Rana aurora Baird and Girard, 1852(b)
Northern Red-Legged Frog
Christopher A. Pearl1
1. Historical versus Current Distribution. Biochemical, ecological, and life-history differences between northern red-legged frogs (historically known as Rana aurora aurora) and California red-legged frogs (historically known as Rana aurora draytonii) support recognition of these taxa as distinct species (Hayes and Miyamoto, 1984; H.B. Shaffer and G.M. Fellers, in preparation). Northern red-legged frogs occur along the California coast north of Elk Creek (Mendocino County) into southwestern Oregon, where their historical range broadens eastward through the Rogue River drainage into the lower elevations of the Cascade Range (Fitch, 1936; Dunlap, 1955; Dumas, 1966; Nussbaum et al., 1983; Jennings and Hayes, 1994a; G.M. Fellers, personal communication). Northern red-legged frogs utilize wetlands between sea level and 1,200 m in elevation, west of the Cascade crest through northwestern Oregon, western Washington, and southwestern British Columbia. Northernmost populations occur near the northern end of Vancouver Island and Sullivan Bay, British Columbia (Green and Campbell, 1984; Stebbins, 1985). Oregon’s Willamette Valley (and potentially the Rogue Valley) appear to be the most reduced and fragmented portions of the range, potentially the result of intensive land use and establishment of a variety of non-native predators (Nussbaum et al., 1983; St. John, 1987; Blaustein and Wake, 1990).
2. Historical versus Current Abundance. Several herpetologists have suggested that the abundance of northern red-legged frogs in Oregon’s Willamette Valley has declined (Nussbaum et al., 1983; St. John, 1987; Blaustein and Wake, 1990). Recent surveys suggest reproductive populations remain on much of the valley floor (C. A. P., Oregon Department of Fish and Wildlife, unpublished data). Northern red-legged frogs are relatively widespread in portions of western Washington (K.R. McAllister et al., 1993; Richter and Azous, 1995; Adams et al., 1999), although analysis of present occurrence at historical sites has not been conducted. They also were widespread historically in southwestern British Columbia and remain common in at least segments of that range (R. Haycock, S. Orchard, personal communications).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Northern red-legged frogs often make extensive movements to breeding wetlands from summer habitats (Nussbaum et al., 1983; Hayes et al., 2001). Males generally reach breeding sites before females, sometimes as early as October, but usually arrive in larger numbers in November–December in Oregon and northern California (Storm and Pimentel, 1954; Storm, 1960; Twedt, 1993).
ii. Breeding habitat. Oviposition generally occurs in vegetated shallows of wetlands with little flow (Storm, 1960; Licht, 1971), but egg masses can be deposited in water up to 5 m (Calef, 1973b). Breeding sites can be permanent or temporary, with inundation usually necessary into June for successful metamorphosis in the Willamette Valley (Storm, 1960; Nussbaum et al., 1983). Breeding is initiated when water temperatures exceed 6–7 ˚C (usually in January), and can extend through March (Storm, 1960; personal observations).
i. Egg deposition sites. Egg masses are usually attached to herbaceous vegetation in areas with little or no flow (Storm, 1960; Calef, 1973b).
ii. Clutch size. Females deposit an average of 530–830 eggs/mass, with a range between 200–1,100 (Storm, 1960; Calef, 1973a; Licht, 1974).
i. Length of larval stage. In Oregon and Washington, eggs generally hatch after about 30–45 d (often in March–April) and reach metamorphosis 11–14 wk later in June–July (Storm, 1960; Calef, 1973a; Licht, 1974; H.A. Brown, 1975b).
ii. Larval requirements.
a. Food. Larvae consume a variety of epiphytic algae and can alter species composition and standing crop in laboratory conditions (Dickman, 1968).
b. Cover. Larvae appear to utilize relatively dense vegetation as cover (Nussbaum et al., 1983; personal observations). Weins (1970) found that larvae became conditioned to prefer microhabitats possessing complex background patterns in laboratory trials, potentially suggesting an affinity for vegetated areas.
iii. Larval polymorphisms. Not reported.
iv. Features of metamorphosis. Approximately 5% of embryos survived to metamorphosis at two British Columbia breeding sites (Calef, 1973a; Licht, 1974). Northern red-legged frogs generally transform at 20–25 mm SVL (Storm, 1960; Calef, 1973a). At a northwestern Washington site, developmental periods for larvae averaged 110 d; mean size at metamorphosis was 28.7 mm (H.A. Brown, 1975b).
v. Post-metamorphic migrations. Juveniles often remain around edges of breeding ponds for short periods (days to weeks) before dispersing, but cues for emigration are not well known (Licht, 1986; Twedt, 1993). Dispersal distances of newly transformed juveniles appear to be related to size at metamorphosis (N. Chelgren, Oregon State University, unpublished data). In the fall, juveniles have been observed in riparian areas > 0.5 km from nearest known breeding site (C.A.P. and M. Hayes, personal observations).
