Alpha and beta diversity of arthropods and plants in organically and conventionally managed wheat fields.
Most studies in applied ecology use measures of α-diversity measures, i.e. the mean diversity on a site, to compare biodiversity effects of different management schemes. The total or α-diversity within a region, however, need not be correlated with the mean α-diversity within any site of the region. Thus, analyses of α-diversity alone may misrepresent the contributions of other diversity components (β) to total diversity (γ). We apply a biodiversity-partitioning approach to species richness from a comparison between paired organic and conventional wheat fields in 21 sites from three regions in Germany, where we recorded plants, bees, carabids, staphylinids and spiders in the centre and edge of the fields. Relative values of α- and β-diversity depended on taxon. Both between-site and between-region β-diversity were very high (in total 60-85%). α-Diversity and between-site β-diversity was larger on the edge than in the centre of fields for all taxa. α-Diversity, between-site β-diversity of plants and bees and between-region β-diversity of bees were higher in organic than in conventional fields, providing local as well as larger-scale species richness benefits. α-Diversity did not differ between management types for the epigaeic arthropods. Lower between-site β-diversity was found for spiders in organic fields than in conventional fields, resulting in higher total species richness in conventionally managed wheat. Similarity in composition of landscapes surrounding the study fields was correlated with similarity in species composition for epigaeic arthropods in conventional fields. For this group of organisms the variability of landscapes in the sample contributed to increasing β-diversity. Synthesis and applications. β-Diversity accounts for the major part of species richness in agro-ecosystems. Implementing an agri-environment scheme such as organic agriculture may result in either an additional increase of total diversity, as could be shown for plants and bees, or in a decrease in total diversity as was the case for the spiders. Therefore, β-diversity needs to be included in the evaluation of different management schemes for conservation. For plant and bees it is recommended to implement agri-environment schemes in contrasting landscapes and in different regions to maximize total species richness benefits.