Frequency-dependent assembly processes determine the coexistence and relative abundance of tropical plant species.

Testing the extent to which ecological communities are structured by deterministic (niche-based) assembly processes, resulting in predictable species abundance and composition, is a fundamental goal of ecology. Here we use a 10-year dataset of 55,156 lianas comprising 86 species in an old-growth tropical forest in Panama to test whether community assembly is consistent with niche-based assembly processes. We find that species diversity and community composition was maintained because species conformed to four general requirements of coexistence theory: (1) species have negative conspecific frequency-dependent feedback that control their local population size; (2) species have a stronger negative effect on their own population than that of heterospecifics; (3) the equilibrium frequencies of species correspond to their relative abundance; and (4) species have positive invasibility. These results indicate that coexistence through deterministic niche-based processes controls local population sizes and prevents any one species from displacing others. Rare species persisted because particularly strong negative feedbacks maintained them at their relatively low equilibrium abundances, thus preventing them from going extinct. Furthermore, we show that it is necessary to use population demography to test coexistence theory because stem mortality alone does not reflect species demography. These findings have broad implications for species coexistence and diversity maintenance in tropical forests and possibly other ecosystems.