Fish and Wildlife Service, Interior.
Notice of 12-month petition finding.
We, the U.S. Fish and Wildlife Service (Service), announce a 12-month finding for a petition to list the Sierra Nevada distinct population segment of the mountain yellow-legged frog (Rana muscosa) under the Endangered Species Act of 1973, as amended. After review of all available scientific and commercial information, we find that the petitioned action is warranted, but precluded by higher priority actions to amend the Lists of Endangered and Threatened Wildlife and Plants. Upon publication of this 12-month petition finding, this species will be added to our candidate species list. We will develop a proposed rule to list this population pursuant to our Listing Priority System.
The finding announced in this document was made on January 10, 2003. Comments and information may be submitted until further notice.
You may send data, information, comments, or questions concerning this finding to the Field Supervisor (Attn: MYLF), Sacramento Fish and Wildlife Office, U.S. Fish and Wildlife Service, 2800 Cottage Way, Room W-2605, Sacramento, California 95825. You may inspect the petition, administrative finding, supporting information, and comments received, during normal business hours by appointment, at the above address.Start Further Info
FOR FURTHER INFORMATION CONTACT:
Peter Epanchin, Susan Moore, or Chris Nagano at the above address (telephone, (916) 414-6600; fax, (916) 414-6710).End Further Info End Preamble Start Supplemental Information
Section 4(b)(3)(B) of the Endangered Species Act of 1973, as amended (Act) (16 U.S.C. 1531 et seq.), requires that, for any petition to revise the List of Threatened and Endangered Species that contains substantial scientific and commercial information that listing may be warranted, we make a finding within 12 months of the date of the receipt of the petition on whether the petitioned action is: (a) Not warranted, or (b) warranted, or (c) warranted but that the immediate proposal of a regulation implementing the petitioned action is precluded by other pending proposals to determine whether any species is threatened or endangered, and expeditious progress is being made to add or remove qualified species from the List of Threatened and Endangered Species. Section 4(b)(3)(C) of the Act requires that a petition for which the requested action is found to be warranted but precluded shall be treated as though resubmitted on the date of such finding, i.e., requiring a subsequent finding to be made within 12 months. Such 12-month findings are to be published promptly in the Federal Register. Start Printed Page 2284
Camp (1917) described the mountain yellow-legged frog as two subspecies of Rana boylii: R. b. sierrae in the Sierra Nevada, and R. b. muscosa in southern California. On the basis of the similar morphological (body structure) characteristics of the two subspecies, the small number of sites where both were found, and breeding experiments, R. b. muscosa and R. b. sierrae were split from the R. boylii group and combined under a single species, R. muscosa (Zweifel 1955). Genetic studies also have concluded that R. muscosa and R. boylii are distinct species (Case 1978; Davis 1986; Green 1986a, 1986b; Hillis and Davis 1986; Macey et al. 2001).
The body length (snout to vent) of the mountain yellow-legged frog ranges from 40 to 80 millimeters (mm) (1.5 to 3.25 inches (in)) (Jennings and Hayes 1994). Females average slightly larger than males and males have a swollen, darkened thumb base (Wright and Wright 1949; Stebbins 1951; Zweifel 1955, 1968). Dorsal (upper) coloration in adults may be variable, exhibiting a mix of brown and yellow, but it also can be grey, red, or green-brown, and usually patterned with dark spots (Stebbins 1985; Jennings and Hayes 1994). These spots may be large (6 mm (0.25 in)) and few, smaller and more numerous, or a mixture of both (Zweifel 1955). Irregular lichen or moss-like patches (to which the name muscosa refers) also may be present on the dorsal surface (Zweifel 1955; Stebbins 1985). The belly and undersurfaces of the hind limbs are yellow or orange, and this pigmentation on the abdomen may extend forward to the forelimbs (Wright and Wright 1949; Stebbins 1985). This species may produce a distinctive mink or garlic-like odor when disturbed (Wright and Wright 1949; Stebbins 1985). Although the species lacks vocal sacks, it can make both terrestrial and underwater vocalizations, which have been described as a flat clicking sound (Zweifel 1955; Stebbins 1985; Ziesmer 1997). The mountain yellow-legged frog has smoother skin, generally heavier spotting and mottling dorsally, and darker toe tips than the foothill yellow-legged frog (R. boylii) (Zweifel 1955; Stebbins 1985).
Eggs of the mountain yellow-legged frog are laid in globular clumps, which are often somewhat flattened, roughly 2.5 to 5 cm (1 to 2 in) across (Stebbins 1985). When eggs are close to hatching, egg mass volume may average 198 cubic cm (78 cubic in) (Pope 1999a). Eggs have three firm jelly-like transparent envelopes surrounding a grey-tan or black vitelline (egg yolk) capsule (Wright and Wright 1949).
The larvae (tadpoles) of this species generally are mottled brown in dorsal coloration with a golden tint and a faintly-yellow venter (underside) (Zweifel 1955; Stebbins 1985). Total tadpole length reaches 72 mm (2.8 in), its body is flattened, and the tail musculature is wide, about 2.5 centimeters (cm) (1 in) or more, before tapering into a rounded tip (Wright and Wright 1949). The mouth has a maximum of 7 labial (lip) tooth rows (2-3 upper and 4 lower) (Stebbins 1985). Larvae often take 2 to 4 years or more to reach metamorphosis (transformation from larvae to frogs) (Wright and Wright 1949; Cory 1962b; Bradford 1983; Bradford et al. 1993; Knapp and Matthews 2000).
The mountain yellow-legged frog is restricted to two disjunct areas in California and a portion of Nevada. One area is in the Sierra Nevada and the other area is in the San Gabriel, San Bernardino, and San Jacinto mountain ranges of southern California (Los Angeles, San Bernardino, Riverside, and San Diego counties) (Zweifel 1955; Jennings and Hayes 1994). The southern California population is isolated from the Sierra Nevada population by the Tehachapi mountain range, with a distance of about 225 kilometers (km) (140 miles (mi)) between the two populations.
In the Sierra Nevada, the historic distribution of the mountain yellow-legged frog was more or less continuous from the vicinity of La Porte in southern Plumas County southward to Taylor and French Joe Meadows in southern Tulare County (Jennings and Hayes 1994). Records for this species in the Sierra Nevada document its occurrence on the east and west sides of the crest in all major drainages from Plumas to Tulare counties, with a single record from Kern County (Zweifel 1955; Jennings and Hayes 1994; Knapp 1996). Except for historic populations in extreme western Nevada in Washoe and Douglas counties, on Mt. Rose near Lake Tahoe, possibly Edgewood Creek, and elsewhere around Lake Tahoe, the species is confined to California (Zweifel 1955). The elevational range for the mountain yellow-legged frog in the Sierra Nevada ranges from approximately 1,370 meters (m) (4,500 feet (ft)) at San Antonio Creek, near Dorrington in Calaveras County, to over 3,650 m (12,000 ft) at Desolation Lake in Fresno County, though populations typically are encountered in the upper half of that elevation range (Zweifel 1955; Mullally and Cunningham 1956; Stebbins 1985).
