Rana boylii

MtDNA data suggest that R. aurora, R. cascadae, and R. muscosa form a clade within the R. boylii species group (Macey et al. 2001). Crother (2017) state that molecular study of geographic variation of this rapidly disappearing species should prove illuminating.

There is substantial evidence that the foothill yellow-legged frog is biogeographically divided into multiple clades with little or no gene flow between the clades. Earlier studies provided strong evidence that there are deep genetic divisions in this taxon (Dever 2007, Lind et al. 2011, Peek 2010). Subsequent, more in-depth and largerscale genetic studies (McCartney-Melstead et al. 2018, Peek 2018) confirmed the certainty and depth of the phylogenetic (evolutionary history) structural divisions of the foothill yellow-legged frog using population genomics (comparison of DNA sequences of populations) (USFWS 2021).

This species has a fairly large range in California and western Oregon. The population suffered steep declines starting in the 1970s and continues to decline at a slower pace. It is threatened by habitat alteration (especially that caused by dams), impacts of airborne agrochemicals, the effects of invasive species, infection with the chytrid fungus, and because recolonization abilities may be greatly restricted by local extirpation patterns.

This species is endemic to the western United States in southern Oregon and California. The current range includes Pacific drainages from Linn County, Oregon south to Tulare County, California (Dodd 2023, GBIF 2025). Using Global Biodiversity Information Facility (GBIF) (2025) records from 2000-2025, range extent is estimated to be 259,077 km² (RARECAT 2025).

The historical range extended farther north to the upper reaches of the Willamette River system, Oregon (west of the Cascades crest) and south to the upper San Gabriel River, Los Angeles County, California (Stebbins 2003, Dodd 2023). The species occurred at least formerly in a disjunct location in northern Baja California. Two specimens (identified by R. C. Stebbins and R. G. Zwiefel) were collected in 1965 at an elevation of 2,040 meters at the lower end of La Grulla Meadow, Sierra San Pedro Martir, Baja California, Mexico (Loomis 1965). However, subsequent searches have not detected the species in that area (Grismer 2002, Stebbins 2003). The species apparently has disappeared from portions of the historical range, especially in western Oregon (Frost 2020) and southern California (see Hayes and Jennings 1988).

This species is represented by a large number of extant occurrences (subpopulations). It has been extensively searched for and commonly found in much of the northern half of the range. Since 1993, it has been found at more than 200 sites in California (Fellers 2005).

Major threats include habitat loss and fragmentation and alteration of natural flow regimes as a result of dam construction (http://www.fs.fed.us/psw/topics/wildlife/herp/rana_boylii/), introduced incompatible aquatic animals, chytrid fungus, aerial drift of pesticides, riverine and riparian impacts of non-selective logging practices, and other habitat degradation and disturbance caused by livestock grazing and in-stream mining. Climate change presumably will have negative effects on this species if it results in increased stream dessication and reduced availability of suitable habitat (Dodd 2023).

On the main stem of the Trinity River, northern California, unnatural flow regimes and loss of habitat caused by dam construction are the greatest threats (Ashton et al. 1997). Potential breeding habitat was reduced by 94% after dam construction (Lind et al. 1996). Controlled flows allowed encroachment of riparian vegetation and retarded cobble/gravel bar formation. Since dam construction, water releases have been reduced to 10-30% of pre-dam flows, based on both total yearly volume and magnitude of periodic high flows (Lind et al. 1996). High flow releases from dams in late spring sometimes result in scouring of egg masses, whereas receding high flows, if poorly timed, can leave egg masses "high and dry" (Lind et al. 1996). Source: Ashton et al. (1997).

Adults, larvae, and/or eggs are vulnerable to an array of non-native predators such as predatory fishes, bullfrogs, and crayfish (Lind et al. 1996, Kupferberg 1996, Ashton et al. 1997, Lind et al. 2003, Fellers 2005, Paoletti et al. 2011, Adams et al. 2017), but the population effects of these species and of potential non-native competitors are not well known. Dam-controlled flows and lack of winter flooding may result in stable pool areas with established aquatic vegetation (Lind et al. 1996, Kupferberg 1996), and this may increase suitable habitat for exotic species such as bullfrogs (Ashton et al. 1997). Decreased flows may force frogs into permanent pools where they are more susceptible to predation (Hayes and Jennings 1988). Kupferberg (UC Berkeley) found that bullfrog larvae perturbed aquatic community structure and exerted detrimental effects on Rana boylii populations in northern California but had only a slight impact on Pseudacris regilla (Froglog, September 1993). Interspecific matings between male R. boylii and female bullfrogs have been observed; these interactions with non-native bullfrogs might reduce the reproductive output of R. boylii (Lind et al. 2003).

Logging and erosion from road cuts have resulted in periodically high levels of stream siltation in some areas of northern California (Ashton et al. 1997). High levels of silt may inhibit the attachment of the egg mass to the substrate (Ashton et al. 1997). Excessive accumulation of silt on the egg masses may have adverse effects on embryo development (Jennings and Hayes 1994). Silt also reduces the interstitial spaces available for use by tadpoles, reduces algal growth on which the tadpoles feed (Power 1990), and can have a significant negative impact on adult frog food resources (e.g., aquatic macro-invertebrates; Petts 1984) (Ashton et al. 1997).

In the Sierra Nevada foothills of California, air-borne pesticides (that move east on the prevailing winds blowing across the highly agriculturalized Central Valley) are likely to be the primary threat (LeNoir et al. 1999, Sparling et al. 2001, Fellers 2005, Sparling and Fellers 2007). Davidson et al. (2002) found evidence that airborne agrochemicals have played a significant role in the decline; habitat destruction, climate change, and UV-B radiation appeared to be contributing factors in the decline of this species.

Ashton et al. (1997) mentioned the potential for spills of toxic materials into streams along roads along the Trinity River in northern California. Bury (1972) found that spilled diesel fuel had negative impacts on R. boylii tadpoles and partially transformed individuals but apparently little impact on adults.

Chytrid fungus has been implicated in the rapid decline of this species in southern California that began in the 1970s (Adams et al. 2017). In laboratory experiments, Davidson et al. (2007) found that chytrid infection reduced growth of newly metamorphosed Rana boylii by approximately one-half.and that exposure to the pesticide carbaryl may increase susceptibility to chytrid infection.

Recolonization abilities may be greatly restricted by local extirpation patterns. For example, dams eliminate habitat and cause local extirpations, and they also interfere with normal dispersal and movements (Fellers 2005).