雅思阅读：蝴蝶消失和生物多样性

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　　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."

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