Population fragmentation of spotted frogs in the Owyhee Mountains

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This report is the third and final based on a study conducted from 1997-1999 on population fragmentation in the Great Basin population, Owyhee subpopulation of Columbia spotted frogs. It includes a synopsis of the 1999 field data and looks at the three years collectively to reveal short-term trends and population demographics. The two previous reports described the importance of understanding the movement behavior of this candidate species in determining population structure. Subsequently, by assessing population structure, the short-term viability of the population can be assessed. This study focused on the observations of movements of individuals within the subpopulation using an intensive mark-recapture method with passive integrated transponder (PIT) tags. The goal for the first year (1997) was to PIT -tag as many frogs as possible so that over the next two years movements could be detected and population structure assessed. The second year (1998), the Rock Creek drainage was the focus, and as many individuals as possible were PIT –tagged and areas of greatest spotted frog density were monitored. The final year (1999), the Rock Creek drainage was again surveyed, but in addition adjacent watersheds were surveyed to search for individuals previously PIT -tagged in the Rock Creek drainage. Movements of individuals to and from a hibernaculum were also monitored in 1999 and 2000. During the short-term (1997-1999), the Owyhee subpopulation numbers steadily decreased. In some occurrences (Stoneman Creek, for example), numbers plummeted, while in others (Rock Creek), the decline was less dramatic, but steady. Climatic conditions combined with fragmentation and disturbance of suitable habitat patches and movement corridors by livestock were factors negatively influencing population numbers. Heavy grazing during fall migration was linked to the unusually skewed gender ratio and subadult loss at the Sam Noble Springs occurrence. Forty-nine separate occurrences were documented in Owyhee and Twin Falls counties; 11 of those were proposed to be included in a long-term monitoring plan being developed by the USFWS. Long-term monitoring of a number of occurrences is needed to assess and determine the long-term trajectory of the Owyhee Mountain subpopulation. Based on current knowledge of the frog’s life history and its habitat, management strategies are recommended (listed) for land use managers to help foster successful frog persistence over the long term.

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Reference Code: U00ENG02IDUS

Full Citation: Engle, J. C., and J. C. Munger. 2000. Population fragmentation of spotted frogs in the Owyhee Mountains. Final report. Cooperative Cost Share Project, Bureau of Land Management and Boise State University. 16 pp.

Location: ANIMAL EF: RANA LUTEIVENTRIS