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Several rows of water-filled tubs covered with black mesh.

U researchers grew American toads and wood frogs in NRRI's backyard, in 45 tubs that replicated the conditions of the praire potholes on the Great Plains.

A deep look at shallow ponds

Prairie pothole study offers clues to the survival of amphibian life in wetlands

By June Kallestad

eNews, January 26, 2006

Nestled into the frozen mud of snow- and ice-covered wetlands, leopard frogs await the spring thaw on the Great Plains of the United States. Glaciers carved these wetlands into the landscape roughly a million years ago, creating an amazing and diverse freshwater resource known as the "prairie potholes."

In warmer seasons, these scattered shallow wetlands are alive with plant and animal life. The potholes--some as much as 10 feet deep and able to hold water for extended periods of time--also play an important role in managing the hydrology of the area, storing excess water during heavy rains and spring snowmelt. Yet they are also threatened by things like increased agricultural development and exotic species, exposure to UV radiation, pesticides, and climate change.

The Natural Resources Research Institute (NRRI) at the University of Minnesota, Duluth (UMD), is in the final stages of a three-year study of those threats; in particular, their effect on the amphibian community. NRRI is collaborating on the project with the U.S. Geological Survey and South Dakota State University, with a $746,433 grant from the Environmental Protection Agency. NRRI ecotoxicologist Pat Schoff is supervising the experiments that will help resource managers understand how these sensitive wetlands will react to changing conditions in the coming decades.

"Animals that live in the prairie potholes have evolved to accommodate fluctuating water levels, but we fear that the coming changes will be too dramatic for them to adapt," says Schoff. "They're already exposed to many environmental stressors, and adding the stressors we expect to see with global climate change may push them past the point of recovery."

A map of the prairie pothole study in the Midwest.
The prairie pothole region within North Dakota, South Dakota, and Minnesota. The green box indicates a prime study area.

The project involves three levels of experimentation: an extensive study covering the entire prairie pothole region (see map), a study focusing on a smaller area, and a mesocosm study, which was completed this fall in NRRI's backyard. The latter replicated the conditions of the prairie potholes in a small, controlled environment. American toads and wood frogs were grown from eggs in 45 tubs treated with varying levels of atrazine (a common herbicide used in the pothole region) and varying water levels to simulate the faster drying conditions that warmer temperatures produce. The scientists carefully documented the amphibians' growth and metamorphosis to determine whether the highly stressed conditions caused the toads' development to speed up from tadpole to frog, and produce smaller-than-normal adults.

Ongoing this winter are more experiments in the NRRI laboratories. UMD graduate student Angela Rohweder is carefully separating male frog specimens from females collected from prairie potholes and the mesocosms, and looking for female reproductive (ovarian) tissue in the males. (A previous study found that atrazine could alter the reproductive anatomy of amphibians.) Her data will be combined with the random sampling of atrazine levels in the pothole region to get a clearer picture of its affect on the frogs.

"Egg formation has been known to occur in male frogs if they are exposed to female hormones during their development," explains Schoff. "What we don't know is if, or at what levels, atrazine plays a role. Ultimately, we're concerned about the amphibians' ability to survive in this region."