SNFPA Draft Supplemental Environmental Impact Statement

June 2003

Chapter 3: Affected Environment

3.2.2.6. Foothill Yellow-Legged Frog (Rana boylii)

The information below was extracted and summarized from the following reference: USDA Forest Service. unpublished. Draft Conservation Assessment for the Foothill Yellow-Legged Frog (Rana boylii). USDA Forest Service, Pacific Southwest Region, Vallejo, CA. This section updates and supplements the information found in FEIS Volume 3, Chapter 3, Part 4.4, pp. 207-208. Detailed references can be found in that document.

Habitat Requirements

The foothill yellow-legged frog has been found primarily within shallow channels with riffles that have at least cobble-sized. Streams and rivers with this species include those with both permanent and intermittent flows, low gradient and high gradient greater than 4 percent, alluvial and bedrock channels. The species is also occasionally found in other in riparian habitats including moderately vegetated backwaters, isolated pools and slow moving rivers with mud substrates.

The ability to withstand and recover from disturbance is crucial for any organism living in the highly variable environment of a river. The life history strategy of the Foothill yellow-legged frog has been shaped by the wet winters and dry summers typical of the Mediterranean climate in the Sierra Nevada. To avoid disturbance of its most vulnerable life stages (eggs and larvae) breeding is timed late enough in the spring to avoid extreme discharge fluctuation. However breeding must occur early enough to allow tadpoles sufficient time to metamorphose, and juveniles time to grow, before the onset of the next wet season. Breeding sites are not continuously distributed along the streams and rivers occupied by this species as the frogs select channels with particular morphological traits. Understanding that the frogs are concentrated in both time and space during breeding, is critical to their conservation. The potential loss of adults and recruits due to any number of risk factors (e.g. dam releases, all terrain vehicles, mining, grazing, etc.) during breeding would be much worse than losses caused by the same activities at times of the year when frogs and tadpoles are more widely dispersed.

Previous literature reports breeding to occur from late March through May, with egg deposition for any single population being concentrated to a two-week period. More recent reports indicate that breeding activity can be spread over several weeks in the Coast Range and up to 31 days in the Sierra. Duration of the breeding season appears to be determined by weather. In cold rainy springs the breeding season is longer than in dry warm springs.

Egg masses usually contain about 900 eggs, but the number of eggs can range from 100 to over 1,000 per mass. For survival to hatching, eggs must remain inundated and attached to substrates despite falling and/or rising water levels. In wide shallow channels, stage height and near bank velocities are less sensitive to changes in discharge than in deeper more confined channels. Breeding sites with greater than average success to hatching have significantly greater width:depth ratios than channels where hatching success is low.

In the coast range, adults are frequently seen when congregated at breeding sites in April, May, and June. Later in the summer adults are scarcely observed along the main stems of larger rivers (the Trinity and Eel Rivers). This may indicate movement into the vegetation, movement into tributaries, or reduced diurnal activity.

Status

The Foothill yellow-legged frog is listed as a sensitive species on the Region 5 Regional Forester's Sensitive Species List. In addition, the frog is a California Species of Special Concern. Jennings and Hayes recommend endangered status in southern and central California south of the Salinas River, Monterey County, and threatened status in the "west slope drainages of the Sierra Nevada and southern Cascade Mountains east of the Sacramento-San Joaquin River axis." In the Coast Ranges north of the Salinas River the Foothill yellow-legged frog stills occurs in significant numbers in some coastal drainages but is also at risk due to anthropogenic and environmental threats.

Risk Factors

For a summary of risk factors, reference the SNFPA Chapter 3, Part 4.4, pp. 207-211.

USDA Forest Service · Pacific Southwest Region