Figure 1. Conceptual framework explaining the variability in the species abundance distribution as a function of two gradients—environmental favorability and species stress tolerance. The size of the triangles represents the population size of the respective tolerance group. The mechanisms hypothesized to exercise the strongest control over the species abundance distribution, i.e. reproduction, immigration, and competition, are given in the triangles. Immigration contributes to population density in both groups but it is expected to be the dominant factor only for sensitive species under harsh conditions.
