by chance. Given the parsimonious hypothesis that closely related taxa are more ecologically similar (i.e., phylogenetic niche conservatism), our results suggest that abiotic filtering tends to be a more prominent force in the structuring of bacterial communities along the gradient. Several studies have suggested that for most macroorganisms, ecological traits are phylogenetically conserved (Prinzing et al., 2001; Blomberg et al., 2003; Cavender-Bares et al., 2004). It is important to emphasize that although this statement may be correct for macrorganisms, the generality of niche conservatism for microorganisms and in particular bacteria is unknown. Observed phylogenetic clustering in microbial communities could also be the result of radiation events combined with dispersal limitation (Horner-Devine and Bohannan, 2006). As we discuss below, alternative explanations for the patterns we observe relate to the phylogenetic and spatial scale of our analyses (Swenson et al., 2006, 2007). Scaling issues are relevant to all of the biodiversity patterns we examined.
In contrast to bacteria, plant communities did not show a uniform phylogenetic structure across the gradient. Plant communities tended