[Photo] Beaver by C. Stephens. Beaver ponds created favourable conditions for tadpoles.

Beavers may be helping to halt the decline of some amphibian populations, a study suggests.

Researchers, surveying streams in the forests of Alberta, Canada, found significantly more frogs and toads where beaver dams were present.

They believe the beaver "ponds" may be providing favourable conditions for developing tadpoles.

The findings may aid amphibian conservation efforts, the team reports in journal Biological Conservation.

The University of Alberta scientists surveyed frogs and toads at 15 beaver-obstructed stream sections and nine free-flowing sections.

Three species of amphibian are common to this area: the boreal chorus frog, the wood frog and the western toad, which is currently listed on the IUCN Red List of threatened species.

An analysis of the "amphibian chorus" - where males croak to entice females - yielded a dramatic result.

"We recorded large numbers (approximately 5,000) of male frogs and toads on streams that had beaver dams, but we didn't record any on the free-flowing unobstructed streams," said Dr Cam Stevens, an ecologist and lead author on the paper.

"We were expecting a difference - but this was a noticeably big difference."

The team also used pitfall traps on the streams compare numbers of young frogs and toads. It found about six times more wood frogs, 29 times more western toads and 24 times more boreal chorus frogs at the beaver bonds compared with the beaver-free streams.

Dr Stevens said: "The beavers create an environment that seems to allow tadpoles to develop and grow."

Beaver dams were providing relatively still, warm, nutrient-rich water, he said, which made for perfect conditions for amphibian young.

'Surrogate species'

The researchers say their finding that the beaver is a "surrogate species" may aid future conservation efforts.

By encouraging beaver populations in the boreal forests of Canada and North America amphibian numbers could be boosted, explained Dr Stevens.

"The challenge will be to promote modest levels of beaver activity even where conflicts with human interests might occur, such as areas designated for tree harvesting and landscapes with high road densities," he added.

There is a global decline in amphibians; nearly a third now face extinction. A number of factors such as habitat loss and degradation, climate change and infectious disease have been put forward as possible causes.

In 2005, a $400m (??200m) global plan was established to rescue frogs, toads, and salamanders from oblivion.