A meta analysis done by a group of scientists reported that organic farming increased species richness by about 30%. Their analysis affirms that organic farming usually has large positive effects on average species richness compared with conventional farming. Given the large areas of land currently under agricultural production, organic methods could undoubtedly play a major role in halting the continued loss of diversity from industrialized nations. The detailed analysis is available online on Journal of Applied Ecology   published on 7th February 2014.

As a part of their studies they tried to find out that by how much does organic farming increase biodiversity compared with conventional agriculture? Do the effects of organic farming depend on the organism or functional group, land-use intensity and structure, and crop type? Has the reported effect size of organic farming on biodiversity decreased or remained stable over time? 

They found large differences in the effect of organic farming on different taxonomic and functional groups. Among taxonomic groups, plants benefited the most from organic farming, probably because of restricted herbicide use. Arthropods, birds and microbes also showed a substantial positive effect. Disaggregating organisms into functional groups showed a variety of responses: among functional groups, the largest effect size was found for pollinators while decomposers showed little effect. Accordingly, most functional groups – herbivores, pollinators, predators and producers – were more diverse in organic farming, with the exception of decomposers.

The crop types showed varying responses, with large positive effect sizes in cereals and mixed farming, and moderate positive effect sizes for all others. 

Organic farming is therefore a tried and tested method for increasing biodiversity on farmlands and may help to reverse the continued declines of formerly common species in developed nations.

The group recommends that more studies are needed in tropical, subtropical and Mediterranean climates at farm scale or landscape scale for a more balanced and globally relevant assessment of organic farming effects on biodiversity, ecosystem services, food production and agricultural sustainability.

Reference

Tuck, S. L., Winqvist, C., Mota, F., Ahnström, J., Turnbull, L. A., Bengtsson, J. (2014), Land-use intensity and the effects of organic farming on biodiversity: a hierarchical meta-analysis. Journal of Applied Ecology. doi: 10.1111/1365-2664.12219