Abstract
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.
Highlights
Beta diversity, or community differentiation among sites, is a key aspect of global patterns in biodiversity, including the richness of tropical forests (e.g., Condit et al 2002, Tuomisto et al 2003) and coral reefs (e.g., Dornelas et al 2006, Cornell et al 2007, Belmaker et al 2008)
It is the community divergence associated with unit changes in environmental gradients that should increase at higher levels of productivity, as some studies suggest (e.g., Harrison et al 2006, Qian and Ricklefs 2007, Qian 2008, Qian et al 2009)
There was no consistent relationship of soil fertility to beta diversity, ; intermediately fertile gabbro soils showed the lowest levels of topographic beta diversity in two of three analyses, and there was no trend toward an influence of soil fertility on elevational beta diversity (Fig. 1)
Summary
Community differentiation among sites, is a key aspect of global patterns in biodiversity, including the richness of tropical forests (e.g., Condit et al 2002, Tuomisto et al 2003) and coral reefs (e.g., Dornelas et al 2006, Cornell et al 2007, Belmaker et al 2008). A third, non-exclusive possibility is that high productivity is associated with increased specialization along environmental gradients, perhaps because productivity intensifies the competitive pressure that leads to habitat specialization in either evolutionary or ecological time (MacArthur 1965) In this case, it is the community divergence associated with unit changes in environmental gradients (e.g., soils, elevation, or ocean depth; ‘structured’ beta diversity, or ‘turnover’ sensu Anderson et al in 2011) that should increase at higher levels of productivity, as some studies suggest (e.g., Harrison et al 2006, Qian and Ricklefs 2007, Qian 2008, Qian et al 2009)
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