1. Manipulative experiments are often used to identify causal linkages betweenbiodiversity and productivity in terrestrial and aquatic habitats. 2. Most studies have identified an effect of biodiversity, but their interpretation hasstimulated considerable debate. The main difficulties lie in separating the effect of speciesrichness from those due to changes in identity and relative density of species. 3. Various experimental designs have been adopted to circumvent problems in theanalysis of biodiversity. Here I show that these designs may not be able to maintain theprobability of type I errors at the nominal level (α= 0·05) under a true null hypothesisof 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 effectsof identity and density of species from those due to number of species. Simulationsshow that the proposed experiments may have increased capacity to control for type Ierrors when effects of density and identity of species are also present. These designs haveenough flexibility to be useful in the experimental analysis of biodiversity in variousassemblages and under a wide range of environmental conditions
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