Warlick, AJ, Johnson, D., Gelatt, T., Converse, SJ. 2022. Environmental drivers of demography and potential factors limiting the recovery of an endangered marine top predator. Ecosphere 13:e4325.
Abstract
Understanding what drives changes in wildlife demography over time is fundamental to the conservation and management of depleted or declining populations, though making inference about the intrinsic and extrinsic factors that influence survival and reproduction is challenging as they can change over time and space. Here we use almost 20 years of mark-resight data from 2000-2018 to examine the effects of environmental variability on age-specific survival and natality for the endangered western distinct population segment of Steller sea lions (Eumetopias jubatus) in Alaska, USA. Though this population has been studied extensively over the last four decades, the causes of divergent abundance trends that have been observed across the range of this population remain unknown. We developed a Bayesian multi-event mark-resight model that accounts for female reproductive state uncertainty. Results indicated that survival rates for male pups (0.44; 0.36-0.53), female yearlings (0.63; 0.49-0.73), and male yearlings (0.62; 0.51-0.71) born in the western portion of the range, estimated here for the first time, were lower than those estimated for male pups (0.69; 0.65-0.74), female yearlings (0.76; 0.71-0.81), and male yearlings (0.71; 0.65-0.78) born in the eastern portion of the range. Additionally, pup mass had a positive effect on pup survival in the eastern portion of the range and a negative effect in the western portion of the range. Local and basin-scale oceanographic features such as the Aleutian Low, the Arctic Oscillation Index, the North Pacific Gyre Oscillation, chlorophyll concentration, upwelling, and wind in certain seasons exhibited correlations with vital rates. However, strong inference is challenging given that relationships between ocean conditions and an adaptive top predator in a highly dynamic ecosystem are exceedingly complex. This study provides the first demographic rate estimates for the western portion of the population range where abundance estimates continue to decline. This work can inform ongoing research and management and will advance efforts to identify factors driving regionally divergent abundance trends with implications for population-level responses to future climate variability.