Do plants need niches? Some recent developments in plant community ecology.

Species-rich plant communities appear to defy the competitive exclusion principle, showing relatively few obvious niche differences between coexisting species. Here we explore alternatives to the potentially endless search for new niche axes. Spatial aggregation in populations, non-transitive competition, episodes of density-independent mortality and various non-equilibrium theories allow trophically similar species to coexist for extended periods. In perennial plants or annuals with a seed pool, asynchrony between species in recruitment permits coexistence by the 'storage effect'. There is increasing evidence that species-specific herbivores and pathogens regulate populations of tropical trees to low levels at which competitive exclusion does not occur. The wide variety of alternatives to niche differentiation lead us to question whether plants need occupy different niches to coexist.