The future of Arctic marine mammals: tenuous at best

Climate change is transforming the Arctic marine realm. Can mammals adapt?

The startling reduction of Arctic sea ice, caused by global warming, is unleashing a cascade of environmental changes that pose unprecedented challenges to the survival of marine mammals around the North Pole, and their fates hinge on the degree to which they can adapt. In the March special issue of Ecological Applications, scientists examine the ongoing and potential impacts of climate change on these marine denizens of the far north.

KRISTIN LAIDRE

Swimming into an uncertain future. Of all Arctic marine mammals, narwhals may prove to be the most sensitive to the effects of climate change.

Present-day species of Arctic marine mammals have existed for tens to hundreds of thousands of years, persisting through several periods of major climate change. “What’s different this time is the rapid rate of change,” says the issue’s co-editor Sue Moore of the National Oceanic and Atmospheric Administration’s Alaska Fisheries Science Center. Although summer sea ice has been a given for about 800,000 years, recent studies indicate that it is melting at least 3 times faster than climate models have predicted. “Now, following last year’s record sea-ice retreats, researchers at the U.S. National Snow and Ice Data Center are warning the North Pole could be ice-free this summer,” Moore notes.

One sure outcome is an upheaval in the population dynamics of seven core Arctic species—polar bear, walrus, bowhead whale, beluga whale, narwhal, ringed seal, and bearded seal—that depend on sea ice for resting, breeding, hunting, and avoiding predators. Disappearing ice will rob these groups of vast expanses of habitat, although some populations could survive around Greenland and the Canadian Arctic Archipelago, where climate models predict that refugia—small “islands”—of sea ice will persist, report Moore and Henry Huntington, an Arctic researcher and private consultant with Huntington Consulting, in their summary paper (PDF Size: 512 KB).

Future changes in marine food webs represent “the really big element of uncertainty,” Huntington says. “Like a marble on top of a cone, it could go in many directions.” Bodil Bluhm (PDF Size: 3.4 MB) and Rolf Gradinger of the University of Alaska Fairbanks evaluated large-scale biotic-change scenarios based on published observations and environmental forecasts. “As melting opens up formerly shaded areas of the water column to sunlight, it’s reasonable to anticipate expanded plankton growth—as long as additional nutrients are available, which is probable in the central Arctic,” Bluhm says. The benefits of increased primary production could ripple up the food chain to a spectrum of pelagic feeders, from zooplankton to the whales and seals that migrate seasonally to the far North. If this happens as predicted, seafloor-dwelling invertebrates—prey for many ice-associated mammals—could lose out as nutrients from plankton blooms remain in the water column rather than largely settling to the seafloor, as they currently do, Bluhm explains.

Furthermore, increasing inputs of sediment-laden freshwater from the Arctic’s huge rivers are creating conditions unfavorable to both benthic and pelagic species. “If these trends prevail, generalist, open-water-feeding animals could gain advantages, while opportunities for animals requiring prey associated with the seafloor and sea ice would decrease,” she concludes.

Habitat loss, shifts in food webs, rising water and air temperatures, and more severe weather could impact the health of Arctic marine mammals, according to findings (PDF Size: 204 KB) by Alaska veterinary pathologist Kathy Burek and colleagues. The expected growth in marine transport, natural resource extraction, and human habitation means that animals will be exposed to more pathogens and chemical pollution as well as heightened risks of ship strikes, oil spills, and possibly acoustic injury, the researchers warn. With higher levels of phytoplankton growth, fish populations—and commercial fishing—also could surge, Huntington says, likely leading to increased interactions and resource competition between humans and fish-feeding mammals.

A team led by Kristin Laidre of the University of Washington found that species will vary in their response to climate-related change. The researchers reviewed (PDF Size: 928 KB) approximately 350 studies to derive a sensitivity index for 11 marine mammals on the basis of parameters such as population size and growth potential, geographic range, and diet and habitat diversity. “All these species are in trouble,” Laidre says, “but some will be able to adapt more readily. Also, some subpopulations will do better. For example, unlike ice-obligate Pacific walruses, their Atlantic cousins haul out onto land to rest and breed.” The index rankings showed that polar bears, hooded seals, and narwhals are most threatened.

As the tide of climate change sweeps in, opportunities will arise to restrict the secondary effects of human activities on Arctic marine mammals, writes Timothy Ragen, executive director of the U.S. Marine Mammal Commission, which sponsored the special issue. “[However,] reductions in greenhouse gas emissions appear to be the only approach that can ensure the long-term conservation of Arctic marine mammals and Arctic ecosystems,” he concludes. —NOREEN PARKS