Biodiversity loss and ecosystem functioning: Distinguishing between number and identity of species

Given currently high rates of extinction, it is critical to be able to predict how ecosystems will respond to loss of species and consequent changes in community structure. Much previous research in this area has been based on terrestrial systems, using synthetically assembled communities. There has beer! much less research on inter-trophic effects in different systems, using in situ removal experiments. Problems with the design of early experiments have made it difficult to determine whether reductions in ecosystem functioning in low diversity treatments were due to the number of species present or merely to the reduced likelihood of including particular ("key") species, or functional groups. We established a field experiment, using cage enclosures, to test whether the number or identity of grazing gastropods present would affect the diversity and productivity of intertidal algal communities. We found that there was no relationship between ecosystem functioning and diversity per se, but that different species had idiosyncratic effects. This appears to be a common feature of intertidal systems, which often contain strongly interacting species. In this case, the limpet Patella ulyssiponensis was the most influential primary consumer. Additional treatments enabled us to test the potential for other grazers to compensate for its loss. Early results (after four months) suggested that compensation was possible, implying potential redundancy of P. ulyssiponensis. This effect disappeared after 13 months, however, highlighting the context dependence of so-called redundancy and underlining the importance of long-term field-based experiments in this area.