Predators and scavengers: Polar bears as marine carrion providers

Abstract

Scavenging is a foraging strategy widely used across the animal kingdom and apex predators provide a large amount of energy in a food web by provisioning carrion. In the harsh environmental conditions of the Arctic, apex predators such as polar bears Ursus maritimus can provide scavenging opportunities for many species. Carrion can act as a buffer when food resources are low, and some terrestrial species use the marine environment for cross-ecosystem resource subsidies. We present an overview of scavenging as a foraging strategy in the Arctic marine environment and examine the contribution of prey provided by polar bears to the Arctic scavenging assemblage. As obligate predators of seals, polar bears contribute a substantial amount of carrion to the marine ecosystem, particularly to the sea ice surface where it is accessible for seasonal scavenging opportunities. We estimated that an average polar bear kills approximately 1001 kg of marine mammal biomass annually and given preferential feeding of blubber and abandonment of carcasses, we estimate that 30% of the biomass is left as available carrion. Consequently, polar bears provision approximately 7.6 × 106 kg year–1 of carrion biomass for scavengers across their range, equivalent to 3.93 × 107 MJ of energy. Eleven vertebrate species are known to scavenge polar bear kills, and an additional eight are possible scavengers. While foraging associations with polar bear kills for some species are better understood, others are scarce or undocumented. We provide an overview of what is known about the role of polar bears as carrion providers, the network of scavenging species on the sea ice, and the possible consequences of trophic downgrading in this ecosystem and recipient ecosystems.