California Red-legged Frog

Rana aurora draytonii

Description

The California red-legged frog is one of two sub-species of the red-legged frog found on the Pacific coast. The California red-legged frog is the largest native frog in the western United States, ranging from 1.5-5.1 in (3.8-12.9 cm) in length. The abdomen and hind legs of adults are largely red; the back is characterized by small black flecks and larger irregular dark blotches with indistinct outlines on a brown, gray, olive, or reddish background color. Dorsal spots usually have light centers. Dorsolateral folds are prominent on the back. Tadpoles range from 0.6-3.1 in (1.5-7.9 cm) in length and the background color of the body is dark brown and yellow with darker spots.

Several morphological and behavioral characteristics differentiate California red-legged frogs from northern red-legged frogs. Adult California red-legged frogs are significantly larger than northern red-legged frogs by 1.4-1.6 in (3.6-4.0 cm). Dorsal spots of northern red-legged frogs usually lack light centers common to California red-legged frogs, but this is not a strong diagnostic characteristic. California red-legged frogs have paired vocal sacs and call in air, whereas northern red-legged frogs lack vocal sacs and call underwater. Female California red-legged frogs deposit egg masses on emergent vegetation so that the egg mass floats on the surface of the water. Northern red-legged frogs also attach their egg masses to emergent vegetation, but the mass is submerged.

Behavior

California red-legged frogs breed from November through March with earlier breeding records occurring in southern localities. Northern red-legged frogs breed in January to March soon after the ice melts. California red-legged frogs found in coastal drainages are rarely inactive, whereas those found in interior sites may hibernate.

Egg masses that contain about 2,000-5,000 moderate-sized, dark reddish brown eggs are typically attached to vertical emergent vegetation, such as bulrushes or cattails. California red-legged frogs are often prolific breeders, laying their eggs during or shortly after large rainfall events in late winter and early spring. Eggs hatch in six to 14 days. In coastal lagoons, the most significant mortality factor in the pre-hatching stage is water salinity. One hundred percent mortality occurs in eggs exposed to salinity levels greater than 4.5 parts per thousand. Larvae die when exposed to salinities greater than 7.0 parts per thousand. Larvae undergo metamorphosis three and one-half to seven months after hatching. Of the various life stages, larvae probably experience the highest mortality rates, with less than 1% of eggs laid reaching metamorphosis. Sexual maturity normally is reached at three to four years of age, and California red-legged frogs may live eight to 10 years.

The diet of California red-legged frogs is highly variable. Larvae probably eat algae; adults consume invertebrates. Vertebrates, such as Pacific tree frogs and California mice, represented over half of the prey mass eaten by larger frogs. Juvenile frogs were found to be active diurnally and nocturnally, whereas adult frogs were largely nocturnal. Feeding activity likely occurs along the shoreline and on the surface of the water.

Habitat

The California red-legged frog occupies a fairly distinct habitat, combining both specific aquatic and riparian components. The adults require dense, shrubby, or emergent riparian vegetation closely associated with deep, still, or slow moving water. The largest densities of California red-legged frogs are associated with deep-water pools with dense stands of overhanging willows and an intermixed fringe of cattails. Well-vegetated terrestrial areas within the riparian corridor may provide important sheltering habitat during winter. California red-legged frogs estivate in small mammal burrows and moist leaf litter. California red-legged frogs have been found up to 98 ft (30 m) from water in adjacent dense riparian vegetation for up to 77 days.

California red-legged frogs disperse upstream and downstream of their breeding habitat to forage and seek estivation habitat.

Estivation habitat is essential for the survival of California red-legged frogs within a watershed. Estivation habitat, and the ability to reach estivation habitat can be limiting factors in California red-legged frog population numbers and survival.

Estivation habitat for the California red-legged frog is potentially all aquatic and riparian areas within the range of the species. Such habitat includes any landscape features that provide cover and moisture during the dry season within 300 ft (91 m) of a riparian area. This could include boulders or rocks and organic debris such as downed trees or logs; industrial debris; and agricultural features, such as drains, watering troughs, spring boxes, abandoned sheds, or hayricks. Incised stream channels with portions narrower than 18 in (45.7 cm) and depths greater than 18 in (45.7 cm) may also provide estivation habitat.

