The Role of Pesticides, Introduced Fish, and Habitat Change in the Decline of the Cascades Frog (Rana cascadae) in California
A number of factors have been proposed as potential causes of amphibian decline in protected areas. In California, a large amount of research has been conducted on the potential effects of both pesticides and introduced fish. Changes in habitat that has historically supported amphibians have also been proposed as a potential contributor to declines. In this study, I examined the role of habitat change, introduced fish, and pesticides in explaining the disappearance of Cascades frogs (Rana cascadae) from sites in California where they have been previously documented.
Examination of these factors was made at 61 sites (waterbodies) located throughout the range of the Cascades frog in California. At each site, visual encounter surveys were conducted to determine the current site status (presence or absence) of fish and Cascades frogs. Supporting habitat features were also examined at each site. Data was collected on littoral zone substrate, shoreline substrate, near-shore water depth, maximum water depth, elevation, and precipitation. The total amount of pesticides applied upwind from each site was calculated using a geographic information system and California Department of Pesticide Regulation records for the years 1991-2000. Non-parametric Mann-Whitney and Chi-square tests were used to evaluate the difference in study variables at present and absent sites. Logistic regression was used to model habitat, fish, and pesticide variables in order to assess the role of these factors in explaining the decline.
The presence of fish was not found to significantly affect the distribution of Cascades frogs. Cascades frogs were found to be almost as common at sites with fish as they were at sites without fish. Descriptive statistics indicated that habitat sampled at present and absent sites was very similar overall and only small differences were detected. Upwind pesticide application was very similar overall and only small differences were detected. Upwind pesticide application was significantly greater at absent sites and was the most important study variable included in the multivariate model. While other factors not being evaluated may also contribute to declines, these results indicate that upwind pesticide application may be especially important in explaining the loss of Cascades frogs from sits where they were formerly present.
Cholodenko, Laura (2006) The Role of Pesticides, Introduced Fish, and Habitat Change in the Decline of the Cascades Frog (Rana cascadae) in California. Masters thesis, Department of Biological Sciences, California State University, Sacramento.
Laura joined the lab in Fall of 2003 and finished her degree in December 2006.
This work was done in collaboration with Dr. Carlos Davidson from California State University, San Francisco.