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Where Disease May Mean Good Health—The Role of Parasites in Natural Ecosystems

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trematode cercaria
Parasite: This free-swimming stage, a trematode cercaria, leaves infected snails to encyst on a fish brain. View is 0.267 mm across. Photograph courtesy of Todd Huspeni, UCSB.
The ecology of parasites has long had implications for human health, and human alterations to the environment have affected the success of parasites worldwide. Humans have altered the world in ways that favor diseases. For example, deforestation, damming, fish farming, and rice farming have increased malaria transmission by creating mosquito-breeding habitats.

Paradoxically, healthier, less degraded ecosystems tend to have more parasites with complex life cycles (that is, they pass through multiple hosts to complete their life cycle) than do altered ecosystems, because these parasites depend on functioning ecosystems. The types of change most likely to affect parasite communities are alterations in host communities resulting from climate change and environmental degradation. Environmental degradation can include introduced species, habitat fragmentation, pollution, and overharvesting. In turn, because parasites, particularly those transmitted through predation (when an organism eats an infected host), have the potential to organize their host communities, changes to parasite communities can profoundly alter natural systems.

Recently, the University of California, Santa Barbara (UCSB), was awarded $2.2 million over the next 5 years by the National Science Foundation to study the role of parasites in natural ecosystems. The money will largely fund student research under the direction of three coprincipal investigators, all specialists in parasite ecology: Armand Kuris, a professor of zoology at UCSB; Andrew P. Dobson, a professor of zoology at Princeton; and myself, a marine biologist with the USGS' Western Ecological Research Center and an adjunct professor of biology at UCSB.

Because salt marshes have proved to be a model system for understanding the ecology of parasites with complex life cycles, we will study parasites of the abundant horn snail, Cerithidea californica, found in salt marshes. The snail acts as a hub in the life cycle of 17 parasitic trematode (worm) species.

A trematode castrates the snail it infects and each day produces scores of free-swimming stages of offspring that leave infected snails to search out a second, intermediate host, such as a fish, clam, crab, or snail. In the second host, the trematode can greatly alter host behavior to increase the chance of transmission to a final host. Wading birds, shorebirds, and seabirds prey selectively on second, intermediate hosts and become parasitized by adult worms. In the final host, the small worms live in the gastrointestinal tract, mate, and produce eggs that pass into the marsh with the host's feces, where they encounter snails and complete their life cycle. Each of the 17 trematode species that use C. californica has a different life cycle.

Shark Inlet, Morro Bay, California Deveraux Lagoon, near Santa Barbara, California
Wetlands: Scientists are studying wetlands to find better ways to assess their condition. Are these sites as healthy as they are beautiful? Above left, Shark Inlet, Morro Bay, CA. Above right, Devereux Lagoon, near Santa Barbara, CA. Photographs by Kevin Lafferty, USGS.

The new research ties in with current research by the USGS and UCSB funded by the U.S. Environmental Protection Agency to develop monitoring tools using parasites to evaluate the health of salt marshes. Mathematical models, molecular tools, laboratory experiments, field experiments, and large-scale comparative field studies will all be used in the investigation. In addition to work at two UCSB natural reserves, Carpinteria Salt Marsh and Coal Oil Point, our research will take us to estuaries in California's Morro Bay and Mugu Lagoon, along the Pacific Coast of Baja California, Mexico, and to Japan.

Related Web Sites
Using Parasites to Monitor Ecosystem Health
U.S. Geological Survey (USGS)

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in this issue: Fieldwork cover story:
Gas-Hydrate Research Wells Completed

Moloka'i Coral Reef Sediment

Research Role of Parasites in Ecosystems

Outreach Public Art Project

Prairie Restoration

Marine Science Day

Marine Environmental Careers Symposium

Students Visit Woods Hole

Congressional Briefing on Wetlands

Woods Hole Science Fairs

Talks—DOE and College of William and Mary

Meetings Netherlands Sediment-Transport Collaboration

Sediment-Transport Modeling

Tampa Bay Estuary Tour

Awards Monterey Bay Research Award

Staff & Center News Japanese Land-Management Team Visits St. Pete

Western Region Retirements

Woods Hole Visitor

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