Description
The California Red-legged Frog is the largest native frog to California, reaching lengths up to 3.25-5.5 inches. It has prominent dorsolateral folds, which are yellow or orange-colored in juveniles, long legs with incompletely webbed toes and eyes that are oriented outwards (Stebbins 2003). Coloring is reddish -brown or brown, gray, or olive, with small black flecks and spots on the back and sides and dark banding on the legs. The hind legs are red underneath extending onto the belly and sides of older individuals. Dark blotches on the back, typically have light coloring in the center. There is a dark mask on the head and a stripe extending from the shoulder to the front of the upper jaw (Stebbins 2003).
Larvae range in length from 0.55-3.15 inches in total length and have up to 2-3 upper and 3-4 lower tooth rows (Stebbins 2003). Newly hatched tadpoles generally are blackish in color, gradually changing to a brown background color with darker marbling or spots as they develop (Storer 1925).
Similar Species
The Northern Red-legged Frog (Rana aurora) and California Red-legged Frog are nearly morphologically identical and used to be considered the same species, however, recent genetic work conducted by Shaffer et al. (2004) has shown these are distinct species. Northern Red-legged Frog and California Red-legged Frog do hybridize where they co-occur in Mendocino County, however south of Mills Creek, only California Red-legged Frog are present.
The Foothill Yellow-legged Frog (Rana boylii) is another native ranid frog to California, but differs in that the dorsolateral folds are less distinct, lacks the reddish coloration of California Red-legged Frog and inhabits streams and rivers.
The American Bullfrog (Lithobates catesbieana) is a large non-native ranid frog that is often found occupying perennial ponds in urban and agricultural areas. Bullfrogs are easily distinguishable by their large size, green snout, lack of a light-colored lip stripe, large tympanum (especially on males) and lack of dorsolateral folds.
The Sierran Treefrog (Pseudacris sierrae) is a small chorus frog that is usually found occupying habitats where California Red-legged Frog are found. This species is easily differentiated by its small size, green color (although background coloration on this species is highly variable), dark eye stripe or "mask", and presence of toe pads.
Geographic 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 Redding, Shasta County southward to northwestern Baja California, Mexico (Jennings and Hayes 1985, Hayes and Krempels 1986). The current distribution of this species includes only isolated localities in the Sierra Nevada, northern Coast and Northern Traverse Ranges. It is still common in the San Francisco Bay area and along the central coast. It is now believed to be extirpated from the southern Transverse and Peninsular Ranges (USFWS 2002).
Habitat
There are four primary constituent elements (PCEs) that are considered to be essential for the conservation or survival of a species. The PCEs for the California Red-legged Frog include: aquatic breeding habitat; non-breeding aquatic habitat; upland habitat; and dispersal habitat (USFWS 2006).
Aquatic breeding habitat consists of low-gradient fresh water bodies including natural and manmade (e.g., stock) ponds and pools in perennial streams (Jennings and Hayes 1994) marshes, lagoons, and dune ponds. Aquatic breeding habitat must hold water for a minimum of 20 weeks in most years. This is the average amount of time needed for egg, larvae, and tadpole development and metamorphosis so that juveniles can become capable of surviving in upland habitats (USFWS 2006). Optimal habitat is characterized by dense, shrubby riparian vegetation associated with deep (>2.3 feet), still, or slow-moving water (Hayes and Jennings 1988, Jennings 1988). Arroyo willow (Salix lasiolepis) seems to provide the most suitable riparian habitat structurally, although cattails and bulrushes also can provide suitable habitat (Jennings 1988). Although California Red-legged Frog are found in ephemeral streams and ponds, populations cannot be maintained where all surface water disappears (Jennings and Hayes 1994).
Aquatic non-breeding habitat may or may not hold water long enough for this species to hatch and complete its aquatic life cycle, but it provides shelter, foraging, predator avoidance, and aquatic dispersal for juvenile and adult California Red-legged Frog. These waterbodies include plunge pools within intermittent creeks; seeps; quiet water refugia during high water flows; and springs of sufficient flow to withstand the summer dry period. California Red-legged Frog can use large cracks in the bottom of dried ponds as refugia to maintain moisture and avoid heat and solar exposure (Alvarez 2004). Non-breeding aquatic features enable California Red-legged Frog to survive drought periods, and disperse to other aquatic breeding habitat (USFWS 2006).
