Food vs. habitat and the assembly of grazer communities: Conflicting evidence from traits, phylogenies, experiments, and field surveys

Background/Question/Methods

The study of community assembly processes currently involves (a) long standing questions about the relative importance of environmental filtering vs. niche partitioning in a wide range of ecosystems, and (b) more recent questions about methodology. The rapidly growing field of community phylogenetics has generated debate about the choice between functional traits and phylogenetic relationships for understanding species similarities, and has raised additional questions about the relative value of experimental and observational approaches. In this study, we use traits, a phylogeny, and field surveys to understand the forces structuring communities of herbivorous marine amphipods living in seagrass and macroalgae. In addition, we compare our field results to a recent mesocosm experiment which tested the effects of both trait and phylogenetic diversity on coexistence using the same species and system. 

Results/Conclusions

We found that environmental filtering is the dominant process in macroalgae habitats, and that niche partitioning is the dominant process in seagrass habitats. This pattern is indicated by both phylogenetic relationships and trait distances, but the type of niche partitioning in seagrass habitats can only be deciphered using trait data. Species coexisting in the field differed not in their feeding niche but in traits related to microhabitat use, providing novel evidence of the relative importance of competition for food vs. enemy free space in structuring communities of phytophagous invertebrates. With respect to methodology, these results conflict with those obtained in mesocosms, where feeding trait diversity did promote coexistence and phylogenetic diversity had no effect. This contrast arises because a greater range of traits (some of which have strong phylogenetic signal) contribute to community assembly in the field, whereas the traits relevant in experimental mesocosms are more labile. This conflict highlights a mismatch between the processes that drive community assembly in the field and the processes we isolate in experimental tests, as well as potential pitfalls of using phylogeny as a single proxy in both contexts.