Polar Bears Face Rapid Tipping Point
Polar bears face 'tipping point'
BBCNews.com,
May 26, 2010
Climate change
will trigger a dramatic and sudden decline in the number of polar bears,
a new study has concluded.
The research is
the first to directly model how changing climate will affect polar bear
reproduction and survival.
Based on what is
known of polar bear physiology, behaviour and ecology, it predicts
pregnancy rates will fall and fewer bears will survive fasting during
longer ice-free seasons.
These changes
will happen suddenly as bears pass a 'tipping point'.
Details of the
research are published in the journal Biological Conservation.
Educated guesses
Until now, most
studies measuring polar bear survival have relied on a method called
"mark and recapture".
This involves
repeatedly catching polar bears in a population over several years,
which is cost and time-intensive.
Because of that,
the information scientists have gathered on polar bear populations
varies greatly: for example, datasets span up to four decades in the
best studied populations in Western Hudson Bay and Southern Beaufort
Sea, but are almost non-existent for bears in some parts of Russia.
Even more
difficult is measuring how survival and reproduction might change under
future climatic conditions.
"Some populations
are expected to go extinct with climate warming, while others are
expected to persist, albeit at a reduced population size," says Dr Peter
Molnar of the University of Alberta, Edmonton, Canada.
However, these
projections are essentially educated guesses, based on experts judging
or extrapolating how current population trends might continue as the
climate changes.
"So we've looked
at the underlying mechanisms of polar bear ecology to assist our
understanding of what will happen in a warming world," Dr Molnar told
the BBC.
Fasting and
mating
Dr Molnar,
Professor Andrew Derocher and colleagues from the University of Alberta
and York University, Toronto focused on the physiology, behaviour and
ecology of polar bears, and how these might change as temperatures
increase.
"We developed a
model for the mating ecology of polar bears. The model estimates how
many females in a population will be able to find a mate during the
mating season, and thus get impregnated."
Male polar bears
find females by wandering the ice, sniffing bear tracks they come
across. If the tracks have been made by a female in mating condition,
the male follows the tracks to her.
The researchers
modelled how this behaviour would change as warming temperatures
fragment sea ice.
They also
modelled the impact on the bears' survival.
Southern
populations of polar bears fast in summer, forced ashore as the sea ice
melts.
As these ice-free
seasons lengthen, fewer bears are expected to have enough fat and
protein stores to survive the fast.
By developing a
physiological model that estimates how fast a bear uses up its fat and
protein stores, the researchers could estimate how long it takes a bear
to die of starvation.
"In both cases,
the expected changes in reproduction and survival were non-linear,"
explains Dr Molnar.
"That is, as the
climate warms, we may not see any substantial effect on polar bear
reproduction and survival for a while, up until some threshold is
passed, at which point reproduction and survival will decline
dramatically and very rapidly."
The US Endangered
Species Act lists the polar bear as "Threatened".
The latest US
assessment of the conservation status of polar bears included the only
two previous studies to assess the impact of climate change, but these
extrapolated population trends, rather than directly modelling how the
ecology of polar bears may alter.
The new study by
Dr Molnar's team offers a way to improve these predictions, and suggests
the potential for even faster declines than those found by the US
assessment.
"Canada has about
two-thirds of the world's polar bears, but their conservation
assessment of polar bears didn't take climate change seriously," says Dr
Molnar, a flaw noted by the IUCN/SSC Polar Bear Specialist Group last
year.
"Our view is that
the Canadian assessment should be redone, properly accounting for
climate change effects.
"The status of
polar bears is likely much more dire than suggested by the Canadian
report," he adds.
"For instance,
for a while we will only see small changes in summer fasting season
survival in Western Hudson Bay.
"[But] eventually
mortality will dramatically increase when a certain threshold is
passed; for example, while starvation mortality is currently negligible,
up to one-half of the male population would starve if the fasting
season in Western Hudson Bay was extended from currently four to about
six months."
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