Pseudacris regilla Baird and Girard, 1852(b)
James C. Rorabaugh1
Michael J. Lannoo2
1. Historical versus Current Distribution. The distribution of Pacific treefrogs (Pseudacris regilla) includes southeastern Alaska (Waters, 1992), southern British Columbia (including Vancouver Island; Cook, 1980; Weller and Green, 1997), most of California, Oregon, Washington, and Nevada, the western half of Idaho (Nussbaum et al., 1983; Stebbins, 1985; Leonard et al., 1993), the western extremes of Montana (Marnell, 1997) and Arizona, southwestern Utah, and most of Baja California (Stebbins, 1985; Hollingsworth and Roberts, 2001). Pacific treefrogs have been introduced to the Queen Charlotte Islands, British Columbia (Reimchen, 1991), and probably to California City and Soda Springs in southern California (Stebbins, 1985), and in Arizona to plant nurseries near Phoenix and livestock waters in the Virgin Mountains, Mohave County (Rorabaugh et al., in review). Pacific treefrogs occur from below sea level to an altitude of 3,536 m (11,600 ft) in California (Brattstrom and Warren, 1955; Bezy and Goldberg, 1997). They occur to at least 1,585 m (5,200 ft) in Washington and to 2,247 m (7,370 ft) in Oregon (Leonard et al., 1993). In the Sierra Nevada, California, their distribution extends above timberline, indeed to all zones below Alpine–Arctic regions (J. Grinnell, C.L. Camp and J.R. Slevin, in Wright and Wright, 1949; Cochran and Goin, 1970; Stebbins, 1985; Leonard et al., 1993; Behler and King, 1998; G. Fellers, personal communication). In the Sonoran and Mojave deserts, including the arid regions of Baja California, Pacific treefrogs are generally restricted to springs, oases, rivers, and agricultural areas (Stebbins, 1985; Bezy and Goldberg, 1997; Hollingsworth and Roberts, 2001; Rorabaugh et al., in review). Reports of Pacific treefrogs from the mountains of central Arizona (Hobbs, 1932) are based on misidentified specimens of mountain treefrogs (Hyla eximia; Stebbins, 1951).
The current range of Pacific treefrogs is similar to their historical range. They are not considered to be declining in Canada (Weller and Green, 1997). Their range has not changed in 50–75 yr in Glacier National Park, Montana (Marnell, 1997). In the high Sierra Nevada of California, Pacific treefrogs are not declining, or they are declining much less dramatically than other species, such as yellow-legged frogs (Rana muscosa), and Yosemite toads (Bufo canorus; Bradford et al., 1994a; see also accounts, this volume). Surveys in Yosemite National Park suggest declines (G.M. Fellers and C.A. Drost, personal communication in Bradford et al., 1994a). In the Central Valley of California during 1990–'92, Pacific treefrogs were present in 27 of 28 counties in which they had been found historically (Fisher and Shaffer, 1996). At U.S. Naval holdings on Kitsap and Toandos peninsulas in western Washington, Pacific treefrogs are the most widespread lentic-breeding amphibian (Adams et al., 1999). Banta (1961) suggested that Pacific treefrogs were eliminated from a reach of the lower Colorado River, Nevada and Arizona, when the waters rose behind Davis Dam to form Lake Mohave.
2. Historical versus Current Abundance. Brattstrom and Warren (1955) comment that Pacific treefrogs are one of the most abundant amphibians in western North America. Slevin (in Wright and Wright, 1949) notes that Pacific treefrogs are probably the most abundant anuran in California. Klauber (in Wright and Wright, 1949) referred to the Pacific treefrog as “the common frog of the coastal areas.” Wright and Wright (1949) found one pond in Olympic Park, Washington, that contained between 500–800 egg masses. Working in Wallowa County, Oregon, Ferguson (1954b) observed that Pacific treefrogs were recorded from ten widespread locations and commented that they were common over most of the county. In the Willamette Valley, Oregon, Jameson (1957) suggests that the distribution of breeding sites serves as a limiting factor to both the occurrence and abundance of Pacific treefrog populations. He further notes that "Most of the semipermanent ponds and roadside ditches in the level flood plain of the Willamette Valley are utilized by these frogs." Jewett (1936) notes that around the Portland, Oregon, area in Multnomah County, Pacific treefrogs are "one of our commonest amphibians…found in numbers during March and April along the borders of sloughs and marshes." In a survey of Glacier National Park, Manville (1957) noted that while Pacific treefrogs were considered common, the Park collection contained only one specimen. Similarly, in Stevens County, Washington, Blanchard (1921) considered Pacific treefrogs to be undoubtedly common, although he collected only three specimens. Svihla and Svihla (1933b) working in Whitman County near Pullman, Washington, noted, "In the springtime, practically every roadside pool around Pullman resounds with the calls of these tree-toads. Egg masses are abundant in these pools by the middle of April." In the Willamette Valley, Oregon, Jameson (1957) observed that some breeding choruses occasionally contain over 500 males. Jameson (1956b, 1957) documented isolation among Pacific treefrog populations. His interpretation was that rapid selection could occur, but that isolation puts populations at risk.
