Abstract:
We evaluated the effects of herbage removal from livestock grazing on
Oregon spotted frog (Rana pretiosa) habitat use by monitoring frog locations in
grazed and matched ungrazed treatments across a range of grazing intensities at
Jack Creek, Fremont-Winema National Forest, Oregon. Thirteen cattle exclosures
were deployed along Jack Creek in the summer of 2003. Movements were
monitored using radio telemetry on adult frogs (N = 24 frogs) within treatments
sites from July through October 2004. Individual frogs were located 1 to 28 times
([mean]=11.7) and were tracked from 1 to 74 days [mean]=35.8 days). A 10-pinpoint
intercept frame was used to estimate relative differences in removed vegetation
cover. This difference between ungrazed treatments and paired grazed controls
was used as a measure of grazing pressure at treatment sites, and examined in
relation to frog habitat preference. Pin measurements ranged from an average
monthly difference of -1.2 to 2.97 pin hits between an exclosure and its control.
As pin differences increased by one unit, the odds of finding a frog in the
exclosure increased by a factor of 1.62 (F1,4 = 3.90, p = 0.05) with an approximate
95% confidence interval of 1.00 to 2.74. Based on the proportion of time frogs
spent inside exclosures, there was evidence that as grazing pressure increased,
frogs preferred ungrazed livestock exclosures.
Secondary objectives of this study were to describe migration routes and
identify overwintering sites of the R. pretiosa population on Jack Creek. From
August to mid-December 2003, frogs (N = 36) were tracked from 5 to 92 days ([mean]
= 49.5 days) and located 2 to 39 times ([mean] = 23.2 times). In mid-October,
individuals were located in sheltered areas along the creek such as willow root
complexes and abandoned beaver runs. Frogs also were found in deep (130 cm)
pools associated with active springs and individuals in these areas were active
within the pools under 5 cm of ice.
Using mark-recapture techniques and deployment of cattle exclosures, we
examined 1) frog migration and 2) grazing effects on frog habitat use for the Jack
Creek R. pretiosa population in 2003-2004. Ultimately, a more complete
understanding of the natural history of this species and how anthropogenic
activities affect amphibians such as R. pretiosa will aid managers in mitigating
potential adverse affects, especially in riparian systems, and contribute to recovery
and restoration strategies.
