Niche and neutral models predict asymptotically equivalent species abundance distributions in high-diversity ecological communities

Abstract

A fundamental challenge in ecology is to understand the mechanisms that govern patterns of relative species abundance. Previous numerical simulations have suggested that complex niche-structured models produce species abundance distributions (SADs) that are qualitatively similar to those of very simple neutral models that ignore differences between species. However, in the absence of an analytical treatment of niche models, one cannot tell whether the two classes of model produce the same patterns via similar or different mechanisms. We present an analytical proof that, in the limit as diversity becomes large, a strong niche model give rises to exactly the same asymptotic form of SAD as the neutral model, and we verify the analytical predictions for a Panamanian tropical forest data set. Our results strongly suggest that neutral processes drive patterns of relative species abundance in high-diversity ecological communities, even when strong niche structure exists. However, neutral theory cannot explain what generates high diversity in the first place, and it may not be valid in low-diversity communities. Our results also confirm that neutral theory cannot be used to infer an absence of niche structure or to explain ecosystem function.

Fig. 1.

Theoretical and empirical SADs for the 50-ha tropical forest plot on BCI. Species are binned into log₂ abundance categories using Preston's method (33). Each line represents a predicted SAD for a community with K equal-sized niches, J = 21,060 individuals, m = 0.075, and θ = 52.1 (see text). Predicted SADs from the niche model for K < 16 were virtually identical to the neutral model (K = 1) and are not shown separately. The points represent the observed SADs from six censuses of the BCI plot from 1982 to 2005.

Fig. 2.

Theoretical SADs for a community with J = 21,060 individuals, m = 0.075, θ = 5.0, and varying number of equal-sized niches (K), where K = 1 corresponds to the neutral model. Species are binned into log₂ abundance categories using Preston's method.