How species richness and total abundance constrain the distribution of abundance
The species abundance distribution (SAD) is one of the most intensively studied distributions in ecology and its hollow-curve shape is one of ecology’s most general patterns. The form of the SAD has been predicted by many models, including those based on niche differentiation, stochastic population dynamics, and the structure of abundance across species’ ranges. Though potentially influenced by all of these, the ability to distinguish between which factors are driving the form of the SAD depends on the presence of sufficient variation in the shape of the possible SADs. If most of the possible SADs have similar shapes, it will be difficult to determine what processes generated them. We use one of the largest and most diverse compilations of data in community ecology to find whether the general shapes of SADs can be explained solely by the constraining influence of two primary constraints, i.e. species richness and total abundance. Using a conceptually simple approach, we compare empirical SADs to the average and majority of all possible SAD shapes based on those constraints. We also use this approach to reveal general constraining properties of species richness and total abundance for distributions of wealth and abundance, in general.
We find that most of the feasible distributions for a given species richness and total abundance exhibit the canonical hollow-curve shape and that the central tendency of the feasible set is strongly correlated with empirical SAD patterns within and among sites (most R2 values > 75%) of tree, butterfly, mammal, and bird communities and metagenomic samples of diverse prokaryotic communities and indoor fungal communities. However, empirical SADs are often more hollow than the bulk of feasible set, revealing exceptional unevenness across many systems with respect to the average and majority of possible SAD shapes. Our findings reveal the sole constraining influence of species richness and total abundance, provide a general explanation for the ubiquitous hollow-curve SAD, a naïve but powerful expectation for the shape of the SAD, and reveal that while the majority of information in the SAD may be contained in total abundance and species richness, that the SAD may still contain information on ecological and statistical process above and beyond these constraints.