Beyond the stranded polar bear: consequences of melting Arctic sea ice for animal movement

Minimum sea ice extent in 2017. The historical minimum extent is shown with an orange line.

Rapid declines in sea ice are likely to have profound effects on the movement of animals, with warming temperatures melting ice bridges and opening new watery routes.

A longer melt season and reductions in the extent and thickness of sea ice are leading to an Arctic Ocean covered more and more by open water rather than meters-thick sea ice. Arctic sea ice hit a record low maximum extent (at the end of winter) in 2017 and remained well below historical levels throughout the summer. Since 1979, when satellite data were first available, the sea ice minimum (at the end of the summer) has declined by 13.2% per decade.

For some animals, sea ice is a bridge facilitating dispersal and long distance migration across the Arctic. While images of stranded and swimming polar bears probably come to mind first, the loss of this sea ice can severely restrict the movement of many other species as well. For example, population genetic analysis showed that Peary caribou (Rangifer tarandus pearyi) in the Canadian Archipelago use sea ice to disperse to other islands, migrate between calving and wintering grounds, and escape extreme weather. A longer ice-free season is predicted to constrain these inter-island migrations, leading to a long-term reduction in gene flow and an increase in inbreeding depression.

For other species, sea ice represents a frozen barrier that hinders movement. Reductions in sea ice may actually lead to greater movement over the Arctic for these species. Sea birds dispersing across the Arctic require access to open water, and whales require access to openings in the sea ice to breathe. McKeon et al. reviewed evidence that sea ice melt allows for the exchange of tetrapods (four-limbed vertebrates) between the Atlantic and Pacific oceans via the Northwest Passage. Vagrant bowhead whales (Balaena mysticetus) from the two oceans, for example, were tracked moving within the same area in the passage after the populations were assumed to be separated by ice.  The ecological and evolutionary consequences of a reduced ice barrier are largely unknown.

Partly due to the challenge of collecting data in an expansive and often inaccessible area, our current knowledge of the impacts of sea ice loss on animal movement is just the tip of the iceberg. As the Arctic continues to warm twice as fast as the global average, there is an urgent need to understand and predict the consequences of melting sea ice.

Resources

Heide-Jorgensen, M. P., Laidre, K. L., Quakenbush, L. T., and Citta, J. J. 2012. The Northwest Passage opens for bowhead whales. Biology Letters 8(2): 270–273.

Jenkins, D.A., Lecomte, N., Schaefer, J.A., Olsen, S.M., Swingedouw, D., Côté, S.D., Pellissier, L. and Yannic, G. 2016. Loss of connectivity among island-dwelling Peary caribou following sea ice decline. Biology Letters 12(9): 20160235.

McKeon, C., Weber, M.X., Alter, S.E., Seavy, N.E., Crandall, E.D., Barshis, D.J., Fechter‐Leggett, E.D. and Oleson, K.L.L. 2016. Melting barriers to faunal exchange across ocean basins. Global Change Biology 22(2): 465-473.

Markus, T., Stroeve, J.C. and Miller, J., 2009. Recent changes in Arctic sea ice melt onset, freezeup, and melt season length. Journal of Geophysical Research: Oceans 114(C12).