Scale and species numbers

One of the main tasks confronting community ecologists is to explain why a particular site harbours a certain number of species. The site might range from a drop of water to the whole Earth, and the species might be drawn fro m a very restricted taxon or include all living organisms. The common probl em, however, is to understand the relative importance of speciation and ext inction and, more locally, of immigration and loss. speciation is the ultim ate motor driving biodiversity and ecologists need to know the factors infl uencing rates of speciation, and whether there is a feedback, positive or n egative, between species numbers and the generation of new tare. However, t he relative importance of speciation and other factors determining species numbers varies crucially across different scales of enquiry. Here, we explo re some of these issues as we move from a macro- to microscale perspective, focusing on a limited number of studies that we believe make important adv ances in the field.