Organic farming increases biodiversity at every level of the food chain - all the way from lowly bacteria to mammals. This is the conclusion of the largest review ever done of studies from around the world comparing organic and conventional agriculture.

Previous studies have shown that organic farming methods can benefit the wildlife around farms. But "the fact that the message is similar all the way up the food chain is new information", says agricultural scientist Martin Entz of the University of Manitoba in Winnipeg, Canada.

The study reviewed data from Europe, Canada, New Zealand and the US. Neither of the two groups of researchers who did the study - one from English Nature, a government agency which champions wildlife conservation, and one from the Royal Society for the Protection of Birds - has a vested interest in organic farming.

"It's good to have independent people add their voice to the debate," says Nic Lampkin, director of the Organic Centre Wales, part of the University of Wales Aberystwyth.

Typically, each of the 76 studies reviewed measured biodiversity in groups of organisms ranging from bacteria and plants to earthworms, beetles, mammals and birds. Of 99 separate comparisons of groups of organisms, 66 found that organic farming benefited wildlife, eight concluded it was detrimental and 25 produced mixed results or suggested no difference between the farming methods.

Mixed farming

According to the researchers, organic farming aids biodiversity by using fewer pesticides and inorganic fertilisers, and by adopting wildlife-friendly management of habitats where there are no crops, including strategies such as not weeding close to hedges, and by mixing arable and livestock farming.

Mixed farming particularly benefits some bird species. Lapwings, for example, nest on spring-sown crops, but raise their chicks on pasture. Intensive agriculture has been blamed for the 80% decline in lapwing numbers in England and Wales since the 1960s. One of the reviewed studies from the UK also points to benefits for bats. Foraging activity was up 84% on organic farms and two species, the greater and lesser horseshoe bats, were found only on organic farms.

The studies might even have underestimated the benefits to wildlife, says Phillip Grice of English Nature. Some looked at farms shortly after they turned organic, so wildlife numbers may just have started increasing.

Some argue that farms that adopt a few organic practices, swapping chemical weeding for mechanical, for example, may help wildlife flourish just as much as completely organic farms. And it is possible that farmers who switched to organic farming may have been predisposed towards environmentally friendly methods. So the biodiversity on their farms may have been higher than average before conversion. The current studies are not detailed enough to answer these questions.

Journal reference: Biological Conservation (vol 122, p 113)

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