A sampling theory for asymmetric communities

TL;DR

This paper develops an analytical model for asymmetric community dynamics that extends Hubbell's neutral theory, providing tools to distinguish neutral from non-neutral ecological communities through species abundance distributions.

Contribution

It introduces the first analytical model of asymmetric community dynamics that generalizes Hubbell's neutral theory and derives approximate sampling distributions for practical analysis.

Findings

Mass-effects can promote coexistence in asymmetric communities.

Unimodal species abundance distributions can occur in asymmetric communities.

Multiple modes indicate non-neutral dynamics.

Abstract

We introduce the first analytical model of asymmetric community dynamics to yield Hubbell's neutral theory in the limit of functional equivalence among all species. Our focus centers on an asymmetric extension of Hubbell's local community dynamics, while an analogous extension of Hubbell's metacommunity dynamics is deferred to an appendix. We find that mass-effects may facilitate coexistence in asymmetric local communities and generate unimodal species abundance distributions indistinguishable from those of symmetric communities. Multiple modes, however, only arise from asymmetric processes and provide a strong indication of non-neutral dynamics. Although the exact stationary distributions of fully asymmetric communities must be calculated numerically, we derive approximate sampling distributions for the general case and for nearly neutral communities where symmetry is broken by a single species distinct from all others in ecological fitness and dispersal ability. In the latter case, our approximate distributions are fully normalized, and novel asymptotic expansions of the required hypergeometric functions are provided to make evaluations tractable for large communities. Employing these results in a Bayesian analysis may provide a novel statistical test to assess the consistency of species abundance data with the neutral hypothesis.