under a wide array of scenarios, but it is ultimately a simplification of complex natural processes and population dynamics that relies on a series of assumptions. Most notably, it focuses on the independent-aged female component of the population (following Regehr et al, 2021) because females are the most important reproductive drivers of polygynous marine mammal populations (e.g., Laidre et al, 2008) and because estimates of K and rmax from the PCoD model were derived specifically for independent-aged females (Johnson et al, 2023). Harvest assessments incorporating more complex age-structured matrix models have been developed for other species (e.g., Regehr et al, 2017;Regehr et al, 2021b) but require a higher quality of sex and age-specific harvest and population data than is currently available for the Pacific walrus population.…”

“…The copyright holder for this preprint (which this version posted June 2, 2024. ; https://doi.org/10.1101/2024.05.29.596516 doi: bioRxiv preprint the Pacific Walrus PCoD framework (Johnson et al, 2023) and are specific to the four climate/disturbance scenarios. Because the PCoD model produces distributions of Kt and rmax,t for the middle and end of the 21 st century, we developed a sampling protocol to estimate each of these parameters in each year of the harvest sustainability simulation.…”

“…Pacific walrus carrying capacity (Kt) and maximum intrinsic population growth (rmax,t) form the basis for population growth or decline in the theta-logistic model, and they are expected to change in relation to declining sea ice availability and other anthropogenic factors. The degree of change can be predicted using the population consequences of disturbance/climate change (PCoD) framework developed by Johnson et al (2023) 1; Fig. S2).…”

“…We felt our assumption was appropriate because walruses have high juvenile and adult survival coupled with a low reproductive rate. Distributions for Kt and rmax,t were generated using the Pacific Walrus PCoD framework (Johnson et al, 2023)…”