By Matt Walker
Editor, Earth News |
A male in prime condition is spotted tracking a female
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".
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We may not see any substantial effect on polar bear reproduction and survival until some threshold is passed.
At that point reproduction and survival will decline dramatically and very rapidly
Peter Molnar University of Alberta, Edmonton, Canada
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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."
Adult male polar bears gather near Churchill, Manitoba waiting for the sea ice to reform
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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.
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WATCHING POLAR BEARS
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"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.
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SOURCES
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"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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