Masters Thesis

A radio telemetric study of the movement patterns of adult northern red-legged frogs (Rana aurora aurora) at Freshwater Lagoon, Humboldt County, California

The movement ecology of the adult northern red-legged frog (Rana aurora aurora) is essentially unknown throughout its range. Systematic studies, particularly those that investigate terrestrial movements of these animals, are urgently needed since this species has been listed federally and by the state of California as a Species of Special Concern. I conducted a radio-telemetry study to determine the extent of the terrestrial movements and habitat use of 12 adult R. a. aurora, 11 females and 1 male, at Freshwater Lagoon, Humboldt County, California, from March through July 1999. The beaded-chain belt method of attaching radio transmitters was tested to evaluate its efficacy on adult R. a. aurora. There was considerable variability in the movements and movement patterns of the frogs. A majority of the frogs moved away from the periphery of the lagoon, the assumed breeding site, some to considerable distances (range of furthest distances = 30-270 m). Most of the distances moved per day were 5 m or less (range = 0-87.5 m). Observed range length (the map distance between the two relocation points farthest from each other in an individual's observed range) varied from 5 to 221 m. Those frogs who made long-distance moves did not, for the most part, do so synchronously, and there was no apparent pattern to these movements on a seasonal basis or in response to daily weather conditions. The frogs, except for one individual, were detected on land during 90% of the survey period, and they tended to stay 5 m or less from water (lagoon, stream, ditch, or seep). Overall distances from water did not appear to decrease as precipitation and soil moisture decreased. Although there was considerable individual variability, the frogs were found much more often in the closed canopy thicket/forest habitat than in the other habitat types, which, in descending order of use, were forb patch, emergent vegetation, grassland, and man-made objects. Sword ferns may be important at the microhabitat level in providing cover for R. aurora. Six of the frogs appeared to use intermittent streams, especially those upslope from the lagoon, within the study area as corridors for at least part of their travels. Conservation efforts should take into consideration that R. a. aurora uses upland habitats. Any terrestrial buffer zones should maintain key vegetative components such as dense patches of shrub and herbaceous vegetation. Movement surveys of R. aurora should be conducted year-round and for subsequent years. The beaded-chain belt method of attaching transmitters proved to be a relatively safe and effective technique for use on large adult R. a. aurora.

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