Land-use intensity and the effects of organic farming on biodiversity: a hierarchical meta-analysis

The benefits of organic farming to biodiversity in agricultural landscapes continue to be hotly debated, emphasising the importance of precisely quantifying the effect of organic vs. conventional farming. We conducted an updated hierarchical meta-analysis of studies that compared biodiversity under organic and conventional farming methods, measured as species richness. We calculated effect sizes for 184 observations garnered from 94 studies, and for each study we obtained three standardised measures reflecting land-use intensity. We investigated the stability of effect sizes through time, publication bias due to the ‘file-drawer’ problem, and consider whether the current literature is representative of global organic farming patterns. On average, organic farming increased species richness by about 30%. This result has been robust over the last 30 years of published studies and shows no sign of diminishing. Organic farming had a greater effect on biodiversity as the percentage of the lan...