Factors influencing biodiversity within organic and conventional systems of arable farming (OF0165)

Summary

This is the final report of Defra project OF0165. The main attached report starts with a more detailed and comprehensive Executive Summary, from which these paragraphs have been extracted.
Previous studies suggest widespread positive responses of biodiversity to organic farming. Many of these studies, however, have been small-scale. The purpose of this project was to test the generality of responses to arable organic farming (i.e. cereal-growing farms) in England through a multi-taxa study of a large number of farms. Abundance and diversity of higher plants, spiders, carabid beetles, wintering birds and bats were measured on matched pairs of organic and conventionally managed farms. Extent and potential quality of non-crop habitat were also measured. Two key issues addressed by the project were (a) whether biodiversity differences between organic and conventional systems arise from amount and management of non-crop habitat or from differences in crop management systems and (b) the importance of duration under organic management.
Plants and invertebrates were examined in 89 pairs of cereal fields (target fields). Birds and bats were studied at a larger spatial scale, extending over several fields on each study farm. Virtually all suitable organic farms in England were studied. The farm pairing procedure was purely geographical and not based on any attributes of either system. Target fields were stratified by cereal type (spring or winter sown) and by age since conversion.
Habitat and management comparisons were carried out at landscape, farm and field scales using data collected in the field and existing landscape datasets (Land Cover Map 2000 and CS2000 field survey data). Within England, organic farms tended to be located to the south of the wheat-growing region, in areas with more grassland than conventional farms. It is difficult to disassociate many landscape level variables from farming system. Organic arable farms were more often mixed farms than their conventional counterparts, leading to smaller field sizes, livestock-proof hedges, diverse rotations and greater extents of grassland. Hedges on organic farms occurred at higher density (length per unit area) and were taller, wider and less gappy than those on conventional farms. All these factors are likely to enhance many components of biodiversity. By contrast, conventional farms were more likely to contain stubble and naturally regenerated set-aside which can be beneficial to wildlife.
This study has confirmed that organic systems do generally support higher levels of biodiversity but that the differences are often quite small. Some, but not all, differences in biodiversity between systems appear to be a consequence of differences in habitat quantity. Plants and invertebrates appear to be responding mainly to crop management practices that are intrinsic to the system. Birds and bats benefit from increased quantities of various non-crop habitats and higher diversity of habitats on organic but this does not account for all differences between systems in these groups. Few relationships with duration of organic management were detected. This may be partly because it was not possible to account for pre-conversion management.
We highlight the potential value to biodiversity on conventional farms of non-crop habitat management (especially hedges), mixed farming and the incorporation of ‘organic’ field margins. High priority areas for future related research include the potential biodiversity benefits of organic livestock farming, long-term controlled studies on responses to conversion, and the effect of extent of organic management at larger scales.