ROCKY MOUNTAIN NEWS (Denver, Colorado) 22 October 08 Study: Frogs are princes to toads (Bill Scanlon)
When American toads pal around with gray tree frogs they are much less likely to get the parasites that cause them to grow malformed legs, a new study has found.
It's an argument for biodiversity - and a warning about what could happen when species in an ecosystem disappear, say the study's co-authors.
The study, led by University of Colorado biologist Pieter Johnson, showed that when the toad tadpoles were raised in tanks with tiny parasitic worms, almost half emerged with leg malformations. But when gray tree frogs were added to the tanks, the parasitic infections dropped almost in half.
The gray tree frogs act as sponges for the trematode parasites, Johnson said. Those parasites later were killed by the immune systems of the frog tadpoles, which can battle the parasites much easier than can American toads.
"In the absence of parasites, the toads and frogs are pure competitors," Johnson said. But when the parasites are in the ecosystem, "the adage 'the enemy of my enemy is my friend' comes into play," Johnson said. The frog tadpoles essentially shield the toads from infection.
Both tree frogs and American toads live in broad stretches of the American Midwest and the Eastern Seaboard, often in the same wetlands, Johnson said.
"This is one of the first experimental studies to definitively show that an increase in diversity of host species actually can reduce parasite transmission and disease," said Johnson, who is a member of CU's Ecology and Evolutionary Biology Department.
The study, which appears in this month's issue of Ecology Letters, was funded by the National Science Foundation.
Johnson noted that there have been large declines across the globe in the diversity of wildlife species. He said the study has implications especially for species that are susceptible to parasites.
He noted that ticks carrying Lyme disease spread it faster to humans when there aren't a lot of other mammalian species around to serve as hosts.
The same phenomenon could hold true for prairie dogs and bubonic plague; birds and West Nile virus; and tick-borne encephalitis, Johnson said.
Study: Frogs are princes to toads