11 Jan 07

Sections of rainforest used to grow coffee maintain more intact ecosystems than areas cleared for intensive agriculture such as rice-growing or pasture, a study by scientists from Oxford, New Zealand and Germany has found.

Nearly six million hectares of tropical rainforest are cut down each year to make way for agriculture, displacing thousands of plant and animal species. Working in Ecuador, researchers from the Universities of Oxford, Canterbury (New Zealand) and Goettingen (Germany) have determined for the first time how the forest food web - the fragile network of interactions among species - is altered by this environmental onslaught. Their findings are published in Nature.

The researchers recorded the fate of over 7,000 nests of wild bees and wasps in undisturbed rainforest and in agricultural habitats. Conversion to intensive agriculture caused a dramatic change in the food web, allowing a single species (a parasitic wasp) to dominate. In contrast, plantations where coffee was grown beneath the forest canopy had similar food webs to undisturbed forest.

Author Dr Owen Lewis, from the Department of Zoology at Oxford University, stresses that the findings about coffee-growing are no cause for complacency: 'Like many tropical countries, Ecuador has a mixture of remnant forest patches and agricultural habitats. If the whole landscape were to be converted to coffee plantations, then we would almost certainly see more dramatic changes.'

The domination of the parasites in intensively-farmed areas, causing high mortality of bees and wasps, is bad news for farmers as well as insects. Bees are important pollinators of crops and wild plants, and wasps provide biological control of agricultural pests. Many poor farmers in Ecuador cannot afford fertilisers and pesticides, and are therefore dependent on the free services provided by wild bees and wasps. 'Converting sections of forest to agriculture in a non-sustainable way is likely to prove self-defeating,' says Dr Lewis.

'In coffee plantations, by contrast, it appears that perhaps agriculture and biodiversity can go hand in hand. The main factor limiting this use of land compared to more intensive farming is the low price that farmers receive for coffee, about US$78 per hectare. The increasing popularity of fair trade coffee among consumers may alleviate this problem, allowing farmers to make a sustainable living while maintaining the integrity of the web of life.'

Early studies of food webs simply recorded 'who eats whom', but recently scientists have begun to analyse the frequency of feeding interactions between species, producing more detailed 'quantitative' food webs. The research just published in Nature is the first time that food webs have been measured in this way across a variety of habitats, and thus provides a detailed insight into the effects of habitat modification on interactions among species.

Author Dr Jason Tylianakis, from the University of Canterbury, says: 'It was already known that this loss of natural habitats causes the extinction of many species, but until now it was unclear what happens to the species that don't go extinct, but have to remain in the agricultural wasteland where forest once stood.'

Dr Lewis believes that the methods pioneered in this research will be useful in other contexts: 'Studying food webs has great potential for understanding how the web of life will be affected by a variety of human actions, such as climate change and the introduction of alien species.'

He cautions that the team's findings should not be seen as an endorsement of converting rainforest to plantations. 'Conservation of rainforest is a much wider issue. Our research only suggests that if small areas of rainforest are to be used for growing crops, some crops are significantly better than others.'