Conceptual Framework (IMAGE) University of Texas at Arlington Caption This is the 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. Credit UTA Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.