What's really happening on the Barro Colorado Island 50 ha forest plot?

John Alroy 

1. Over the decades, Barro Colorado Island (BCI) has incubated major conceptual and empirical advances in ecology. For example, the theoretical zero-sum multinomial (ZSM), log series, and Poisson log normal (PLN) models have been used to describe abundance distributions and quantify diversity in the BCI 50 ha forest plot.

2. A new model called the compound exponential-geometric series (CEGS) better predicts distributions and quantifies alpha and beta diversity in 1 ha subplots. Distributions are analysed by fitting the four models to each of the subplot's data sets and then predicting counts in adjacent subplots. The PLN performs second-best while the ZSM and log series have almost no predictive power. Meanwhile, only CEGS detects plausible species-area relationships.

3. CEGS assumes nothing specifically about regional speciation rates, density dependence, or competition. It results when recruitment rates are on a per-species basis, whereas death rates are per-capita. It is very much not neutral because it posits that each species has a randomly allocated underlying abundance. It could be interpreted as implying that ecological traits govern demographic rates and in turn structure stem counts. A series of new predictions follows from the model.

4. The model deserves further testing with ecological data, including other forest plot inventories.

5. Synthesis: Ecologists often advocate so-called neutral models in which tree species have identical recruitment and mortality rates, but the actual data for the best-studied tree plot in the world suggest high variation in rates. When a better model is used, internally consistent and sensible species richness estimates are obtained.