Mountain yellow-legged frogs rarely are found more than 1 m (3.3 ft) from water (Stebbins 1951; Mullally and Cunningham 1956; Bradford et al. 1993). At the lower elevations in the Sierra Nevada, the species usually is associated with rocky stream beds and wet meadows surrounded by coniferous forest (Zweifel 1955; Zeiner et al. 1988). At higher elevations, the species occupies lakes, ponds, tarns, and streams (Zweifel 1955; Mullally and Cunningham 1956; Stebbins 1985). The borders of alpine (above treeline) lakes and montane (mountain) meadow streams used by mountain yellow-legged frogs are frequently grassy or muddy; this differs from the sandy or rocky shores that are inhabited by the amphibian in lower elevation streams (Zweifel 1955). Adults typically are found sitting on rocks along the shoreline, usually where there is little or no vegetation (Mullally and Cunningham 1956). Although the species may use a variety of shoreline habitats, both larvae and adults are less common at shorelines which drop abruptly to a depth of 60 cm (2 ft) than at open shorelines that gently slope up to shallow waters of only 5-8 cm (2-3 in) deep (Mullally and Cunningham 1956; Jennings and Hayes 1994). Mountain yellow-legged frogs also use stream habitats, especially in the northern part of their range. Streams utilized by adults vary from those having high gradients with numerous pools, rapids, and small waterfalls, to those with low gradients with slow flows, marshy edges, and sod banks (Zweifel 1955). Aquatic substrates vary from bedrock to fine sand, rubble (rock fragments), and boulders (Zweifel 1955). Mountain yellow-legged frogs seem to be absent from the smallest creeks, probably because these have insufficient depth for adequate refuge and overwintering habitat (Jennings and Hayes 1994).
Both adults and larvae overwinter for up to 9 months in the bottoms of lakes that are at least 1.7 m (5.6 ft) deep; however, overwinter survival may be greater in lakes that are at least 2.5 m (8.2 ft) deep, under ledges of stream or lake banks, or in rocky streams (Bradford 1983; V. Vredenburg et al. (in press)). In some instances, frogs have been found to overwinter in underwater bedrock crevices between 0.2 m (0.7 ft) and 1 m (3.3 ft) below the water surface (Matthews and Pope 1999) and the use Start Printed Page 2285of such crevices appears to allow them to survive in shallower water bodies that freeze to the bottom in winter (Pope 1999a). In lakes and ponds that do not freeze to the bottom in winter, mountain yellow-legged frogs may overwinter in the shelter of bedrock crevices as a behavioral response to the presence of introduced fishes (V. Vredenburg et al. (in press)).
Adult mountain yellow-legged frogs breed in the shallows of ponds or in inlet streams and are often seen on wet substrates within 1 m (3 ft) of the water's edge (Zweifel 1955). Adults emerge from overwintering sites immediately following snowmelt and will move over ice to get to breeding sites (Pope 1999a; V. Vredenburg in litt. 2002). Mountain yellow-legged frogs in the Sierra Nevada deposit their eggs underwater in clusters, which they attach to rocks, gravel, vegetation, or under banks (Wright and Wright 1949; Stebbins 1951; Zweifel 1955; Pope 1999a). Clutch size varies from 15 to 350 eggs per egg mass (Livezey and Wright 1945; V. Vredenburg et al. (in press)). In laboratory breeding experiments, egg hatching times ranged from 18 to 21 days at temperatures ranging from 5 to 13.5 Celsius (°C ) (41 to 56 Fahrenheit (°F)) (Zweifel 1955). Field observations are similar (Pope 1999a).
The time required to develop from fertilization to metamorphosis is believed to vary between 1 and 4 years (Storer 1925; Wright and Wright 1949; Zweifel 1955; Cory 1962b; V. Vredenburg et al. (in press)). Since larvae must overwinter at least two or three times before metamorphosis, successful breeding sites are located in, or connected to, lakes and ponds that do not dry in the summer, and that are sufficiently deep so as to not completely freeze through in winter (Bradford 1983). Larval survival to metamorphosis is possible in lakes that do not dry out during the summer. Knapp and Matthews (2000) found the number of larvae was larger in fishless water bodies deeper than 2 m (6.5 ft). Bradford (1983) found that mountain yellow-legged frog die-offs sometimes result from oxygen depletion during winter in lakes less than 4 m (13 ft) deep. However, larvae may survive for months in nearly anoxic (oxygen-deficient) conditions when shallow lakes are frozen to the bottom. Recent studies have reported populations of mountain yellow-legged frogs overwintering in lakes less than 1.5 m (5 ft) deep that were assumed to have frozen to the bottom, and yet healthy frogs were documented to emerge the following July (Matthews and Pope 1999; Pope 1999a). Radio telemetry indicated that the mountain yellow-legged frogs were utilizing rock crevices near shore, crevices, holes, and ledges where water depths ranged from 0.2 m (0.7 ft) to 1.5 m (5 ft) (Matthews and Pope 1999). The granite surrounding these overwintering habitats may insulate the mountain yellow-legged frogs from the extreme winter temperatures, providing that there is an adequate supply of oxygen either in the water or air (Matthews and Pope 1999).
Larvae maintain a relatively high body temperature by selecting warmer microhabitats (Bradford 1984). During winter, larvae remain in warmer water below the thermocline (thermally stratified water); after spring overturn (thaw and thermal mixing of the water), they continue to behaviorally modulate their body temperature by daily movements: during the day, larvae move to warm, shallow, nearshore water, and during the late afternoon and evening, they retreat to the warmer waters off shore (Bradford 1984).
The time required to reach reproductive maturity is thought to vary between 3 and 4 years after metamorphosis (Zweifel 1955). Longevity of adults is unknown, but adult survivorship from year to year is very high, so they are undoubtedly long-lived amphibians (Matthews and Pope 1999; Pope 1999a). Although data currently are limited, evidence exists that mountain yellow-legged frogs display strong site fidelity and return to the same overwintering and summer habitats from year to year (Pope 1999a).
In aquatic habitats, mountain yellow-legged frog adults typically move only a few hundred meters (few hundred yards) (Matthews and Pope 1999; Pope 1999a), but distances of up to 1 km (0.62 mi) have been recorded (V. Vredenburg et al. (in press)). Adults tend to move between selected breeding, feeding, and overwintering habitats during the course of the year. Though adults are typically found within 1 m (3.3 ft) of water, overland movements of over 65 m (215 ft) have been recorded (Pope 1999a); the furthest reported distance of a mountain yellow-legged frog from water is 400 m (1,300 ft) (V. Vredenburg et al. (in press). Almost no data exist on the dispersal of juvenile mountain yellow-legged frogs away from breeding sites (Bradford 1991). However, juveniles that may be dispersing to permanent water have been observed in small intermittent streams (Bradford 1991). Mountain yellow-legged frog population dynamics are thought to have a metapopulation structure (Bradford et al. 1993; Drost and Fellers 1996; Knapp and Matthews 2000). In describing the metapopulation concept, Hanski and Simberloff (1997) stated: “* * *the two key premises in this approach to population biology are that populations are spatially structured in assemblages of local breeding populations and that migration among the local populations has some effect on local dynamics, including the possibility of population reestablishment following extinction.”
Adult mountain yellow-legged frogs are thought to feed preferentially upon terrestrial insects and adult stages of aquatic insects while on the shore and in shallow water (Bradford 1983). Feeding studies on Sierra Nevada mountain yellow-legged frogs are limited. Remains found inside the stomachs of mountain yellow-legged frogs in southern California include a wide variety of invertebrates, including beetles, ants, bees, wasps, flies, true-bugs, and dragonflies (Long 1970). Larger frogs take more aquatic true bugs (insects in the taxonomic order Hemiptera) probably because of their more aquatic behavior (Jennings and Hayes 1994). Adult mountain yellow-legged frogs have been observed eating Yosemite toad (Bufo canorus) and Pacific treefrog (Pseudacris regilla) larvae (Mullally 1953; Zeiner et al. 1988; Pope 1999b; Feldman and Wilkinson 2000) and can be cannibalistic (Heller 1960). Mountain yellow-legged frog larvae graze on benthic detritus, algae, and diatoms along rocky bottoms in streams, lakes, and ponds (Bradford 1983; Zeiner et al. 1988). Larvae have also been observed cannibalizing conspecific (of the same species) eggs (Vredenburg 2000). In addition, larvae have been seen feeding on the carcasses of dead metamorphosed frogs (V. Vredenburg et al. (in press)).