Upland habitats include areas within 200 feet of aquatic and riparian habitat and are comprised of grasslands, woodlands, and/or vegetation that provides shelter, forage, and predator avoidance. These upland features provide breeding, non-breeding, feeding, and sheltering habitat for juvenile and adult frogs (e.g., shelter, shade, moisture, cooler temperatures, a prey base, foraging opportunities, and areas for predator avoidance). Upland habitat can include structural features such as boulders, rocks and organic debris (e.g. downed trees, logs), as well as small mammal burrows and moist leaf litter (USFWS 2006).
Dispersal Habitat includes accessible upland or riparian habitats between occupied locations within 0.7 mi of each other that allow for movement between these sites. Dispersal habitat includes various natural and altered habitats such as agricultural fields, which do not contain barriers to dispersal. Moderate to high density urban or industrial developments, large reservoirs and heavily traveled roads without bridges or culverts are considered barriers to dispersal (USFWS 2006). Short-distance dispersal movements are generally straight-line movements (Bulger et al. 2003). Overland dispersal movements through upland habitats typically occur at night during wet weather (USFWS 2002, Bulger et al. 2003, Fellers and Kleeman 2007). During dry weather, California Red-legged Frog tend to remain very close to a water source however, overland dispersal may occur in response to receding water (USFWS 2002). California Red-legged Frog have been documented to disperse up to 2.9 km (1.8 mi) (Fellers and Kleeman 2007).
Reproduction
Breeding takes place at night in permanent ponds or the slack water pools of streams during the winter and early spring (late November-through April) (Storer 1925, USFWS 2002). Males usually appear at the breeding sites 2 to 4 weeks before females who are attracted to calling males. Females lay egg masses containing about 2,000 to 5,000 eggs, which hatch in 6 to 14 days, depending on water temperatures (USFWS 2002). Larvae metamorphose in 3.5 to 7 months, typically between July and September (Storer 1925, Wright and Wright 1949, USFWS 2002). Juvenile frogs are about one inch (25.4 millimeters) long at metamorphosis and commonly sun themselves during the day at the edge of the riparian zone next to the breeding site. As they grow, they gradually shift from diurnal and nocturnal periods of activity, to largely nocturnal activity (Hayes and Tennant 1986). During periods of rainfall, both juveniles and a few adults may disperse away from breeding sites and may be found some distance (up to 0.5 mile) away from the nearest water (Jennings, unpubl. data). Sexual maturity is usually attained by males at 2 years of age and females at 3 years of age. However, during extended periods of drought, frogs may take 3-4 years to reach sexual maturity (Jennings and Hayes 1985).
Diet
Adult California Red-legged Frog consume a wide variety of prey including aquatic and terrestrial insects, crustaceans and snails (Stebbins 1985, Hayes and Tennant 1986), as well as worms, fish, tadpoles, smaller frogs (e.g. Pseudacris sierrae), and occasionally small mammals (USFWS 2002). Aquatic larvae are herbivorous, grazing on algae (Jennings et al. in litt. 1992). Feeding generally occurs along the shoreline of ponds or other watercourses and on the water surface.
Conservation
California Red-legged Frog was listed as Federally Threatened May 23, 1996 (61 FR 25813-25833). Critical Habitat for the California Red-legged Frog was designated on March 17, 2010 (75 FR 12815 12959). A Recovery Plan for the California Red-legged Frog was published by the USFWS on May 28, 2002.
California Red-legged Frog populations are threatened by numerous human activities that often act synergistically and cumulatively with natural disturbances (i.e. droughts or floods) (USFWS 2002). Human activities which negatively affect California Red-legged Frog include; agriculture, urbanization, mining, overgrazing, recreation, timber harvesting, nonnative plants, impoundments, water diversions, degraded water quality, and introduced predators.
California Red-legged Frog is infrequent or absent in habitats where introduced aquatic predators such as green sunfish (Lepomis cyanellus), Louisiana red-swamp crayfish (Procambarus clarkii) and bullfrogs (Lithobates catesbeiana) are present (Hayes and Jennings 1986, 1988), probably because the larval stages are susceptible to such predators (Jennings and Hayes 1994).
Over 90 percent of the historic wetlands in the Central Valley have been lost due to conversion for agriculture or urban development (USFWS 1978, Dahl 1990). This has resulted in a significant loss of frog habitat throughout the species' range (USFWS 2002). Habitat along many stream courses has also been isolated and fragmented, resulting in reduced connectivity between populations and lowered dispersal opportunities.
