Colloquium paper: microbes on mountainsides: contrasting elevational patterns of bacterial and plant diversity

Abstract

The study of elevational diversity gradients dates back to the foundation of biogeography. Although elevational patterns of plant and animal diversity have been studied for centuries, such patterns have not been reported for microorganisms and remain poorly understood. Here, in an effort to assess the generality of elevational diversity patterns, we examined soil bacterial and plant diversity along an elevation gradient. To gain insight into the forces that structure these patterns, we adopted a multifaceted approach to incorporate information about the structure, diversity, and spatial turnover of montane communities in a phylogenetic context. We found that observed patterns of plant and bacterial diversity were fundamentally different. While bacterial taxon richness and phylogenetic diversity decreased monotonically from the lowest to highest elevations, plants followed a unimodal pattern, with a peak in richness and phylogenetic diversity at mid-elevations. At all elevations bacterial communities had a tendency to be phylogenetically clustered, containing closely related taxa. In contrast, plant communities did not exhibit a uniform phylogenetic structure across the gradient: they became more overdispersed with increasing elevation, containing distantly related taxa. Finally, a metric of phylogenetic beta-diversity showed that bacterial lineages were not randomly distributed, but rather exhibited significant spatial structure across the gradient, whereas plant lineages did not exhibit a significant phylogenetic signal. Quantifying the influence of sample scale in intertaxonomic comparisons remains a challenge. Nevertheless, our findings suggest that the forces structuring microorganism and macroorganism communities along elevational gradients differ.

Fig. 1.

Variation in taxon richness (A) and phylogenetic diversity (B) across the elevation gradient. Data are presented as the fraction of total richness and phylogenetic diversity across the gradient. Solid symbols indicate sample richness (core or quadrat), and open symbols indicate the pooled richness at each elevational site (n = 5 per site). At the sample level, Acidobacteria richness and phylogenetic diversity linearly decrease with elevation (regression analysis, r² = 0.22, P < 0.05; r² = 0.23, P < 0.05, respectively), whereas angiosperm richness and phylogenetic diversity patterns are hump-shaped (regression analysis, r² = 0.53, P < 0.0005; r² = 0.47, P < 0.005, respectively). Model choice was based on Akaike information criteria.

Fig. 2.

Variation in community phylogenetic relatedness along the elevation gradient as measured with the NRI (A) and NTI (B). Positive index values indicate phylogenetic clustering, and negative values indicate phylogenetic overdispersion. Observed community phylogenetic structures unlikely to arise by chance (P < 0.05) are depicted by solid symbols. All microbial communities are clustered, with >50% being significantly clustered. Angiosperm communities are not uniformly clustered or dispersed across the gradient, but rather become increasingly overdispersed with increasing elevation. This trend in increased overdispersion with elevation is significant when measuring relatedness with the NRI (solid line; r² = 0.70, P < 0.001).

Fig. 3.

Compositional and phylogenetic similarity of Acidobacteria communities (A) and angiosperm communities (B), as a function of the elevation separating the communities. The compositional (blue) and phylogenetic (black) similarity for both angiosperm and Acidobacteria communities significantly decrease with increasing elevational separation (Mantel test, P < 0.001). Lines represent best-fit regressions of similarity versus change in elevation (see Materials and Methods). The slope of the decay of phylogenetic similarity between Acidobacteria communities is significantly steeper than predicted by a null model constrained by the decrease in taxon turnover (red) (P < 0.05). The slope of the decay in phylogenetic similarity across the angiosperm communities is not significantly different from the null prediction given species turnover. Fig. S7 illustrates within-site similarity as a function of elevational distance.