Concerns
have been raised that female polar bears are running out of eligible
males because of the way the creatures are being hunted, which could
trigger the sudden collapse of endangered populations. |  | Female polar bear with her cub: The study raises the possibility of female polar bears not being able to find a mate |
Even though the bear is deemed vulnerable by conservationists,
management policies in Canada - where 60 per cent of the world's
population live - encourage hunters to select for males in order to
conserve females while maximising the number of bears that may be
harvested for the fur trade, recreational hunting and Inuit
communities, where bear hunts are a tradition.
"However, prolonged sex-selective harvest has reduced the numbers of
adult males in all Canadian polar bear populations, leading to
female-biased sex-ratios," said Péter Molnár of the University of
Alberta, Edmonton, one of a team that reports on the lack of male bears
today in the journal Proceedings of the Royal Society Biological
Sciences. This raises the concern that males
could become depleted to the point where females cannot find mates.
Using data on around 500 bears - about one fifth of the total
population in Lancaster Sound, Canada - the team shows that the current
numbers of males remain high enough fertilise all females.
Molnár say that, given the trends, the findings are "a cause for
concern," though he stressed the team only looked at conditions that
would cut mating success, rather than the overall impact on population
growth.
However, his team warns that "a sudden and rapid reproductive collapse
could occur if the sex ratio drops below a critical threshold. This
threshold depends on local bear densities, and must therefore be
evaluated separately for each population." As a
result, they believe that the current harvesting methods should err on
the safe side. "Currently observed high litter production rates despite
reduced male numbers should not be taken as evidence that populations
are secure." Prof Stephen Buckland of the
University of St Andrews comments that this work does raise questions
about the wisdom of the harvesting strategy in Canada.
He notes that the work "also has implications for polar bear
populations threatened by climate change, where the combination of
habitat loss and any significant harvest, however structured, may lead
to rapid reductions in population sizes." The
polar bear is a vulnerable species at high risk of extinction, not
least because predicted decreases in the polar sea ice due to global
warming. Local long-term studies show that seven out of 19
subpopulations are declining or already severely reduced..
The new study focuses on a phenomenon called the Allee effect, in which
individuals of many plant and animal species suffer reduced fitness at
low population densities, which increases their extinction risk. This
has been seen at work in saiga antelopes, African wild dogs, African
elephants and moose and is thought to have helped drive the passenger
pigeon to extinction. The team fears that the
bear may see the same collapse as has been well documented with the
saiga antelope: "Despite heavy sex-selective poaching and a continuing
depletion of adult males, female fertilisation rates remained
unaffected for a long time..but eventually collapsed in a sudden and
nonlinear fashion when males were depleted below a critical threshold." |