Species: Mountain yellow-legged Frog Rana muscosa
Family: Ranidae Order: Anura Class: Amphibia
Species note author: Carlos Davidson
Date: March 1996.
The mountain yellow-legged frog (Rana muscosa) is a medium sized ranid frog native to the high Sierra Nevada and southern California transverse range mountains. The frog is a California Species of Special Concern and was a federal category 2 candidate for listing under the Endangered Species Act (before the Fish and Wildlife Service abolished the category 2 candidates).
I. Natural History
Taxonomy. The mountain yellow-legged frog was original separated from the foothills yellow-legged frog (Rana boylii) by Zweifel (1955). There are morphological differences between populations of mountain yellow-legged frogs in the Sierra Nevada, and in Southern California (Zweifel 1955). There may also be call differences between the two populations (Tim Zismer per comm, Davidson 1995). Genetics work underway will likely support treating southern California populations as a distinct species (Mark Jennings, per comm). This paper will focus exclusively on mountain yellow-legged frogs in the Sierra Nevada.
Distribution. Rana muscosa is a near endemic to California, occurring outside the state only around Lake Tahoe in Nevada (Jennings and Hayes 1995). The historic range of the species is down the Sierra spine from near La Porte (Plumas County), south to Taylor and French Joe Meadows (Tulare County) with disjunct populations to the North in Butte County, and a possible single historic record to the south in Kern County (Jennings and Hayes 1995). Elevations range from 1370 meters to over 3650 meters (Zweifel 1955, Mullally and Cunningham 1956).
Habitat. Rana muscosa lives in high mountains lakes, ponds, tarns and steams. It prefers open shorelines that gently slope up to shallows of a few inches (Mullally and Cunningham 1956). Shallows are used by tadpoles to absorb heat, and are used by adults as oviposition sites (Jennings and Hayes 1995). In the few lakes where frogs and introduced fish co-occur shallows may provide refuges for the frogs.
Population dynamics. Almost no data exists on natural fluctuations of population size or population age structure (Bradford 1991). Similarly time for egg development, time to sexual maturity and adult life span are unknown (Jennings and Hayes 1995). Eggs are laid right after the ice clears from lakes in clusters of about 200-300 eggs (Zweifel 1955). Larvae are reported to overwinter at least once and often twice before metamorphosis (Cory 1962, Bradford 1983). Roland Knapp (per comm) has observed cases of a third and possibly forth overwintering by larvae.
Adults and tadpoles overwinter in lakes, and therefore require lakes deep enough not to freeze solid. Adults in shallow lakes are also susceptible to winterkill due to oxygen depletion. Larvae, however, can withstand several months in nearly anoxic conditions (Bradford 1983). In 1978 winterkill was responsible for the mortality of all adults (except a single individual) in 21 of 26 lakes surveyed, while tadpoles survived in all lakes (Bradford 1983).
Bradford (1991) observed a large scale die off in a lake due to red-legged disease (caused by the bacterium Aeromonas hydrophila), and another die off due to predation. Large population fluctuations in Rana muscosa are probably common due to predation, disease, or winterkills. However each of these causes is concentrated on only a single age class (predation on metamorphs, and disease and winterkill mainly on adults).
Movement/Migration. Almost no data exists on dispersal for mountain yellow-legged frogs (Bradford 1991). Juveniles have been observed in small intermittent streams, and may be dispersing to permanent water (Bradford 1991). Frogs may avoid cross even short distances of dry ground (Mullally and Cunningham 1956).
Feeding. Diet of adults consists of beetles, flies (Diptera), ants, bees (Apoidea), wasps (Hymenoptera and true bugs (Hemiptera: Jennings and Hayes 1991).
Predators. Bradford (1991) reported predation by Brewers Blackbirds (Euphagus cyanocephalus) killed all metamorphs of the year at a single lake. Coyote (Canis latrans) and western terrestrial garter snakes are known to prey on tadpoles and postmetamorphs (Jennings and Hayes 1995). Introduced fish (trout Salmo spp. and charr Salvelinus spp.) have been observed to prey on mountain yellow-legged frogs, and the two seldom occur together (Grinnel and Storer 1924, Bradford et al. 1993, Knapp in press).
II. Status
A significant number of populations of Rana muscosa have disappeared from the Sierra Nevada, but the exact extent of decline is unclear due to the lack of systematic surveys (Jennings and Hayes 1995). In a Sierra wide survey of historic Rana muscosa sites, Bradford et al. (1994a) found populations in the western portion of Sequoia National Park (Kaweah River drainage) had largely disappeared, while extant populations remained else where in Sequoia and Kings Canyon parks (Kern, Kings, and San Joaquin River drainages). Throughout the species range, outside Sequoia and Kings Canyon Parks a 1988-1991 resurvey of 24 historic sites from 1955-1979 found frogs present at only 3 sites (Bradford et al. 1994a). Since 1970 few frogs have been observed in the extreme northern part of the species' range (Butte and Plumas counties) (Jennings and Hayes 1995).
III. Possible Causes Of Decline
1. Introduced fish
The impact of introduced fish has emerged as one of the leading hypothesis to explain declines of ranid frogs in the western U.S. (Hayes and Jennings 1986, Fisher and Shaffer in press, Knapp in press). Predation on frogs by introduced fish is well documented (Needham and Vestal 1938, Bradford 1989). A number of recent studies in the Sierra Nevada and Central Valley found that native amphibians are largely absent from sites with introduced fish, and conversely sites with amphibians present seldom have introduced fish (Zardus 1977, Bradford 1989, Jennings and Hayes 1986, Bradford et al. 1993, Fisher and Shaffer in press). Non-native salmonids (trout Salmo spp. and charr Salvelinus spp.) have been introduced throughout the Sierra in lakes and streams that were historically fishless (Christenson 1977, Knapp in press). . Jennings and Hayes (1986) argued that direct predation by fish may be responsible for the decline of amphibians in the Sierra foothills and Central Valley.
