Lotteries in communities of sessile organisms.

Traditional niche theory has its formal roots in the equilibrium solutions to Lotka-Volterra type equations. In recent years, this theory has lost much of its former pre-eminence for those interested in communities of sessile organisms, as witnessed by two recent reviews in TREE. Of the alternative theories that are now emerging, the so-called lottery theories have in common the assumption that the population dynamics of species are partly governed by random factors that are intrinsic to the system. They are thus genuinely stochastic community theories, in that randomness is taken as a system's property rather than as a 'disturbance' that is imposed from the outside. Analyses of these theories suggest, contrary to classical results, that coexistence of very similar species is possible, that environmental variability generally enhances such coexistence and that convergent, rather than divergent, evolution may be a result of interspecific competitive interactions.