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Last Updated: Tuesday, 17 April 2007, 17:16 GMT 18:16 UK
'Fewer leaves' behind frog demise
In decline: Strawberry frog

A decline in the amount of leaves on the ground could be behind the rapid demise of frog species, a study of a rainforest in Costa Rica has suggested.

Until now, the prime suspect for the amphibians' population crash was a deadly fungal infection.

By studying data over a 35-year period, researchers found that lizards, which were not susceptible to the infection, had also declined by a similar rate.

The study appears in Proceedings of the National Academy of Sciences (PNAS).

Writing in the paper, the team said the global decline of amphibian populations ranked "among the most critical issues in conservation biology".

Of particular concern, the scientists wrote, were "enigmatic" declines - where there had been a rapid fall in species populations but no obvious human cause, such as the destruction of habitat.

One of the prime suspects for the enigmatic decline of frogs was chytrid fungus (Batrachochytrium dendrobatidis), deadly to amphibians.

A paper, published in the journal Nature last January, looked at biodiversity hotspots in Central and South Amercia and found that changes to the local climate had created perfect conditions for the spread of the frog-killing fungus.

Lack of litter

But the PNAS paper found another potential culprit - the lack of leaf-litter on the forest floor.

WHAT ARE AMPHIBIANS?
Amphibians on a leaf (Conservation International)
Group includes frogs, toads, salamanders and caecilians
First true amphibians evolved about 250m years ago
Adapted to many different aquatic and terrestrial habitats
Present today on every continent except Antarctica
Undergo metamorphosis, from larvae to adults

The international team of scientists examined data of amphibian and common reptile populations in La Selva, a protected area of rainforest in Costa Rica.

Between 1970 and 2005, the data showed that the number of amphibians had declined by about 75%, which supported the idea that frogs were being wiped out by the chytrid fungus.

However, the data also showed a similar fall in the area's reptiles, which were not susceptible to the fungus.

Over the same period, the data showed that there had been a 75% reduction in the density of leaves falling to the ground from the rainforest's canopy.

Leaf litter provides a vital habitat, offering food and shelter, for the amphibians and lizards.

The team, from Florida International University, the University of Costa Rica and San Diego State University, suggested shifts in the area's climate had led to a decline in the habitat needed to sustain the creatures.

"The increasingly warm and wet conditions of the past two decades could negatively influence standing litter mass by affecting rates of litterfall or litter decomposition," the authors wrote.

This is a very interesting set of work that points out the complicated nature of species decline
Dr Paul Pearce-Kelly,
Zoological Society of London

Dr Paul Pearce-Kelly, a senior curator at the Zoological Society of London, said the findings made an important contribution towards understanding what was behind the decline of the world's frog populations.

"This is a very interesting set of work that points out the complicated nature of species decline," he said.

"We shouldn't forget that any kind of change affecting one species can leave it weakened and predisposed to being more vulnerable to disease and other impacts.

"The environment, regardless of whether it is a protected forest system or not, is highly vulnerable to temperature changes."


SEE ALSO
Beavers 'helping frogs survive'
15 Jan 07 |  Science/Nature
Warming link to amphibian disease
25 Oct 06 |  Science/Nature
Bullfrog linked to fungus spread
24 May 06 |  Science/Nature
Climate culprit for frog deaths
11 Jan 06 |  Asia-Pacific

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