Dispersal Limitation,Recruitment Limitation And Species Coexistence

One of the main focus of community ecology is to explain the maintenance of species diversity within communities. Contrary to classical niche theory, the neutral theory proposed by Hubbell in 2001 has attracted much attention by ecologists because of its mathematical simplicity and predictive power. However, the basic assumption of neutral theory is the ecological equivalence among individuals of different species, i.e. individuals irrespective of species identity share same probability of birth, death, migration and speciation, is obviously at odds with empirical observations. Theoretical studies have also demonstrated that a slight difference in species per capita birth or death rates can result in a much shorter species coexistence time and quite different species abundance distributions (SADs) compared with the predictions of the neutral theory. To support neutral theory, some ecologists proposed that dispersal and recruitment can largely delay competitive exclusion so that slow speciation can compensate species extinction, and the neutral assumption can be replaced by assuming that different species can have different demographic rate, given that different species share same per capita fitness through life-history tradeoffs. However, the effect of dispersal and recruitment limitation on species coexistence remain unclear, and the effects of dispersal and recruitment limitation are often confounded, especially in field studies. Meanwhile, almost all the studies on fitness equivalence assumption are spatial implicit, failing to take dispersal and recruitment limitation into account. Here we used computer simulations to investigate the effects of dispersal and recruitment limitation on delaying competitive exclusion in both neutral and nearly neutral communities in a spatially explicit context. Through spatially implicit theoretical analysis and spatially explicit simulations, we studied whether equalizing birth-death tradeoffs necessarily lead to neutral communities and what a role dispersal limitation and self-recruitment plays in maintaining species diversity within such communities. Finally, we studied the effect of self-recruitment on the species diversity in trade-off communities.The main conclusions in this paper are:1. Our simulation results revealed that the effects of even slight competitive asymmetry can only be offset by extremely strong dispersal and recruitment limitation. For the first time, we demonstrated that dispersal limitation is more important to the general shape of both SADs and species area relationships (SARs), whereas recruitment limitation tends to have a bigger impact on offsetting competitive asymmetries and producing neutral patterns.2. When dispersal limitation and birth-death tradeoffs operate simultaneously as they must do in natural communities, we are surprised to find that they interfere with each other in such a way that dispersal limitation favors more fecund species. Fitness equivalence is no longer guaranteed by a perfect tradeoff and, contrary to popular belief, dispersal limitation is found to impede, rather than facilitate, the stochastic coexistence of species. Still, more species can coexist than allowed through demographic equivalence, providing a potentially alternative explanation for biodiversity maintenance in nature.3. In a birth-death tradeoff community, without self-recruitment, we are surprised to find that dispersal and recruitment limitation favors lower fecund species and impede the new species into the community, leading a lower species diversity in the community.