Understanding the consequences of changing biodiversity on rocky shores: How much have we learned from past experiments?

Manipulative experiments are increasingly used to establish causal connections between biodiversity and productivity or other measures describing the functioning of ecological systems. Many studies have detected significant effects of changing the number of species in experimental treatments on productivity and other response variables, but interpretation of these results is complicated by difficulties in separating effects of number, identity and relative density of species. A variety of experimental designs has been proposed to circumvent these problems, but no approach has been developed to address all these problems simultaneously. Rocky shore ecology has contributed relatively little to this debate, despite a long tradition of experimental manipulations of species in this environment. Here, I illustrate exemplars of biodiversity and biodiversity-like experiments in rocky shore habitats, to derive lessons for future experimental analyses of biodiversity in marine coastal systems. It is found that the principles underlying modern experiments on biodiversity have been available for a long time, being developed in studies of competition among intertidal gastropods more than 20 years ago, but have been largely overlooked by aquatic and terrestrial ecologists. Building on these principles, I propose a design of experiment that can discriminate among effects of number, identity and density of species in highly diverse assemblages. Possible applications of this approach to the analysis of biodiversity and its effects on the ability of assemblages to withstand disturbances are discussed.