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Research

Study Shows Parasites Outweigh Predators


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Ryan Hechinger examines specimens under a microscope
Above: UCSB researcher Ryan Hechinger examines specimens under a microscope to identify parasites. Photograph by Kevin Lafferty, USGS. [larger version]
In a study of parasitic and free-living (non-parasitic) species in three estuaries on the Pacific coast of California and Baja California, a team of researchers from the University of California, Santa Barbara (UCSB), the U.S. Geological Survey (USGS), and Princeton University determined that parasite biomass in those habitats exceeds that of top predators, in some cases by a factor of 20. Their findings, which could have significant biomedical and ecological implications, appeared in the July 24 issue of the science journal Nature (URL http://dx.doi.org/10.1038/nature06970).

According to Armand Kuris, professor of zoology in UCSB's Department of Ecology, Evolution, and Marine Biology and a lead author of the paper, the study's findings have a potential impact on the perceived role of parasites in an ecosystem. From an ecological perspective, parasites serve both as regulators to prevent species from becoming numerically dominant and as indicators of the health of a particular ecosystem. The study shows for the first time that parasites might drive the flow of energy in ecosystems.

"The total amount of energy flow in ecosystems due to infectious processes must be enormous—even greater than we'd expect, given the large parasite biomass," Kuris said. "I expect the amount of energy going into host-tissue repair and replenishment is also huge. An implication of our study is that we should pay more attention to the energetics of disease."

Tidal habitats of San Quintín
Above: Tidal habitats of San Quintín (Baja California, Mexico), one of three estuaries where researchers quantified parasitic and free-living biomass, finding that parasites have far more biomass than is typically thought. Photograph by Kevin Lafferty, USGS. [larger version]

Biomass is the amount of living matter that exists in a given habitat, expressed either as the weight of organisms per unit area or as the volume of organisms per unit volume of habitat. Until now, scientists have believed that because parasites are microscopic in size, they make up a small fraction of biomass in a habitat, while free-living organisms such as fish, birds, and other predators make up the vast majority.

The researchers quantified the biomass of free-living and parasitic species in the three estuaries and demonstrated that parasites have substantial biomass in these ecosystems. "Parasites have as much, or even more, biomass than other important groups of animals—like birds, fishes, and crabs," said Ryan Hechinger, a researcher at UCSB's Marine Science Institute and co-lead author of the paper.

birds
Above: Certain parasitic groups dominate the parasite biomass, such as trematode worms, whose biomass exceeds that of birds by threefold to ninefold. Photograph by Kevin Lafferty, USGS. [larger version]

The article grew out of a 5-year study supported by a $2.2-million grant from the National Science Foundation and the National Institutes of Health through the agencies' joint Ecology of Infectious Diseases program. In addition to Kuris, principal investigators include Kevin Lafferty, a marine ecologist with the USGS; and Andrew Dobson, a professor of ecology and evolutionary biology at Princeton University. Other important collaborators included Leopoldina Aguirre-Macedo, of the Centro de Investigación y Estudios Avanzados Unidad Mérida, and Mark Torchin, a staff scientist with the Smithsonian Tropical Research Institute.

snail Cerithidea californica
Above: The snail Cerithidea californica is commonly parasitized by larval trematodes that castrate their hosts; the trematodes average 22 percent of the total soft-tissue weight of individual infected snails. C. californica and its larval trematode parasitic castrators were considerable components of animal biomass in the three estuaries. Photograph by Kevin Lafferty, USGS. [larger version]

The researchers quantified parasites and free-living organisms in the Carpinteria Salt Marsh in California (United States) and in the Bahia San Quintín and Estero de Punta Banda estuaries in Baja California (Mexico). Their study included 199 species of free-living animals, 15 species of free-living vascular plants, and 138 species of parasites.

"The reason we wanted to complete this study is because a lot of work we've done has suggested that parasites are important in ecosystems. But no one's actually looked at them as a group throughout an ecosystem," said Lafferty. "Also, no one's considered parasites from the perspective of how much they weigh because it's always been assumed they weigh almost nothing. Now we know that's not true.

"For example, in an estuary there are more kilograms of trematode worms—parasites—than kilograms of birds," he noted. "If you could see the trematodes with binoculars, you might not bother bird watching."

Said Hechinger: "No one debates whether it's important for ensuring human welfare to understand how ecosystems work. How can we possibly understand something without accounting for its major parts? Because our findings indicate that parasites control a massive amount of biomass, it would seem future research can't ignore them."

According to Kuris, understanding the enormity of parasite biomass and the burden it places on available hosts could lead to new strategies in the management of infectious diseases. Treatment protocols might put greater emphasis on enhancing the host's ability to defend itself against parasitic disease and slow the rate of energy uptake by the parasites and pathogens.

The full citation for the article is: Kuris, A.M., Hechinger, R.F., Shaw, J.C., Whitney, K.L., Aguirre-Macedo, Leopoldina, Boch, C.A., Dobson, A.P., Dunham, E.J., Fredensborg, B.L., Huspeni, T.C., Lorda, Julio, Mababa, Luzviminda, Mancini, F.T., Mora, A.B., Pickering, Maria, Talhouk, N.L., Torchin, M.E., and Lafferty, K.D., 2008, Ecosystem energetic implications of parasite and free-living biomass in three estuaries: Nature, v. 454, no. 7203, p. 515-518, doi:10.1038/nature06970 [URL http://dx.doi.org/10.1038/nature06970].

About the author: Andrea Estrada is a writer in the Office of Public Affairs at the University of California, Santa Barbara.


Related Sound Waves Stories
Parasites, the Thread of Food Webs?
August 2006
Biologists Count Parasites to Assess Health of Marsh
July 2006

Related Web Sites
Ecosystem energetic implications of parasite and free-living biomass in three estuaries
Nature

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Parasites Outweigh Predators

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Awards National Wetlands Research Center Staff Receive Awards

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