Small‐scale environmental heterogeneity and the analysis of species distributions along gradients

Abstract. The observed distribution of a species along an environmental gradient is strongly affected by environmental variability within a quadrat. Because a quadrat does not represent a point along an environmental gradient, but rather a range of conditions, it is likely to contain species not typically associated with the mean conditions in the quadrat. Systematic relationships exist between a species' true distribution, the observed distribution as a function of mean quadrat environment, and the frequency distribution of the environment within that quadrat. The observed species habitat breadth increases and the observed maximum abundance decreases as within‐quadrat environmental heterogeneity increases. If species distributions or beta diversities are to be compared among species or coenoclines, they should be correctedforintra‐quadratheterogeneity.Wederive simple corrections for environmental heterogeneity. The distributions of hardwood forest understory species along a soil acidity gradient in the North Carolina piedmont are presented as an example.