This species is known from the Pacific drainages from the upper reaches of the Willamette River system, Oregon (west of the Cascades crest), south to the upper San Gabriel River, Los Angeles County, California, including the coastal ranges and Sierra Nevada foothills, in the USA. There is a disjunct population at La Grulla Meadow, Sierra San Pedro Martir, Baja California, Mexico. It has apparently disappeared from portions of its historical range, especially in southern California (see Hayes and Jennings 1988). It occurs from sea level up to 2,040m asl.
Habitat and Ecology
This species inhabits partially shaded, rocky streams at low to moderate altitudes, in areas of chaparral, open woodland, and forest (Nussbaum, Brodie and Storm 1983; Hayes and Jennings 1988). It seeks cover at the bottom of a pool when startled. Its breeding and non-breeding habitats are the following, in order of decreasing favourability: (1) partially shaded, small perennial streams, 30-1,000m asl, with at least some cobble-sized rocks, riffle areas and a stream depth rarely greater than 1m; (2) intermittent, small, partly shaded, rocky streams displaying seasonal riffle habitat; (3) large (consistently greater than 1m in stream depth), partly shaded, perennial streams with rocky or bedrock habitat; and (4) open perennial streams with little or no rocky habitat. Breeding takes place in pools of streams, and eggs are usually attached to gravel or rocks at the edge of pools or streams (Nussbaum, Brodie and Storm 1983). In northern California, eggs were found attached to cobbles and boulders at lower than ambient flow velocities, near confluences of tributary drainages in wide, shallow reaches, and most breeding sites were used repeatedly (Kupferberg 1996).
This species has probably been extirpated from the Tehacahapi Mountains southwards, and there have also been severe declines in the central Sierra foothills of California (Drost and Fellers 1996). It is now rare or absent in Oregon (Leonard et al. 1993), moderately common in north-western California and the northern Sierra foothills, and rare or absent in the central and southern Sierra foothills. This species was first recorded in Mexico three decades ago, but almost nothing is known about its biology there.
Threats to this species include stream scouring (it may negatively impact frogs in stream bed hibernation sites), introduced aquatic species, non-selective logging practices, and stabilization of historically fluctuating stream flows. However, because causes of declines are uncertain, it is difficult to assess the degree to which this species is threatened. S.J. Kupferberg (pers. comm.) found that bullfrog (Rana catesbeiana) larvae perturbed aquatic community structure and exerted detrimental effects on R. boylii populations in northern California but had only a slight impact on Pseudacris regilla.
Some populations of this species occur in national forests in California and Oregon, but this does not necessarily provide adequate protection. It also occurs in a few national, regional and state parks, and on properties owned by The Nature Conservancy. In Mexico it occurs within the San Pedro Martir National Park, which is a relatively well-preserved area. This species is protected by Mexican law under the "Special Protection" category (Pr).
Georgina Santos-Barrera, Geoffrey Hammerson, Gary Fellers 2004. Rana boylii. In: IUCN 2014