As early as 1924 Grinnell and Storer (1924) noted that mountain yellow-legged frogs tended not to occur in lakes with fish. But mountains yellow-legged frogs have declined in fishless lakes in the last 30 years, arguing against direct fish predation as the cause of decline (Bradford 1991, Bradford et al. 1993, Bradford 1994a). For example, return surveys in 1989 of 27 fishless sites which had Rana muscosa populations in 1978-79 and found frogs present at one site (Bradford et. al 1994a). The resurveys found no significant change in abundance of Pseudacris regilla.
Bradford et al. (1993) showed that the spatial distribution of yellow-legged frogs has become fragmented due to the presence of introduced trout in streams that may have once served as dispersal and recolonization routes. Predation by introduced fish may have eliminated frogs from many larger lakes, and made remaining smaller populations vulnerable to local extinction by preventing dispersal and recolonization. This scenario assumes that mountain yellow-legged frogs have metapopulation or source-sink population dynamics (Sjogren 1994), yet to date this has not been investigated. Similarly the dispersal behavior of yellow-legged frogs is unknown, and therefore it is unclear if the spatial isolation observed by Bradford et al. (1993) is biologically relevant.
The allopatric distribution of amphibians and introduced fish may be the result of habitat alteration that favors fish rather than predation excluding frogs from sites with fish (Jennings and Hayes 1986, Fisher and Shaffer in press). For example in the Central Valley, draining of marshes and the creation of permanent warm bodies of water may have created excellent habitat for introduced fish, and unsuitable habitat for some native amphibians. To date, there has been little investigating of habitat differences between Sierra lakes with frogs and lakes without frogs.
2. Airborne contaminants
The decline of mountain yellow-legged frogs and Yosemite toads (Bufo canorus) in apparently "pristine" habitat inside national parks has raised the possibility that airborne contaminants may be responsible (Drost and Fellers 1994, Cory 1994). It has been shown that air currents in the San Joaquin valley are capable of carrying pesticides up into the Sierras. Cory (1994) points to greater declines in the western and southern Sierra and suggests that heavy spraying of the chemical Mollinate in the mid-1970s in the San Joaquin Valley may be responsible.
3. Increased UV-B radiation
The hypothesis that amphibian declines may be caused by increases in UV-B radiation due to ozone thinning is consistent with the apparent global nature of declines (Wake 1991, Blaustein and Wake 1990) and that many declines have taken place in "pristine" high mountain habitat (e.g., Boreal toads (Bufo boreas boreas) in the Rocky Mountains (Carey 1993), the Yosemite Toad and Rana muscosa in the Sierra Nevada (Kagarise Sherman and Morton 1993, Bradford et al. 1994a)).
Blaustein et. al (1994) found reduced hatching success for boreal toads and Cascades frog (Rana cascadae) eggs exposed to UV-B light (from roughly 80 percent hatchling success with UV-B filtered out to 60% without the filter). It is unclear if the observed 25 percent reductions in hatching success are sufficient to cause population declines in species with such high fecundity. No differences between shielded and unshielded eggs were found for Pacific treefrogs (Pseudacris regilla). The three species also were found to have different levels of photolayse activity, a DNA repair enzyme, with the treefrogs having higher levels than either the boreal toad or Cascades frog. Long et. al (1995) found synergistic affects of UV-B light and low pH on egg development in Rana pipiens. To date, no UV-B work has been done for Rana muscosa.
4. Acid deposition
Bradford et al. (1992) examined Rana muscosa egg and tadpole tolerances to low pH in the laboratory and compared the tolerances to the peak observed acidity in Sierra lakes of pH 5.0. They found that down to a pH of 5.0, there were no differences in egg development or tadpole survival (although there may have been sublethal stresses). In a related study water chemistry was compared at sites with frogs and apparently suitable sites without frogs, and neither pH nor acid neutralizing capacity was found to be statistically different (Bradford et. al 1994b). Therefore acid deposition is not a likely cause of the decline of Rana muscosa.
5. Climate change
The 1987-1992 drought, which was severe even by historic standards, has been discounted as a cause of decline because some declines preceded the drought (Bradford et. al 1994), and Rana muscosa primarily uses permanent bodies of water (Zweifel 1955, Mullally and Cunningham 1956). Drought may act synergistically with other factors. For example, Pound and Crump (1994) hypothesize that the disappearance of the golden toads (Bufo perigienes) in Costa Rica may have been due to the combination of airborne contaminants and drought with resulted in higher toxic concentrations.
6. Research activity
Researchers may affect populations by handling animals, toe-clipping or introducing pathogens. However in the Tablelands area where Bradford et al. (1994a) found complete disappearance of frogs, the visual surveys used involved no contact with animals or water.
IV. Viability
Many small populations of Rana muscosa may be subject to demographic and genetic factors affecting viability, however where the species occurs it not infrequently occurs in large numbers (Knapp per comm) and therefore it seems unlikely that demographic and genetic factors are critical for overall species viability. Even in small populations, environmental stochasticity in the form of winter kills, disease or predation may be more important than demographic and genetic factors. Without information on dispersal it is not possible to assess the importance of metapopulation dynamics to species viability. Given the rapid and unexplained declines, viability for Rana muscosa is probably contingent on processes that we do not yet understand such as toxicants, UV-B radiation, or the interruption of metapopulation dynamics by introduced fish.
Literature Cited
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