D. Juvenile Habitat. After dispersing from breeding habitats, juveniles tend to occupy relatively moist, densely vegetated riparian microhabitats during the summer (Twedt, 1993; personal observations). Movements away from these microenvironments may be related to elevated moisture levels (Licht, 1986). Hayes and Hayes (2003) have recently documented juvenile growth rates.
E. Adult Habitat. Most northern red-legged frog adults appear to leave breeding sites relatively soon after the breeding period and may move substantial distances (> 300 m) from breeding pools in mesic forests and riparian areas (Nussbaum et al., 1983; Licht, 1986; Gomez and Anthony, 1996; Hayes et al., 2001). Summer habitats of adults in the mid elevations of the Oregon Cascade range include streambanks and moist riparian areas (Hayes et al., 2001; personal observations). At one northern California breeding lagoon, adults tended to use microhabitats adjacent to standing water rather than remaining in standing water (Twedt, 1993).
Greater numbers of adult northern red-legged frogs have been trapped in coniferous stands of moderate moisture than in drier stands in the Oregon and Washington Cascades (Aubry and Hall, 1991; Bury et al., 1991). One study found red-legged frog adults more frequently in older managed forest stands (Aubry, 2000), but other terrestrial studies have not documented clear preferences for any stand age in managed and unmanaged forests (Aubry and Hall, 1991; Bury et al., 1991; Bosakowski, 1999). Conclusions of the aforementioned terrestrial studies are limited by low captures in pitfall traps and variable juxtaposition of sampled stands relative to breeding sites.
F. Home Range Size. Unknown, but adults are wide ranging (see "Adult Habitat" and "Breeding migrations" above).
G. Territories. Northern red-legged frogs generally are not considered territorial, but males can act aggressively toward one another at breeding sites, and egg masses are often deposited in a dispersed fashion, in contrast to other northwestern lentic ranid frogs (Calef, 1973b; Nussbaum et al., 1983; personal observation).
H. Aestivation/Avoiding Dessication. Not documented.
I. Seasonal Migrations. See "Breeding migrations" and "Post-metamorphic migrations" above.
J. Torpor (Hibernation). Unknown, but likely in northern ranges and higher elevations. Adults in southern and coastal ranges can remain active through winter (Nussbaum et al., 1983; Twedt, 1993).
K. Interspecific Associations/Exclusions. Northern red-legged frogs often share breeding sites with northwestern salamanders (Ambystoma gracile), long-toed salamanders (A. macrodactylum), Pacific chorus frogs (Hyla regilla), rough-skinned newts (Taricha granulosa), and introduced American bullfrogs (Rana catesbeiana). In moderate elevations of the Cascade Range (about 600–1000 m), northern red-legged frogs may share breeding ponds and co-occur along streams with Cascade frogs (Rana cascadae; Nussbaum et al., 1983; Hayes, 1996; B. Bury and D. Major, unpublished data). Historically, northern red-legged frogs occurred syntopically with Oregon spotted frogs (Rana pretiosa) in lowland western Oregon, Washington, and southwestern British Columbia (Licht, 1971, 1974, 1986; Nussbaum et al., 1983; K.R. McAllister et al., 1993). Negative interactions have been suggested between northern red-legged frogs and introduced bullfrogs and sport-fish in Oregon (Hayes and Jennings, 1986; Kiesecker and Blaustein, 1997a, 1998).
L. Age/Size at Reproductive Maturity. Males can become sexually mature the breeding season following metamorphosis, but the majority breed only after 2 yr of age (Licht, 1974; see also Hayes and Hayes, 2003). Females usually reproduce after they reach 3 yr old, although a small portion may be able to breed in the second season after transforming (Licht, 1974).
M. Longevity. Poorly known in field situations, but reportedly can exceed 10 yr in captivity (Cowan, 1941).
N. Feeding Behavior. Northern red-legged frogs consume a variety of small insects, arachnids, and mollusks (Fitch, 1936; Licht, 1986). Larger adults are able to take larger food items, including juvenile conspecifics and salamanders (Licht, 1986; Rabinowe et al., 2002). Young frogs are believed to forage in moist areas close to water, pursuing food farther away only during wet periods (Licht, 1986).
O. Predators. Larval northern red-legged frogs are eaten by fish, rough-skinned newts, northwestern salamanders, giant water bugs (Belostomatidae), larval diving beetles (Dytiscidae), and anisopteran odonates (Calef, 1973a; Licht, 1974). Garter snakes (particularly common garter snakes [Thamnophis sirtalis]) and adult northern red-legged frogs consume juvenile frogs, and herons and raccoons also prey on adults (Licht, 1974, 1986; Gregory, 1979; personal observations). Predation by introduced game-fish and American bullfrogs may represent important threats to northern red-legged frogs throughout their range (Hayes and Jennings, 1986; Twedt, 1993; Kiesecker and Blaustein, 1997a, 1998).
