Are communities saturated? On the relationship between alpha, beta and gamma diversity

A popular way to suggest a regional influence on local species diversity ha s been to plot local versus regional diversity. The form of these curves ha s been interpreted as evidence for or against "community saturation" due to species interactions. This interpretation, however, is unwarranted. Using the concepts of alpha, beta and gamma diversity, I show that local-regional richness curves are determined by the way total diversity is partitioned b etween its alpha and beta components, which itself is a matter of scale. Ch anging the scale of the local community amounts to changing the scale at wh ich the heterogeneity of the interactions between organisms and their envir onment manifests itself, and hence the balance between alpha and beta diver sity. Community saturation may occur because of physical limitations, but t here are no theoretical grounds for the belief that species interactions se t an absolute upper limit to diversity at any scale. A distinction between different meanings of the concept of "saturation" is proposed to clarify th is issue. I argue that the challenge now is to understand the relationship between alpha and beta diversity at multiple scales, and the processes that determine it.