Isolated populations are now more vulnerable to extinction through stochastic environmental events (i.e. drought, floods) and human-caused impacts (i.e., grazing disturbance, contaminant spills) (Soulé 1998). Isolated populations suffer from increased predation by nonnative predators, changes in hydroperiod due to variable wastewater outflows, and increased potential for toxic runoff.
References
Bulger J. B., J. S. Norman, And R. B. Seymour. 2003. Terrestrial Activity and Conservation Of Adult California Red-Legged Frogs Rana aurora draytonii In Coastal Forests And Grasslands. Biological Conservation, 110:85.-95.
Dahl, T. E. 1990. Wetland losses in the United States, 1978s to 1980s. U.S. Fish and Wildlife Service, Washington, DC.
Hayes, M. P. and M. R. Jennings. 1986. Decline of ranid frog species in western North America: Are bullfrogs (Rana catesbeiana) responsible? Journal of Herpetology 20(4):490-509.
Hayes, M. P. and D. M. Krempels. 1986. Vocal sac variation among frogs of the genus Rana from western North America. Copeia 1986(4):927-936.
Hayes, M. P., and M. R. Jennings. 1988. Habitat correlates of distribution of the California Red-legged Frog (Rana aurora draytonii) and the Foothill Yellow-legged Frog (Rana boylii): implications for management. Pages 144-158 In: R. C. Szaro, K. E. Severson, and D. R. Patton (technical coordinators). Management of Amphibians, Reptiles, and Small Mammals in North America. Proceedings of the Symposium, July 19-21, 1988, Flagstaff, Arizona. U.S. Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado. General Technical Report (RM-166):1-458.
Hayes, M. P. and M. R. Tennant. 1985. Diet and feeding behavior of the California Red-legged Frog Rana aurora draytonii (Ranidae). The Southwestern Naturalist 30(4):601-605.
Jennings, M. R. 1988. Natural history and decline of native ranids in California. Pages 61-72 In: H. F. De Lisle, P. R. Brown, B. Kaufman, and B. McGurty (editors). Proceedings of the Conference on California Herpetology. Southwestern Herpetologists Society, Special Publication (4):1-143.
Jennings, M. R. and M. P. Hayes. 1985. Pre- 1900 overharvest of California Red-legged Frogs (Rana aurora draytonii): The inducement for bullfrog (Rana catesbeiana) introduction. Herpetological Review 32(1):94-103.
Rathbun, G. B. and T. G. Murphy. 1996. Evaluation of a radio-belt for ranid frogs. Herpetological Review 27(4):197-189.
Shaffer, H. Bradley, G. M. Fellers, S. Randal Voss, J. C. Olive and Gregory B. Pauly (2004 Species boundaries, phylogeography and conservation genetics of the red-legged frog (Rana aurora/draytonii) complex. Molecular Ecology 13(9): 2667-2677)
Soulé, M. 1998. Viable Populations for Conservation. Cambridge University Press, Great Britain.
Stebbins, R. C. 2003. A Field Guide to Western Reptiles and Amphibians. 3rd Edition, revised. Houghton Mifflin Book Co., Boston. New York, NY.
Storer, T. I. 1925. A synopsis of the amphibia of California. University of California Publications in Zoology 27:1-342.
U. S. Fish and Wildlife Service (USFWS). 1978. Concept plan for waterfowl wintering habitat preservation, Central Valley, California. Region 1, Portland, OR.
U. S. Fish and Wildlife Service (USFWS). 1996. Endangered and threatened wildlife and plants: determination of threatened status for the California Red-legged Frog. Federal Register 61(101):25813-25833. May 23.
U. S. Fish and Wildlife Service (USFWS). 2002. Recovery plan for the California Red-legged Frog (Rana aurora draytonii). U.S. Fish and Wildlife Service, Portland, OR.
U. S. Fish and Wildlife Service (USFWS). 2006. Designation of Critical Habitat for the California Red-Legged Frog, and Special Rule Exemption Associated With Final Listing for Existing Routine Ranching Activities; Final Rule. Federal Register 71(71): 19243-19346. April 13.
Wright, A. H., and A. A. Wright. 1949. Handbook of frogs and toads of the United States and Canada. Comstock Publishing Company, Ithaca, N.Y.
Classification
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Rana - Brown Frogs
- Rana draytonii - California Red-legged Frog
