OU Biology experiment reveals how some trees can make frogs sick
Biologists know that frogs and salamanders worldwide are threatened by habitat loss and parasitic diseases.
Now, researchers at Oakland University have discovered that shifts in forest composition also pose a significant threat specifically to Michigan frogs, due to the loss of ash and elm trees and their replacement by red maple trees. This is because tree leaves are an important source of tadpole food, and maple leaves are a poor-quality food source with few nutrients and contain high levels of toxic phenolic chemicals.
In their article selected for inclusion in the December 2016 Journal of Animal Ecology, OU researchers showed that the species composition of leaf litter can also have complex effects on tadpole parasites.
In an experiment conducted by former student Jeff Stephens and his advisor Professor Tom Raffel, Ph.D., they showed that different species of leaf litter can lead to higher or lower infection levels by the snail-borne flatworm parasite Ribeiroia ondatrae. This parasite is best known for causing frog leg deformities.
They found that leaf litter quality had complex and opposing effects parasitic infection in tadpoles. High-quality litter increased the number of parasites attacking tadpoles but also accelerated tadpole development, allowing them to leave the water sooner and escape from parasitism. Leaves with higher levels of toxic phenolics, like the red maple, slowed down tadpole development without greatly influencing snails, leading to both increased exposure and susceptibility to parasites.
“We’ve known for ages that leaf litter was an important resource for tadpoles, but until recently biologists assumed it didn’t matter which tree species the litter came from,” says Raffel. “Jeff’s work has shown just how wrong we were. First, we learned that poor quality litter stunts tadpole growth and development, and now we find that litter composition also influences parasite infection.”
Because of the complex effects of litter quality on tadpole infection, it is difficult for the scientists to predict exactly how changing forest composition will affect tadpole infections in natural ponds. This will depend on the relative strength of positive versus negative effects of leaf litter on tadpole parasitism.
Nevertheless, Raffel thinks these results have important implications for our understanding of how nutrition affects disease.
“One really interesting result was that tadpoles who ate high-quality food were more resistant to infection than tadpoles that ate low-quality food,” he said. “To me, this serves as a cautionary tale about potential interactions between malnutrition and infectious diseases.”
This research was funded in part through a National Science Foundation research grant awarded to Professor Raffel.
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