The Status of Rana boylii in Oregon : Analyses of Selected Biotic and Abiotic Variables at Historical Locations for Rana boylii (Baird)
Abstract
One hundred and five historic localities for the foothill yellow-legged frog (Rana boylii Baird) were obtained from extensive search of verifiable sources. This pool yielded a subset of 91 identifiable localities that could be surveyed. Surveys, conducted in 1997- 1998, coupled searches for frogs with the scoring or measurement of biotic and physical variables. Surveys of localities lacking frogs (LLF) were repeated two times to ensure a high probability of detection if frogs were present. Data collected were designed to facilitate comparison between localities with frogs (L WF) and LLF to reveal potential causes of disappearances.
Rana boylii were found at 39 out of 91 localities surveyed. Localities with frogs differed significantly in substrate features, disturbance regimes, and their exotic vertebrate species compositions from LLF. Two substrate differences were significant: LWF had more coarse substrate which was less embedded than LLF . Likewise, two elements of human-related disturbance were prominent: LWF had less grazing than LLF, and most sites downstream from large impoundments were LLF. Bullfrogs and exotic fishes were observed significantly more often at LLF.
Foothill yellow-legged frogs may no longer exist at 57% of the localities at which they were historically recorded in Oregon. Physical and biotic data suggest that the LLF have sustained changes unfavorable to yellow-legged frogs, and that the pattern of change is complex. Prominent among physical changes was increased embeddenedness, a change that occurs because bedload fines increase. Several human related disturbance factors can contribute to increasing bedload fines, and the relative importance of these factors may be drainage-specific. Distinguishing which factors are most important will be difficult, but grazing and impoundments which frequently increase fine bedloading, may play a prominent role because their distribution between L WF and LWF appeared to be strongly asymmetric. Interpretation of the importance of exotic aquatic vertebrates remains ambiguous. In particular, these data cannot disentangle extirpation of the foothill yellow-legged frog that has resulted from unfavorable habitat changes that simply favored the exotics from the potentially direct effects of exotics on foothill yellow-legged frogs via predation, competition, or some combination of these.
