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Unravelling A Rainforest Food Web

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Unravelling a rainforest food web
Is your enemy's enemy your friend? Not if you're an insect in the tropical rainforest in Belize. Researchers at Oxford University and Imperial College London investigating what happens to one species when another species with a shared natural predator disappears have found that the population of the remaining species increases.

Dr Owen Lewis in Oxford's Department of Zoology, with Dr Becky Morris and Professor Charles Godfray from Imperial College, studied different species of leaf-miner insects in Belize. Leaf-miners in the larval stage live within leaves, munching their way through the inside of the leaf as they grow. At maturity they develop into beetles, moths or flies. The larvae of leaf-miners are often preyed on by parasitoid wasps which develop inside the leaf-miner, eating it alive and eventually killing it.

The scientists removed one species of leaf-miner from experimental plots within the rainforest to see what would happen to the remaining species of leaf-miner. As they had predicted, the parasitoids decreased in number as a result of one of their food sources being taken away, and so the remaining leaf-miners increased.

'Instinctively you might expect that if you shared an enemy with another species and that species were removed, then your enemy - in this case the parasitoid - would focus on you, and your species would suffer as a result,' said Dr Lewis. 'But in fact, though that might happen in the very short term, removing a source of the parasitoid's food can in the long run decrease the number of parasitoids, to the benefit of other species.

'Even though two species of insect might never physically meet each other, they can influence each other through this process of 'apparent competition'. The presence of the other species actually increases the number of enemies, and the species which copes better with parasitism will do best. Eventhough the two species with the same enemy are not competing in any direct sense, the structural changes you see in the populations mirrors what you get when two species are in direct competition with each other, for example for food.'.

The theory of apparent competition is well developed, and researchers have predicted for years what would happen in the circumstances the scientists created. However, this study, the results of which were published in Nature, is the first convincing real-life demonstration of the theory.

The role of apparent competition may be especially important in tropical forests, because the diversity of the habitat means that direct competition for food is rare. Studying the effects of apparent competition is a way of examining how the very diverse communities of living organisms in rainforests interact and are structured.

'One of the important lessons from our research is about the effects human actions can have on ecological food webs,' said Dr Lewis. 'Activities like logging for timber can remove hosts or prey in a way which changes the food web, with repercussions far beyond the set of species that are directly affected. We must consider the indirect impacts of removing or introducing a species before carrying out interventions such as logging or biological control.'.

Posted By: Tyler


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