Proposed Endangered Status for the Southern California Mountain Yellow-Legged Frog

This material is excerpted and rearranged from the Federal Register: December 22, 1999, Volume 64, Number 245: Proposed Endangered Status for the Southern California Distinct Vertebrate Population Segment of the Mountain Yellow-Legged Frog Full Text (pdf)

Pictures of mountain yellow-legged frog from CalPhotos

The Fish and Wildlife Service proposes to list the southern California distinct vertebrate population segment of mountain yellow-legged frog (Rana muscosa) as endangered, pursuant to the Endangered Species Act of 1973, as amended.

Background: The mountain yellow-legged frog, Rana muscosa, is a true frog in the family Ranidae. Mountain yellow-legged frogs from the high Sierra and the mountains of southern California are somewhat similar to each other, yet distinct from the foothill yellow-legged frog, R. boylii. The range of the mountain yellow-legged frog is divided by a natural geographic barrier, the Tehachapi Mountains, which isolates Sierran frogs from those in the mountains of southern California. The distance of the separation is about 140 miles.

Differences are based on limb size, calls, allozyme variation, and habitats. Frogs in southern California are typically found in steep gradient streams in the chaparral belt, even though they may range up into small meadow streams at higher elevations. In contrast, Sierran frogs are most abundant in high elevation lakes and slow-moving portions of streams. The rugged canyons of the arid mountain ranges of southern California bear little resemblance to the alpine lakes of the Sierra Nevada.

Description: Mountain yellow-legged frogs are moderately sized, about 1.5 to 3 inches The pattern is variable, ranging from discrete dark spots that can be few and large, to smaller and more numerous spots with a mixture of sizes and shapes, to irregular lichen-like patches or a poorly defined network. The body color is also variable, usually a mix of brown and yellow, but often with gray, red, or green-brown. Some individuals may be dark brown with little pattern. The throat is white or yellow, sometimes with mottling of dark pigment. The belly and undersurface of the high limbs are yellow, which ranges in hue from pale lemon yellow to an intense sun yellow.

Habit: Southern California mountain yellow-legged frogs are diurnal, highly aquatic frogs, occupying rocky and shaded streams with cool waters originating from springs and snowmelt. They feed on small, streamside arthropods. They do not occur in the smallest creeks. The coldest winter months are spent in hibernation, probably under water or in crevices in the bank.

Current Range: In southern California, mountain yellow-legged frogs can still be found in the San Gabriel Mountains in the upper reaches of Prairie Creek/Vincent Gulch, Devil's Canyon, and Alder Creek/East Fork, on the East Fork of the San Gabriel River, and Little Rock Creek on the Mojave River, in City Creek in the San Bernardino Mountains and in Dark Canyon in the San Jacinto Mountains.

Historic Range: In southern California, the elevation range was between 600 to 7,500 feet. Representative localities include Eaton Canyon, Los Angeles County, 1,220 feet, and Bluff Lake, San Bernardino County, 7,560 feet. An isolated southern population existed on Mt. Palomar in northern San Diego County, but this population appears to be extinct. Flooding during the winter of 1969 was thought to be the major factor in the loss of mountain yellow-legged frogs from Evey Canyon (west of Mt. Baldy Road) in the San Gabriel Mountains.

Decline: The entire population of mountain yellow-legged frogs in the San Gabriel and the San Jacinto Mountains (8 more or less isolated sites) is probably fewer than 100 adult frogs. The City Creek population in the San Bernardinos has not been counted. Prior to the late 1960s, mountain yellow-legged frogs were abundant in many southern California streams, but they are now absent from more than 99 percent of their previous range in southern California. This decline is part of a well-known larger pattern of declines among native ranid frogs in the western United States. Nowhere have the declines been any more pronounced than in southern California, where, besides declines in mountain yellow-legged frogs, the California red-legged frog has been reduced to a few small remnants, and the foothill yellow-legged frog (R. boylii) may be extinct. The chronology of the decline is not well documented, but it appears that a precipitous decline occurred over the last three or four decades. The decline went largely unnoticed and was not studied.

Mechanisms of Decline: The mechanisms causing the declines of western frogs are not well understood and are certain to vary somewhat among species, but the two most common and well-supported hypotheses for widespread declines of western ranid frogs are: (1) Past habitat destruction related to unregulated activities such as logging and mining and more recent habitat conversions for water development, irrigated agriculture, and commercial development and (2) alien predators and competitors such as introduced rainbow trout. Other environmental factors that could have adverse effects over a wide geographic range include pesticides, certain pathogens, and ultraviolet-B (beyond the visible spectrum) radiation, but their role, if any, in amphibian declines is not well understood.

Fairly heavy camping and day use coincides with frog habitat along the East Fork, San Gabriel River (dispersed camping), Prairie Fork Creek (campground, recently burned and presently closed by the FS), and Little Rock Creek (trail, rock climbing). Local habitat changes caused by recreational suction dredging for gold, human use around campgrounds and picnic grounds, heavily used trails, and large quantities of trash and toxic materials dumped into the stream bed may harm the habitat and contribute to local extinctions wherever these activities intersect with mountain yellow-legged frogs.

Because the declines have been so precipitous, and have spared only a small number of frogs in a few localities, the factors, and their interactions, that caused the decline may never be fully understood. In the case of the mountain yellow-legged frog, the only factor listed above that can be ruled out as a likely cause of decline is habitat destruction related to activities such as logging, mining, irrigated agriculture, and commercial development. The range of the mountain yellow-legged frog in southern California is mainly on public land administered by the U.S. Forest Service (FS). Most of the rugged canyons and surrounding mountainous terrain have been altered little and look much the same today as they did when mountain yellow-legged frogs were collected in the early decades of the 1900s.

The arroyo toad (Bufo microscaphus californicus), a federally listed endangered species, is present in the San Gabriel Mountains, but there is no benefit to the mountain yellow-legged frog because the two species occupy different areas in the San Gabriel Mountains and the arroyo toads are not known to occur elsewhere in the limited range of the mountain yellow-legged frog.

The Angeles and San Bernardino National Forests manage all known locations of mountain yellow-legged frogs in southern California. However, the FS does not include Rana muscosa on its list of sensitive species, although the Angeles and San Bernardino National Forests manage the frog as if it were sensitive. Nevertheless, the FS does not have a management plan for the mountain yellow-legged frog or an adaptive management strategy that addresses the specific conservation and recovery needs of the species.

Even though all the causes of decline may never be fully understood, the available information suggests a high probability that this frog may be extinct in southern California within a few decades.

Discussions about disappearance (Please note all these references concern the Sierran subspecies)

Westward Frog's Species Tour of Rana muscosa - Mountain Yellow-legged Frog

California Air Resources Board Abstract on Aquatic Animals in Acidic Waters of the Sierra Nevada

USDA Pacific Southwest Research Station, Forest Service Frogs or Fish??? "There is no question that the decline of the mountain yellow-legged frog is dramatic, and there is no question that trout are playing an important role in mountain yellow-legged frog declines."

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Updated 22 March 2001.