Rana muscosa

Yellow-legged frog populations now recognized as Rana sierrae formerly were included in Rana muscosa. Vredenburg et al. (2007) examined phylogeography of Rana muscosa as defined by Stebbins (2003) and determined that R. muscosa occurs in the southern Sierra Nevada and in mountains to the south and that populations in the Sierra Nevada north of this range comprise a distinct species (Rana sierrae).

Note that the full range of Rana muscosa includes the southern Sierra Nevada and is larger than that of the distinct population segment listed under the U.S. Endangered Species Act (mountains of southern California south of the Sierra Nevada).

Occurs in the southern Sierra Nevada and mountains of southern California; numerous population declines and local extirpations have occurred and are ongoing, some in apparently pristine habitats; introduced trouts are a major factor in the decline; oher threats include disease, small population sizes that make the populations vulnerable to extirpation from catastrophic events, and climate change.

Rana muscosa occurs in the southern Sierra Nevada of California and in mountains to the south in southern California. In southern California south of the Sierra Nevada, the historical range extended from Palomar Mountain in San Diego County northward and westward through the San Jacinto, San Bernardino, and San Gabriel Mountains of Riverside, San Bernardino, and Los Angeles counties; these formed four isolated clusters of montane populations (Vredenburg et al. 2007). Additionally, the species occurred as an isolated cluster of populations on Breckenridge Mountain, south of the Kern River in Kern County, and in the Sierra Nevada (west of the crest) in Tulare, Inyo and Fresno counties, extending north to Mather Pass (Vredenburg et al. 2007). The mountain ridges that separate the headwaters of the South Fork Kings River from the Middle Fork Kings River, from Mather Pass to the Monarch Divide, form the northern border of the range.

Rana muscosa is now extirpated on Palomar and Breckenridge mountains and in much of the former range elsewhere in southern California and the southern Sierra Nevada (USFWS 2006, 2012; Vredenburg et al. 2007). In the mountains of southern California, it exists as highly isolated populations (USFWS 2012). Historical elevational range in the mountains of southern California was 1,220-7,560 feet (370-2,290 meters (Stebbins 1985; USFWS 2002, 2012).

Historically, Rana muscosa was documented in approximately 166 localities in creeks and drainages in the mountains of southern California (Jennings and Hayes 1994). Vredenburg et al. (2007) determined that Rana muscosa occurred historically at only 79 localities (they defined each historic locality as a circle centered on the point representing their interpretation of the locality description and with a radius of 1 km). Currently the species exists as nine populations in the San Gabriel, San Bernardino, and San Jacinto mountains (USFWS 2012). An experimental reestablishment area exists in the San Jacinto Mountains (USFWS 2012).

California Department of Fish and Game (2011) determined that Rana muscosa currently occupies 129 localities within 17 HU12 watersheds in the Sierra Nevada and 16 localities across 8 watersheds in the Transverse and Peninsular ranges of southern California.

Threats in the mountains of southern California include the following (USFWS 2012): Metapopulation dynamics have been severely interrupted by the presence of predatory nonnative trout in waterways (trout have eliminated and replaced populations rangewide, fragmented the remaining habitat, isolated extant populations in marginal habitat, and currently inhibit natural dispersal, recolonization, and recruitment in the historical range). Physical isolation of populations has caused genetic isolation (inbreeding has been detected in three populations, and genetic bottlenecks have been detected in all populations). A virulent fungal pathogen has been detected in all populations and appears to be inhibiting recruitment of the juvenile life stage. Catastrophic natural events such as wildfires and flooding greatly increase the likelihood that the small, isolated populations will become extirpated.

USFS has protected and managed the majority of southern Rana muscosa habitat, but recreational activities continue to impact habitat at three of nine southern R. muscosa populations (Vincent Gulch, Dark Canyon, and Fuller Mill Creek). Illegal marijuana cultivation has impacted four occupied sites since listing (Devil's Canyon, Bear Gulch, Vincent Gulch, and City Creek). Repeated impacts from roadwork activities (sedimentation, contamination, and introduction of invasive plant species) have continued in two occupied southern R. muscosa occurrences. Long-term fire suppression caused the increase of fuel loads and wildfire risk rangewide. Fire thoroughly burned two occupied sites (Devil's Canyon and East Fork City Creek) resulting in a decline of R. muscosa at City Creek. The remaining seven R. muscosa populations in the mountains of southern California remain at an extreme risk of fire and could be exposed to impacts from fire management activities in the future. This is the most significant risk to the habitat of southern R. muscosa. Source: USFWS (2012).

The widespread introduction of nonnative trout contributed to the decline of this species through predation. All populations in the mountains of southern California are now isolated in marginal habitat in the headwaters of tributaries. Nonnative trout occupy the downstream waters (dispersal and migratory routes) at five of nine occupied localities (South Fork Big Rock Creek, Bear Gulch, Vincent Gulch, Fuller Mill Creek, and Dark Canyon) and upstream at one (Tahquitz-Willow Creek). Nonnative trout have already fragmented and reduced available habitat. They currently prevent recolonization of historically occupied areas, disrupting metapopulation dynamics, and as such, increase the vulnerability of populations to wildfire and flooding, and increase the likelihood of inbreeding. Source: USFWS (2012).

The chytrid fungus (Bd), has been identified as having potentially catastrophic effects (localized extinction) on mountain yellow-legged frog populations. Populations in southern California have low infection rates, indicating that some adults are persisting and are likely capable of reproducing. The offspring of these individuals will likely be vulnerable to mortality caused by chytridiomycosis until they reach adulthood but are particularly susceptible immediately following metamorphosis. Therefore, while Bd poses a significant risk to the small and isolated populations, persistent individuals may be able to replenish these populations with time if enough survive to reproductive maturity. Additional information is needed regarding the effects of Bd on Rana muscosa populations in the southern California mountains, particularly with consideration of reintroduction, augmentation, and translocation efforts occurring. Other pathogens could have negative effects on these populations, although they currently appear to have little to no impact on the wild populations. Source: USFWS (20102).

Populations in the southern Califronia mountains face a high extinction risk even from natural environmental fluctuations due to the vulnerabilities associated with few, small, isolated populations. Genetic variability is low in all populations, and each population appears to be bottlenecked. Inbreeding thus far has been minimal but is evident in three of the nine populations. Finally, metapopulation dynamics are severely inhibited, possibly preventing the natural recovery of populations through recolonization. Source: USFWS (2012).

Impacts from ongoing and future climate change (e.g., increased average temperatures, more frequent high temperature events, potentially decreased precipitation, and decreased snowpack leading to decreased stream flows in snow-fed waters) are likely to have negative effects on yellow-legged frogs through habitat reduction and alteration, including problems associated with increased drought and increased fire frequency. UV-B radiation, acid precipitation, and contaminants may potentially impact yellow-legged frog populations, but the past and present effects of these factors are poorly known. Source: USFWS (2012).