Reporter 400, 21 April 1997

The mosquitoes are coming - if we're lucky

Popular images of global warming hold out the prospect of Mediterrean climates in northern Europe - growing grapes in Scotland - with a sting in the tail of rising levels of skin cancer. Sunnier climes, it is believed, will bring pests which thrive in such conditions, like the mosquito or the tsetse fly. Such pests are well known and much research has been conducted into their control.

But a four-year project by Professor Bryan Shorrocks and Dr Andrew Davis into animal communities has shown that the effects of global warming are nowhere near as foreseeable as was thought. The distribution of plants and animals following global temperature rises cannot be predicted on projected climate changes alone, they believe. Thus, higher temperatures could also bring infestations of as-yet unidentified pests to northern Europe.

"Researchers in medicine and agriculture have tended to assume that all pests are known - and that they will simply migrate into Britain from southern Europe," says Dr Davis. "So it's predicted that we will become hosts to insects like the European Corn Borer - a major pest in Spain, Italy, Greece and Turkey.

"Other organisms have been disregarded because they don't at the moment transmit diseases or pose any economic threat. But a species that is rare at say 15°C - because of the interaction between the species as the effects of warming are felt - may become common.

"An insect which you may find today, say, in very small numbers in a field of crops, eating perhaps a few grammes of material a day, is no threat. But imagine its numbers increasing to thousands or tens of thousands, and you're talking about kilos of grain being destroyed. We have no idea what these insects are because they've not been thought important, they haven't been looked for."

The Leeds research involved setting up ten laboratory ecosystems of linked cages, each housed in incubators at different temperatures corresponding to the average temperatures found between Leeds and southern Spain - from 10 to 25°C. A number of species of fruit-fly were used as the experimental organisms and individual ecosystems were established with these species either alone, in pairs, with all three together and with one of their parasites, the wasp Leptopilina boulardi.

It was found that the temperatures in which each species survived depended on a whole range of factors - including dispersal, whether or not the flies could move between temperatures, on interactions, which other species were present and how many other species there were. When parasitic wasps were added to the incubators, the numbers of the fruit-fly it preys upon declined, as might be expected.

But this reduction in numbers of one particular species also allowed the populations of other species of fruit-fly to rise due to the lack of competition for resources. This happened even at temperatures where the wasp itself was absent because, as the incubators were linked, its activities at other temperatures changed the numbers of individuals of different species dispersing between temperatures.

When the 'global' temperature was increased by setting the incubators to 5°C higher, the density of species changed, even any one temperature. One species of fruit-fly became much more abundant at 15°C after 'global warming' and actually became the most common species at that temperature in place of another. This effect has been undetected by researchers whose predictions have been based on climate changes alone and experiments using single species.

Grapes might grow in Scotland but the overall outcome of global warming might not be that simple - nor that attractive.

The work was funded by a £241,000 contract under the Biotechnology and Biological Sciences Research Council's £8m Biological Adaptations to Global Environmental Change programme.WWW More

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