Examining the relationship of pesticides and amphibians in Costa Rican artificial wetlands

Abstract

Destruction of natural wetlands is among the largest threats to amphibians. While artificial wetlands have been proposed as a surrogate habitat for displaced amphibians, this has never been rigorously tested in the field. Artificial wetlands expose amphibians to many potential stressors, including pesticides. We assessed variables at the community, population, and individual levels for amphibians in artificial wetlands along a gradient of pesticide exposure in Guanacaste, Costa Rica. Our objective was to determine if pesticide exposure predicted measureable differences in health, population density, and community composition. We did this by assessing the general amphibian community and by a more in-depth study of one species, Rhinella marina. The community study detected thirteen species through call sampling and capture. Community composition was similar among sites, but sites with high pesticide application rates had lower community similarity through time. Higher population densities were associated with lower pesticide application rates for two species. Variable species responses are likely due to differences in life history. Pesticides significantly predicted body condition for one of the six species assessed, Leptodactylus fragilis, with higher body condition associated with lower pesticide application rates. For Rhinella marina, pesticide application rate significantly predicted the presence of insect head capsules in toad gut contents. Pesticide application rate also significantly predicted infection intensity for several parasites: Rhabdias nematodes, intestinal trematodes, haemogregarines, and microfilaria. Higher haemogregarine burdens were associated with higher pesticide application rates, while burdens for the other three parasites showed the opposite trend. All individuals tested were negative for Bd and ranavirus. Taken together, this study suggests that pesticide exposure may be important for amphibians, but importance varies among species. Contrary to expectations, Rhinella marina from high pesticide sites were not immunosuppressed. Lower infection intensities of several important parasite species may result in overall better health in sites with higher pesticide application rates. However, lower population densities and species loss from the communities in higher pesticide sites indicate that pesticides could negatively affecting amphibians in other ways. More research is needed to determine the cause of these community level changes, and to make management recommendations to improve artificial wetlands as amphibian habitat.