The distribution of the Sierra Nevada mountain yellow-legged frog is restricted primarily to publicly managed lands at high elevations, including streams, lakes, ponds, and meadow wetlands located on national forests, including wilderness and non-wilderness on the forests, and national parks. Approximately 210 known mountain yellow-legged frog populations (or populations within metapopulations) exist on the national forests within the Sierra Nevada, though not all of these populations may be reproducing successfully. In the national parks of the Sierra Nevada, there are 758 known sites with mountain yellow legged-frogs, most of which occur within 59 different basins that have multiple breeding populations that are connected hydrologially, so that populations in each basin function as Start Printed Page 2286metapopulations). Within these 758 sites, 330 populations exist for which we have evidence of successful reproduction. Overall, we estimate that 22 percent of the remaining mountain yellow-legged frog sites within the Sierra Nevada are found within the national forests (including those with and those without evidence of successful reproduction), while 78 percent are found within the national parks (including those with and those without evidence of successful reproduction). These percentages represent the number of sites within the national forests and the national parks of the Sierra Nevada; they do not represent the number of individuals present at each site. The methods for measuring the numbers of populations and metapopulations in the national forests and the national parks have not been standardized and, therefore we must use caution when we compare national forests numbers to national park numbers. However, the remaining populations of mountain yellow-legged frogs are more numerous and larger in size in the national parks than in the national forests.
National forests with extant populations of mountain yellow-legged frogs include the Plumas National Forest, Tahoe National Forest, Humboldt-Toiyabe National Forest, Lake Tahoe Basin Management Unit (managed by the U.S. Forest Service (USFS)), Eldorado National Forest, Stanislaus National Forest, Sierra National Forest, Sequoia National Forest, and Inyo National Forest. National parks with extant populations of mountain yellow-legged frogs include Yosemite National Park, Kings Canyon National Park, and Sequoia National Park.
Grinnell and Storer (1924) first observed declines of mountain yellow-legged frog populations. Since then, a number of researchers have reported that the mountain yellow-legged frog has disappeared from a significant portion of its historic range in the Sierra Nevada (Hayes and Jennings 1986; Bradford 1989; Jennings and Hayes 1994; Bradford et al. 1994a; Jennings 1995, 1996; Stebbins and Cohen 1995; Drost and Fellers 1996; Knapp and Matthews 2000). The observed declines of mountain yellow-legged frog populations in the 1970s were small relative to the declines observed during the 1980s and 1990s. Rangewide, it is estimated that mountain yellow-legged frog populations have undergone a 50 to 80 percent reduction in size (Bradford et al. 1994a; Jennings 1995; Stebbins and Cohen 1995; Drost and Fellers 1996; Jennings 1996; Knapp and Matthews 2000). The most pronounced declines have occurred north of Lake Tahoe in the northernmost 125 km (78 mi) portion of the range, and south of Sequoia and Kings Canyon National Parks in Tulare County in the southernmost 50 km (31 mi) portion, where only a few populations remain (Fellers 1994; Jennings and Hayes 1994). Based on available USFS survey and observation data, there appear to be very few or no known large populations north of the Plumas National Forest.
Mountain yellow-legged frogs historically occurred in Nevada on the slopes of Mount Rose in Washoe County and probably in the vicinity of Lake Tahoe in Douglas County (Linsdale 1940; Zweifel 1955; Jennings 1984). In 1994 and 1995, mountain yellow-legged frog surveys were conducted by Panik (1995) at 54 sites in the Carson Range of Nevada and California, including eight historic locations; no mountain yellow-legged frogs were observed. A few scattered and unconfirmed sightings were reported in Nevada in the late 1990s, but any populations remaining in this State are likely to be extremely small and the species is thought to be extirpated from Nevada (R. Panik, Western Nevada Community College, in litt., 2002).
The number of extant populations of the mountain yellow-legged frogs in the Sierra Nevada is greatly reduced. Remaining populations are patchily scattered throughout nearly all their historic range (Jennings and Hayes 1994; Jennings 1995, 1996). At the northernmost portions of the range in Butte and Plumas counties, few populations have been seen or discovered since 1970 (Jennings and Hayes 1994). Declines have also been noted in the central and southern Sierra (Drost and Fellers 1996). In the southern Sierra Nevada (Sierra, Sequoia, and Inyo National Forests; and Sequoia, Kings Canyon, and Yosemite National Parks), there are relatively large populations (e.g., breeding populations of over 20 adults) of mountain yellow-legged frogs; however, in recent years, some of the largest of these populations have been extirpated (Bradford 1991; Bradford et al. 1994a; R. Knapp, Sierra Nevada Aquatic Research Laboratory, in litt. 2002). Mountain yellow-legged frog populations are more numerous and larger in size in the national parks of the Sierra Nevada than in the surrounding USFS lands (Bradford et al. 1994a; Knapp and Matthews 2000).
Between 1988 and 1991, Bradford et al. (1994a) resurveyed sites known historically (between 1955 and 1979) to have contained mountain yellow-legged frogs. They resurveyed 27 historic sites on the Kaweah River, a western watershed within Sequoia National Park, and did not detect mountain yellow-legged frogs at any of these locations. They resurveyed 21 historic sites within the Kern, Kings, and San Joaquin River watersheds in Sequoia and Kings Canyon National Parks, and detected mountain yellow-legged frogs at 11 of these sites. Frogs were detected at three locations out of 24 historic sites outside of Sequoia and Kings Canyon National Parks. Rangewide, their resurvey effort detected mountain yellow-legged frogs at 14 of 72 historic sites, representing an 80 percent population decline. On the basis of these results, Bradford et al. (1994a) estimated a 50 percent population decline in Sequoia and Kings Canyon National Parks, with more pronounced declines elsewhere in the mountain yellow-legged frog's range.
Drost and Fellers (1996) surveyed for mountain yellow-legged frogs at sites documented by Grinnell and Storer (1924) in the early part of the 20th Century. The frog was reported to be the most common amphibian where they surveyed in the Yosemite area (Grinnell and Storer 1924). Drost and Fellers (1996) repeated Grinnell and Storer's 1924 survey and reported mountain yellow-legged frog presence at only 2 of the 14 sites where this animal had been previously detected. These two positive sightings consisted of a single larva at one site and a single adult female at another site. Drost and Fellers (1996) identified and surveyed 17 additional sites with suitable mountain yellow-legged frog habitat, and these surveys resulted in the detection of three additional populations.
For the 86 historically occupied mountain yellow-legged frog sites documented between 1915 and 1959 and resurveyed by Bradford et al. (1994a) and Drost and Fellers (1996), an 80 percent decline occurred in the number of historical frog populations. Of the 86 historic sites, only 16 remained occupied at the time of resurvey.
Knapp and Matthews (2000) surveyed more than 1,700 high elevation (averaging 3,400 m (11,150 ft)) lakes and ponds in the Sierra National Forest's John Muir Wilderness Area and in Kings Canyon National Park, encompassing a total of approximately 100,000 hectares (ha) (247,000 acres (ac)). They found a strong negative correlation between introduced trout and the distribution of mountain yellow-legged frogs. In the summer of 2002, Knapp (in litt. 2002) resurveyed 302 water bodies determined by 1995 to 1997 surveys to be occupied by mountain yellow-legged frogs, and Start Printed Page 2287resurveyed 744 of over 1,400 sites where frogs were not previously detected. Knapp found no change in status at 59 percent of these sites, but found that 41 percent of the sites had gone extinct, while 8 percent of previously unoccupied sites were colonized. These data indicate an extinction rate that is 5 to 6 times higher than the colonization rate within this study area. This high rate of extinction over a 5- to-7-year time frame suggests the species may become extinct within a few decades (assuming that the rate of extinction and recolonization observed over this time period accurately reflects the long-term rates). The documented extinctions appeared to occur nonrandomly across the landscape, are spatially clumped typically, and involve the disappearance of all or nearly all mountain yellow-legged frog populations in a watershed (R. Knapp in litt. 2002). The colonization sites also appeared to be nonrandomly distributed, occurring primarily in watersheds with large mountain yellow-legged frog populations (R. Knapp in litt. 2002).
