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 contribution of experimental versus observational approaches to understanding evolutionary constraints on community assembly. In a series of three studies, we used traits, a phylogeny, and field surveys to identify the forces structuring communities of herbivorous marine amphipods and isopods living in adjacent seagrass and macroalgae. First, we sequenced portions of 3 genes of peracarid crustaceans from Bodega Harbor: mitochondrial cytochrome c oxidase subunit I (COI) and 16S rRNA, and nuclear 18S rRNA and built a phylogeny. We also collected trait data on these species ranging from feeding preferences and rates, to phenology, to body size and tube building ability (Best and Stachowicz 2013).
Once this data was available we took both experimental and observational approaches to testing the relative importance of traits and phylogeny for determining community diversity and coexistence of particular species. First, we performed an experiment in outdoor mesocosms (Best, Caulk and Stachowicz 2013) in which we orthogonally manipulated diversity of feeding traits and phylogenetic diversity of grazer communities and measured the effect of each on coexistence, invasion, and evenness of assemblages. We found that feeding trait diversity, but not phylogeny affected these aspects of coexistence; this was because feeding traits had no phylogenetic signal. However, field data painted a very different picture (Best and Stachowicz 2014). Environmental filtering was the dominant process in macroalgae habitats, but niche partitioning is the dominant process in seagrass habitats, and the strength of these assembly mechanisms varies with seasonal fluctuations in environmental conditions and resource availability. These patterns are indicated by both phylogenetic relationships and trait distances, but the type of resources being partitioned in seagrass habitats can only be deciphered using trait data. Species coexisting in seagrass 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. habitat in structuring communities of phytophagous invertebrates. With respect to methodology, the results for seagrass habitats conflict with those obtained in experimental 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 much stronger phylogenetic signal than feeding traits) contribute to community assembly in the field. This highlights a mismatch between the processes that drive community assembly in the field and the processes we isolate in experimental tests, and illustrates that using phylogeny as a single proxy in both contexts may impede the synthesis of observational and experimental results. Currently we are examining habitat selection of each of these species under a range of conditions to investigate whether these species coexist in part by partitioning enemy-free space in the field.
We have also used sessile marine invertebrate communities to examine the processes underlying coexistence using both experimental studies and model parameterization (Edwards and Stachowicz 2010, 2011, 2012). These suggest how traditional competition – colonization tradeoffs, combined with temporal niches and stochasticity in patterns of larval settlement likely all contribute to the maintenance of diversity among a group of relatively similar suspension feeding marine invertebrates, all of which are primarily limited by available space.
Best, R.J. and J.J. Stachowicz. 2014. Phenotypic and phylogenetic evidence for the importance of food and habitat in the assembly of communities of herbivorous marine amphipods. Ecology 95:775–786 [-pdf-]
Best, R.J. and J.J. Stachowicz. 2013. Phylogeny as a proxy for ecology in seagrass amphipods: which traits are most conserved? PLoS ONE 8(3): e57550 [-pdf-]
Best R.J., N.C. Caulk, and J.J. Stachowicz. 2013. Trait versus phylogenetic diversity as predictors of competition and community composition in herbivorous marine amphipods. Ecology Letters 16:72-80. [-pdf-]
Edwards, K.F. and J.J. Stachowicz. 2012. Temporally varying larval settlement, competition, and coexistence in a sessile invertebrate community. Marine Ecology Progress Series 462: 93–102.[-pdf-]
Edwards, K.F. and J.J. Stachowicz. 2011. Spatially stochastic settlement and the coexistence of benthic marine animals. Ecology 92:1094-1103. [-pdf-]
Edwards, K.F. and J.J. Stachowicz. 2010. Multivariate tradeoffs, succession, and phenological differentiation in a guild of colonial invertebrates. Ecology 91: 3146Ð3152.[-pdf-]