From the Arctic to the Mojave Desert, terrestrial and marine habitats are rapidly changing. These changes impact animals that are adapted to specific ecological niches, sometimes displacing them or reducing their numbers. From their privileged vantage point, satellites are particularly well-suited to observe habitat transformation and help scientists forecast impacts on the distribution, abundance and migration of animals.
In a press conference Monday at the American Geophysical Union meeting in San Francisco, three researchers discussed how detailed satellite observations have facilitated ecological studies of change over time. The presenters discussed how changes in Arctic sea ice cover have helped scientists predict a 30 percent drop in the global population of polar bears over the next 35 years. They also talked about how satellite imagery of dwindling plant productivity due to droughts in North America gives hints of how both migratory herbivores and their predators will fare. Finally, they also discussed how satellite data on plant growth indicate that the concentration of wild reindeer herds in the far north of Russia has not led to overgrazing of their environment, as previously thought.
Long-term polar bear declines
Polar bears depend on sea ice for nearly all aspects of their life, including hunting, traveling and breeding. Satellites from NASA and other agencies have been tracking sea ice changes since 1979, and the data show that Arctic sea ice has been shrinking at an average rate of about 20,500 square miles (53,100 square kilometers) per year over the 1979-2015 period. Currently, the status of polar bear subpopulations is variable; in some areas of the Arctic, polar bear numbers are likely declining, but in others, they appear to be stable or possibly growing.
“When we look forward several decades, climate models predict such profound loss of Arctic sea ice that there’s little doubt this will negatively affect polar bears throughout much of their range, because of their critical dependence on sea ice,” said Kristin Laidre, a researcher at the University of Washington’s Polar Science Center in Seattle and co-author of a study on projections of the global polar bear population. Eric Regehr of the U.S. Fish and Wildlife Service in Anchorage, Alaska, led the study, which was published on December 7 in the journal Biology Letters.
“On short time scales, we can have variable responses to the loss of sea ice among subpopulations of polar bears,” Laidre said. “For example, in some parts of the Arctic, such as the Chukchi Sea, polar bears appear healthy, fat and reproducing well — this may be because this area is very ecologically productive, so you can lose some ice before seeing negative effects on bears. In other parts of the Arctic, like western Hudson Bay, studies have shown that survival and reproduction have declined as the availability of sea ice declines.”
Regehr, Laidre and their colleagues’ results are the product of the International Union for Conservation of Nature’s (IUCN) Red List assessment for polar bears. To determine the level of threat to a species, IUCN requests scientists to project what the species population numbers will be after three generations. Using data collected from adult females in 11 subpopulations of polar bears across the Arctic, Regehr and Laidre’s team calculated the generation length for polar bears—the average age of reproducing adult females—to be 11.5 years. They then used the satellite record of Arctic sea ice extent to calculate the rates of sea ice loss and then projected those rates into the future, to estimate how much more the sea ice cover may shrink in approximately three polar bear generations, or 35 years.
Lastly, the scientists evaluated different scenarios for the relationships between polar bear abundance and sea ice. In one of them, the bear numbers declined directly proportionally with sea ice. In the other scenarios, the researchers used the existing, albeit scarce, data on how polar bear abundance has changed with respect to sea ice loss, using all available data from polar bear subpopulations in the four existing polar bear eco-regions, and projected forward these observed trends. They concluded that, based on a median value across all scenarios, there’s a high probability of a 30 percent decline in the global population of polar bears over the next three to four decades, which supports listing the species as vulnerable on the IUCN Red List.
“It is difficult to predict what population numbers will be in the future, especially for animals that live in vast and remote regions,” Regehr said. “But at the end of the day, polar bears need sea ice to be polar bears. This study adds to a growing body of evidence that the species will likely face large declines as loss of their habitat continues.”