While parasite infection is now recognized as a major cause of such deformities, the environmental factors responsible for increases in parasite abundance had largely remained a mystery until the study was undertaken, Johnson said.
"One of our main goals was to understand how parasites are going to respond to land-use changes and ecosystem alterations," he said. "What we found is that nitrogen and phosphorus pollution from agriculture, cattle grazing and domestic runoff have the potential to significantly promote parasitic infection and deformities in frogs."
The trematode has a complex life cycle that involves three host species, he said. In addition to the infectious stage in snails and the cyst stage in frogs, the parasites rely on predators including wading birds to complete their life cycle by consuming infected frogs and spreading the parasite back into the ecosystem through defecation.
The research team built 36 artificial ponds in central Wisconsin similar to farm stock tanks -- a common breeding site of frogs and salamanders -- and stocked each with selected numbers of snails and tadpoles of the green frog. In addition to adding nutrients, the researchers took on the role of birds in the trematode life cycle by adding parasite eggs to the tanks, then measuring the subsequent ecological responses.
In ponds with added nutrients, snail biomass increased by 50 percent and the snails increased parasite egg production by up to eight-fold, he said. The infection rate in frogs rose by two- to five-times in those tanks, Johnson said.
As few as 12 trematode larvae, known as cercariae, can kill or deform a single tadpole by burrowing into their limb regions and disrupting normal leg devel
|Contact: Pieter Johnson|
University of Colorado at Boulder