At most localities, Pacific treefrogs are probably as common today as they were historically; indeed, Pacific treefrogs are usually the most abundant amphibians where they occur (G. Fellers, personal communication). Leonard et al. (1993) describe them as the most common frog in the Pacific Northwest. In the Sierra Nevada above 2,440 m, they are, by far, the most commonly encountered amphibians (Jennings et al., 1992). Pacific treefrogs are also common in artificial ponds in northwestern Idaho (Monello and Wright, 1999). Fellers (personal communication) has observed 1,500–2,000 egg masses at individual sites.
Pacific treefrog population sizes vary due to climatic conditions. Near Reno, Nevada, treefrog populations were eliminated at a breeding pond ten times from 1905–'89 due to flooding or drought, but were recolonized by individuals that avoided climatic extremes by taking refuge in moist, cool retreats (Weitzel and Panik, 1993). Fewer male treefrogs call during cold, windy nights, as compared to warm, calm nights (Brenowitz and Rose, 1999); thus, any assessments of relative abundance based on call counts must take into account weather conditions.
South of Reno, Nevada, breeding Pacific treefrogs at a semipermanent pond, 21 m in length, peaked at a mean of 60 frogs (53–66; Weitzel and Panik, 1993). In northern Idaho, breeding males at five ponds numbered 360 during 1 yr, 160 the next (Schaub and Larsen, 1978). Calling male treefrogs space themselves in wetland habitats about every 75 cm, limiting the number of calling males in an area (Whitney and Krebs, 1975). In June 1992 at a site in northern California, Pacific treefrog tadpole density was 16.25 ± 7/m2 (mean ± 1 SD) and ranged from 0–350. One month later, mean density had dropped to 8.5 tadpoles/m2. Tadpole density was judged to be relatively high in 1992 (Kupferberg, 1997a).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. From November–July, Pacific treefrogs move from the cool, moist terrestrial retreats they use as overwintering sites to aquatic breeding sites (Weitzel and Panik, 1993). In western Oregon and Washington, breeding migrations are triggered by warm (5–10 ˚C) winter rains (Nussbaum et al., 1983; Weitzel and Panik, 1993). In northern Idaho, no adult frogs were found in breeding sites from mid June to early April (Schaub and Larsen, 1978). In Los Angeles County, California, Pacific treefrog females were present in large numbers and actively approached males only when wet bulb air temperatures ranged from 12.9–14.7 ˚C (Straughan, 1975). Individuals probably home to the same general area every year and then migrate to the nearest pond or to a pond with the loudest chorusing (Schaub and Larsen, 1978). A few male Pacific treefrogs enter breeding ponds first and begin calling, which attracts other males and females (Weitzel and Panik, 1993).
Gary Fellers (personal communication) observes that the peak of egg laying is from late winter to late spring, but that egg laying lasts well into the summer in the mountains. Leonard et al. (1993) indicate a breeding season from January–July, with this latter date observed in a population at 2,200–2,620 m in the Sierra Nevada (Livezey, 1953). Pacific treefrogs breed from November–July in southern California (Brattstrom and Warren, 1955; Perrill, 1984; Hollingsworth and Roberts, 2001) and were heard calling from November–June on the Colorado River, Arizona to California (Rorabaugh et al., in review). South of Reno, Nevada, Pacific treefrogs breed from February–May, with breeding peaking in March (Weitzel and Panik, 1993). In Glacier National Park, Montana, frogs breed in April–May, when water temperatures are 10–12 ˚C (Marnell, 1997). Jameson (1957; see also Schaub and Larsen, 1978) notes that males will call as late as August at elevations above 610 m, and that during the height of the breeding season, males will call 24 hr/d, with the greatest intensity during the hours following sunset. In the Willamette Valley, Washington, females stay at the breeding sites an average of 9.6 d (1–27) and males stay on average 33 d (1–90). In northern Idaho, males spent 10–13 d at breeding ponds (Schaub and Larsen, 1978). However, in Nevada, males stayed at the breeding ponds for the entire breeding season (February–May; Weitzel and Panik, 1993).
After breeding, frogs leave the pond and move about on the ground or in low shrubbery (Leonard et al., 1993), or move back to cool, moist retreats used for overwintering or aestivation; however, a few individuals remain at the breeding pond (Weitzel and Panik, 1993). Breeding males may switch wetlands during the breeding season and can move at least 400 m to adjacent ponds (Schaub and Larsen, 1978).
