Logic dictates that climate change and habitat degradation affect the world's plants and animals. But the combined impact of these sometimes competing forces has never been assessed for any species¿until now. A study described in this week's issue of Nature examined the fate of British butterflies and suggests that their numbers are dwindling, despite the fact that a warmer climate should benefit them.

Christopher D. Thomas of the University of Leeds in England and his colleagues studied 46 butterfly species in Britain, where spring and summer temperatures have increased over the past 25 years. Such warming should benefit the butterflies by enabling faster growth rates for larvae, for example. The team discovered, however, that three quarters of the insects instead experienced declines in their numbers over the past three decades. It seems that habitat degradation, driven by intensification in agriculture, has eroded 70 percent of the butterflies' natural habitat since 1940 and negated any positive effects warming may have had.

What is more, the scientists note, not all species were affected equally. The numbers of butterflies classified as sedentary or as habitat specialists, which are less likely to move about and live in a variety of habitats, declined the most. The researchers conclude that this trend could result in "biological communities with reduced numbers of species and [that are] dominated by mobile and widespread habitat generalists."

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