Cradle to grave
Study provides insight into evolution and extinction of vanished elephant seal colony
Posted August 7, 2009
An extinct southern elephant seal colony that once existed in huge numbers along sandy and rocky beaches in Antarctica has provided new insight into how quickly a species can respond to the emergence of a new habitat as climate changes — and just as quickly disappear.
That’s one of the findings in a paper published in the journal PLoS Genetics in July by scientists who studied DNA sequences from the organic remains of seals found along a nearly 300-kilometer stretch of coastline in Victoria Land, just north of the U.S. Antarctic Program’s McMurdo Station.
Mark de Bruyn, lead author of the study and now with Bangor University in the U.K, said the findings showed that a very large, genetically diverse breeding population of southern elephant seals existed in the Ross Sea region around 7,000 to 400 years ago.
“I believe the research is quite novel, as it tracks the population from inception to extinction,” de Bruyn said via e-mail while conducting fieldwork in Borneo. “The short timeframe is also noteworthy. Most of the ancient DNA work to date has focused on much older timeframes.”
One exception, he added, has been work by David Lambert from Massey University in New Zealand, whose research has included studying “microevolution” in Adélie penguins over the last 6,000 years.
The Victoria Land colony would have represented the southernmost extent of southern elephant seals in the world, according to Brenda Hall, a geologist with the University of Maine and a co-author on the paper. Hall, whose research interests include reconstruction of paleoclimate conditions in Antarctica, led the field team that collected the remains of the seals.
The closest southern elephant seal colony today lives on the subantarctic Macquarie Island, which lies about halfway between Antarctica and Australia. The Victoria Land colony originated from Macquarie, home to about 80,000 seals, de Bruyn said. He estimated the Victoria Land colony had as many as 220,000 breeding individuals at its height, larger than any extant, or existing, colony today.
Climate change, the scientists say, allowed the colony to both thrive and later collapse.
It appears the ice sheet along the coast began to recede about 8,000 years ago as the interglacial climate warmed — the time period between ice ages, the most recent being the Holocene. In addition, the sea ice that would have blocked access to the beaches appears to have disappeared or declined enough for long periods of time each year to allow the seals to breed and molt on land, Hall said.
The colony then began to decline about 1,000 years ago, according to the researchers, indicating yet another change in the climate.
“Our main conclusion is that things have cooled off in that part of the western Ross Sea over the last 500 to 1,000 years and the sea ice has re-expanded,” Hall said. “We also see some evidence of glacier re-expansion at that time as well.”
Hall said it is unclear just how much warmer the climate was before it shifted again. She said the next logical step would be to create a model to determine what sort of climate might have existed to create relatively sea ice-free conditions favored by the southern elephant seals, massive pinnipeds whose male bulls can tip the scale at 4,000 kilograms.
“The ocean had to be warmer. How much warmer, I don’t know,” she said.
The Ross Sea cooling period roughly coincides with the so-called Little Ice Age in Europe, which brought colder winters to both Europe and North America for several centuries. There is still quite a bit of scientific debate on the timing of the Little Ice Age and its global connectedness.
“The timing of this cooling in the Antarctic is not exactly the same as the Little Ice Age,” Hall said, adding that her team and colleagues in New Zealand are just beginning to work on that problem. “It may be a bit more complicated than that.”
There is also some debate, she said, about the current climate trend in the Ross Sea area. Most scientists agree that parts of West Antarctica and the Antarctic Peninsula are warming, but disagreement exists on East Antarctica — a much larger ice sheet that mostly sits on bedrock above sea level, unlike its partner on the other side of the Transantarctic Mountains.
From her fieldwork and observations dating back to the 1990s — and that of University of Maine colleague George Denton, who first visited Antarctica in 1958 — it appears that some glaciers have retreated in the region, Hall said. She estimated that coastal glaciers might have receded as much as 500 to 600 meters, based on 1950s aerial photography.
“It may have been warming a little bit on the coast,” Hall said, but added that a paucity of data makes it difficult to know if the warming is part of a long-term trend or the workings of a natural cycle.
A little warmer or colder, one wouldn’t mistake the Victoria Land coast today as a place for a beach vacation, let alone as a location for weeks of fieldwork. Hall and her field team, including co-principal investigator Paul Koch from the University of California, Santa Cruz, picked over every ice-free spot from the mouth of Taylor Valley near McMurdo Station north to near Terra Nova Bay on their hunt for the extinct seals.
The ancient remains of the animals mostly included skin and hair, along with some bones and skeletons. At first, Hall said, locating the bits of organic material is a bit like a needle-in-the-haystack search.
“It’s not at all obvious. People have walked over these beaches for decades without finding it,” she said. “But once you know what to look for, it’s very obvious. Usually it’s under rocks, so you have to get down on your hands and knees and start flipping over a lot of rocks.
“There’s a bit of an art to it, I guess, and a bit of luck to it as well,” she added.
Rus Hoelzel, another co-author of the paper from Durham University in the U.K., said in a press release from his institution that while the study looks at the past, it has implications for how future environmental change may affect marine and terrestrial systems.
“We’ve shown how a highly mobile marine species responded to the gain and loss of new breeding habitat. The new habitat was quickly adopted, probably because seals migrate annually into Antarctic waters to feed,” he said. “However, when the ice returned and the habitat was lost, only a small proportion returned to the original source population. The Antarctic population crashed and much diversity was lost.”
If climate does warm in the Ross Sea and sea ice again declines as it did in the past, it’s possible the southern elephant seals may return to their abandoned breeding grounds in Antarctica, according to de Bruyn.
“But how this warming impacts the present day colonies on Antarctic and sub-Antarctic islands is unknown,” he said. “It could potentially become too warm on these islands for viable colonies, and, of course, the distribution of food resources would be critical to their survival … We don’t know how this is going to change in the future.
“Of course, other less mobile species are unlikely to be able to track habitat availability resulting from climate change as efficiently as the elephant seal,” de Bruyn added. “In no way should this research be interpreted as warming being a good thing for southern elephant seals. We don’t understand the dynamics well enough yet.”
NSF-funded research in this story: Brenda Hall, University of Maine, Award No. 0439979; and Paul Koch, University of California, Santa Cruz, Award No. 0439906.
About the Sun