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Hit by Climate Change, Dwindling Antarctic Seal Population Grows More Diverse

As sea waters in the South Atlantic warm, the amount of krill available for seals drops, leading to a smaller yet more genetically varied population
A mother and offspring Antarctic fur seal


A mother and offspring were part of the study that analyzed the Antarctic fur seal population of South Georgia Island. The study found that climate change led to a 30 percent decline in the female population from 2003 to 2012.
Credit: Jaume Forcada

Although climate change continues to stir up opportunities and challenges for animals across the world, new research published today in Nature shows the ups and down this change is creating for one species in particular. (Scientific American is part of Nature Publishing Group.)

The study analyzed the Antarctic fur seal population of South Georgia Island, which was observed over the last three decades, to see how climate change is affecting the species. The researchers found a 30 percent decline in the female population from 2003 to 2012. They also found, however, that the declining population would most likely become genetically more diverse as climate change continues. The reason, they say, is that genetically similar females are being excluded from breeding, as reflected by an estimated 17 percent decline in genetic similarity among the female population in the past two decades. “We’ve found that the seals have been significantly affected by climate change,” says Jaume Forcada, who directed the study and serves as marine mammal leader for the British Antarctic Survey (BAS), in a prepared video statement. “This is because the availability of Antarctic krill—the seals’ main food source—has decreased, putting the population under stress.”

The krill population has suffered due to rising sea-surface temperatures. As the amount of krill declines, only the more genetically diverse, or heterozygous, fur seals that are fitter and stronger survive. “The heterozygosity, which is a good indicator of fitness, is selected for within a generation by the fact that nonheterozygous individuals don’t survive to reach breeding age,” says Iain Staniland, marine mammal ecologist at the BAS and part of this research team. “In recent decades, when the climate has changed and food has been scarce, only the very fittest have made it through the difficult first years of life.”

For example, the scientists found that female fur seals that made it to breeding age were 5 percent heavier at birth than those who didn’t. They also found that the females were about a year older on average when they started breeding and had a larger body size compared with the situation two decades earlier when the population was much more stable.

Heterozygosity is not a trait that can be passed down directly to offspring, however, according to Staniland, so many pups are born with a high degree of genetic similarity and cannot survive the changing environment. Therefore, the overall population of the seals continues to decline, despite increased diversity. “They are not adapting to the changes in the environment,” he says.

Forcada stresses that heterozygosity in itself is not an evolutionary response, yet it could maintain higher levels of genetic variation, giving a greater chance of subsequent evolutionary adaptation that could help “buffer” the impacts of long-term climate change.

This new research is valuable because few studies have examined species’ response to environmental changes in such detail for such a long period of time, according to Staniland. “If we had just looked at the levels of heterozygosity in the population, then we would have seen an encouraging increase in overall fitness and the true picture of what was happening would have been missed,” he says. “Because we followed lots of different individuals over their life times, Dr. Forcada was able to show that actually the population was struggling and failing to adapt. With investment and developing technologies we hope to be able to see if the fur seal population response seen in this study is typical,” he notes, adding that he doesn’t know if the seals’ response to climate change is typical but that there would be reason to worry about any animal populations facing such environmental stress.

“Although some species have been known to evolve very quickly in response to the effects of climate change,” Forcada said in the video statement, “these seals are not adapting, they are not evolving. For them, the future is uncertain. Understanding these changes in the long term is one of the best things we can do to anticipate how the entire ecosystem is going to respond to future changes in the climate.”

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