The golden toad of Costa Rica may have been driven to extinction by climate change, scientists believe.

The toad's demise has been revealed by research into the changing populations of species in Costa Rica.

The scientists concluded that rising temperatures may have been to blame.

The disappearance of the toad is part of a pattern of change that is affecting not only amphibians but also reptiles and birds as well.

Changing climate has allowed toucans to colonise the cloud forests

The researchers say 20 out of 50 species of frogs and toads have disappeared from a 30 square kilometre study area.

The ecology of the region, in the highland forests near Monteverde, is governed by the frequent formation of clouds and mist.

The mist frequency "has declined dramatically since the mid-1970s", because of the warming of the oceans and the atmosphere.

Clouds in the mountains of Costa Rica now form at higher altitudes than they did previously, so the frequency with which the forests are mist-clad has declined.

The researchers believe that changes of this sort may have triggered disease outbreaks, including those known as chytrid fungi. The fungi infect the moist skins of frogs, toads and salamanders.

However, the researchers say that populations of forest lizards have disappeared in much the same way as amphibian populations. But the chytrid fungi are thought unlikely to attack reptiles.

So climate change, not a particular disease organism, may prove to be the common denominator in the declines.

Losses and gains

The changing pattern of cloud and mist formation is having other effects as well.

The researchers also looked at populations of two classes of birds - those that normally breed in the cloud forest, and those that avoid it.

They found that while the number of species in the first group remained comparatively stable, that in the second group increased.

In one area, 15 colonising species have managed to establish breeding populations.

There were only three pairs of golden-crowned warblers, for instance, when they first nested there in 1994. But by last year the number had risen to 20 pairs.

And a species of toucan which normally breeds only in lowlands and foothills has established itself in the mountains as well.