Site-suitability difference and the expectation of species richness difference in stochastic two-subcommunities models

Abstract

Understanding how community assembly during the initial faunization phases determines the difference in species richness is a fundamental question in ecology. However, there are few predictions for when and to what degree the differences in species richness between subcommunities will emerge. Here, we investigate the expectation of richness difference in a pair of subcommunities, assuming that species may have different and independent presence probabilities. We then introduce several indices and examine how expected richness difference is determined by the indices. We found that (i) species differences (the average of species presence probabilities in two subcommunities divided by the total presence probability) have inconsistent effects on richness difference; (ii) the degree of spatial heterogeneity (average of differences in species presence probabilities in two subcommunities across species) may, but not always, have a good predictive ability for richness difference; and (iii) the absolute difference in average presence probabilities (site-suitability difference) has robust predictive ability for richness difference unless richness difference is very small. This work provides a theoretical framework for predicting and analyzing species richness difference from presence-absence data based on null models.