Traditional tests of neutral theory compare the observed local community to predictions that assume a neutral regional pool. Here, we instead use regional-scale abundance data to inform neutral theory predictions for the tropical tree community in the 50 ha Barro Colorado Island (BCI) plot. We consider a set of potential metacommunities differing in their spatial scale and use them to inform the standard spatially-implicit neutral model. We assess whether or not our neutral model can describe the local community composition of BCI across all 35 years since its first census. Through a combined analytical and simulation approach, we predict both average properties and expected variation around them, allowing us to avoid false assumptions of standard statistical methods. Knowledge of species regional abundance enables us to use local abundances to explicitly test for neutral processes acting at the local scale, where prior analyses confound spatial scales. It also enables us to carry out both community- and species-level analyses. As a result, for the first time, we can test neutral predictions of species-level abundances (as opposed to community-level measures, such as the species abundance distribution).
Results/Conclusions
Overall, when informed by regional abundance data, neutral theory provides a poor description of the species abundance distribution (SAD) of the tree community in the 50 ha BCI plot. Indeed, based on this community-level metric, we can reject the standard neutral model in later census years. Species-level analyses enable us to reject the neutral model for all census years and potential metacommunity spatial scales. Specifically, a greater proportion of species differ significantly from their neutral expectations than in our neutral simulations (p < 0.05 in all cases). We also highlight 12 species that deviate from neutral theory predictions across all census years, regardless of parameter values, which may be particularly worthy of further study. Surprisingly, we find little connection between which species abundance bins drive the rejection of the species abundance distribution and the extent of species-level departures from neutral predictions among species whose abundance falls in those bins. It seems species-level properties and community-level properties provide different windows into how a community may be departing from neutral processes.