While the parasite hypothesis may conflict with conventional ideas about infectious disease and human health (malaria, for example, is caused by a parasite), the worms the scientists are investigating are not just any kind of parasite.
For one, these worms, known as trematodes, must sequentially infect certain hosts to complete their lifecycle. Snails to crabs to birds might be a typical sequence for one species, snails to fish to birds for another. These trematodes also stand apart from other parasites in that they cause negligible disease for their highest trophic level hosts, usually birds. The worms' lifecycle thus typically begins in a snail and ends in a bird, with the intermediate host animals being primary variables among worm species.
Intrigued by the prospects of developing a new tool for monitoring changes in wetland ecology, NOAA's California Sea Grant recently awarded support to parasite mavens Armand Kuris of the Department of Ecology, Evolution and Marine Biology at the University of California, Santa Barbara, and Kevin Lafferty of the Biological Resources Discipline at the U.S. Geological Survey to collect California horn snails from 30 coastal salt marshes between Marin County and Imperial Beach at the U.S.-Mexico border.
"The horn snail is a mobile data recorder," Lafferty said. "It is a hub for more than 20 trematode species." If any one requisite intermediate host is missing, the parasite cannot reproduce and hence will be underrepresented in the resident snail population, he explained.
A survey of the trematode population in resident snails thus becomes a clever means of reconstructing the food web in the area, as the trematodes reflect the predator-prey relationships that must be occurring to support their reproductive lifecycle.
"Trematodes require all of the pieces of the puzzle to complete their lifecycle," Lafferty said. "When we see a lot of parasites in an estuary, we know it is in good shape. For example, an estuary with high infection rates tells you that it is visited by many birds, and many types of birds."
The goal of the Sea Grant project is to establish a baseline snail-trematode count in marshes, particularly those slated for restoration. By comparing worm statistics before and after a restoration project, which could include activities such as digging channels or removing non-native plants, the biologists believe that wetlands managers will have a tool for gauging restoration success and its gaps.
If, for example, a certain trematode species is missing, it could indicate that its hosts are lacking appropriate habitats. "The trematode information provides a novel way to see what we need to alter to improve habitats," Kuris said.
The feasibility of the snail-as-data-logger idea was established at a case-study site at the Carpinteria salt marsh in Santa Barbara. There, the biologists showed the trematode community did indeed become measurably more vibrant after restoration, due to an increase in the number of birds foraging on infected fish and benthic invertebrates.
To further validate the method, UCSB graduate student Ryan Hechinger conducted four month-long bird surveys at the study site, using video cameras to capture images of as many birds as possible. The results proved encouraging as the video-based estimates of the bird community were in close agreement with those from the snail-trematode analysis. "The more birds there were at a site, the more parasites," Hechinger said. "The more kinds of birds, the more kinds of trematodes, just as we predicted."
Hechinger hopes to produce a manual for resources managers that will explain how to collect snails, identify the trematodes inside them, and then translate this i nformation into information on resident populations of birds, fishes and benthic invertebrates.
"We think counting trematodes is an effective tool for assessing the biodiversity of salt marshes in California," Lafferty said. "We are interested in developing similar techniques for other ecosystems, such as coral reefs and kelp forests."
Wetlands being surveyed as part of the California Sea Grant project: