Plant species diversity and environmental heterogeneity: spatial scale and competing hypotheses

What is the observed relationship between plant species diversity and spatial environmental heterogeneity? Does the relationship scale predictably with sample plot size? What are the relative contributions to diversity patterns of variables linked to productivity or available energy compared to those corresponding to spatial heterogeneity? Observational and experimental studies that quantified relationships between plant species richness and within-sample spatial environmental heterogeneity were reviewed. Effect size in experimental studies was quantified as the standardized mean difference between control (homogeneous) and heterogeneous treatments. For observational studies, effect sizes in individual studies were examined graphically across a gradient of plot size (focal scale). Relative contributions of variables representing spatial heterogeneity were compared to those representing available energy using a response ratio. Forty-one observational and 11 experimental studies quantified plant species diversity and spatial environmental heterogeneity. Observational studies reported positive species diversity-spatial heterogeneity correlations at all points across a plot size gradient from similar to 1.0 x 10(-1) to similar to 1.0 x 10(11) m(2), although many studies reported spatial heterogeneity variables with no significant relationships to species diversity. The cross-study effect size in experimental studies was not significantly different from zero. Available energy variables explained consistently more of the variance in species richness than spatial heterogeneity variables, especially at the smallest and largest plot sizes. Species diversity was not related to spatial heterogeneity in a way predictable by plot size. Positive heterogeneity-diversity relationships were common, confirming the importance of niche differentiation in species diversity patterns, but future studies examining a range of spatial scales in the same system are required to determine the role of dispersal and available energy in these patterns.