The species richness–resource availability relationship is hump-shaped

Abstract

Ecological theory shows that, as resource availability increases, the number of related species (S) rises from zero at first, reaches a peak (optimum), and then falls to zero again to form a unimodal (hump-shaped) curve. Although rarely demonstrated, I show support for the unimodal, S-environment model exists among studies of soil nutrients and pH, substrate water, air temperature, evapotranspiration, sunlight, fire frequency (as a surrogate for resource turnover), herbivory, and plant density and productivity (as surrogates for resource availability). The rising left-hand side of the curve is due to a positive response to the controlling variable (e.g., soil nutrients) and the falling right-hand side to metabolic suppression by supraoptimal levels (e.g., protein denaturation by heat) or the retarding effect of secondary variables (e.g., increasing self shading). Statistically significant shape outcomes depend on range of the variable tested, scale of the study, taxonomy and life form of the targeted species assemblage, extent that species are distributed along the gradient, type of curve hypothesized, and extent to which the study continues to zero S. Interpretations should consider whether the left tail of the curve will terminate at the origin (0,0). Mechanistic explanations for the unimodal pattern may involve species interactions, such as individual fitness at the microscale optimizing at moderate abundance in the species mix, the inevitable increasing presence of inhibitory secondary effects, and existence of more resource-use generalists than specialists. Six reasons for lack of support for the unimodal hypothesis are noted. Support for the unimodal model is more likely the greater the range of the variable tested and the greater its causative link to S. The concept of ‘prediction’ in ecology needs to go beyond the tradition of (curvi)linear relationships and accept that most relationships in nature are (must be) unimodal.