News Feature Mixed boost for biodiversity

Abstract

Organic agricultural methods are being promoted in many countries as a means of boosting biodiversity but data are thin on the ground. A new detailed study finds that the benefits vary between different groups of species. Nigel Williams reports. Organic agricultural methods are being promoted in many countries as a means of boosting biodiversity but data are thin on the ground. A new detailed study finds that the benefits vary between different groups of species. Nigel Williams reports. One issue of great concern to many researchers is the rapidly vanishing biodiversity in the face of intensive agricultural methods. Organic techniques offer an alternative, but evidence of their effects has often been based on studies that were poorly designed, limited in scope in terms of the species examined, or local in scale. Meta-analysis of published studies differing in methodology and scale suggests that biodiversity responses to organic farming vary across studies and organism groups. Organic farming appears to be associated with increased species richness and abundance for plants, predatory invertebrates and birds. But a new study led by R.J. Fuller at the British Trust for Ornithology and colleagues, reported in Biology Letters of the Royal Society (published online), uses data from an integrated study of plants, carabid beetles, spiders, birds and bats conducted on a large sample of the organic farms growing cereals in England to address two issues.First, they tested whether responses to organic farming in terms of species number, diversity and abundance are specific to particular groups of species. Second, they assessed whether organic farms differ from non-organic farms in habitat extent, composition and management and, therefore, whether any differences in biodiversity are potentially linked to habitat heterogeneity and availability. Organic farms of at least 30 hectares with joining organic fields containing arable land were paired with non-organic farms on the basis of proximity, crop type and cropping season. Data were collected from 89 pairs of farms but pairs were not geographically adjoined. The results confirm the earlier evidence suggesting organic farming is associated with higher levels of biodiversity. But the striking result was that plants were far more consistent and pronounced in their response compared with the other species groups. For other groups, even where significant differences were detected, the results were variable with wide confidence intervals, the team found. In addition to biodiversity analysis, the study revealed that organic farms differ from non-organic farms in habitat extent, composition and management. The widely different practices, such as the pattern of crop rotation, cutting of hedges, time of crop sowing and presence of livestock, for example, added to the differences of organic production. However, the exclusion of synthetic pesticides and fertilizers from organic systems is fundamental. Given this, the authors asked, why did the magnitude of differences in species density and abundance vary so much between the different groups they looked at?The striking result was that plants were far more consistent and pronounced in their response compared with other species groups The striking result was that plants were far more consistent and pronounced in their response compared with other species groups One factor may be the differential impacts of timing and special scales on the colonization traits of organisms. Plants are more directly and immediately affected by both pesticide and fertilizer inputs, but have the ability to recolonize from the seed bank immediately following conversion to organic management and freedom from pesticides. For other groups, such as the spiders, carabid beetles, birds and bats studied, recolonization is affected by proximity of populations able to move into the new environment. “Many organic farms are isolated units, embedded in non-organic farmland managed with conventional levels of pesticide and fertilizer inputs, often coupled with relatively low levels of habitat heterogeneity, which inevitably affects species colonization,” the authors say. Also, most existing organic farms probably offer insufficient resources to affect population sizes of species with large spatial needs, notably birds. The authors suggest that extension of organic farming is a potential means of re-establishing heterogeneity of farmland habitats, and thereby enhancing farmland biodiversity. “Strategies aimed at increasing both the total extent of organic farming and the size and contiguity of individual organic farms, could help to restore biodiversity in agricultural landscapes,” the authors report.