We know biodiversity matters for the way ecosystems function, but we are only just beginning to understand the interplay between ecological and evolutionary processes that shape this relationship, especially in complex food webs and under rapidly changing environmental conditions.
For example, why do we see species with some combinations of traits (e.g., feeding traits, temperature tolerance, dispersal strategies) but not others? What does that mean for what combinations of species will make up a community and how it will function? How important are fine-scale differences among isolated populations of the same species, relative to between species? What kinds of habitats will protect both species and genetic diversity across changing landscapes? To answer these questions, ecologists and evolutionary biologists increasingly need to collaborate across disciplines, use new tools (e.g., genomic data and analysis, large scale modeling), and combine observations and experiments. I have worked on these types of questions in a range of aquatic systems, from the Pacific ocean, to large Swiss lakes, to Arizonan ponds.
How are changing temperature and precipitation impacting the availability of permanent water, and the kinds of species that can move across the landscape?
In Northern Arizona, warming temperatures (especially at lower elevations) are leading to streams that are ephemeral rather than permanent, and natural springs with decreasing flows. New projects in the lab are exploring the extent to which anthropogenic aquatic habitats can support diverse food webs across the landscape, and facilitate the dispersal of aquatic and terrestrial wildlife. Answering these questions begins with understanding dispersal, feeding, and tolerance traits of a range of aquatic invertebrates, and making use of high resolution satellite imagery to understand variation in habitat availability across space and time.
How do ecology and evolution interact?
Evolution can happen much more quickly than ecologists have traditionally appreciated – making it a constant source of changes to the trait diversity contributing to community composition and function. For example, work with stickleback fish shows divergent adaptations to marine, lake, or river habitats that can appear in little over 100 years (under 100 generations). These trait differences between lake and stream fish can impact how they feed and recycle nutrients, with impacts on aquatic ecosystems from plant growth to invertebrate production. Even more interesting is that these ecological changes can then feed-back to alter which types of sticklebacks do best in subsequent generations (potentially leading to further evolution). Although sticklebacks are frequently used as a model system for basic
questions in ecology and evolution, these findings also have direct implications for conservation questions: in central Europe, multiple lineages have been introduced by humans, and are now both interacting with native fish and hybridizing with each other, impacting broad scale trajectories for trait diversity in Swiss fish communities.
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.