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By Peter Rejcek
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
, now at Griffith University in Australia, 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.”
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