Immigration and the maintenance of local species diversity

Explaining the maintenance of high local species diversity in communities g overned by competition for space has been a longstanding problem in ecology We present a simple theoretical model to explore the influence of immigrat ion from an external source on local coexistence, species abundance pattern s, and ecosystem processes in plant communities. The model is built after c lassical metapopulation models but is applied to competition fog space betw een individuals and includes immigration by a propagule rain and an extinct ion threshold for rare species. Our model shows that immigration can have a huge effect on local species diversity in competitive communities where co mpetition for space would lead to the exclusion of all but one species if t he community were closed. Local species richness is expected to increase st rongly when immigration intensity increases beyond the threshold required f or the successful establishment of one or a few individuals. Community stru cture and species relative abundances are also expected to change markedly with immigration intensity. Increasing immigration causes total space occup ation by the community to increase but primary productivity on average to e ither decrease or stay constant with increasing diversity, depending on the relation between immigration and local reproduction rates. These results s tress the need for a regional perspective to understand the processes that determine species diversity, species abundance patterns, and ecosystem func tioning in local communities.