Broadly, my research focuses on questions of how marine communities function after they have been altered by man. Specifically, I am interested in looking at how changes in biodiversity can alter food web structure, function, and productivity. I am concerned both with removal of species due to habitat degradation, overfishing, and climate change as well as the addition of species from biological invasions.
As a scientist, I believe that satisfying answers to these issues can only come when we examine nature from multiple viewpoints. In my work I seek to combine rigorous quantitative and qualitative models, tightly controlled laboratory manipulations, more realistic field experiments, and examinations of large-scale long-term community data sets using structural equation modeling. Each of these elements alone can only reveal a piece of the larger puzzle. But I believe that by bringing them together using common statistical frameworks, ecologists can do much to get more complete answers to any question we ask.
Feedbacks between biodiversity and ecosystem function
Ecology is replete with examples of researchers asking superficially similar questions - e.g., How does species diversity influence herbivory? How does herbivory influence species diversity? - and arriving at relationships that seem in conflict with one another.
Rather than conflict, I see researchers looking at two sides of the same coin. When one looks at just what each researcher is assuming to be the cause and response in a relationship, however, one finds that they are often looking at different points in time of a larger feedback relationship. This is not surprising given the number of feedbacks in ecology - e.g., predator-prey cycling, plant-soil feedbacks, etc.
Currently, I am examining feedbacks between species richness and herbivory in the kelp forests of Southern California. Within these forests, the primary source of food for sea urchins, the giant kelp Macrocystis, can be removed by large storms, which are predicted to increase in frequency due to climate change. What role will a diverse healthy reef community play in mediating the increase in urchin grazing? How will the resulting grazing alter the community? Are there different community cycles that can be generated depending on the initial diversity?
Consequences of decreasing predator diversity for top-down control
My previous work has concentrated on the consequences of decreasing predator diversity in marine ecosystems. Overfishing, habitat degradation, pollution, and more have all led to declines in marine species diversity. Most marine extinctions at both the local and global scale are of predators. Within kelp forests of the Channel Islands, fewer predator species correlated with less kelp and more herbivores. Experimentally, I have shown that this may be due to different predator species each causing different herbivores to reduce consumption of kelp.
Within communities of marine fouling organisms, I have found similar patterns. There, predator diversity is negatively correlated with the amount of their sessile invertebrate prey. This appears to be due to some complementarity, but also a trade-off in interspecific and intraspecific interactions. Over time, communities with more species of predators have lower cover and biomass, and are able to filter less plankton from the water.
Consequences of enhanced exotic species diversity
While diversity within the oceans is threatened, exotic species may actually increase diversity at local scales. Harbors and marinas are a notable hotspot of exotic species diversity. How are these increases in exotic species altering ecosystem function? My works has shown that communities with high diversity of either native or exotic species filter water and regenerate nutrients in a similar manner. Low diversity communities, dominated by a single exotic species, behave idiosyncratically. Increases in exotic species diversity may make up for some lost function of natives that cannot survive in urban marine environments. However, increased exotic species diversity also increases the rick of an out of control exotic dominating an ecosystem and changing its function in unpredictable ways.