Marine Mammals and Sea Ice Loss: Convergence of Ecological and Evolutionary Time

The ecological effects of diminishing sea ice cover on marine mammals are the subjects of several investigations, but the evolutionary implications have received little attention. We suggest that the rapid environmental change in the Arctic "coupled with the evolutionary history of marine mammals" argues that evolution (sensu stricto) will likewise be rapid.

Landmasses and sea ice cover have limited the movements and gene flow among marine mammals and, thereby, shaped their evolution. The evolution of walruses, at least 10 species of seals, and polar bears involved adaptation to seasonal sea ice as a substrate on which they rest, give birth, nurse young, and molt. Antarctic sea ice provided a refuge from predation and effectively acted as an extension of the terrestrial substrate. Arctic sea ice, however, is largely surrounded by landmasses and served both as extensive substrate and a barrier to gene flow.

On an ecological time scale, rapid declines in arctic sea ice cover–especially summer ice cover–will alter competitive and predator-prey relationships as sub-arctic marine mammals colonize the Arctic. Which species dominate in the new communities will depend on niche breadth, the fate of carbon in the altered regime, and stochastic events. Even species that can be described as overall generalists may be vulnerable, however, as a result of narrow constraints on a single niche axis. Ringed seals, for example, are broad in distribution and diet, but depend on specific snow conditions to raise young. Our knowledge of marine mammal ecology has advanced substantially in the past few decades due to technological advances in radio telemetry and in remote sensing. Those technologies have allowed us to investigate the seasonal importance to marine mammals of specific habitat features including sea ice, snow cover, bathymetry, and biological productivity.

Evolutionary responses to sea ice reductions will include extinctions, adaptive evolution, and introgressive hybridization. The rapid pace of ice loss relative to the long generation times of marine mammals may favor hybridizations and extinctions. Hybridizations also will be favored by the karyotypic uniformity of marine mammals. Adaptive responses will be limited by the pace of environmental change but may be favored by high selection coefficients. Changes in gene frequencies (evolution) among arctic marine mammals are likely to take place on a scale more typically thought of as ecological.