Phylogenetic and functional composition of Arctic herbivore assemblages

To understand trophic interactions we need to understand both the diversity and linkages in a vertical dimension (between trophic levels) and also the diversity in a horizontal dimension (within a trophic level). A trophic level may consist of a large range of taxa and encompass multiple ways of interacting with other trophic levels and the wider environment. It is rare for the horizontal dimension of diversity to be analysed in a trophic context, and efforts to date have largely looked only at species diversity within trophic levels. However, incorporating phylogenetic and functional dimensions of diversity can both broaden and deepen our understanding of ecological mechanisms and hence trophic interactions. In this study we present a phylogenetic and functional assessment of a whole trophic group at a biome scale. We will focus on vertebrate herbivores of the Arctic biome. A previous study mapped the species richness of arctic herbivores, finding it peaked in the western Nearctic and was positively associated with plant productivity and predator diversity. In this study we created a phylogeny of all vertebrate arctic herbivores and used this to form the basis of a spatial analysis of the phylogenetic diversity and structure of this trophic level across the Arctic biome. We also used functional traits of the Arctic herbivores to classify how the species of this trophic level interact with their environment. Results from our study show that both phylogenetic and functional diversity are correlated with species diversity across the arctic. However, assemblages of herbivores that differ in terms of species composition are more similar in terms of phylogenetic and functional composition. These together suggest that regional species differentiation does not reflect strong evolutionary divergence or functionally different herbivore communities. However, the ratio of phylogenetic diversity to functional diversity suggests that assemblages are functionally-dispersed. This may be due to competition within arctic herbivore assemblages in structurally simple ecosystems. Our study demonstrates the power of combining spatial analysis of phylogenetic and functional diversity, in order to increase our understanding of trophic ecology and interactions.