A recent review of the current status of 255 previously documented mountain yellow-legged frog locations (based on Jennings and Hayes (1994)) throughout its historic range concluded that 83 percent of these sites are no longer occupied by this species (Davidson et al. 2002). Each national forest and national park is discussed individually below.
Lassen National Forest: Historically, mountain yellow-legged frogs occurred on the Lassen National Forest within multiple watersheds, including Butte Creek, the West Branch Feather River, and the Middle Fork Feather River (M. McFarland, in litt. 2002). The last confirmed mountain yellow-legged frog sighting on the Lassen National Forest was made in 1966 in the area of Snag Lake in the West Branch Feather River watershed. Since 1993, the Lassen National Forest has conducted or funded informal and formal systematic amphibian surveys to assess the relative distribution and abundance of amphibian species, including the mountain yellow-legged frog. On the Lassen National Forest, mountain yellow-legged frogs have not been detected or confirmed during any of these surveys (M. McFarland in litt. 2002).
Plumas National Forest: Based on resurvey efforts, Jennings and Hayes (1994) noted that the mountain yellow-legged frog was extirpated at a number of locations in the Plumas National Forest. As survey efforts continue by the Plumas National Forest, more mountain yellow-legged frog populations are being documented. However, most of the estimated 55 populations are small, consisting of only a few individuals (T. Hopkins, USFS, pers. comm., 2002). The species appears to have disappeared from a significant number of historic locations, and the abundance of the species at known sites appears to be quite low.
Tahoe National Forest: Mountain yellow-legged frogs were present historically throughout the Tahoe National Forest and the surrounding areas of Sierra, Nevada, and Placer counties. Jennings and Hayes (1994) conclude that, based on their re-surveys of historic locations, 1992, the species had been extirpated in a number of locations by 1992.
The Tahoe National Forest has been conducting some amphibian surveys. Approximately four or five extant populations exist in which mountain yellow-legged frog breeding has been documented (A. Carlson, USFS, pers. comm. 2002). Extant mountain yellow-legged frog populations on the Tahoe National Forest have been observed in both stream and pond habitats. One extant breeding population inhabits an old mining tailing pond that has been restored naturally to a forested wetland condition with an abundance of bankside and emergent vegetation (A. Carlson, pers. comm. 2002). The largest Tahoe National Forest population observed in recent surveys consists of fewer than 10 individuals. The species appears to have disappeared from a significant number of historic locations within the Tahoe National Forest and is in low abundance where it still persists (A. Carlson, pers. comm. 2002).
Lake Tahoe Basin Management Unit: Historic sightings of the mountain yellow-legged frog in the Lake Tahoe Basin Management Unit are numerous, indicating that the species was abundant in the Lake Tahoe area (J. Reiner, USFS, pers. comm. 2002). Today, only one known population of mountain yellow-legged frogs remains on this national forest, although in 1997, the USFS saw evidence of limited breeding in the Desolation Wilderness (J. Reiner, pers. comm. 2002; J. Reiner and M. Schlesinger, USFS, in litt. 2000). The known population is small, as some adults were seen in 1999 but were not detected during 2002 surveys, though larvae were detected. The habitat at this site is a meadow and stream complex that is large (approximately 24 ha (60 ac)) and in good condition (J. Reiner, pers. comm. 2002).
Humboldt-Toiyabe National Forest: Only the westernmost portion of the Humboldt-Toiyabe National Forest is within the historic range of the mountain yellow-legged frog (Stebbins 1985). A distributional map of mountain yellow-legged frogs produced by Jennings and Hayes (1994) indicates historic collections of this species within the Humboldt-Toiyabe National Forest in California. Resurveys of locations where mountain yellow-legged frogs occurred indicate that the species had become extirpated by 1992 at a number of locations in Humboldt-Toiyabe National Forest (Jennings and Hayes 1994). Surveys in California are ongoing. Approximately four populations (all in California) exist on this national forest (C. Milliron, California Department of Fish and Game (CDFG), in litt. 2002; L. Murphy, USFS, pers. comm. 2002). Chytrid fungus (see Factor C, Disease, below) has been documented at one of these populations (C. Milliron, in litt. 2002).
Eldorado National Forest: The mountain yellow-legged frog is distributed across the Eldorado National Forest with populations or metapopulations (multiple breeding populations within the same basin that have hydrologic connectivity between them) within the headwaters and headwater tributaries of several watersheds, including the Rubicon River, the South Fork American River, the North Fork Cosumnes River, and the North Fork Mokelumne River (J. Williams, USFS, in litt. 2002).
Numerous surveys for mountain yellow-legged frogs have been conducted on this national forest by the USFS, the CDFG, and several contractors between 1990 and 2002. Reproducing populations have been found at a variety of locations in high elevation areas of this national forest. Surveys for amphibians within the Eldorado National Forest in 1992 resulted in no detections of mountain yellow-legged frogs, though this may be a function of the limited area and habitat type that was surveyed (Martin 1992). Jennings and Hayes (1994) indicate both extirpated populations and extant populations on the Eldorado National Forest. Intensive surveys by CDFG and USFS in 2001 and 2002 resulted in an estimated 18 extant populations or metapopulations of mountain yellow-legged frogs on the Eldorado National Forest, although both the mean number of populations and population size are generally low relative to historic reports (J. Williams, in litt. 2002). Currently, approximately four populations exist with between 25 and 50 mountain yellow-legged frogs; these are the largest populations on the Eldorado National Forest (J. Williams, in litt. 2002). Start Printed Page 2288
Stanislaus National Forest: A 1992 survey (Martin 1992) in the Stanislaus National Forest located mountain yellow-legged frogs at only 2 of 16 locations surveyed, and at these locations, the numbers of adults detected were small (under five). Jennings and Hayes (1994) indicate that the species has been extirpated from a number of historic locations. There are approximately 80 extant populations of mountain yellow-legged frogs on the Stanislaus National Forest; of these, only about 8 appear to have more than 10 adults, and only 2 populations are known to have 25 to 30 adults (L. Conway, USFS, pers. comm. 2002).
Yosemite National Park: From 1914 to 1920, Grinnell and Storer conducted a biological survey along a transect across the Sierra Nevada. They documented mountain yellow-legged frogs at 14 sites throughout Yosemite National Park and noted the species was abundant in this area. Numerous frogs were found in lakes and streams at high elevations (Grinell and Storer 1924). “Hundreds of frogs” were found at Young Lake and frogs were “very numerous” at Westfall Meadow (Camp1915, as cited in Drost and Fellers 1994). Large numbers of specimens were collected; for example, 25 were taken at Vogelsang Lake (Grinnell 1915, as cited in Drost and Fellers 1994).
The mountain yellow-legged frog was documented at several additional locations in Yosemite National Park from 1957 to 1960 (Heller 1960). At Johnson Lake, Mullally and Cunningham (1956) reported a mountain yellow-legged frog population decline between 1950 and 1955, though they did not quantify the decline. They attributed this decline to the unusually long and cold winter of 1951-1952. Some of Yosemite's “densest aggregations of frogs ever noted” by Mullally and Cunningham (1956) were in lakes near Ostrander Lake south of Glacier Point; they attributed the absence of frogs in Ostrander Lake to the presence of non-native trout.
