Amphibian populations at Yellowstone - the world's oldest national park - are in steep decline, a major study shows.
The authors link this to the drying out of wetlands where the animals live and breed, which is in turn being driven by long-term climate change.
The results, reported in the journal PNAS, suggest that climate warming has already disrupted one of the best-protected ecosystems on Earth.
The park covers some 9,000 sq km (3,500 sq miles) in the western United States.
It lies mostly within the state of Wyoming, but spills over into Montana and Idaho. The area has been protected for more than a century; US congress granted Yellowstone national park status on 1 March 1872.
 |  There is a pretty substantial signal of climate change in this region  Sarah McMenamin, Stanford University |
Visitors flock there to see its geysers, hot springs and bubbling mud pots, fuelled by ongoing volcanism. The park's vast forests and grasslands are also home to grizzly bears, wolves and bison.
But it is to much less conspicuous inhabitants - frogs, toads and salamanders - that scientists look for early indications of environmental degradation.
Four amphibian species are native to the park: the blotched tiger salamander (Ambystoma tigrinum melanostictum), the boreal chorus frog (Pseudacris triseriata maculata), the Columbia spotted frog (Rana luteiventris) and the boreal toad (Bufo boreas boreas).
The lower Lamar Valley in northern Yellowstone harbours countless small, fishless ponds - ideal for amphibian breeding and larval development.
Downward trend
Between 1992 and 1993, researchers surveyed 46 of these "kettle" ponds, which are re-filled in spring by groundwater and snow melt running down from the hills.
 The "kettle" ponds are ideal habitats for amphibians |
When a team from Stanford University in California repeated this survey between 2006 and 2008, the number of permanently dry ponds had increased four-fold.
Of the ponds that remained, the proportion supporting amphibians had declined significantly.
In addition, three of the four native amphibian species had suffered major declines in numbers. The number of species found in each location - the "species richness" - had also dropped off markedly.
Amphibians lay jelly-coated eggs that are unsuited to development on land, so they must return to water in order to spawn.
"They go through an aquatic period and a terrestrial period during their lives so they are very susceptible to changes in both types of environment," said co-author Sarah McMenamin, from the department of ecology and evolution at Stanford.
Changing landscape
The scientists saw no change in numbers of the boreal toad - which the IUCN conservation body lists as threatened. But this species was rare in 1992, as it is now, making it difficult to extract population trends.
 The tiger salamander is among the affected species |
Ms McMenamin, along with her colleagues Elizabeth Hadly and Christopher Wright, also analysed monthly temperature and precipitation data from Yellowstone, as well as satellite images of the park taken between 1988 and 2008.
These data revealed that decreasing rainfall and increasing temperatures during the warmest months of the year have significantly altered the landscape.
Drought is now more common and more severe than at any time in the past century, the researchers say.
"There is a pretty substantial signal of climate change in this region," Ms McMenamin told BBC News.
 Yellowstone is famous for its geysers, such as Old Faithful |
"Snow pack during the winter is decreasing - which other studies have documented - and the regional aquifer is drying up as a result of these large-scale climate changes.
"These ponds are changing, the environment is changing, the landscape is drying up and the amphibians no longer have a place to breed. It's disturbing."
Amphibian populations are in crisis worldwide: pollution, diseases such as chytrid fungus and rana virus, invasive species, UV radiation and habitat destruction all contribute to the problem.
Climate change might affect amphibian populations in numerous ways. In addition to drying out aquatic breeding habitats - preventing spawning - it could also make the land environment inhospitable to them.
Evidence suggests that a warming climate could also predispose amphibians to infection, particularly to chytrid fungus.
Paul.Rincon-INTERNET@bbc.co.ukOriginal here
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