Parasites in Aquatic Ecosystems
Although arguably the most common life history strategy on earth, parasitism has historically been omitted from much of food web ecology. One current research focus aims to assess how parasites influence the structure and dynamics of wetland food webs over time. The images at right show a pond for which we constructed a food web including infectious agents and a visualization of the web using FoodWeb3D from the Pacific Ecoinformatics and Computational Ecology Lab (www.foodwebs.org).
Ecological Stoichiometry and Disease
Ecological stochiometry can link processes within individuals (feeding, assimilation, growth) to processes on the ecosystem scale (nutrient cycling, productivity, food web dynamics). Current research in this area aims to determine how host resource stoichiometry, and mismatches in the elemental composition of hosts and parasites, influences disease. Artificial pond ecosystems (‘mesocosms’) are shown at right in a field at the Hopland Research and Extension Center. Mesocosms are useful for ecological stoichiometry experiments because we can carefully manipulate ratios of nutrients in a controlled manner.
We have also assessed the contributions of parasites to energy flow in wetlands by measuring the biomass and productivity of trematode worms, alongside the biomass of free-living animals. Our results suggest that parasites make significant direct contributions to the flow of energy through some wetlands. Experimental studies also indicate that parasites might be an important prey resource for pond predators. A histological section of a trematode-infected freshwater snail is shown to the left (nearly all of the visible tissue is trematode). Parasites have been found to play important roles in ecosystem energetics in some estuaries and wetlands, but their role in energy flow through most other ecosystems remains unknown.
Invasions in Wetlands
Invasive species have become major drivers of ecosystem change, a problem that is particularly prominent in freshwaters. We have combined regional field surveys with outdoor mesocosm experiments to disentangle the individual and combined effects of two common wetland invaders, the western mosquitofish (shown at right) and the American bullfrog. Ongoing work aims to evaluate how environmental variables, such as productivity, influence the magnitude of invader impacts on native communities.
Research
We’ve also introduced nonnative mosquitofish to a wetland in a controlled manner, allowing us to see how well small-scale experiments ‘scale-up’ to ecosystems. The wetland was divided in half by a fence, allowing us to measure changes in the wetland community before and after the introduction of mosquitofish. The divided wetland is shown to the left.
Behavioral Ecology and Host-Parasite Interactions
While parasite-induced mortality and pathology of hosts are often obvious, more subtle effects of parasitism might also have important consequences for ecological communities. We are examining how parasites influence host behavior, both before and after infection. Prior to infection, the “threat” of disease might induce anti-parasite behaviors, similar to the manner in which predators induce certain behavioral changes in prey. This work is being done using trematodes (larval stage shown at left) and amphibian larvae.
Tropical Amphibian Declines
I’ve recently received funding to begin a project on the eastern slope of the Andes Mountains in Ecuador which will aim to disentangle some of the environmental drivers of amphibian declines in that area. Stream-breeding glass frogs (Centrolenidae; shown at right) used to be abundant in many high-elevation streams, but land use changes, nonnative trout and disease may be contributing to their decline.