Between 1988 and 1991, Bradford et al. (1994a) randomly selected and surveyed four mountain yellow-legged frog populations documented in Yosemite between 1955 to 1979. Although they did not resurvey all of the mountain yellow-legged frog populations previously reported from within the park, they reported that the four resurveyed populations were extirpated (Bradford et al. 1994a). In 1992 and 1993, Drost and Fellers (1996) revisited 38 of the original 40 sites surveyed by Grinnell and Storer from 1914 to1920, and surveyed other sites with potential mountain yellow-legged frog habitat. The mountain yellow-legged frog had declined by approximately 80 percent from the locations documented by the 1924 study (Drost and Fellers 1996). A distribution map of mountain yellow-legged frogs produced by Jennings and Hayes (1994) also documents extinctions and indicates a population decline of this species from Yosemite National Park. Colwell and Beatty (2002) surveyed 35 lakes with appropriate mountain yellow-legged frog habitat within the Tuolumne and Merced River drainages of Yosemite National Park in 1992 and 1993; only 3 lakes were found to have mountain yellow-legged frogs.
Currently in Yosemite National Park, 251 mountain yellow-legged frog sites exist, most of which occur within 23 different basins that have multiple breeding populations with habitat that is connected hydrologically, so that the populations in each basin function as a metapopulation (R. Knapp in litt. 2002). Six sites have populations with over 100 adult mountain yellow-legged frogs each, 1 site has a population with between 51 and 100 adults, and 41 sites have populations between 10 and 50 adults each. In addition, 203 sites have fewer than 10 adults each. Of the 251 mountain-yellow legged frog sites in the park, evidence of breeding has been found in 71 populations.
Inyo National Forest: Jennings and Hayes (1994) document the extirpation of some mountain yellow-legged frog populations from the Inyo National Forest. In 1994, 15 known locations had mountain yellow-legged frog populations (Parker 1994). Currently, 7 basins within the Inyo National Forest have known extant mountain yellow-legged frog populations or populations that function as metapopulations (C. Milliron, in litt. 2002). Some of these populations are stable, consisting of several hundred individuals representing all age classes (L. Sims, USFS, in litt. 2002). Chytrid fungus (see Factor C, Disease, below) has been documented at an additional population location that is now extinct (C. Milliron, in litt. 2002).
Sierra National Forest: In 1955, Mullally and Cunningham (1956) reported encountering mountain yellow-legged frogs along Paiute Creek “very sparingly” at approximately 2,300 m (7,700 ft), with frogs becoming more abundant at higher elevations. The “densest populations” were found above 3,050 m (10,000 ft) in the Humphrey's Basin area, and a “great many, including tadpoles” were noted at and near Pine Creek Pass, with frogs also seen at Golden Trout and Desolation Lakes.
Jennings and Hayes (1994) indicated that the mountain yellow-legged frog has become extirpated at a number of historical locations in the Sierra National Forest. Knapp and Matthews (2000) report on mountain yellow-legged frog population declines associated with fish stocking within the John Muir Wilderness Area of the Sierra National Forest (see Factor C, Disease, below). In 1995 and 1996, Knapp and Matthews (2000) surveyed 669 lakes, ponds, and other water bodies in the John Muir Wilderness Area. Mountain yellow-legged frog adults were found in 4 percent of these water bodies, and frog larvae in 3 percent (Knapp and Mathews 2000). In 2002, Knapp conducted resurveys at the 28 water bodies that had been occupied by mountain yellow-legged frogs in 1997, and also at 118 of the 641 sites where frogs were not detected in 1997. Knapp found no change in mountain yellow-legged frog status at 39 percent of these 28 previously occupied water bodies, but found that the frogs at 61 percent of the 28 previously occupied sites had gone extinct, while colonization had occurred at 10 percent of 118 previously unoccupied sites (R. Knapp in litt. 2002).
Although not all potential mountain yellow-legged frog habitats have been surveyed within the Sierra National Forest, approximately six subwatersheds have extant metapopulations (H. Eddinger, USFS, in litt. 2002). These subwatersheds are in the upper headwaters of the South Fork Merced River, South Fork San Joaquin River, and North Fork Kings River. They include the Mono Creek Basin, the Bear Creek Basin, the Paiute Creek Basin, the Humphreys Creek Basin, the Big Creek Basin, and the Dinkey Creek Basin.
Sequoia and Kings Canyon National Parks: Relatively few records exist for mountain yellow-legged frog prior to 1955 in the Sequoia and Kings Canyon National Parks. From 1955 to 1979, the species is known to have occurred in at least 21 sites scattered throughout Sequoia and Kings Canyon National Parks, although historic abundance is not known (Bradford et al. 1994a). In 1978-1979, the headwaters of seven creek systems were surveyed for mountain yellow-legged frogs in the national parks. Frogs were found at 27 sites greater than 200 m (660 ft) apart (Bradford et al. 1994a). A distributional map of mountain yellow-legged frogs produced by Jennings and Hayes (1994) indicates numerous historic sightings and collections of the species within both national parks, as well as numerous extinctions. The species was already noted to have disappeared from Start Printed Page 2289approximately half of previously occupied locations in Sequoia and Kings Canyon Parks by the late 1980s (Bradford et al. 1994a). On the basis of surveys, Bradford et al. (1994a) estimate that mountain yellow-legged frogs have been extirpated from half of their historic locations in Sequoia and Kings Canyon National Parks. For example, Fellers (1994) surveyed in Sequoia and Kings Canyon National Parks and did not detect the mountain yellow-legged frog in the Kaweah watershed where the species was located historically.
In 1997, Knapp and Matthews (2000) surveyed 1,059 lakes, ponds, and other water bodies in Kings Canyon National Park. Mountain yellow-legged frog adults were found in 31 percent of these water bodies, and frog larvae in 20 percent (Knapp and Mathews 2000). Some significant frog populations remain in Sequoia and Kings Canyon National Parks, but extensive declines have been described. In 2002, Knapp (in litt. 2002) resurveyed 274 water bodies occupied by mountain yellow-legged frogs in 1997, and he also resurveyed 626 of the 785 sites where frogs were not detected in 1997. Knapp found no change in status at 60 percent of the 274 previously occupied sites, but found that 39 percent of the 274 previously occupied sites had gone extinct, while colonization had occurred at 7 percent of 626 previously unoccupied sites.
Currently in Sequoia and Kings Canyon National Parks, 507 mountain yellow-legged frog sites are known, most of which occur within 36 different basins that have multiple breeding populations that are hydrologically connected, so that the populations within each basin function as a metapopulation. Fifty-four sites have populations of more than 100 adult mountain yellow-legged frogs, 25 sites have populations between 51 and 100 adults, 132 sites have populations between 10 and 50 adults, and 296 sites have fewer than 10 adults. Of the 507 mountain yellow-legged frog sites in Sequoia and Kings Canyon National Parks, breeding evidence has been found at 259 populations (R. Knapp in litt. 2002).
Sequoia National Forest: Jennings and Hayes (1994) indicate that the mountain yellow-legged frog has been extirpated from a number of historical locations in the Sequoia National Forest. Mountain yellow-legged frogs were collected on several historic locations of the Kern Plateau in Sequoia National Forest (Jennings and Hayes 1994). Today, two known extant populations exist on the Sequoia National Forest (S. Anderson, USFS, in litt. 2002).
All of the recent mountain yellow-legged frog sightings from the Sequoia National Forest have been of single frogs or very small populations. In 1992, mountain yellow-legged frogs were not detected during amphibian surveys conducted at 17 sites in Sequoia National Forest (Martin 1992). The species appears to be severely reduced in numbers and range in the Sequoia National Forest.
Distinct Vertebrate Population Segment
Under the Act, we must consider for listing any species, subspecies, or, for vertebrates, any distinct population segment (DPS) of these taxa if there is sufficient information to indicate that such action may be warranted. To implement the measures prescribed by the Act, we, along with the National Marine Fisheries Service (National Oceanic and Atmospheric Administration-Fisheries), developed a joint policy that addresses the recognition of DPSs for potential listing actions (61 FR 4722). The policy allows for a more refined application of the Act that better reflects the biological needs of the taxon being considered, and avoids the inclusion of entities that do not require the Act's protective measures.