P. Anti-Predator Mechanisms. Tadpoles reduce activity when exposed to chemical cues of injured conspecifics in lab trials (Wilson and Lefcort, 1993), and ammonium (NH4+) may be a component of exudates from disturbed larvae (Kiesecker et al., 1999).
Larval northern red-legged frogs reduced activity and distanced themselves from native odonate predators in lab trials and were larger at metamorphosis when the predator was present (Barnett and Richardson, 2002). A laboratory study found that northern red-legged frog tadpoles reared in the presence of predators fed conspecifics or cues of injured conspecifics transformed earlier and smaller than controls (Kiesecker et al., 2002). One field study did not detect northern red-legged frog tadpole avoidance of injured conspecifics (Adams and Claeson, 1998).
Adults possess long rear legs and exceptional leaping ability, which, in tandem with cryptic coloration and stationary evasion behavior, can make juveniles and adults difficult for terrestrial predators to apprehend (Gregory, 1979).
Q. Diseases. An iridovirus has been reported from northern California (Mao et al., 1999). Lefcort and Blaustein (1995) reported altered behavior when larvae were exposed to the yeast Candida humicola, which may be transmitted through water and fecal material (Richards, 1958).
R. Parasites. The trematodes Megalodiscus microphagus and Prosthopycoides lynchi have been documented in the digestive tracts of Oregon northern red-legged frogs (Macy, 1960; Martin, 1966a). Johnson et al. (2002) report moderate levels of morphological abnormalities in juvenile northern red-legged frogs relative to other Pacific northwestern amphibians. These malformations potentially are associated with infection by the trematode Ribeiroia ondatrae (Johnson et al., 2002).
4. Conservation. Thorough field studies documenting declines in northern red-legged frogs are lacking, but a broad array of potential stressors may affect this species. Non-native American bullfrogs are established throughout much of the lowland range of northern red-legged frogs west of the Cascades, and bullfrogs have been hypothesized as displacing northern red-legged frogs here (Nussbaum et al., 1983; St. John, 1987; Kiesecker and Blaustein, 1997a, 1998). In Oregon, northern red-legged frog larvae have been found to compete poorly with bullfrog larvae when food resources are concentrated (Kiesecker and Blaustein, 1998; Kiesecker et al., 2001b). However, other studies conducted in Washington have failed to identify direct effects of competition on northern red-legged frog larvae or exclusion from wetlands supporting bullfrogs (Richter and Azous, 1995; Adams, 2000). Negative associations of nonnative fish appear to be of greater importance (Adams, 1999, 2000), and the interactive effects of fishes and bullfrogs may be greater than either separately (Kiesecker and Blaustein, 1998).
Water quality degradation and altered hydrological regimes associated with agricultural and urban land uses are also of concern in the Puget Trough and Willamette Valley (Nussbaum et al., 1983; Platin, 1994; Richter and Azous, 1995; De Solla et al., 2002b). However, northern red-legged frogs persist in some urbanized habitats in the region (Richter and Azous, 1995, 2000). Laboratory investigations of nitrogenous by-products from agricultural fertilizers suggest northern red-legged frog larvae are intermediate among Willamette Valley amphibians in their sensitivity to ammonium sulfate, but may be susceptible to ammonium ions derived from related compounds (Schuytema and Nebeker, 1999b; Nebeker and Schuytema, 2000). Larval northern red-legged frogs are relatively susceptible to nitrite, a form that is usually short-lived in aerobic field conditions (Marco et al., 1999). De Solla et al (2002a) found organochlorine pesticides and PCB residues in eggs of northern red-legged frogs, but levels were not high relative to reference sites. Hatching rates of northern red-legged frogs in their lowland range do not appear to be reduced by ambient UV-B radiation, although embryos may be susceptible to future increases in UV-B irradiance (Blaustein et al., 1996; Ovaska et al., 1997; Belden and Blaustein, 2002a). Modeling studies suggest stressors that impact juvenile red-legged frogs have the greatest potential to influence population fluctuations (Biek et al., 2002).
Northern red-legged frogs are considered a Species of Special Concern in California (California Department of Fish and Game, 1999), Sensitive-Vulnerable in Oregon’s Willamette Valley, and Sensitive-Unknown elsewhere in Oregon (Oregon Natural Heritage Program, 1995).
1Christopher A. Pearl
USGS Forest and Rangeland Ecosystem Science Center
3200 SW Jefferson Way
Corvallis, Oregon 97331
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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