By Daniel Cressey
A study suggesting that organic agriculture gives better pest control and larger plants than conventional farming is sure to reignite longstanding debates about the merits of organic versus conventional agriculture. It also highlights an often-neglected aspect of biodiversity.
"Organic agriculture promotes more balanced communities of predators," says David Crowder, author of the new study published June 30 in Nature.
"Our study does not tell farmers they should shift to organic agriculture. What our study suggests is that organic agriculture is promoting these more balanced natural enemy communities and they may have better, organic pest control."
Much focus is put on species numbers or 'richness'. But the research by Crowder, an insect ecologist at Washington State University in Pullman, and his colleagues, shows the importance of "evenness"--the relative abundance of different species. Evenness quantifies not just the presence of different species, but whether one is dominant or whether there is an equal distribution of numbers between species.
The team looked at the bugs, nematodes and fungi that attack the hated Colorado potato beetle (Leptinotarsa decemlineata).
They conducted a meta-analysis of data collected on these denizens of Washington potato fields and found that although organic and conventional farms did not differ markedly in the richness of beetle eaters, the evenness of predators differed "drastically". Organic fields--where only a limited number of man-made chemicals can be used--had far greater evenness than those where pesticides were applied regularly.
Furthermore, the team set up an experimental field in which they manipulated the evenness of predators. Increasing the evenness led to what the researchers call a "powerful trophic cascade," resulting in fewer potato-munching beetles and larger potato plants.
Although the work of Crowder and his group does not address the issue of yields from organic versus conventional farms, their study found that the increased evenness of organic farms compared with that of conventional farms led to 18 percent lower pest densities and 35 percent larger plants. Bigger plants generally mean greater potato yields.
At least as important as what the research says about organic farming is what it says about species evenness.
"Almost all the studies that have been done have looked at the number of species in an ecosystem," says Crowder. "Very few studies have looked at the relative abundance. We think our study is really one of the first to highlight that evenness is also important."
Understanding evenness can be extremely useful to those studying biodiversity, agrees Marc Cadotte, a community ecologist at the University of Toronto at Scarborough. This knowledge can shed light on, for example, the processes maintaining species abundance, and can also be crucial for determining how ecosystems will respond to challenges, such as those posed by climate change.
This paper, says Cadotte, demonstrates that different agricultural practices have distinct effects on evenness, and that manipulating evenness leads to the cascading reactions identified by Crowder and his team.
"Evenness is a critical component of biodiversity," says Cadotte. "Much research has emphasized species richness, maybe at the detriment of studying evenness."
Meanwhile, Crowder says that the next step is to discover what it is about organic agriculture that promotes evenness, and to determine whether this finding also applies in systems other than potato fields.