雅思阅读推荐：蝴蝶消失和生物多样性

Death in the rainforest: fragile creatures give the world a new climate warning

A red-eyed tree frog in Costa Rica

A protected rainforest in one of the world\'s richest biodiversity hotspots has suffered an alarming collapse in amphibians and reptiles, suggesting such havens may fail to slow the creatures\' slide towards global extinction.

Conservationists working in a lowland forest reserve at La Selva in Costa Rica used biological records dating from 1970 to show that species of frogs, toads, lizards, snakes and salamanders have plummeted on average 75% in the past 35 years.

Dramatic falls in amphibian and reptile numbers elsewhere in the world have been blamed on habitat destruction and the fungal disease chytridiomycosis, which has inflicted a devastating toll across central and South America. But scientists hoped many species would continue to thrive in dedicated reserves, where building, land-clearance and agricultural chemicals are banned.

The new findings suggest an unknown ecological effect is behind at least some of the sudden losses and have prompted scientists to call for urgent studies in other protected forest areas. The researchers, led by Maureen Donelly at FloridaInternationalUniversity, believe climate change has brought warmer, wetter weather to the refuge, with the knock-on effect of reducing the amount of leaf litter on the forest floor. Nearly all of the species rely on leaf litter to some extent, either using it for shelter, or feeding on insects that eat the leaves.

The study revealed sharp declines among two species of salamander, whose numbers fell on average 14.52% every year between 1970 and 2005. Frog species slumped too, with numbers of the mimicking rain frog falling 13.49%, the common tink frog 6.69%, and the strawberry poison frog 1.18% a year. Lizards suffered similar falls, with one species, the striped litter skink, down 10.03% each year, and orange-tailed geckos declining by 8.05% every year.

The researchers also analysed weather records for the region, which revealed a rise of more than 1C in temperature over the 35-year period and a doubling of the number of wet days. The study was published in the journal Proceedings of the National Academy of Sciences yesterday. "All of the falls recorded elsewhere have been in high, mountainous regions and those have mostly been driven by the spread of fungus. All of the tests we\'ve done for the fungus here have been negative," said Steven Whitfield, a co-author of the study.

"Our best guess is that the declines are related to a drop in leaf litter on the forest floor. Most of the species use leaf material as a place to hide, but because it\'s moist, it\'s also a place to shelter when it\'s dry and warm. Many of these species also feed on the insects that eat the leaf matter, so if that disappears, so does their food and shelter."

The scientists say it is crucial to extend the study to other protected forests, such as those in Peru, Ecuador and Colombia, to assess the populations of amphibians and reptiles there.

"If we are to design effective conservation strategies, we need to know what\'s causing these declines. If it\'s down to a link between climate and leaf litter, then we need to better understand that," said Mr Whitfield.

Amphibians are considered delicate sentinels of environmental change. Sudden population collapses were first noticed during the 1980s, during which more than 120 species are thought to have become extinct.

In Britain, the common toad population is "seriously threatened", with natterjack toads having declined by 75% in the past century. Numbers of great crested newts in the country have dropped by 60% since the 1960s.

Last year, English Nature announced that the chytridiomycosis had arrived in Britain following the escape of infected bullfrogs imported from North America. The organisation destroyed 11,000 frogs in an attempt to contain the spread.

The collapse of amphibian populations prompted 50 of the world\'s leading conservation experts to call last year for an urgent mission to save them from extinction. In a letter to the US journal Science, the conservationists proposed a $400m (￡217m) plan called the Amphibian Survival Alliance, which would dispatch teams to collect endangered amphibians for captive breeding.

John Fa, director of conservation science at the Durrell Wildlife Conservation Trust in Jersey, said: "Something needs to be done in terms of saving these species, and maybe that is captive breeding, but it\'s difficult. You have to know that even if you\'re able to breed up the numbers, will they survive in the environment when you put them back?"

Declined at least 75% in the past 35 years, even in protected areas: Common salamander; strawberry poison frog; Bransford\'s litter frog; broad-headed rain frog; Noble\'s rain frog; mimicking rain frog; common tink frog; Warszewitsch\'s frog; orange-tailed gecko; leaf litter lizard; striped litter skink.