Most efforts in understanding how biodiversity influences ecosystem functioning have come from experimental work performed under highly controlled conditions of laboratories and mesocosms. Of the biodiversity and ecosystem functioning (BEF) studies examining the effects of marine diversity of one trophic level on functions at other trophic levels, none were performed in subtidal field habitats. Several studies have highlighted the importance of pursuing BEF studies under natural field conditions, as the probability of detecting a diversity effect increases in natural versus artificial systems. In this study, we addressed this need by investigating the effects of sea urchin diversity on the percent of substrate grazed in a natural subtidal rocky habitat in the Galapagos. We manipulated in cages the richness and composition of the following urchins species: Eucidaris galapagensis, Lytechinus semituberculatus, and Tripneustes depressus, which co-occur, and are the most common urchins in the central archipelago. There were three levels of species richness with all possible 2-species combinations. Because Tripneustes individuals are generally 2X larger than the other two species, and big species seem to have disproportionate effects in BEF studies, we also compared treatments with and without Tripneustes to test if it was the solely responsible for any significant effect.
Results/Conclusions
The experiment ran for five weeks at ~10 m depth at Caamaño, a wave-exposed site in the central archipelago. We found that richness, species composition, and the influence of Tripneustes had a significant effect on the amount of substrate grazed. In addition, both non-transgressive (NTO) and transgressive overyielding (TO) occurred. All 2-species assemblages grazed more than their expected values (NTO, based on the average of the single species treatments), and the assemblages of Eucidaris+Lytechinus and of Lytechinus+Tripneustes grazed more than the best performing component urchin species when alone (TO). We conclude that in spite of the influence of a large key species (Tripneustes), resource use complementarity and/or positive interactions between urchins also affected grazing intensity. We suggest that differences in urchin abilities to forage under high flow conditions and differences in urchin feeding preferences may explain our results. To the best of our knowledge, this study is the first BEF study performed in a natural subtidal habitat, and this is relevant as a our natural approach allowed us to examine the influence of additional factors (flow, feeding preferences) that are usually lacking in artificial environments and that ultimate govern natural ecosystems.