Trait-based approaches to species abundance distributions

Background/Question/Methods
The species abundance distribution (SAD) is one of the most common descriptors of community structure. Typically communities are dominated by a few common species and many rare ones. Despite the ubiquity of these patterns, until recently there was little process-based theory to explain them. Traditional niche models usually do not permit coexistence of more than a few species, so are unable to generate realistic SADs. Neutral models, based on the interplay between ecological drift and speciation, can generate realistic SADs but the assumption of strict neutrality is questionable. Here I take a metacommunity perspective where local abundance is driven by the interplay between immigration and selection (competition), where species are defined by functional traits that affect fitness.

Results/Conclusions
This niche-based framework not only produces realistic SADs but also predicts the relation ship between traits and abundance. These models can analyzed numerically and further understood using analytical approximations. The strength of immigration relative to selection is a key parameter that shapes SADs, while the emergent shape of the fitness landscape and the traits of the metacommunity species pool determine patterns of trait diversity. Whether environmental factors affect birth rate or mortality is also important in determining the abundance of rare species. This theory provides a simple alternative framework to explain SADs in different communities and makes testable predictions about the traits of common versus rare species.