Does trait conservatism guarantee that indicators of phylogenetic community structure will reveal niche-based assembly processes along stress gradients?

Question: Is the strength of phylogenetic trait conservatism in the species pool reflected in the ability of α- and β-scale phylogenetic diversity indicators to detect niche-based assembly processes along stress gradients? Methods: We used a simple community assembly model to explore the power of indicators of phylogenetic community structure to detect assembly processes. The model assumes that with declining stress the influence of niche complementarity on species fitness increases, while that of environmental filtering decreases. We separately incorporated two trait-independent stochastic processes — mass and priority effects — in simulating species occurrences and abundances along a hypothetical stress gradient. We measured power for α- and β-scale indices of phylogenetic diversity (PD) as the proportion of simulations yielding a significant correlation between these indices and the community position along the stress gradient. For α-scale indices, we ran simulations where species richness was constant along the gradient, or either increased, decreased or varied randomly with declining stress. We used four models of trait evolution, differing in the degree of phylogenetic trait conservatism, to examine how variation in the influence of phylogeny on trait variation impacted the power of PD indices. Results: None of the PD indices gave high power to detect assembly processes for either the mass effects or priority effects model, even when trait variation was strongly conserved. Those measures of α PD that gave moderate power in the priority-effects model with strong trait conservatism were greatly affected by variation in species richness. Consequently, none of the indices examined met the necessary selection criteria. Additional analyses revealed that trait conservatism at the species pool level was poorly maintained at the meta-community scale, with phylogenetically-determined species composition having only moderate power to detect functionally-determined species turnover. This lack of a strong phylogenetic signal in trait variation at the meta-community scale is the most likely explanation for the poor ability of PD indices to detect assembly processes in our study. Conclusion: Phylogenetic diversity indices seem to have limited ability to detect trait-based assembly processes along ecological gradients, even when trait conservatism at the species pool level is strong. Consequently, PD is unlikely to provide a means of avoiding the tough decisions aro