Distribution

The historical range of the California red-legged frog extended along the coast from the vicinity of Point Reyes National Seashore, Marin County, California, and inland from the vicinity of Redding in California's Shasta County, to northwestern Baja California in Mexico.

California red-legged frogs are known to occur in 243 streams or drainages in 22 counties, primarily in the central coastal region of California. Monterey, San Luis Obispo, and Santa Barbara counties support the greatest number of currently occupied drainages. Historically the California red-legged frog was known from 46 counties, but the taxon is now extirpated from 24 of those counties. In seven of the 22 occupied counties, California red-legged frogs are known from a single occurrence. The most secure aggregations of California red-legged frogs are found in aquatic sites that support substantial riparian and aquatic vegetation and lack exotic predators, such as bullfrogs, bass, and sunfish. Only three areas within the entire historic range of the California red-legged frog may currently support more than 350 adults: Pescardero Marsh Nature Preserve, Point Reyes National Seashore, and Rancho San Carlos. The San Francisco Airport drain-age location, identified as containing over 350 individuals, is now thought to be nearly extirpated. Threats, such as expansion of exotic predators, proposed residential development, and water storage projects, occur in the majority of drainages known to support California red-legged frogs.

Threats

The California red-legged frog has sustained a 70% reduction in its geographic range in California as a result of several factors acting singly or in combination. Habitat loss and alteration, overexploitation, and introduction of exotic predators were significant factors in the California red-legged frog's decline in the early to mid-1900s. It is estimated that California red-legged frogs were extirpated from the Central Valley floor before 1960. Remaining aggregations of California red-legged frogs in the Sierra foothills became fragmented and were later eliminated by reservoir construction, continued expansion of exotic predators, grazing, and prolonged drought.

Habitat loss and alteration are the primary factors that have negatively affected the California red-legged frog throughout its range. For example, in the central valley of California, over 90% of historic wet-lands have been diked, drained, or filled primarily for agricultural development and secondarily for urban development. Wetland alterations, clearing of vegetation, and water diversions that often accompany agricultural development make aquatic sites unsuitable for California red-legged frogs. Urbanization with its associated roadway, stream channelization, and large reservoir construction projects has significantly altered or eliminated California red-legged frog habitat, with the greatest impact occurring in southern California. The majority of extant localities are isolated and fragmented remnants of larger historical populations.

Current and future urbanization poses a significant threat to the California red-legged frog. Sixty-five drainages are associated with urbanization threats. Proposed urban developments include the East County Area Plan in Alameda County, which involves development of up to 52,000 acres (20,000 hectares), and projects currently proposed in the Ruby Hills/Arroyo Del Valle watershed and south Livermore valley; reservoir canyon ponds in Santa Clara County; Alamo, Shadow, and Brookside Creeks in Contra Costa County; the Carmel River in Monterey County; and the Santa Ynez River in Santa Barbara County. In Santa Cruz County, a proposed commuter rail project linking Santa Cruz to Watsonville could increase urban development in southern portions of the county.

Loss of habitat and decreases in habitat quality will occur as a result of on-site degradation of the stream environment and/or riparian corridor, or through modification of instream flow. Where streams or wet-lands occur in urban areas, the quality of California red-legged frog habitat is degraded by a variety of factors. Among these factors are introduction of exotic predators, elimination of stream bank vegetation, collecting, and loss of upland habitat.

Water projects, which accompany urban and agricultural growth, have had a negative effect on California red-legged frogs and their habitat. The timing and duration of water releases from reservoirs, particularly on the central California coast, can render a stream unsuitable for California red-legged frog reproduction and maintain populations of exotic predators in downstream areas that would normally be dry in summer. Reservoirs are typically stocked with predatory species of fish and bullfrogs. These species often disperse into surrounding California red-legged frog habitat disrupting natural community dynamics. California red-legged frogs generally are extirpated from downstream portions of a drainage one to five years after filling of a reservoir. In some larger drainages, however, isolated California red-legged frog populations have persisted upstream.

Water diversions, groundwater well development, and stock pond or small reservoir construction projects degrade or eliminate habitat. Diverting water from natural habitats to these projects disrupts the natural hydrologic regime. During periods of drought, reduced availability of water within natural drainages combined with drawdown from the impoundments, disrupts reproduction, foraging, estivation, and dispersal.

