Living on the drifting sea ice: polar bears walk on a food conveyor belt

Submitted by editor on 6 May 2015. Get the paper!
Figure caption: Polar bears travel and hunt on the sea ice, a platform that can drift many kilometers per day. To understand their home range, and that of other marine species, we need to account for drift. Photo by Andrew Derocher


By Marie Auger-Méthé


Many animals confine their movement to a much smaller area than would be expected based on their mobility. These animals often return to the same locations to forage, mate, or give birth. The broad area encompassing these frequently used locations is referred to as an animal’s home range. The home range can be used to identify the potential areas containing the conditions and resources necessary for the survival and reproduction of a species, and thus the conditions that promote occupancy. Home range size can represent how much space is required by individuals of a species to thrive.

The home range concept is fundamental to our ecological understanding of terrestrial animals and has played an important role in the conservation and management of these species. However, the home range concept is less prevalent in marine studies. Using the polar bear as an example, we demonstrate that using standard home range tools on marine species can be misleading, especially if drift is ignored.

Polar bears return to the same geographic region annually and follow similar movement patterns between seasonally important regions. However, aspects linked to the amount of prey encountered and energy expended cannot be related to this geographic area without incorporating ice drift. Sea ice can move many kilometers per day. The average daily ice drift experienced by the bears in our study was 4.5 km per day. The main preys of polar bears, ringed seals, are linked to the sea ice. Ringed seals must maintain breathing holes and birth lairs to prevent them from freezing closed, and thus are tied to a specific piece of ice from freeze-up to melting. Thus, for polar bears, the area of visited sea ice will represent the resource aspect of their home ranges better than the geographic area.

We developed a technique to estimate the amount of sea ice visited by bears. We show that the traditional geographic home range can underestimate both the movement of bears and the amount of space, and thus potential resources, encountered. Moreover, area of  encountered ice increased with ice drift, indicating that bears living on highly mobile ice might be exposed to higher energetic costs, but potentially larger energetic gains, than bears inhabiting stable ice. The methods and concepts presented here can serve as a foundation for new approaches to study the space use of species living in moving environments.

Full paper here.