University of Minnesota
The leopard frog is disappearing from wetlands.
The tadpole's tale
Some environmental pollutants may take a tricky route to damage
By Deane Morrison
For more than 10 years, researchers like UMD senior research associate Lucinda Johnson have been trying to determine whether poor health and malformations are connected to reduced frog populations worldwide. Loss of habitat poses the greatest threat, says Johnson, but other factors operate in an ecological underworld; these culprits may only be identified when their accomplices are also rounded up.
A glimpse of what this murky network may look like appeared recently in the journal Nature, when Johnson and other researchers reported that a common herbicide could indirectly lead to an increase in the number of frog parasites in a wetland ecosystem. The paper is available online.
The researchers studied atrazine, a herbicide widely used in the United States. The study showed that atrazine in pond water could lead to a higher population of snails, which harbor parasites that also infect frogs.
"To my knowledge, this is the first time this indirect effect has been quantified," says Johnson. By influencing the snails' environment, the chemical may be "eventually removing a check and balance on the snails.
Parasites lost, parasites regained
The trematode worm that infects the frogs gets passed to frog-eating birds like herons and egrets. Inside the birds, the worms develop to adulthood. The adults produce eggs that are released into water with the birds' feces. The eggs hatch, develop into larvae, and burrow into snails. After further development, they burrow their way out again and swim in search of tadpoles. They infect them, the tadpoles turn into frogs, and the cycle continues.
"This represents a plausible mechanism that could explain the observed phenomenon of [leopard frog] decline," says Johnson. But, she adds, it doesn't necessarily mean that atrazine is actually killing frogs. "This is one of many potential mechanisms that could help explain the decline," she explains.
According to a 2003 EPA report, atrazine is estimated to be the most heavily used herbicide in the United States, being applied primarily on corn and sugarcane and on residential lawns in Florida and the Southwest. On a per unit area basis, the heaviest uses occur in parts of Delaware, Iowa, Illinois, Indiana, Ohio, and Nebraska. Atrazine, according to a report in Scientific American, is found in 57 percent of U.S. streams, the product of runoff from agricultural fields. Once in a stream, it may be carried to lakes, ponds, and other wetlands.
This side of parasites
The parasite in question is a tiny worm called a trematode. Like many other parasites, it lives a nomadic life, infecting one "host" animal after another in a perpetual cycle (see sidebar).
In one part of the life cycle, worm larvae are released from snails' bodies and swim off in search of a tadpole. When they find one, they often enter through the opening where body waste is expelled and tend to cluster at the base of one or both of the tadpole's developing hind legs. The presence there of a particular kind of trematode has been linked to hind limb deformities, but larvae also take up residence in the tadpole's internal organs.
For the study, Johnson and her colleagues collected leopard frogs from 18 wetlands near St. Cloud, Minnesota. The researchers found a positive correlation between the amount of atrazine in a wetland and the number of parasites in that wetland's frogs.
Their colleague Jason Rohr, of the University of South Florida, then set up outdoor tanks and turned them into mini wetlands by adding water, tadpoles, insects, algae, and other plants and animals. In some tanks, the water contained a level of atrazine comparable to levels found in the environment.
To Johnson, the central message of the "Nature" study is that big questions involving one or more sources of stress on species can only be addressed by "multiscale and multidimensional" studies involving studies in the field, the laboratory, and model ecosystems like the outdoor tanks.
Four weeks later, the tanks containing atrazine had more periphyton (a nice word for plant scum), which is a major snail food, and significantly more snails. More to the point, the tadpoles in the atrazine tanks had more parasites and lower counts of infection-fighting immune cells than those in tanks containing no atrazine.
A statistical analysis showed that of all the factors that could be at work, the presence of atrazine in water was the best predictor of higher parasite loads and reduced immune cell counts. Also, higher levels of phosphorus, a component of fertilizer, appeared to play a complementary role when atrazine was present.
Atrazine will break down in a matter of weeks if exposed to air and sun, but if buried in sediment it could persist in a wetland until the next breeding season, says Patrick Schoff, a colleague of Johnson's at UMD's Natural Resources Research Institute and a coauthor of the study. He is conducting a separate investigation of whether atrazine can disrupt the endocrine systems of frogs, leading males to produce egg cells in their testes.
To Johnson, the central message of the Nature study is that big questions involving one or more sources of stress on species can only be addressed by "multiscale and multidimensional" studies involving studies in the field, the laboratory, and model ecosystems like the outdoor tanks. She and her colleagues are currently involved in a study of how human land use and the drying-up of wetlands affect frogs.