tion in sampling extent (i.e., the geographic distance between the furthest sampled elevations), which often occurs among studies, and sampling intensity (or effort) along the gradient within a taxonomic group, which often occurs within studies. As a result, the disparity in elevational richness patterns observed between bacteria and plants is likely caused by differences in how ecological and evolutionary processes have operated across the gradient (although see below for a discussion on the potential influence of scaling effects).
As expected, for both bacteria and plants (Forest et al., 2007) we found that the patterns of phylogenetic diversity mirrored those of taxon richness (Fig. 7.1B). However, a more detailed look at the phylogenetic structure of the bacterial and plant communities revealed another significant difference. At all elevations bacterial communities had a tendency to be more phylogenetically clustered than expected by chance (Fig. 7.2). This observation is consistent with results reported by Horner-Devine and Bohannan (2006) who found that bacterial communities in a wide range of environments tended to be phylogenetically more closely related than expected