'Structured' beta diversity increases with climatic productivity in a classic dataset

Despite a surge of interest in the measurement of beta diversity, there remain only a modest number of well-supported explanations for variation in naturally occurring levels of beta diversity. Among the few emerging generalizations is that beta diversity tends to increase with productivity; it remains to be determined whether the mechanism(s) involves habitat specialization or random factors in community assembly. We examined this question using the classic dataset of Whittaker (1960), who first defined beta diversity in a study of plant communities along multiple abiotic gradients related to productivity. With increasing productivity along climatic gradients (elevation or topography), though not a soil fertility gradient, we found increases in the levels of 'structured' beta diversity, i.e., the turnover associated with each of the other gradients, consistent with greater habitat specialization. 'Unstructured' beta diversity, i.e., the among-site variation not associated with gradients, varied idiosyncratically among different combinations of environmental factors. These results were robust to the use of either presence-absence or relative abundance data. We conclude that habitat specialization along gradients may tend to increase either with productivity itself, or with regional (gamma) diversity, which tends to be higher in more productive climates and conceivably 'spills over' in ecological or evolutionary time to enhance structured beta diversity.