Effects of biodiversity on ecosystem stability: distinguishing between number and composition of species

Abstract

Declines in biodiversity have caused concern because of ethical and aesthetic reasons, but also because of the consequences for the goods and services provided by natural ecosystems. Consequently, ecologists have focused for decades on testing the idea that systems with more species are more stable. The results, however, have been complex and inconsistent. In particular, it is still unclear whether high stability in species-rich communities is due to the number of species per se (species richness) or to the increased likelihood of including particular species or functional types (species composition). In this thesis, I evaluated the contribution of species richness and species identity to the stability of marine hard-bottom communities. Combining observational and manipulative experimental methods, I conducted three field studies in intertidal and shallow subtidal habitats of Helgoland Island, NE Atlantic. First, I conducted an observational study to test whether intertidal communities containing many species are more stable (i.e. do vary less over time) than communities containing fewer species. Species covers were estimated every 6 months for 24 months and an index of stability was calculated for total community cover across time (S = mean SD-1). Second, I conducted a synthetic-assemblage experimentin which I increased the diversity of field-grown sessile suspension-feeding invertebratesto determinate whether assemblages containing several functional groups consume a greater fraction of resources than is caught by any of the functional types grown alone...