Storm damage repair and flood control maintenance on streams are current threats to California red-legged frogs. Routine flood control maintenance includes vegetation removal, herbicide spraying, shaping of banks to control erosion, and de-silting of the creek, all of which degrade California red-legged frog habitat.

Routine road maintenance, trail development, and facilities construction activities associated with parks in or adjacent to California red-legged frog habitat can result in increased siltation in the stream. If this siltation occurs during the breeding season, asphyxiation of eggs and small California red-legged frog larvae can result.

Placer mining may threaten California red-legged frog habitat because of heavy siltation. The siltation resulted from upstream gold mining. Deep holes in streams created by instream placer mining also may provide habitat for exotic predatory fish. Creeks, streams and rivers are open to suction dredging throughout the year in 13 of 22 counties within the current range of the California red-legged frog.

Road-killed California red-legged frogs have been documented at several locations in San Mateo and Santa Cruz Counties. Road kills may deplete frog aggregations in borderline habitat and otherwise protected areas. Where roads cross or lie adjacent to California red-legged frog habitat, they may act as barriers to seasonal movement and dispersal.

Livestock grazing is another form of habitat alteration that is contributing to declines in the California red-legged frog. Cattle have an adverse affect on riparian and other wetland habitats because they tend to concentrate in these areas, particularly during dry seasons. Cattle trample and eat emergent and riparian vegetation, often eliminating or severely reducing plant cover. Loss of riparian vegetation results in increased water temperatures, which encourage bullfrog reproduction. Riparian vegetation loss due to cattle grazing includes the loss of willows, which are associated with the highest densities of California red-legged frogs. Cattle grazing also results in increased erosion in the watershed, which accelerates the sedimentation of deep pools used by California red-legged frogs and adversely affects aquatic invertebrates. Aquatic invertebrates are common prey items of California red-legged frogs.

Grazing effects are not limited to riparian areas. Improper grazing of upland vegetation can expose soils to erosive impacts of raindrops, reduce water infiltration, and accelerate runoff. This can erode topsoil and cut rills and gullies, concentrating runoff, deepening gullies, lowering water tables, and increasing sediment production. Sediment introduced into streams can alter primary productivity and food supply, and fill interstitial spaces in streambed material, thereby impeding water flow, reducing dissolved oxygen levels, and restricting waste removal. Suspended sediments reduce light penetration to plants and reduce oxygen-carrying capacity of the water. Reduction in photosynthesis and primary production decreases productivity of the entire ecosystem.

Livestock grazing can cause a nutrient loading problem in areas where cattle are concentrated near the water, but in other areas it can reduce nutrients through removal of riparian vegetation. Riparian vegetation provides organic material for approximately 50% of a stream's nutrient energy. Detritus from such plants is a principal source of food for aquatic invertebrates. Streamside vegetation also provides habitat for terrestrial insects, another important dietary component for other aquatic or riparian-associated species. Livestock grazing also has been implicated as a contributing factor in the decline and disappearance of California red-legged frogs from the lower Salinas River and the San Francisco peninsula.

In addition to cattle, feral pigs also disturb the riparian zone through their rooting, wallowing and foraging behavior in the shallow margins of water bodies. Feral pigs disturb and destroy vegetative cover, trample plants and seedlings, and cause erosion. At Pinnacles National Monument, soil compaction and possible disturbance of frog eggs caused by feral pigs have been noted in California red-legged frog habitat. Off-road vehicle use adversely affects California red-legged frogs in ways similar to livestock grazing and feral pig disturbance. Off-road vehicles damage riparian vegetation, increase siltation in pools, disturb the water in stream channels and crush eggs, larvae, juveniles, and adults. California red-legged frogs were eliminated in part by off-road vehicle activities at the Mojave River above Hesperia, at Rincon station on the west fork of the San Gabriel River, and in Piru Creek above Pyramid Lake.

Heavy recreational use of parks also can degrade habitat for the California red-legged frog. At Big Basin Redwood Park in Santa Cruz County, heavy recreational use may have contributed to the disappearance of California red-legged frogs from Opal Creek.

