In some regions of the world, organic farms may be a losing proposition for both wildlife and food production.
Organic farms in the United Kingdom protect just 12 percent more biodiversity than conventional farms.
These farms also produce less than half the amount of crop yield as their conventional counterparts.
Biodiversity was higher on organic farms that were surrounded by other organic farms as opposed to those surrounded by conventional farms.
Organic farms have a lot going for them, but they probably won't solve all the world's food and conservation-related problems, suggests a new study.
In a detailed comparison between organic and conventional farms around England, researchers found that the diversity of wild plants and animals was just 12 percent higher on organic farms -- a much smaller difference than previous studies had led scientists to expect. At the same time, acre for acre, organic farms produced half the amount of crop yield.
The global demand for food will double in the next three or four decades, said lead researcher Tim Benton, a conservation ecologist at the University of Leeds in the U.K. To meet those needs without converting all of the world's wild lands into farmland, his study suggests that intensive conventional farming might be the best strategy in some places.
"When you model what the best way is of generating both biodiversity and food yield, you find very often that the optimal strategy is to farm intensively where you can, and in effect set aside blocks of land where you don't farm at all," Benton said. "The difference between organic land and wild land is so large that it is better to farm intensively and set aside land than it is to farm the whole area organically."
Organic farming methods are appealing to many people because they avoid artificial pesticides and chemical fertilizers, while presumably nurturing a greater diversity of life. When scientists have compared organic farms with conventional farms in hundreds of studies, they've found that organic farms support a 30 to 40 percent more diverse array of plants and animals.
The problem with those studies, Benton said, is that organic farms often occupy variable landscapes full of valleys, streams and hillsides, which don't necessarily lend themselves to large-scale industrial farming. And it may be because they offer such a variety of niches that organic-conducive landscapes have higher levels of biodiversity. In other words, they may have simply started out that way.
To find out exactly how much of a benefit organic farming actually provides to biodiversity, Benton and colleagues spent two years carefully choosing nearly 200 fields from 32 farms around the U.K. Half of the fields were in areas dominated by conventional farms. Half were in areas dominated by organic farms.
Fields were paired so that they matched each other according to 32 variables, including climate, topography and soil type. At both the edges and centers of the fields, the researchers counted species of birds, insects, worms, flowers, butterflies, weeds and more.
The results, some of which appeared in the journal Ecology Letters and some of which will appear in two papers yet to be published, were complicated -- with some groups of organisms doing better in one situation and others doing better in another.
Overall, biodiversity was 12 percent higher on organic farms. But to the team's surprise, a whole suite of birds that have evolved to live on farmland did better on conventional farms. Weeds were one of the groups that did best on organic farms. Meanwhile, crop yield dropped by 55 percent on organic farms.
"Organic farms aren't a panacea," said Jude Girard, an avian ecologist at Carleton University in Ottawa, Canada. "They have a really important role to play in farmland biodiversity, but they're not the answer."
The new study also found that diversity was higher on organic farms that were surrounded by other organic farms as opposed to those surrounded by conventional farms. That's an important reminder, Girard said, that boosting both conservation and food production will require cooperation from farmers, managers and politicians to avoid treating farms as isolated plots
It's also important to note that the best strategies for farming might vary tremendously in different parts of the world, said Martin Entz, professor of sustainable agriculture at the University of Manitoba. He just returned from Central America, where large-scale rice farms are driving diversity out of the tropics, while small-scale organic farms are preserving high levels of diversity.
"There are good organic farms and not-so-good organic farms," Entz said. "It depends entirely on the place."