Diverse Ecosystems Show Clue to Human Disease Prevention

The richer the ecosystem, the more protection there is against disease, according to a study that found a variety of frogs, toads or salamanders dwelling in a pond together made a healthier environment for all amphibians.

Ponds with a half dozen amphibian species had a 78 percent decline in parasite transmission, compared with ponds hosting one type of amphibian, a study of 345 wetlands published today in the journal Nature found. The findings may have lessons for preventing human disease, said the study author, Pieter Johnson.

Making environments more diverse might help combat illnesses such as West Nile, said Johnson, an assistant professor of ecology and evolutionary biology at the University of Colorado in Boulder. The West Nile virus typically transmits between birds and mosquitoes several times before finding its way into humans. Boosting bird variety might make it more difficult for the germ to spread and lead to fewer human cases.

“The interesting question becomes, is biodiversity important for combating the cause of disease?” Johnson said in a telephone interview. “That’s the motivation.”

While previous studies have suggested correlations between increased diversity in an environment and lowered disease risks in humans, an experiment to find out would be a challenge, he said. Researchers would have to release a known number of infected mosquitoes to observe how areas with abundant bird species responded, and then compare to areas with fewer types. That sort of study would be “hard, expensive, and ethically problematic,” he said.

California Ponds

The group visited ponds in California, examining 24,215 amphibians and 17,516 snails to track a parasitic flatworm called Ribeiroia ondatrae. The parasite causes missing, mangled, or extra sets of hind legs in amphibians, and uses snails as a transitional host for asexual reproduction. It’s not clear how Ribeiroia causes the malformed legs.

The first species that appear in new ecosystems breed rapidly, and are most prone to infection. Those who appear later in a pond’s lifecycle are typically more resistant to parasites, and their resistance lowers the transmission between the snails and amphibians.

The group also set up experiments in 40 artificial ponds, both indoors and outdoors, to make sure the species diversity, and not something else in the natural environment, accounted for their findings. The artificial results echoed the natural ones.

Johnson said the artificial results allowed him to say with confidence that the increased diversity is what leads to parasite resistance. In agriculture, intercropping, or alternating crops either spatially or over time, is basically the same concept, he said.

The research was supported by the National Science Foundation, the National Geographic Society, and the David and Lucile Packard Foundation.

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