Increasing accuracy of causal inference in experimental analyses of biodiversity

1. Manipulative experiments are often used to identify causal linkages between biodiversity and productivity in terrestrial and aquatic habitats. 2. Most studies have identified an effect of biodiversity, but their interpretation has stimulated considerable debate. The main difficulties lie in separating the effect of species richness from those due to changes in identity and relative density of species. 3. Various experimental designs have been adopted to circumvent problems in the analysis of biodiversity. Here I show that these designs may not be able to maintain the probability of type I errors at the nominal level (alpha = 0.05) under a true null hypothesis of no effect of species richness, in the presence of effects of density and identity of species. 4. Alternative designs have been proposed to discriminate unambiguously the effects of identity and density of species from those due to number of species. Simulations show that the proposed experiments may have increased capacity to control for type I errors when effects of density and identity of species are also present. These designs have enough flexibility to be useful in the experimental analysis of biodiversity in various assemblages and under a wide range of environmental conditions.