Partitioning selection and complementarity in biodiversity experiments

The impact of biodiversity loss on the functioning of ecosystems and their ability to provide ecological services has become a central issue in ecolog y. Several experiments have provided evidence that reduced species diversit y may impair ecosystem processes such as plant biomass production(1-5). The interpretation of these experiments, however, has been controversial(6-12) because two types of mechanism may operate in combination(6,13-15). In the 'selection effect', dominance by species with particular traits affects ec osystem processes. In the 'complementarity effect', resource partitioning o r positive interactions lead to increased total resource use. Here we prese nt a new approach to separate the two effects on the basis of an additive p artitioning analogous to the Price equation in evolutionary genetics(16-19) . Applying this method to data from the pan-European BIODEPTH experiment(4) reveals that the selection effect is zero on average and varies from negat ive to positive in different localities, depending on whether species with lower- or higher-than-average biomass dominate communities. In contrast, th e complementarity effect is positive overall, supporting the hypothesis tha t plant diversity influences primary production in European grasslands thro ugh niche differentiation or facilitation.