Under our DPS Policy, we use two elements to assess whether a population segment under consideration for listing may be recognized as a DPS. The elements are: (1) the population segment's discreteness from the remainder of the species to which it belongs; and (2) the significance of the population segment to the species to which it belongs. If we determine that a population segment being considered for listing is a DPS, then the level of threat to the population is evaluated based on the five listing factors established by the Act to determine if listing it as either threatened or endangered is warranted.
Discreteness. Under our DPS Policy, a population segment of a vertebrate species may be considered discrete if it satisfies either one of the following two conditions: (1) it is markedly separated from other populations of the same taxon as a consequence of physical, physiological, ecological, or behavioral factors. Quantitative measures of genetic or morphological discontinuity may provide evidence of this separation; or (2) it is delimited by international governmental boundaries within which significant differences in control of exploitation, management of habitat, conservation, status, or regulatory mechanisms exist. The proposed DPS, the Sierra Nevada mountain yellow-legged frog, is based on the first condition, the marked separation from other populations.
The range of the mountain yellow-legged frog is divided by a natural geographic barrier, the Tehachapi Mountains, which geographically isolates the populations in the southern Sierra Nevada from those in the mountains of southern California. The distance of the geographic separation is about 225 km (140 mi). The geographic separation of the Sierra Nevada and southern California mountain yellow-legged frogs was recognized in the earliest description of the species by Camp (1917), who treated specimens from the two areas as separate subspecies of R. boylii. Camp (1917) described the two subspecies based on differences in their biogeography and morphology.
Ziesmer (1997) analyzed vocalizations of mountain yellow-legged frogs from 86 locations in Alpine and Mariposa counties in the Sierra Nevada, and vocalizations of mountain yellow-legged frogs from 23 locations in the San Jacinto Mountains of Riverside County in southern California. The vocalizations of Sierra Nevada frogs differed from those of southern California frogs in pulse rate, harmonic structure, and dominant frequency. Ziesmer (1997) concluded that the differences in vocalization supported the hypothesis that mountain yellow-legged frogs from the Sierra Nevada and southern California may represent separate species.
Genetic analyses support the discreteness of the mountain yellow-legged frog populations in southern California from those in the Sierra Nevada. In an allozyme (genetic) study that compared mountain yellow-legged frogs from the central Sierra Nevada with those from southern California, a fairly significant genetic difference was found between the two populations (D. Green, McGill University, in litt. 1993). However, because there were no frog samples from the southern Sierra Nevada for comparison, it was not clear whether the difference reflected two ends of a cline (a character gradient), or distinctions between the Sierra Nevada and southern California populations. Thus, because the data set was incomplete, Green (in litt., 1993) interpreted the results cautiously.
A phylogenetic analysis of mitochondrial deoxyribonucleic acid (DNA) sequences of the mountain yellow-legged frog was performed throughout its distribution (Macey et al. 2001). This study concluded that there are two major genetic lineages of the mountain yellow-legged frog (inclusive of the Sierra Nevada populations and Start Printed Page 2290the southern California populations), with populations in the Sierra Nevada falling into three distinct groups and the fourth being the southern California population (Macey et al. 2001). Though three genetic lineages of mountain yellow-legged frogs have been identified in the Sierra Nevada, more genetic sampling is needed to delineate specific boundaries of the three genetic lineages before they are treated or managed as separate units (Macey et al. 2001). Therefore, this finding treats the three genetic lineages of the mountain yellow-legged frog in the Sierra Nevada as one DPS, discrete from the mountain yellow-legged frog DPS in southern California.
The biogeographic fragmentation within the Sierra Nevada population of mountain yellow-legged frogs occurs between Kings Canyon National Park and a region slightly north of Yosemite National Park, allowing for the central and northern Sierra Nevada populations to share more genetic similarities than the southern Sierra Nevada and southern California populations (Macey et al. 2001). In fact, this study indicates that the southern Sierran group (largely in Fresno County) may be more closely related to the southern California mountain yellow-legged frogs than with those in the central and northern Sierra Nevada (Macey et al. 2001). This research suggests that the initial divergence between the northern and southern populations of mountain yellow-legged frogs occurred 2.2 million years before present. Within each of these groups, Macey et al. (2001) have detected a similar pattern of divergence that suggests the northern Sierra Nevada and central Sierra Nevada mountain yellow-legged frog populations diverged 1.5 million years before present, and the southern Sierra Nevada and the southern California mountain yellow-legged frog populations diverged from each other approximately 1.4 million years before present. Today, these 4 groups are isolated by arid valleys; this isolation is most pronounced between southern California and the southern Sierra Nevada. The biogeographic pattern of genetic divergence as detected in the mountain yellow-legged frogs of the Sierra Nevada has also been observed in four other reptiles and amphibians, suggesting a common event that fragmented their ranges (Macey et al. 2001).
Sierran frogs and southern California mountain yellow-legged frogs also differ ecologically in the types of aquatic habitat they occupy. Mountain yellow-legged frogs in southern California are typically found in steep gradient streams, even though they may range into small meadow streams at higher elevations (Zweifel 1955; Mullally 1959). In contrast, Sierran frogs are most abundant in high-elevation lakes and slow-moving portions of streams (Zweifel 1955; Mullally and Cunningham 1956), habitat that is distinct from the canyons of southern California's arid mountain ranges, which are inhabited by the southern California DPS of the mountain yellow-legged frog.
Significance. Under our DPS Policy, once we have determined that a population segment is discrete, we consider its biological and ecological significance to the larger taxon to which it belongs. This consideration may include, but is not limited to: (1) Evidence of the persistence of the discrete population segment in an ecological setting that is unusual or unique for the taxon; (2) evidence that loss of the population segment would result in a significant gap in the range of the taxon; (3) evidence that the population segment represents the only surviving natural occurrence of a taxon that may be more abundant elsewhere as an introduced population outside its historic range; or (4) evidence that the discrete population segment differs markedly from other populations of the species in its genetic characteristics.
We have found substantial evidence that all but one (there are no introduced populations of mountain yellow-legged frogs outside of its historic range) of these significant factors are met by the population of mountain yellow-legged frogs in the Sierra Nevada. Furthermore, it is significant because a major reduction in abundance of the species as a whole would occur if the Sierra Nevada population were extirpated. The extinction of the Sierra Nevada population of the mountain yellow-legged frog would result in the loss of a genetic entity, a reduction in the geographic range of the species, a loss of the species persistence in a setting ecologically unique relative to the ecological setting of the southern California population, and a reduction in the number of breeding populations. As discussed above, the Sierra Nevada population appears to be genetically distinct from the southern California population of mountain yellow-legged frogs. The mountain yellow-legged frogs of the Sierra Nevada comprise the main distribution of the species at the northern and central limits of the species' range. Loss of the Sierra Nevada population would be significant as it would eliminate the species from the majority of its range and would reduce the species to fewer than 10 small isolated sites in southern California (50 FR 44382). The geographic isolation of the Sierra Nevada population from the mountain yellow-legged frogs in southern California prevents genetic interchange between these populations.
Conclusion. We evaluated the Sierra Nevada population of the mountain yellow-legged frog to determine whether it meets the definition of a DPS, addressing discreteness and significance as required by our policy. We conclude that the Sierra Nevada population of the mountain yellow-legged frog is discrete from the southern California population, on the basis of their geographic separation, differences in vocalization, differences between their habitats, and apparent genetic differences. We conclude that the Sierra Nevada population of the mountain yellow-legged frog is significant because the loss of the species from the Sierra Nevada would result in a significant reduction in the species' range and its population numbers, and would constitute the loss of a genetically discrete population that differs markedly from the southern California population of mountain yellow-legged frogs. Because the population segment meets both the discreteness and significance criteria of our DPS policy, the Sierra Nevada portion of the mountain yellow-legged frog's range qualifies for consideration for listing. An evaluation of the level of threat to the DPS based on the five listing factors established by the Act follows.
