Birth weights for female Antarctic fur seals are falling as warming seas deplete their prime food source, but scientists are also seeing genetic variations crucial for survival.
Climate change has begun to take its toll of one of Antarctica’s top predators. The Antarctic fur seal is being born with a lower weight and tends to breed later than earlier generations − almost certainly in response to the reduced availability of its prime food, krill.
But the fur seal is also changing in other ways. British Antarctic Survey scientists report in Nature that those females that survive to motherhood are more likely to have a higher level of genetic variation − a characteristic known as “heterozygous,” associated with higher fitness in many species.
In a world of environmental change – of warming seas and changing ocean chemistry – this confers a survival advantage, in that the individual is more likely to cope with the stresses of change.
Research such as this is based on long periods of observation, and the scientists gathered data from as far back as 1981 to assess the changes in a population of fur seals in South Georgia, in the southern Atlantic.
They measured age, body length, weight, counted the numbers of sea pups, noted the diet, and recorded climate data. They also took genetic samples from 1,728 seals.
“Compared with 20 years ago, we can see that female fur seals are now born with a lower weight, those that survive and return to breed tend to be the bigger ones, and they have their first pup later in life than they used to,” said the report’s lead author, Jaume Forcada, a marine mammal leader.
“Such changes are typically associated with food stress. An important food source for the seals is Antarctic krill, and decades of data collected at South Georgia show how changes in the seal population have occurred over time with krill availability.
“Even if krill is very abundant, environmental variation determines its availability in the seals’ feeding grounds. This variation is driven by climate, which impacts local atmospheric, sea ice and oceanographic conditions.”
If the climatic conditions are challenging, then krill is harder to find, which makes it tough on fur seals and, directly or indirectly, all other Antarctic predators. But the picture for the moment remains uncertain.
Recently, other researchers pronounced that the population in Antarctica of the Adélie penguin – another greedy consumer of krill – is higher than all previous estimates, which suggests that some species at least are, for the moment, finding enough for supper.
But the krill population is sensitive to a south polar phenomenon called the southern annular mode, or SAM, a seasonal pattern of winds and pressures that changes from time to time.
The team found that the seal population responded to the notorious El Niño cycle in the Pacific, and to the SAM, and that the data from this population clearly showed a response to climate change. Overall, the number of heterozygous seals has increased by 17%, but other indicators of survival fitness are not so encouraging.
“Over the last two decades, the proportion of breeding females that are highly heterozygous has increased, as these individuals are more likely to survive the changing conditions,” said the report’s co-author, Joe Hoffman, reader in population genetics at Bielefeld University, Germany.
“Strong selection by the environment can drive rapid evolution. However, in this case the seals do not appear to be evolving because surviving females do not pass their heterozygosity on to their offspring.
“Therefore, with each new generation the process of selection has to start all over again, with only those individuals that happen to be born heterozygous having a good chance of survival. As the climate continues to change, many fur seal pups are not surviving to adulthood, and the population is declining.” – Climate News Network