The role of the Allee effect in species packing

The species-packing model of May and MacArthur is modified to include a commonly-expected influence of sexual reproduction, namely a systematic diminishing of the rate of increase in a population when it becomes rare (called the “Allee effect”). This effect causes discreteness, i.e., a finiteness to the density of species found along a resource axis. The species separate in a manner that relates to their intrinsic capacities to utilize the resources. Also discussed is the issue of species diversity gradients, and how the question of species discreteness might apply to it. The model with the Allee effect is in reasonable accord with island diversity patterns, but is minimally applicable to longitudinal gradients. Environmental stochasticity is modelled with noise terms governed by widely varying timescales. However, the resulting stochastic extinction is found neither to generate discrete distributions by itself, nor to have substantive effects on the discrete distributions generated by the Allee effect.