The impacts of organic farming on biodiversity

The impact of agricultural intensification on biodiversity is a global issue that will become increasingly severe with continued growth of human populations. There is worldwide interest in developing less intensive, but economically viable and productive, farming systems with reduced environmental impacts.  Organic farms display many of the features that have become scarce on much farmland, including crop rotations incorporating grass leys, exclusion of synthetic pesticides and fertilisers, and dependence on animal and green manures. They therefore offer an opportunity for understanding the mechanisms by which intensification has affected biodiversity; this may give insights as to how wider biodiversity may be restored.  If specific elements of organic farming can be associated with biodiversity benefits, it may be possible to integrate these elements into wider non-organic systems.
 
The aim of this Defra-funded project was to ask, using a large sample of farms over a wide geographic area, two principal questions. First, we asked whether organic and conventional farms differed in terms of biodiversity and habitat, and second, whether any differences could be linked to variation in non-crop habitat or to differences in farming practice. Eighty-nine pairs of organic and conventional farms across England were studied between 2000 and 2003. Data were collected on higher plants, selected invertebrate groups (spiders and carabid beetles), bats and birds, together with detailed data on farm management and the type, quality and extent of crop and non-crop habitats. The project was a partnership between the Wildlife Conservation Research Unit, the British Trust for Ornithology, the Centre for Ecology and Hydrology, and the Royal Agricultural College.

Across all taxa, diversity and overall abundance tended to be higher on organic farms but in most cases the difference was quite small and it was not consistently detected in all analyses. The most striking differences were for plants where both species richness and cover of non-crop plants were consistently higher in organic fields (on average there were >80% more species within organic fields). There were also habitat differences between organic and conventional farms at all scales.  These differences related to crop types (more grass on, and in the vicinity of, organic farms), livestock (more on organic) and boundary attributes (hedges were present at higher density, were cut less frequently, and were taller, wider and less gappy on organic). These attributes of organic farms are expected to be broadly beneficial to wildlife.

In the case of plants and invertebrates there was little evidence that non-crop habitats were driving differences between systems in diversity or abundance; the observed differences were probably caused by factors associated with crop management and were therefore inherent to the system. There was evidence that differences in bird abundance were related to differences in habitat extent. However, for several bird species, effects of system were detected after controlling for habitat. It seems likely that habitat diversity is an important factor affecting bat diversity. The bat transects in the two systems contained overall similar amounts of hedgerow, yet bat activity was greater on organic and relationships with hedgerow density were confined to organic. This suggests that both habitat extent and management system are important to bats.

Our findings suggest that there are considerable benefits associated with the management systems adopted within organic farming, confirming the results of previous, smaller scale studies. Landscape attributes, non-cropped habitat and crop management all affect biodiversity, but in ways that interact and vary between taxa. Despite the overall differences between the two systems, there remained considerable variation in biodiversity within systems, with some conventional farms performing better than some organic. Further analysis would help identify characteristics of farms that are rich in biodiversity regardless of system.