There is considerable geographic variation in call patterns (Snyder and Jameson, 1965; see also Foster, 1967; Allan, 1973; Straughan, 1975; Whitney, 1980, 1981). Calls are given infrequently during the day and reach a maximum about 1 h after sundown (Foster, 1967). Male breeding behavior is detailed by Perrill (1984). Breeding animals displaced 274 m returned home to their breeding wetland (Jameson, 1957). Svihla and Svihla (1933b) noted a post-breeding adult in an alfalfa field moving back to the woods.
ii. Breeding habitat. Includes most aquatic habitats, including lakes, ponds, slow-moving streams, backwaters of large rivers, wet meadows, emergent marshes, forested swamps, reservoirs, muskegs, supratidal pools, golf course ponds, and irrigation ditches (Wright and Wright, 1949; Brattstrom and Warren, 1955; Stebbins, 1985; Waters, 1992; Leonard et al., 1993; Gardner, 1995; Rorabaugh et al., in review). Pacific treefrogs breed in both temporary and permanent waters. In the Pacific Northwest, they are often found breeding in fishless, ephemeral wetlands that dry up before mid summer (Leonard et al., 1993). They are most likely to use shallow, quiet waters for breeding, especially waters with submerged and/or emergent vegetation (Nussbaum et al., 1983). Breeding may occur in weakly brackish waters (Stebbins, 1951; Gardner, 1995). At the Angelo Coast Range Reserve in northern California, Pacific treefrogs breed in wet meadows from early spring to late summer and then switch to breeding in a river as the dry season progresses (Kupferberg, 1997a). In northeastern California, Pacific treefrogs breed in isolated temporary ponds surrounded by semiarid habitats (Watkins, 1996). Within wetlands, breeding populations appear to be clumped, and the location of these aggregations may be the result of frogs being attracted to calling individuals and choruses rather than characteristics of the habitat (Whitney and Krebs, 1975).
In southern California, frogs call at water temperatures of 10–12 ˚C and avoid large lakes or cold, spring-fed streams in which water temperatures remain low. Frogs seldom, if ever, sing at water temperatures above 20 ˚C (Brattstrom and Warren, 1955). In northern Idaho, frogs chorused in water temperatures as low as 2.0 ˚C and air temperatures of 0.5 ˚C. However, they preferred warmer, more open ponds (Schaub and Larsen, 1978).
i. Egg deposition sites. In Glacier National Park, Montana, Pacific treefrogs breed in April–May when water temperatures are 10–12 ˚C (Marnell, 1997). Jameson (1957) observed that within ponds, shallower, more heavily vegetated portions are used for breeding. Leonard et al. (1993) concurred, noting that eggs are laid on submerged aquatic vegetation, including grass, stems, and sticks; however, in shallow water eggs may be deposited on the bottom (Hollingsworth and Roberts, 2001). Egg masses are commonly found at depths ≤ 10 cm and may be found floating, attached to objects at the surface (Stebbins, 1951). Egg masses are observed in waters with temperatures from 3–30 ˚C (average about 11 ˚C; G. Fellers, personal communication).
On Vancouver Island, British Columbia, ambient levels of UV-B radiation, and levels 15–30% above ambient, did not affect the hatching success of Pacific treefrog eggs (Ovaska et al., 1977). Hatching success under ambient levels of UV-B radiation was also unaffected in the Oregon Cascade Mountains (Blaustein et al., 1994c) and in the Santa Monica Mountains, California (Anzalone et al., 1998). Compared to some other anurans, Pacific treefrog eggs are resistant to the effects of UV-B radiation due to differential activity of a photolyase that functions to repair UV-B-induced DNA damage through its action on cyclobutane pyrimidine dimmers (Blaustein et al., 1994c).
Acidified habitats may not be affecting egg survival of Pacific treefrogs in the Sierra Nevada. The estimated extreme pH in Sierra Nevada waters is 5.0; in the laboratory, sensitivity to acidic waters (LC50) for Pacific treefrog eggs averages a pH of 4.3 (Bradford et al., 1994b,c).
