Species richness and patterns of abundance result from the interplay between niche differences, realized in models as intraspecific density-dependence, and so-called neutral processes that slow competitive exclusion when species' fitnesses are similar. Most mechanisms of intraspecific density-dependence do not act instantaneously, and are mediated through delayed changes in either the physical or biotic environment that generate negative feedbacks between population size and reproductive success. The degree of species coexistence supported by niche differences also depends sensitively on the absolute size of the population, as the advantage of rarity must counterbalance the random loss of species through demographic stochasticity. By generalizing approximation methods developed for systems with finite population size and intraspecific density-dependence, I present a method to estimate both equilibrium species richness and species abundance distributions in the presence of delays. This method can also incorporate heterogeneity among species in parameters describing fecundity and survivorship, predicting the key factors leading to higher abundance.

Results/Conclusions

By distributing its effect, delays reduce the predicted coexistence associated with a particular strength of intraspecific density-dependence. Delays also qualitatively change in the shape of species abundance distributions, rendering them consistent with the widely observed hollow curve shape even in the presence of strong intraspecific density-dependence that otherwise generates an unrealistic unimodal shape.