Aims Much recent theory has focused on the role of neutral processes in assembling communities,but the basic assumption that all species are demographically identical has found little empirical support.Here,we show that the framework of the current neutral theory can easily be generalized to incorporate species differences so long as fitness equivalence among individuals is maintained through trade-offs between birth and death.Methods Our theory development is based on a careful reformulation of the Moran model of metacommunity dynamics in terms of a non-linear one-step stochastic process,which is described by a master equation.Important Findings We demonstrate how fitness equalization through demographic trade-offs can generate significant macroecological diversity patterns,leading to a very different interpretation of the relation between Fisher’s a and Hubbell’s fundamental biodiversity number.Our model shows that equal fitness(not equal demographics)significantly promotes species diversity through strong selective sieving of community membership against high-mortality species,resulting in a positive association between species abundance and per capita death rate.An important implication of demographic trade-off is that it can partly explain the excessively high speciation rates predicted by the neutral theory of the stronger symmetry.Fitness equalization through demographic trade-offs generalizes neutral theory by considering heterospecific demographic difference,thus representing a significant step toward integrating the neutral and niche paradigms of biodiversity.
Aims The neutral theory of biodiversity has been criticized for being fragile with even slight deviations from its basic assumption of equal fitness among species.In response to this criticism,Hubbell((2001)The Unified Neutral Theory of Biodiversity and Biogeography.Princeton,NJ:Princeton University Press)proposed that competitive exclusion can be infinitely delayed by dispersal and recruitment limitation,thus making species effectively neutral.But the theoretical foundation for this claim still remains unclear and controversial,and the effects of dispersal and recruitment limitation are often confounded,especially in field studies.This study aims to provide an affirmative theoretical answer to the question of whether dispersal limitation and recruitment limitation can separately or jointly overwhelm the effects of fitness differences among species and lead to neutral community dynamics.Methods Computer simulations were used to investigate the effects of dispersal and recruitment limitation on delaying competitive exclusion in a homogeneous habitat in a spatially explicit context.Important Findings We found that even a slight competitive asymmetry would require extremely strong dispersal and recruitment limitation for neutrality to emerge.Most importantly,when the effects of dispersal and recruitment limitation were set apart,it is found that recruitment limitation is more effective in delaying competitive exclusion,whereas dispersal limitation tends to have a stronger impact on the general shape of both species abundance distributions and species–area relationships.
