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

The observed distribution of a species along an environmental gradient is strongly affected by environ- mental variability within a quadrat. Because a quadrat does not represent a point along an environmental gradi- ent, 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 ob- served 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 corrected for intra-quadrat heterogeneity.We derive simple corrections for environmental heterogeneity. The distri- butions of hardwood forest understory species along a soil acidity gradient in the North Carolina piedmont are presented as an example.

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