Effects of changes in number, identity and abundance of habitat-forming species on assemblages of rocky seashores

Understanding the relationship between variation in biodiversity and the alteration of ecosystem processes and stability has become a central ecological issue during the last decade. A large number of experimental and theoretical studies have focused on the effects of changes in species richness and identity, while the role of variation in species abundance has received less attention in biodiversity experiments. By using an experimental design that effectively separates the effects of species richness and identity, while controlling for variation in species abundance, we examined the consequences of loss of 2 habitat-forming species (the canopy algaCystoseira compressaand the musselMytilus galloprovincialis) in Mediterranean low-shore assemblages of algae and invertebrates. Results revealed significant effects associated with changes in number and identity of habitat-forming species on other organisms. The magnitude and direction of these effects, however, changed as a function of the abundance of manipulated species. Our findings indicate how changes in biodiversity of even 2 species can result in complex effects, stressing the importance of investigating non-random loss of habitat-forming species and the need to consider density-dependent effects in biodiversity experiments, particularly when experiments are undertaken in systems like rocky shores, where density-dependent effects are pervasive.