Organic farming and habitat heterogeneity benefit functional diversity

Studies comparing biodiversity in conventional and organic farming systems are many and varied. They have studied different taxa, at different scales, in different landscapes and have done so by comparing a very wide range of different metrics. While the results of these studies have been diverse, a recent meta-analysis published in Journal of Applied Ecology by Tuck et al.Land-use intensity and the effects of organic farming on biodiversity: a hierarchical meta-analysis” affirmed that organic farming has large positive effects on biodiversity when compared with conventional farming. The effect size does vary, however, with organism group and crop studied, and is greater in landscapes with higher land-use intensity.

Photo taken from the cover of the Virtual Issue: Applied Ecology in Agricultural Systems, December 2013. Photo credit John Finn.
Photo taken from the cover of the Virtual Issue: Applied Ecology in Agricultural Systems, December 2013. Photo credit John Finn.

Evidence of how such positive biodiversity gains relate to real, on-the-ground benefits to ecosystem functioning, however, remains largely lacking, particularly with respect to plants. The paper recently published by Rader et al.Organic farming and heterogeneous landscapes positively affect different forms of plant diversity” has addressed this issue in a novel way. In addition to plant species richness, which has tended to be the “go-to” measure for such assessments, the authors investigated a wider range of functional-level traits in an attempt to assess the community effect of both habitat heterogeneity (via the presence of semi-natural habitats) and farm management intensity (organic vs. conventional). By measuring the functional diversity of a system (the diversity of functional traits within a community), its phylogenetic diversity (the diversity of evolutionary relationships among coexisting species), redundancy value (the extent to which species in a community overlap in their traits) and community composition, this has allowed a far better assessment to be made of the consequences of anthropogenic change on biological communities than by studying species richness alone.

Plant species richness and functional diversity both responded positively to landscape heterogeneity. As found by Tuck et al., these effects tended to be stronger on conventional farms. In short, this means that not only were a wider range of species found in the more heterogeneous environments (higher species richness), but that those species also performed a wider range of functional traits in the community. A higher redundancy value was also found, which essentially means that in heterogeneous habitats, a larger number of plants were found which essentially had the same role. This function, along with the longer phylogenetic branch lengths observed in plants on organic farms, are both widely regarded in ecological theory as providing communities with an important buffer from environmental change.

Therefore, while the conclusions of this study are in broad agreement with those of many others – habitat heterogeneity and organic farming benefit plant diversity metrics – the importance and novelty of this study’s findings should not be understated. The fact that functional metrics, many of which help maintain ecosystem functioning and make communities more robust to environmental change, appear to be benefitting from these types of management further supports the need to increase the availability of non-crop habitats in agricultural systems, particularly on conventional farms. It is only by maintaining the largest possible array of functional traits in such environments that their capacity to provide multiple ecosystem functions can be maintained, many of which are likely to directly benefit the human population.

Guest post by Ailsa McKenzie, Associate Editor, Journal of Applied Ecology

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