Timber harvest threatens California red-legged frogs through loss of riparian vegetation and increased erosion in the watershed, which fills pools with sediment and smothers egg masses. In Santa Cruz County, timber harvest is proposed adjacent to Adams Creek and Whitehouse Creek and occurs periodically on a tributary of Blooms Creek. The proposed timber harvests would occur in three of 18 streams in the county that support California red-legged frogs. In Pescadero Creek at Portola State Park, erosion and siltation caused by severe winter storms and upstream logging operations may have been the cause of the disappearance of California red-legged frogs from this portion of the stream.

Six consecutive years of drought in California severely affected remaining California red-legged frogs in the Sierra foothills. Many sites in intermittent streams that held California red-legged frogs before the drought were completely dry during field surveys conducted between 1985-1992. Sites still holding pools of water had water levels so low that access by predators was enhanced. Livestock grazing at many sites exacerbated effects of the drought by limiting or preventing riparian habitat regeneration. Long-term survival of California red-legged frogs may be compromised by the elimination of refuge areas during times of the year when the stream is dry. However, California red-legged frog populations are undoubtedly capable of recovering from drought, provided other factors have not irreparably degraded their habitat, and provided they have not been completely extirpated from the drainage.

Extensive flooding has been a significant contributing factor in the extirpation of the California red-legged frog from desert drainages of southern California.

A considerable amount of occupied California red-legged habitat exists in the form of isolated patches along stream courses. These patches of suitable habitat represent mere remnants of a much larger historical habitat that once covered whole drainages. Fragments of formerly extensive populations of California red-legged frogs are now isolated from other populations. Populations isolated in habitat fragments are vulnerable to extinction through random environmental events or anthropogenic catastrophes. With only three of 243 known creeks or drainages supporting populations of over 350 adults, all remaining occurrences are considered vulnerable to these threats. Once a local extinction event occurs in an isolated habitat fragment, the opportunity for recolonization from a source population is reduced. Thus, local extinctions via stochastic processes, coupled with habitat fragmentation may represent a substantial threat to the continued existence of the California red-legged frog over much of its range.

Conservation and Recovery

At several parks, the National Park Service has conducted or is planning to conduct status surveys for California red-legged frogs. The forest service has conducted and has ongoing amphibian surveys in many National Forests within the historic range of the California red-legged frog. In Los Padres National Forest, the forest service has altered flow regimes in Piru Creek between Pyramid Lake and Lake Piru to benefit the endangered arroyo southwestern toad. Although no specific studies have been done, these flow regime changes also may benefit the California red-legged frog. The forest service has also designated more than 31 mi (50 km) of Sespe Creek in Los Padres National Forest as "Wild and Scenic" under the National Wild and Scenic Rivers Act of 1968.

The Contra Costa Water District undertook construction of a large reservoir project on Kellogg Creek, Contra Costa County, with the Bureau of Reclamation involved to amend water service contracts and modify water rights to facilitate project construction. A mitigation and monitoring program was proposed to compensate for California red-legged frog habitat losses at Los Vaqueros. The mitigation plan included a bullfrog and exotic fish control program to be carried out for the life of the reservoir project. In addition, Bureau of Reclamation projects have guided water contract renewals as well as road maintenance activities and grazing leases, all of which may affect California red-legged frogs. U. S. Fish and Wildlife Service (FWS) is involved in the development of two Habitat Conservation Plans to potentially protect three localities of California red-legged frogs.

FWS has established five recovery areas within the historical range of the California red-legged frog: the western foothills and Sierran foothills to 5,000 ft (105 km) in elevation in the central valley hydrographic basin; the central coast ranges from San Mateo and Santa Clara counties south to Ventura and Los Angeles counties; the San Francisco Bay/Suisun Bay hydrologic basin; southern California, south of the Tehachapi Mountains; and the northern coastal range in Marin and Sonoma counties. These five units are essential to the survival and recovery of the California red-legged frog.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Sacramento Fish and Wildlife Office
U. S. Fish and Wildlife Service
Federal Building
2800 Cottage Way, Room W-2605
Sacramento, California 95825-1846
Telephone: (916) 414-6600
Fax: (916) 460-4619

Reference

U.S. Fish and Wildlife Service. 23 May 1996. "Determination of Threatened Status for the California Red-Legged Frog." Federal Register 61(101): 25813-25833.