Even highly mobile, large, and long-lived species are sensitive to dynamic and changing conditions as the Arctic warms. A new study reports that population swings in eastern North Pacific grey whales – some of which have resulted in recent mass mortality events – are driven by changing prey biomass and ice cover in the Arctic. Climate change is driving rapid change in Arctic ecosystems, including the highly productive shallow basins of the Pacific Arctic, which are critical marine areas that support seasonal foraging opportunities for various migratory marine species. While climate impacts affect lower-trophic level and short-lived species most directly, it is less clear how long-lived migratory species will respond to rapid polar warming. Eastern North Pacific grey whales travel great distances across the Pacific to summer in these areas in the Pacific Arctic, where they feed on highly abundant benthic crustacean prey. Populations of these whales – a species hunted to near extinction by the early 1900s through commercial whaling – have shown great success in rebounding to estimated pre-whaling levels. However, several unexpected major mortality events over the past 35 years have each led to large declines in grey whale abundance, casting concern on the status and stability of the whale's population.
It's been proposed that variable and changing Arctic conditions may be drivers of grey whale population dynamics. Joshua Stewart and colleagues combined more than 50 years of data on grey whale abundance, calving, body condition, and strandings with a population dynamics model to better understand the factors underlying the carrying capacity for the population. Stewart et al. discovered boom-and-bust cycles in abundance driven by large and rapid changes in their Arctic food supply – and access to it. When low prey biomass coincided with high ice cover, grey whales experienced pronounced mortality events, each reducing the population by ~15 to 20%. According to the findings, in the last 35 years alone, grey whales have experienced three significant population "busts." Moreover, Stewart et al. suggest that, given current trends, future climate conditions in the Arctic will not be favorable for the whales' vital prey species,. "If Stewart et al. are correct, then species with a narrow dietary niche composed of low-trophic-level prey species should be expected to exhibit considerable variation in their demography as environmental conditions change," writes Andrew Read in a related Perspective.
