Increasing nest predation will be insufficient to maintain polar bear body condition in the face of sea ice loss

Abstract

Climate change can influence interspecific interactions by differentially affecting species-specific phenology. In seasonal ice environments, there is evidence that polar bear predation of Arctic bird eggs is increasing because of earlier sea ice breakup, which forces polar bears into nearshore terrestrial environments where Arctic birds are nesting. Because polar bears can consume a large number of nests before becoming satiated, and because they can swim between island colonies, they could have dramatic influences on seabird and sea duck reproductive success. However, it is unclear whether nest foraging can provide an energetic benefit to polar bear populations, especially given the capacity of bird populations to redistribute in response to increasing predation pressure. In this study, we develop a spatially explicit agent-based model of the predator-prey relationship between polar bears and common eiders, a common and culturally important bird species for northern peoples. Our model is composed of two types of agents (polar bear agents and common eider hen agents) whose movements and decision heuristics are based on species-specific bioenergetic and behavioral ecological principles, and are influenced by historical and extrapolated sea ice conditions. Our model reproduces empirical findings that polar bear predation of bird nests is increasing and predicts an accelerating relationship between advancing ice breakup dates and the number of nests depredated. Despite increases in nest predation, our model predicts that polar bear body condition during the ice-free period will continue to decline. Finally, our model predicts that common eider nests will become more dispersed and will move closer to the mainland in response to increasing predation, possibly increasing their exposure to land-based predators and influencing the livelihood of local people that collect eider eggs and down. These results show that predator-prey interactions can have nonlinear responses to changes in climate and provides important predictions of ecological change in Arctic ecosystems.

Fatty acid‐based diet estimates suggest ringed seal remain the main prey of southern Beaufort Sea polar bears despite recent use of onshore food resources

“…This study represents the first attempt to quantify seabird as prey in the diet of polar bears and has only one potentially representative avian species; therefore, we recommend further research to quantify the different seabird species and other potential onshore prey susceptible to polar bear predation and at what rates, as these pieces of evidence would suggest how onshore foraging by polar bears may impact seabird populations. Although our study has suggested that they may only represent a relatively minor component of polar bear diets, devastation of breeding colonies by polar bear predation has been reported (Dey et al, ; Iverson et al, ; Smith et al, ), and if sustained over multiple breeding seasons, creates the potential for localized reproductive failure.…”

“…Observations of predatory activities of individual polar bears in a small number of subpopulations have suggested substantial depredation of colonial nesting seabirds, particularly in recent years (Dey et al, ; Iverson et al, ; Smith et al, , G. Divoky, unpublished data). However, the actual contribution of this onshore resource to the diets of polar bears at the subpopulation‐level has not been previously evaluated.…”

“…Predation on seabirds has also been observed with increasing frequency in a few subpopulations (e.g., western Hudson Bay [WHB], Davis Strait [DS], Foxe Basin [FB]), as polar bear onshore arrival now overlaps with seabird nesting activities to a greater degree (Dey et al, , ; Gormezano & Rockwell, ; Iverson et al, ). There are 50 million or so seabirds that nest in more than 1,600 colonies on Alaska's coasts in the summer season, representing about 87% of total number of seabirds in the United States (U.S.…”

“…For polar bears, the energetic inputs from lipid‐rich, ice‐associated prey species are crucial for the high‐energy lifestyles that come along with living in the Arctic (Rode et al, ). Accordingly, it has been posited that onshore resources are not important from the perspective of energy intake, but few have been included in quantitative diet estimates (Dey et al, ; Pilfold et al, ; Rode et al, ). While bowhead whale has been recently included in both fatty acid and stable isotope‐based diet estimates (Cherry, Derocher, Hobson, Stirling, & Thiemann, ; McKinney et al, ; Rogers et al, ), seabird has not yet been considered.…”

Polar Bear Foraging Behavior During the Ice-Free Period in Western Hudson Bay: Observations, Origins, and Potential Significance

“…The energy efficiency of egg scavenging will depend to a great extent on whether the bear is actively targeting and moving from nest to nest, using either olfactory or visual cues as did the bear eating eider eggs (appendix: 3), or if the bear is simply eating eggs from nests as they are randomly encountered (cf. Dey et al, 2017).…”