"In the absence of parasites, the toads and frogs are pure competitors," Johnson said. "But when trematode parasitism is present in the ecosystem, the adage 'the enemy of my enemy is my friend' comes into play for the toads, which are essentially shielded from infections by the tree frogs." Co-authors on the Ecology Letters study included Richard Hartson from the University of Wisconsin-Madison and Donald Larson and Daniel Sutherland from the University of Wisconsin-La Crosse.
The researchers also ran experiments involving American toad tadpoles coupled with green frog tadpoles, and others involving American toads, eastern tree frogs and green frogs together in the same tanks, said Johnson. In the tanks containing toad tadpoles and green frog tadpoles, the toad tadpoles had similarly high infection rates to those shown when they were the only tadpoles in the tanks.
But when all three tadpole types were raised together, the toad tadpoles were once again buffered from the parasites by the "dilution effect" provided by tree frogs. "Thus, the important determinant of parasite transmission was not total host diversity but the specific composition of the host community," wrote the authors.
The trematode has a complex life cycle involving snails, amphibians and predators. Host snails release parasite larvae into the water, infecting amphibians and causing deformations. Deformed toads and frogs rarely survive long in the wild because of their susceptibility to predators like wading birds, which ingest them and later defecate into wetlands, releasing trematodes to infect other snails and completing the life cycle.
As few as 12 trematode larvae, known as cercariae, can kill or deform a single tadpole by forming cysts in its developing limbs, causing missing limbs, extra limbs and other severe malformations, Johnson said. A 2007 CU-Boulder study led by Joh
|Contact: Pieter Johnson|
University of Colorado at Boulder