Diet energy density estimated from isotopes in predator hair associated with survival, habitat, and population dynamics

Abstract

Sea ice loss is fundamentally altering the Arctic marine environment. Yet there is a paucity of data on the adaptability of food webs to ecosystem change, including predator–prey interactions. Polar bears (Ursus maritimus) are an important subsistence resource for Indigenous people and an apex predator that relies entirely on the under‐ice food web to meet its energy needs. In this study, we assessed whether polar bears maintained dietary energy density by prey switching in response to spatiotemporal variation in prey availability. We compared the macronutrient composition of diets inferred from stable carbon and nitrogen isotopes in polar bear guard hair (primarily representing summer/fall diet) during periods when bears had low and high survival (2004–2016), between bears that summered on land versus pack ice, and between bears occupying different regions of the Alaskan and Canadian Beaufort Sea. Polar bears consumed diets with lower energy density during periods of low survival, suggesting that concurrent increased dietary proportions of beluga whales (Delphinapterus leucas) did not offset reduced proportions of ringed seals (Pusa hispida). Diets with the lowest energy density and proportions from ringed seal blubber were consumed by bears in the western Beaufort Sea (Alaska) during a period when polar bear abundance declined. Intake required to meet energy requirements of an average free‐ranging adult female polar bear was 2.1 kg/day on diets consumed during years with high survival but rose to 3.0 kg/day when survival was low. Although bears that summered onshore in the Alaskan Beaufort Sea had higher‐fat diets than bears that summered on the pack ice, access to the remains of subsistence‐harvested bowhead whales (Balaena mysticetus) contributed little to improving diet energy density. Because most bears in this region remain with the sea ice year round, prey switching and consumption of whale carcasses onshore appear insufficient to augment diets when availability of their primary prey, ringed seals, is reduced. Our results show that a strong predator–prey relationship between polar bears and ringed seals continues in the Beaufort Sea. The method of estimating dietary blubber using predator hair, demonstrated here, provides a new metric to monitor predator–prey relationships that affect individual health and population demographics.

Space‐use strategies drive diet composition of Baffin Bay polar bears

“…The availability of beluga/narwhal and harp/hooded seals is higher in the pack ice as compared to the nearshore areas (Andersen et al, 2009; Finley et al, 1990; Laidre et al, 2004; Richard et al, 1998), which may have contributed to the higher proportions of alternative prey in the diet of offshore females. The higher proportion of alternative prey may also be a signal that offshore bears are eating less overall, a scenario illustrated by concurrent increases in dietary proportions of beluga and a decline in energy density for Southern Beaufort polar bears during periods of low bear survival (Rode et al, 2022).…”

“…The factors driving increased dietary proportions of beluga/narwhal could be attributed to unusual entrapment events of beluga/ narwhal or variation in overall availability during annual whale migrations (Heide-Jørgensen & Acquarone, 2002;Heide-Jørgensen et al, 2003;Laidre et al, 2012). It is also possible that the high proportions of beluga/narwhal indicate that the energy density of the diet decreased during this year (Rode et al, 2022). A high proportion of beluga whales, similar to this study's estimate in 2009, was estimated by QFASA in previous studies of BB polar bears (Galicia et al, 2015(Galicia et al, , 2021Thiemann et al, 2008), as well as in other subpopulations (Florko et al, 2021;Galicia et al, 2021).…”

Giardia and Cryptosporidium in resident wildlife species in Arctic Alaska

“…We cannot determine the source of the single Giardia infection we identified but multiple pathways are plausible. Giardia parasites have been detected in ringed ( Pusa hispida ) ( Hughes-Hanks et al, 2005 ) and bearded seals ( Erignathus barbatus ) ( Dixon et al, 2008 ), two important prey species for polar bears ( Rode et al, 2022 ), as well as bowhead whales ( Balaena mysticetus ) ( Hughes-Hanks et al, 2005 ), which polar bears commonly scavenge on shore after community harvests in Alaska ( Wilson et al, 2017 ). As polar bears spend increasing amounts of time on land, they may be more frequently exposed to pathogens of terrestrial and/or anthropogenic origin, a pattern that has been observed in the southern Beaufort Sea and among other polar bear populations ( Atwood et al, 2017 ; Pilfold et al, 2021 ; Smith et al, 2022 ).…”

Effects of Prey Switching at Different Stages on Life Parameters of Neoseiulus bicaudus

“…The phenomenon of prey switching refers to a predator's inclination towards preying on species with higher abundance in the prey population [1]. This phenomenon is widely observed in nature and represents the adaptation of predators to their environment [2][3][4], which contributes to the stability of predator populations [5]. The fecundity, survival, and development of predators can be adversely affected by prey switching on predators [2,6,7].…”