Date of Award
Master of Science
Natural Resource Ecology and Management
Clay L. Pierce
Amphibians are declining throughout the United States and worldwide due to habitat loss, emergent diseases, and chemical contaminants in the environment. Iowa is a heavily modified landscape where 90% of the historic wetland area has been converted to row crop agriculture. In Iowa, the Conservation Reserve Enhancement Program (CREP) strategically restores wetlands to reduce nitrogen loads in tile drainage effluent. This project examined the quality of amphibian habitat provided by these restored wetlands by comparing amphibian species richness, estimated monthly survival probabilities of adult leopard frogs (Lithobates pipiens), and developmental stress levels in leopard frogs to a suite of environmental stressors including nutrient concentrations, water chemistry, and the presence of parasites and aquatic predators in wetlands. We also measured the concentrations of pesticides, herbicides, and fungicides in water and amphibian tissue samples and compared them to the prevalence of the amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd) within the chorus frog (Pseudacris maculata) population, as well as the abundance of zoospores in water samples and on frog skin.
CREP and reference wetlands offer different qualities of amphibian habitat. CREP wetlands were characterized by higher nitrogen concentrations, more alkaline pH, slightly longer hydroperiods, and greater depths. Differences in structural characteristics may contribute to the increased prevalence of non-native bullfrogs and fish, while high nitrogen concentrations may increase the risk of trematode parasitism for resident amphibians. Unfortunately, these physical characteristics are central to the primary nitrogen removal functions of CREP wetlands, so cannot be easily avoided. Mechanical drawdowns, which are already recommended on an as-needed basis for emergent vegetation management in CREP wetlands, could have an added benefit of reducing the impact of predators such as bullfrogs and fish on native amphibian species. Reference wetlands had higher concentrations of Bd zoospores and a higher incidence of developmental stress, but overall there were few differences in the composition of the amphibian assemblage, or in the population sizes and survival probabilities of leopard frogs between wetland types.
There were no differences in the concentrations of pesticides in water or chorus frog tissue samples or in the abundance of zoospores of the amphibian chytrid fungus between CREP and reference wetlands. While the concentration of zoospores in water samples was not related to the concentration of pesticides in water samples, fungicides and non-fungicides had opposing relationships to the prevalence of Bd in the chorus frog population. Fungicides and non-fungicides also had opposing relationships to zoospore abundance in water and on frog skin. In general, the abundance and prevalence of the amphibian chytrid fungus was positively correlated with total non-fungicide concentrations and either negatively or not correlated with total fungicide concentrations.
CREP and reference wetlands provide important habitat for amphibians in central Iowa. Maintaining some relatively predator-free wetlands within the larger complex of wetlands with a variety of hydroperiods appears to be important for the long term persistence of amphibians in this landscape, especially in light of increasing variability in rainfall due to climate change. Further study on the interactions between combinations of chemicals at ecologically relevant concentrations and other environmental stressors, including emergent diseases, will contribute greatly to our understanding of the effects of land use on amphibians and will aid in the conservation of amphibians.
Rebecca Ann Reeves
Reeves, Rebecca Ann, "Amphibian stress, survival, and habitat quality in restored agricultural wetlands in central Iowa" (2014). Graduate Theses and Dissertations. 14042.