Which species coexist?
For species to exist in the same place, they often need to have similar habitat requirements but different resource use or feeding strategies. In a northern California estuary, we tested the relative importance of these two mechanisms in communities of crustacean grazers, which live on seagrasses and macroalgae and are important for controlling algae blooms and supporting fish production. Using data for several traits and surveys of community composition, we found that different habitats (eelgrass beds, mudflats) have very different assembly mechanisms, possibly due to different levels of temperature stress and resource diversity.
What can trait vs. phylogenetic diversity tell us about communities composition in experimental and field contexts?
Trait data is time consuming to collect, subject to phenotypic plasticity, and dependent on biases about what traits we think might be important. This has led to considerable interest in using phylogenetic proxies for the ecological variation between species. However, the link between phylogenetic relationships and species coexistence varies with both trait lability and ecological context.
Comparing field surveys and experimental mesocosms, our research on crustacean grazers in seagrass meadows showed that competitive interactions at some scales could be predicted by labile feeding traits, while larger scale assembly in the field was predicted by body size and temperature tolerance. Since these traits vary in their correlation with evolutionary divergence time, it is not possible to make general interpretations of phylogenetic community structure across scales. This challenges our use of phylogenies as a proxy for all ecological differences, but also pushes both ecologists and evolutionary biologists to develop better models for how different trait combinations evolve in the first place.
How does multivariate grazer trait variation affect ecosystem function in changing seagrass systems?
To understand the impact of trait diversity on ecosystem function, we need to know not only which species are found in which places, but also what those species contribute to important ecological processes. Globally, seagrass meadows are declining, and one of the main threats is overgrowth by algae. One possible cause of this overgrowth is increased predation on the invertebrate grazers that control that algae.
However, this trophic cascade hypothesis can break down if, across potential grazers, predation susceptibility is not positively correlated with consumption rates of algae. If fish tend to remove grazers that are less important for algae control, this can buffer the system from change.
Correlations between multiple traits also impact our predictions about community effects of global change if species vary in both their ability to control algae and their response to increasing temperature. Using a multi-generation mesocosm experiment, we found that differences between species in their grazing rates on algae were amplified under warmer temperatures. We also found that one important species increased its consumption rate when it was raised at warmer temperatures, suggesting an important role for phenotypic plasticity in response to changing conditions.
What trait combinations promote species invasions?
In bays with heavy shipping traffic (e.g., San Francisco Bay), introduced species now make up a much larger proportion of the community than in more isolated systems (e.g., Bodega Bay). Interestingly, the processes of community assembly in seagrass beds seem to be similar for both native and introduced amphipod species. This suggests that invasive species in this system do not have particular intrinsic advantages in their trait combinations, but rather that anthropogenic disturbance and a constant inflow of propagules might be more important reasons for species turnover in heavily impacted environments.