Birds, insects and plants usually showed an increased species richness in organic
farming systems. However, the number of studies was low in most organism groups
(range 2–19) and there was significant heterogeneity between studies. The effect of
organic farming was largest in studies performed at the plot scale. In studies at the farm
scale, when organic and conventional farms were matched according to landscape
structure, the effect was significant but highly heterogeneous.
4.
On average, organisms were 50% more abundant in organic farming systems, but the
results were highly variable between studies and organism groups. Birds, predatory insects,
soil organisms and plants responded positively to organic farming, while non-predatory
insects and pests did not. The positive effects of organic farming on abundance were
prominent at the plot and field scales, but not for farms in matched landscapes