Susan Brown, National Geographic News 21 Nov 07;
Populations of polar bears can be pushed to the brink of collapse if hunters kill too many males, a new study suggests.
A single male can impregnate several females each breeding season, so wildlife managers generally maintain that fewer males than females are needed to support a healthy population. But the balance can tip too far if there are not enough males to go around.
Hunting has already tilted the balance in some Canadian bear populations, as trophy hunters target males, which can grow to twice the size of females.
A team of biologists has estimated the tipping point for a population of polar bears around Lancaster Sound in the Canadian province of Nunavut. There, hunters are currently allowed to take two males for every female.
The scientists found that the danger point for the sound's polar bears may be closer than was previously believed.
The researchers took into account how long a male is likely to look before finding a mate and how long he spends with her before moving on.
The findings predict that the Lancaster Sound population could reach the brink when the ratio of breeding males to females reaches 2-to-3. The most recent census of bears in the area found a single adult male for each available female.
Once the bears start missing their mates, the number of new cubs would fall rapidly, the team reported.
The minimum proportion of males depends on how crowded an area is with bears. Sparse, scattered populations need more males, they added.
"It just takes so much longer to find those very few females that are running around somewhere," said Péter Molnár of the University of Alberta in Edmonton who led the study.
The work of Molnár's team provides the sort of information managers need as they set limits on hunting, said Scott Schliebe, a polar bear specialist with the U.S. Fish and Wildlife Service in Alaska.
"When we monitor these populations, we're going to have to look at the ratio of males to females to make sure that things aren't tipping."
Molnár's team reported its findings today in the Proceedings of the Royal Society B.
Sex threat to polar bears
Roger Highfield, Science Editor, Telegraph 21 Nov 07;
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.
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."