Previous Federal Action
On February 10, 2000, we received a petition, dated February 8, 2000, from the Center for Biological Diversity and Pacific Rivers Council to list the Sierra Nevada population of the mountain yellow-legged frog as endangered. The petitioners stated that the Sierra Nevada population of the mountain yellow-legged frog qualifies for listing under our DPS Policy. On October 12, 2000, we published a 90-day finding on that petition in the Federal Register (65 FR 60603) concluding that the petition presented substantial scientific or commercial information to indicate that the listing of the Sierra Nevada population of the mountain yellow-legged frog may be warranted; we also requested information and data regarding the species.
This 12-month finding is made in accordance with a court order which requires us to complete a finding by January 10, 2003 (Center for Biological Diversity and Pacific Rivers Council v. Norton and Jones) (No. C 01-2106 SC). This notice constitutes the 12-month finding for the February 10, 2000, petition. Start Printed Page 2291
Summary of Factors Affecting the Species
Section 4 of the Act and regulations (50 CFR part 424) promulgated to implement the listing provisions of the Act describe the procedures for adding species to the Federal lists. We may determine a species (which is defined in section 3 of the Act as including any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature) to be endangered or threatened because of one or more of the five factors described in section 4(a)(1) of the Act. These factors, and their application to the Sierra Nevada DPS of the mountain yellow-legged frog (mountain yellow-legged frog), are as follows:
A. The present or threatened destruction, modification, or curtailment of its habitat or range. A number of hypotheses, including habitat loss, have been proposed for recent global amphibian declines (Bradford et al. 1993; Corn 1994; Alford and Richards 1999). Habitat destruction, however, does not appear to be the primary factor leading to the decline of the mountain yellow-legged frog. The mountain yellow-legged frog occurs at high elevations in the Sierra Nevada, which have not had the types or extent of large-scale habitat conversion and disturbances which have occurred at lower elevations (Bradford et al. 1993; Knapp 1996; Knapp and Matthews 2000). Large scale habitat conversion has not been identified within the range of this species; thus, direct habitat destruction or modification associated with intensive human activities, as measured by urban or agricultural land use within the mountain yellow-legged frogs' range, has not been implicated in the decline of this species (Davidson et al. 2002). However, other human activities have played a role in the modification of mountain yellow-legged frog habitat. These include livestock grazing, non-native fish introductions (see Predation, Factor C, below), timber management, road construction and maintenance, recreation, water diversions, fire management activities, and introduction of environmental contaminants (see Other, Factor E, below). These activities have modified habitat in ways that have fragmented and isolated mountain yellow-legged frog populations, and thereby, may have caused or contributed to the decline of this DPS (Bradford et al. 1993).
Grazing of livestock in Sierra Nevada meadows and riparian areas (aquatic ecosystems and adjacent upland areas that directly affect them) began in the mid-1700s with the European settlement of California (Menke et al. 1996). Following the gold rush of the mid-1800s, grazing rose to a level that exceeded the carrying capacity of the available range and caused significant impacts to meadow and riparian ecosystems (Meehan and Platts 1978; Menke et al. 1996). From 1870 to 1908, within the range of the mountain yellow-legged frog in the high Sierra Nevada, meadows were converted to summer rangelands for grazing cattle, sheep, horses, goats, and in some areas pigs; however, the alpine areas were mainly grazed by sheep (Beesley 1996; Menke et al. 1996). This practice resulted in the degradation of these extremely sensitive areas (Menke et al. 1996).
In general, livestock grazing within the range of the mountain yellow-legged frog was at a high but undocumented level until the establishment of national parks (beginning in 1890) and national forests (beginning in 1905). Within established national parks, grazing by cattle and sheep was replaced by that of packstock, such as horses and burros. Within established national forests, the amount of livestock grazing was gradually reduced and better documented, and the types of animals shifted, with reductions in sheep and increases in cattle and packstock. In general, livestock grazing within the national forests has continued with gradual reductions since the 1920s, except for an increase during World War II. Continuing decreases, motivated by concern towards resource protection, conflicts with other uses, and deteriorating range conditions, continued from the 1950s through the early 1970s but still exceeded sustainable grazing capacity in many areas (Menke et al. 1996; University of California (UC) 1996a). Grazing management that is more sensitive to riparian areas has been implemented and continues to increase since the 1970s (UC 1996a).
Packstock grazing is the only grazing currently permitted in the Sierra Nevada national parks. Packstock grazing also is permitted in national forests within the Sierra Nevada. However, there has been very little monitoring of the impacts of packstock use in this region (Menke et al. 1996). Use of packstock in the Sierra Nevada increased since World War II as a result of increased road access and increases in leisure time and disposable income (Menke et al. 1996). Demand for packstock use and recreational riding in the Sierra Nevada are projected to increase as California's human population increases (USFS 2001).
Observational data indicate livestock negatively impact mountain yellow-legged frog populations by altering frog habitat and trampling individuals (R. Knapp, in litt. 1993a, 1993b, 1994, 2002; Jennings 1996; A. Carlson, pers. comm. 2002; USFS 2002; V. Vrendenburg, in litt. 2002).
Livestock grazing causes changes in wetland systems, including meadows, streams, and ponds; modifies mountain yellow-legged frog habitat by removing overhanging banks that provide shelter; and contributes to the siltation of breeding ponds. Pond siltation may decrease the survivorship of overwintering larvae, subadults, and adult mountain yellow-legged frogs as the overwintering habitats need to be deep enough so that the entire water column does not freeze and underwater caves and crevices are available (Bradford 1983; Pope 1999a).
Grazing of livestock in riparian areas impacts vegetation in multiple ways, including: soil compaction, which increases runoff and decreases water availability to plants; herbage removal, which promotes increased soil temperatures and evaporation rates at the soil surface; and direct physical damage to the vegetation (Kauffman and Krueger 1984; Cole and Landres 1996; Knapp and Matthews 1996). Streamside vegetation protects and stabilizes streambanks by binding soils to resist erosion and to trap sediment (Chaney et al. 1990). A study by Kauffman et al. (1983) indicated that livestock grazing may have weakened the streambank structure through trampling and removal of vegetation, thereby promoting conditions for erosion. Removal of vegetative cover within mountain yellow-legged frog habitat decreases available habitat, exposes frogs to predation (R. Knapp, in litt. 1993b), and increases the threat of dessication (Jennings 1996). Grazing may result in changes to vegetation composition, resulting in an increased density of forested stands and the expansion of trees into areas that were formerly treeless (Cole and Landres 1996).
Livestock grazing can cause a nutrient loading problem due to urination and defecation in or near the water, and can elevate bacteria levels in areas where cattle are concentrated near water (Meehan and Platts 1978; Stephenson and Street 1978; Kauffman and Krueger 1984). The nutrient status of streams can markedly influence the growth of microflora and microfauna and directly and indirectly affect many other characteristics of the stream biota Start Printed Page 2292(Lemly 1998). Growth of filamentous bacteria on the bodies and gills of aquatic insects has been documented in association with nutrient loading in livestock use pastures, along with significantly lower densities of insects at downstream sites. In laboratory and field studies, aquatic insects with this bacterial growth experienced extensive mortality. This indicates that elevated bacteria levels associated with livestock use can negatively influence stream insect populations (Lemley 1998). Adverse effects to aquatic insects within the range of the mountain yellow-legged frog could result in a lowered prey availability, possibly increasing intraspecific competition for limited resources.
