Ecological Plasticity in Seals May be Key to Climate Change Survival
Crockford, S. J. and G. Frederick, G. 2011. Neoglacial sea ice and life history flexibility in ringed and fur seals. pg. 65-90 In T. Braje and R. Torrey (eds), Human and Marine Ecosystems: Archaeology and Historical Ecology of Northeastern Pacific Seals, Sea Lions, and Sea Otters. University of California Press, LA.
It has been known for decades that the Arctic ringed seal, Phoca hispida has two distinct ecotypes: a large-bodied, highly territorial form that gives birth and breeds on the stable, shorefast ice and a smaller, early maturing form that lives and breeds exclusively in the mobile offshore pack ice. Some biologists have insisted that pack-ice breeding individuals make up a significant portion of the total ring seal population, not a minor one, a position that is now supported by a recent genetic study (Davis et al. 2008). Despite this evidence, the role that ecological plasticity might play in the survival of ringed seals to future climate change has been routinely ignored (e.g. Kelly et al. 2010).
For at least 200 years, northern fur seals (Callorhinus ursinus) have used the Pribilof Islands in the eastern Bering Sea as a rookery for giving birth and breeding. About 90% of all fur seals migrate every summer to the Pribilofs, after which females and juveniles travel as far south as California. Biologists consider the strongly migratory nature of the modern fur seal one of its defining life history characteristics. However, it hasn't always this way: from at least 5,000 years ago until just before Europeans arrived, fur seals also maintained rookeries along the west coast of North America from Alaska to southern California (Crockford et al. 2002). These southern populations were not migratory, moving onshore/offshore with the seasons rather than north/south (Newsome et al. 2007): in other words, it appears that fur seals also have two distinct ecotypes. What environmental condition could have made prehistoric fur seals generate a non-migratory ecotype?
In this new work, Crockford and Frederick discuss the phenomenon of ecological plasticity and life history flexibility of ringed seals and fur seals in relation to work completed in 2004, where they identified marine mammal skeletal remains recovered from an archaeological site in the eastern Aleutians, Alaska (Crockford and Frederick 2007). This site (located near the modern fishing port of Dutch Harbor) was occupied by ancient Aleuts during the Neoglacial cold period that prevailed between 5500 to 2000 years ago.
The authors identified almost 6000 bones. For many of these, they estimated the age of the animals represented. This was considered important because newborn animals are usually found in very specific habitats at very specific times of year.
Ringed seals made up 32% of the identified bones and of these, 90% were from 2-6 month old pups. Pups this age are found almost exclusively at the ice edge. The few adult bones recovered were as small as known pack ice ringed seals. Most of the pup bones were developmentally advanced for their small size, suggesting to the authors that they represented pack ice ecotypes also. The interpretation that pack ice must have been present offshore was supported by the fact that many bones of bearded seals were also identified, including newborns and newly-weaned young: bearded seals give birth and nurse their pups exclusively in the pack ice. The authors discuss their finding that the large, shorefast ice ecotypes of ringed seals were over-represented in both biological accounts and museum skeletal collections they consulted-a collection and research bias also note by Davis et al (2008) in their genetic study. Both authors suggest that this phenomenon makes it appear that the large shorefast ecotypes are more common.
Crockford and Frederick suggest that their sample of small-sized ringed seal bones constitute evidence that a pack ice ecotype was available during the Neoglacial and supports modern evidence that pack ice ringed seals make up a significant portion of the total population. They contend (pg. 83) that "having two life history strategies-one that utilizes shorefast ice and another that takes advantage of mobile pack ice-has almost certainly given ringed seals the evolutionary flexibility they have needed to survive in the Arctic over millions of years; such flexibility certainly needs to be factored into modern ecological studies and population estimates." In contrast, in a recent publication on the current status of the ringed seal by American NOAA scientists (Kelly et al. 2010), the phenomenon of pack ice vs. shorefast ice ecotypes is mentioned briefly in passing (pg.36) but apparently considered irrelevant to the discussion of how ringed seals might adapt to future sea ice changes.
In contrast, fur seal bones made up another 40% of the total sample from the Aleutian archaeological site. Of those, a large number represented unweaned pups (< 4 months of age), a strong indicator that pups were being harvested from a nearby rookery (Unalaska Island is about 400km south of the Pribilofs and is too far for ancient Aleuts to have travelled in skin boats). No breeding rookeries exist on or near Unalaska Island today, nor were any known to have existed historically but a rookery must have existed nearby during the Neoglacial.
Crockford and Frederick conclude that the Pribilof Islands must have become inaccessible to fur seals during the Neoglacial due to southward-expanding sea ice-these condition appear to have prompted fur seals to establish non-migratory breeding colonies along the Aleutians and down the west coast of North America.
As the authors point out, migratory northern fur seals must have made major adjustments to their migratory habit during previous ice ages: sea level declines of more than 100 meters turned the eastern Bering Sea (including the Pribilofs) into a Land Bridge at least three times over the last 200,000 years. Not until rising sea levels at the end of the last Ice Age made the Pribilofs accessible, about 10,000 years ago, could fur seals begin using them as a rookery. However, by 5,500 years ago, they were too ice-bound to be suitable.
The ability of fur seals to adjust to such profound changes demonstrates that this migratory marine mammal is much more flexible than they appear and far more adaptable to changing climatic conditions than we give them credit for-perhaps not surprising for an animal that has existed in this area for two million years or more (Boessenecker 2011). Ringed seals have existed for just as long, through as many ice ages: having two distinctly different ecotypes for different types of ice has almost certainly been a significant factor in the ability of ringed seals to survive past fluctuations in sea ice. Such ecological flexibility needs to be factored in to any prediction of what might happen in response to future changes in sea ice conditions.
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