Population Structure of Spotted Frogs in the Owyhee Mountains

Occurring in the Owyhee Mountains of southwestern Idaho is an isolated population of Columbia spotted frogs, Rana luteiventris. This population is part of the portion of this species that is classified by the United States Fish and Wildlife Service as a candidate species, that is, listing as threatened or endangered is warranted, but precluded by other higher priority actions.

The long-term outlook for the Owyhee population is unknown. Several BLM surveys (Munger 1994, 1995, 1997, and in prep.) have yielded a substantial number of sites where spotted frogs occur, on the order of 60 populations, including approximately 25 breeding sites. However, these data only provide a snapshot of the present situation--we do not yet know whether spotted frogs are increasing, decreasing, or are stable in numbers. The best way to determine the population trajectory is to conduct long-term (e.g., 20 years) monitoring at a number of sites. There exists a shorter-term alternative to assessing at least some of the factors that determine the long-term viability of a population: assessment of population structure.

Throughout their range, spotted frogs are closely associated with water. In a desert habitat such as is found in the Owyhee Mountains, this association will tend to lead to fragmented populations. A rich literature on the dynamics of metapopulations (e.g., Gill 1975, Wilcox and Murphy 1985, Hanski 1991, Sjogren 1991, Thomas and Harrison 1992. Stacey and Taper 1992) contains two generalizations: first, smaller subpopulations are more likely to go extinct due to random processes than are large populations. Second, the long-term persistence of a metapopulation consisting of a set of subpopulations will depend on the ability of frogs to colonize one fragment from another and thus "rescue" subpopulations that have gone extinct: the stronger the barriers, the lower the long-term persistence. As an example of these dynamics, Bradford et al. (1993) argued that fragmentation of populations of frogs in the Sierra Nevada by the presence of introduced fish has led to the demise of local subpopulations.

We are pursuing two studies to describe the population structure of spotted frogs. Here we describe preliminary results for one of these studies: the observation of movements of individuals among populations using a combination of intensive marking with PIT (passive integrated transponder) tags and radiotracking. The second study, funded by a BSU faculty grant. will use the study of the genetic structure of spotted frog populations to help understand the metapopulation dynamics of those populations: if subpopulations tend to be genetically disjunct this would argue that they are isolated and more prone to extinction. If they are genetically similar. this would argue that they are not isolated and under less danger of extinction.

Our goal for the first year was to PIT-tag as many frogs as possible so that over the next two years we could detect movements and assess metapopulation structure.