Throughout the range of the mountain yellow-legged frog in the Sierra Nevada approximately 79 currently active grazing allotments exist on USFS-administered lands. Of these grazing allotments, at least 29 have extant mountain yellow-legged frog populations within them. An estimated 13 percent of the approximately 210 known mountain yellow-legged frog populations, or populations that function as metapopulations, on Sierra Nevada national forests occur within active grazing allotments. Many of the mountain yellow-legged frog populations in the Sierra Nevada that occur within active grazing allotments are small. These populations may be more vulnerable to extirpation as a result of grazing-induced habitat modification, and if extirpated they might not be recolonized in situations where they are isolated from other populations and lack habitat connectivity to potential source populations.
In the 60-Lakes Basin of Kings Canyon National Park, packstock use is regulated in wet meadows to protect mountain yellow-legged frog breeding habitat in bogs and lakeshores from trampling and associated degradation (V. Vredenburg, in litt. 2002; H. Werner, NPS, in litt. 2002).
Recreation is the fastest growing use of national forests. As such, its impacts on the mountain yellow-legged frog are likely to continue and to increase (USDA 2001). Recreational activities take place throughout the Sierra Nevada and have significant negative impacts on several plant and animal species and their habitats (USDA 2001a). To further recreational opportunities and angling success, non-native trout stocking programs in the Sierra Nevada started in the late 19th Century (Bahls 1992; Pister 2001). Trout stocking throughout the range of the mountain yellow-legged frog has contributed to the decline of this species (see Predation, Factor C, below). The recreational impact of anglers at high mountain lakes has been severe in the Sierra Nevada, with most regions reporting a level of use greater than that which the fragile lakeshore environments can withstand (Bahls 1992).
Recreation may threaten all life stages of the mountain yellow-legged frog through direct disturbance resulting from trampling by humans, packstock, or vehicles, including off-highway vehicles; harassment by pets; and associated habitat degradation (Cole and Landres 1996; USFS 2001). Studies have not been conducted to determine whether recreational activities are contributing to the decline of the mountain yellow-legged frog, and recreation has not been implicated as a cause of major decline of the mountain yellow-legged frog.
Dams and Water Diversions
Dams and water diversions have altered aquatic habitats in the Sierra Nevada (Kondolf et al. 1996). Numerous reservoirs have been constructed within the range of the mountain yellow-legged frog. These include Huntington Lake, Florence Lake, Lake Thomas A. Edison, Saddlebag Lake, Convict Lake, Cherry Lake, and other reservoirs associated with Hetch Hetchy, Upper and Lower Blue Lakes, Lake Aloha, Silver Lake, Hell Hole Reservoir, French Meadow Reservoir, Lake Spaulding, and others. The extent of the impacts that these projects have had on the mountain yellow-legged frog is not known. The construction of dams probably has affected mountain yellow-legged frogs in the Sierra Nevada by altering their habitat and movements, and also by altering the distribution of predators (reservoirs are often stocked with non-native fish species that incidentally prey on mountain yellow-legged frogs (See Predation, Factor C, below)). Mountain yellow-legged frogs cannot live in or move through the exposed shorelines created by reservoirs, nor can they successfully reproduce in these environments with predatory fishes unless there are shallow side channels or disjunct pools that are free of predatory fishes (Jennings 1996).
Dams may alter the temperature and sediment load of the rivers they impound (Cole and Landres 1996). Dams, water diversions, and their associated structures can alter the natural flow regime with unseasonal and fluctuating releases of water, create habitat conditions unsuitable for native amphibians both upstream and downstream of dams, and act as barriers to movements by dispersing juvenile and migrating adult amphibians (Jennings 1996). Where dams act as barriers to mountain yellow-legged frog movement, they would effectively prevent genetic exchange between populations and the recolonization of sites. Water diversions that remove water from mountain yellow-legged frog habitat may adversely impact breeding success and adult survivorship if the diversion results in a lowering of the water level to the extent that the entire water column freezes in the winter, or to the extent that the habitat is rendered dry. These factors are likely to have contributed to the decline of mountain yellow-legged frogs and probably continue to pose a risk to the species.
Roads and Timber Harvest
Any activity that severely alters the terrestrial environment, including road construction and timber harvest, is likely to result in the reduction and extirpation of amphibian populations in the Sierra Nevada (Jennings 1996). Most of the mountain yellow-legged frog populations are in areas such as national parks or designated wilderness areas where timber is not harvested (Bradford et al. 1994a; Drost and Fellers 1996; Knapp and Matthews 2000). Some of these populations, and others outside of these areas, are located at too high an altitude for timber to be harvested, so this activity is not expected to affect the majority of extant mountain yellow-legged frog populations. There are some mountain yellow-legged frog populations in areas where timber harvests have occurred in the past and others where it may occur in the future. There are also roads within the range of the mountain yellow-legged frog; however, neither of these factors has been implicated as an important contributor to the decline of this species (Jennings 1996).
Fire Management Activities
Mountain yellow-legged frogs are generally found at high elevations in wilderness areas and national parks where vegetation is sparse and fire suppression activities are implemented infrequently. Potential impacts to the species resulting from fire management activities include: Water drafting (taking of water) from occupied ponds and lakes, resulting in direct mortality or rendering the habitat unsuitable for reproduction and survivorship; construction of fuel breaks either by hand or heavy equipment, potentially resulting in erosion and siltation of habitat; fire suppression with water applications or fire retardants; and Start Printed Page 2293increased human activity in the area, potentially disrupting mountain yellow-legged frog behavior.
Fire retardant chemicals contain nitrogen compounds and/or surfactants (a subset of chemical additives usually used to facilitate application). Laboratory tests of these chemicals have shown that they can cause mortality in fishes and aquatic invertebrates by releasing surfactants and ammonia when they are added to water (Hamilton et al. 1996), and similar effects are likely on amphibians. Therefore, if fire retardant chemicals were dropped in or near mountain yellow-legged frog habitat, they could have negative effects on individuals.
In some areas within the range of the mountain yellow-legged frog, long-term fire suppression has changed forest structure and conditions where fire severity and intensity are higher (McKelvey et al. 1996). Prescribed fire has been used by land managers to achieve various silvicultural objectives, including the reduction of fuel loads. In some systems, fire is thought to be important in maintaining open aquatic and riparian habitats for amphibians (Russel et al. 1999). But severe and intense wild fires may reduce the ability of amphibians to survive such a fire. However, amphibians display adaptive behavior that may minimize mortality from fire, by taking cover in wet habitats or taking shelter in subterranean burrows, though the moist and permeable skin of amphibians increases their susceptibility to heat and dessication (Russel et al. 1999). Neither the direct nor indirect effects of prescribed fire or wildfire on the mountain yellow-legged frog have been studied, but because the species generally occupies high elevation habitat, fire is not a likely risk to this species in much of its range.
In summary, historic grazing activities likely modified the habitat of the mountain yellow-legged frog throughout its range. Although grazing pressure has been significantly reduced from historic levels, grazing may continue to contribute to localized degradation and loss of suitable habitat, negatively affecting mountain yellow-legged frog populations. The effects of recreation, dams, water diversions, roads, timber harvests, and fire management activities on the mountain yellow-legged frog are not well studied, and though they may have negatively affected mountain yellow-legged frogs and their habitat, they have not been implicated as primary factors in the decline of this species (Bradford et al. 1993; Bradford et al. 1994a; Jennings 1996; Knapp and Matthews 2000). However, recreation, dams, water diversions, roads, timber harvests, and fire management activities may be factors of secondary importance in the decline of the mountain yellow-legged frog and the modification of its habitat (Jennings 1996).
B. Overutilization for commercial, recreational, scientific, or educational purposes. There is no known commercial market for mountain yellow-legged frogs, nor are there documented recreational or educational use for mountain yellow-legged frogs, although it is likely that they have been handled by cu