Effects of organic farming on plant and butterfly functional diversity in mosaic landscapes

Abstract

Organic farming can benefit plants and butterflies in terms of species richness and abundance, in particular in homogeneous landscapes. Nevertheless, whether organic farming can benefit functional diversity of these two organism groups is not well understood. Organic farming could benefit functional diversity by counteracting simplification and homogenisation of biotic communities caused by earlier agricultural intensification, and therefore contribute to communities more resilient to environmental or land use changes. We analysed species richness and four functional diversity indices (functional richness, evenness, divergence and dispersion) of plants and butterflies on 15 pairs of organic/conventional farms in North-West Spain, whilst accounting for independent and joint effects of landscape context. To better understand links between functional diversity and taxonomic species assemblages, we applied an indicator species analysis to determine whether any particular plant or butterfly species were significantly more abundant on organic or conventional farms. Both butterfly species and functional richness were higher on organic than on conventional farms. In addition, when the farms were surrounded by low proportions of agriculture, butterfly functional evenness was higher on organic farms. In contrast, organic farming did not affect plant species richness or functional diversity. Nevertheless, plants were affected by landscape openness and field size, so that plant functional richness decreased with increasing landscape openness while functional evenness showed a complex relationship to the interaction between field size and landscape openness. Indicator analyses revealed that three plant species, but no individual butterfly species, were significantly related to organic farming. In conclusion, organic farms provide higher-quality habitat for butterflies than conventional farms, either by promoting specific grassland plants benefiting butterflies, or more generally by providing a wider niche space, fostering functionally diverse butterfly communities.