Effects of long-term experimental manipulation of biotic and abiotic factors on the phylogenetic structure of plant communities

Background/Question/Methods

One goal of the field of community phylogenetics is to understand the ecological processes leading to phylogenetic structures within communities. Most studies quantify the phylogenetic structure of natural communities and infer the ecological processes that created those patterns. However, these inferences can be flawed because different processes can produce the same patterns. In this study we take an alternative approach: In a long-term field experiment we manipulate multiple biotic and abiotic ecological processes and quantify subsequent changes in community phylogenetic structure. In the acid grasslands at Silwood Park, England, we manipulated presence-absence of rabbits, insects, mollusks, nutrients, and lime in a fully factorial design and recorded the relative abundance off all plant species after 22 years. Using community composition data and a dated molecular phylogeny of all species in the community we asked: What is the effect and relative importance of rabbits, insects, mollusks, nutrients, and lime in shaping phylogenetic structure of plant communities? 

Results/Conclusions

We found that rabbit grazing and lime had significant but opposite impacts on phylogenetic structure.  Removal of rabbits decreased phylogenetic diversity (PD) and mean pairwise distance (MPD) meaning that individuals within ungrazed communities are more likely to be closely related than in grazed communities. Previous theoretical and empirical studies suggest that herbivory is an important mechanism maintaining species diversity; these results suggest they may also maintain phylogenetic diversity within communities. The addition of lime increased MPD meaning that in the less acidic plots individual plants where less likely to be closely related than compared to individuals within acidic plots. This suggests that the stressful acidic environments are selecting for more closely related species, a result consistent with environmental filtering. Insects and mollusks, however, had no effect on the phylogenetic structure of communities even though previous work from this experiment showed they can have dramatic effects on community composition. The ecological effects of insects and mollusks seem to be idiosyncratic and thus not predictably shaped by evolutionary history. Future analyses will quantify the role of nutrients in shaping communities, calculate the effect sizes of the different treatments, and compare phylogenetic patterns to multivariate community composition results.