Increasing land use drives changes in plant phylogenetic diversity and prevalence of specialists

Abstract

Increased human land use has resulted in the increased homogenization of biodiversity between sites, yet we lack sufficient indicators to predict which species decline and the consequence of their potential loss on ecosystem services. We used comparative phylogenetic analysis to (1) characterize how increasing conversion of forest and grasslands to grazing pasturelands changes plant diversity and composition; (2) examine how changes in land use relate to declines in functional trait diversity; and (3) specifically investigate how these changes in plant composition affect the prevalence of zygomorphy and the possible consequences that these changes may have on pollinator functional groups. As predicted, we found that the conversion to grazing pasturelands negatively impacted species richness and phylogenetic composition. Clades with significantly more represented taxa in grasslands (GL) were genera with a high representation of agricultural weeds, while the composition was biased towards clades of subalpine herbaceous wildflowers in Mixed Forest (MF). Changes in community composition and structure had strong effects on the prevalence of zygomorphic species likely driven by nitrogen-fixing abilities of certain clades with zygomorphic flowers (e.g., Fabaceae). Land conversion can thus have unexpected impacts on trait distributions relevant for the functioning of the community in other capacities (e.g., cascading effects to other trophic levels (i.e., pollinators). Finally, the combination of traits represented by the current composition of species in GL and MF might enhance the diagnostic value of productivity and ecosystem processes in the most eroded ecosystems.

Keywords: Functional diversity; Phylogenetic community structure; Pollinator specialization; Trait composition.

Figure 1. Sampling localities.

Green squares correspond to Grassland (GL) communities and yellow triangles represent Mixed Forest (MF).

Figure 2. Measures of phylogenetic community structure for two land cover types (LCT), Grassland (GL) and Mixed Forest (MF).

GL and MF communities have phylogenetic clustering in species when (A) phylogenetic diversity was compared to that expected from a null phylogeny, SES MPD (P < 0.05), and (B) when accounted on the terminal phylogenetic composition of communities, SES MNTD (P < 0.05).

Figure 3. Linear regression with regression slopes between mean pairwise distance (MPD), mean nearest taxon distance (MNTD) and land conversion index for all species and zygomorphic species.

Triangles, MF sites; circles, GL plant communities. (A–B) represent analyses where all species in each communities are considered; (C–D) represent analyses where only zygomorphic species in each communities are considered. The blue lines are the fit lines spanning the full range of the plot for the two set of data.

Figure 4. Regional plant phylogeny with 891 species.

Green circles correspond to species of the important clades in less altered communities (LUC < 0.05) and yellow circles represents important clades in highly altered communities (LUC > 0.05; see Table 4). Red shading band indicates nitrogen-fixing clade within Rosids, which we speculate is an important factor contributing to why these clades are significantly more represented in the more disturbed areas. Green bands correspond to Ericales and Asterids clades that were more represented in the more protected zones.