  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.
