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 from a very restricted taxon or include all living organisms. The common problem, however, is to understand the relative importance of speciation and extinction and, more locally, of immigration and loss. Speciation is the ultimate motor driving biodiversity and ecologists need to know the factors influencing rates of speciation, and whether there is a feedback, positive or negative, between species numbers and the generation of new taxa. However, the relative importance of speciation and other factors determining species numbers varies crucially across different scales of enquiry. Here, we explore 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 advances in the field.