Eggs can survive temperatures as low as -5 to -7 ˚C for up to 2 hr, and as high as 34 ˚C (Brattstrom and Warren, 1955). Embryos develop and hatch in 1–5 wk (Nussbaum et al., 1983; Leonard et al., 1993; Hollingsworth and Roberts, 2001). In the laboratory, larvae may hatch in 6 d (Pickwell, 1947).
ii. Clutch size. Females lay between 400–750 eggs (maximum 1,250), 1.3 mm in diameter, in clusters composed of 9–80 eggs (average 18; see also Stebbins, 1951; Gaudin, 1965; Nussbaum et al., 1983, Perrill and Daniel, 1983; Leonard et al., 1993; and Marnell, 1997). There may be geographic variation in clutch size. In San Diego, California, mean clutch size was 267.7 (range 62–633; Perrill and Daniel, 1983). Further, Storer (in Wright and Wright, 1949) speculates that size of egg clutches may reflect degree of solitude in mating pairs. Where few pairs mate, egg cluches are larger. Stebbins (1951) notes that towards the close of egg laying, clutch size tapers off to 3 or 4 or even single eggs. Females may produce ≥ 3 clutches/season. Average time between clutch deposition is 37.3 d (range of 13–69; Perrill and Daniel, 1983).
i. Length of larval stage. In western Oregon, 2 mo; 2.5 mo in northern Idaho (Nussbaum et al., 1983). Metamorphosis occurs from June–October, depending on altitude and latitude. In the Pacific Northwest, tadpoles metamorphose in June at low elevations and late August at higher elevations (Leonard et al., 1993). In northwestern Idaho, tadpoles metamorphose from mid July to mid September (Schaub and Larsen, 1978). Dill (1977) reported tadpoles in British Columbia as late as August.
ii. Larval requirements. Pacific treefrog tadpoles typically are found in quiet waters or sluggish, slow streams. At a pond near Reno, Nevada, tadpoles tended to aggregate in deeper (≤ 1.2 m) water. In enclosure experiments, larval survival was lower in permanent ponds as compared to ephemeral wetlands, however, no cause of this difference was evident (Adams, 2000). Compared with other anuran species, including two species of spadefoot toads (Pelobatidae), Pacific treefrog tadpoles are the least heat resistant (H.A. Brown, 1969). Pacific treefrog tadpoles apparently prefer temperatures around 19–20 ˚C, but can tolerate low temperatures of 0–2 ˚C and high temperatures to 33 ˚C, with lethal temperatures about 5 ˚C higher (Brattstrom and Warren, 1955). Weitzel and Panik (1993) note that tadpoles grew rapidly at water temperatures of 12–18 ˚C. Tadpoles are tolerant of weakly saline conditions (Stebbins, 1951; Gardner, 1995).
Tadpoles are highly sensitive to nitrites, which cause reduced feeding, less vigorous activity, disequilibrium, other abnormalities, and increased mortality. Substantial mortality occurred at Environmental Protection Agency recommended nitrite concentrations (1 mg N–NO2/L) for drinking water (Marco et al., 1999). Nitrogen compounds from agricultural runoff or drainage into Pacific treefrog habitats could harm tadpoles (Schuytema and Nebeker, 1999a,b). In the laboratory, the herbicide diuron causes limb deformities and reduced growth in tadpoles, but experimental concentrations were much higher than those found in the field (Schuytema and Nebeker, 1998). Compared to other vertebrates, Pacific treefrog tadpoles are relatively tolerant of the pesticides Guthion and Guthion 2S (Nebeker et al., 1998).
The LC50 pH value for Pacific treefrog tadpoles is 4.23 (Bradford et al., 1994b). The lowest pH likely to occur in surface waters of the Sierra Nevada is 5.0 (Bradford et al., 1992), thus acidification is not likely to cause acute toxicity to Pacific treefrog tadpoles in the Sierra Nevada. This supposition was borne out by data from Sequoia National Park (Soiseth, 1992) and elsewhere in the Sierra Nevada (Bradford et al., 1994c). Ambient levels of UV-B radiation near Victoria, British Columbia, did not affect tadpole survivorship; however, survival in the first 2 mo of development was substantially lower under enhanced (15–30% over ambient) levels of UV-B (Ovaska et al., 1997).
a. Food. Wassersug (1976) considered Pacific treefrog tadpoles to be typical pond tadpoles, possessing a generalized oral morphology. Typical pond tadpoles are generally considered to be non-discriminatory suspension feeders that ingest a variety of prey including green algae, blue-green algae, bacteria, diatoms, protozoa, and a wide variety of organic and inorganic debris (Wassersug, 1975; Wagner, 1986). However, Kupferberg (1997b) demonstrates that Pacific treefrog tadpoles can select those foods that favor optimal growth. In feeding trials, tadpoles selected algae with relatively high protein content. Tadpole diets may be low in protein, and addition of protein to their diets increases size at metamorphosis (Kupferberg, 1997b). Wagner (1986) describes Pacific treefrog tadpoles with a specialized feeding behavior—ingesting pine (Pinus sp.) and fir (Abies sp.) pollen, when it is seasonally available on the water surface. In the presence of pollen, tadpoles altered their behavior to feed at the surface. Tadpoles that consumed diets rich in diatoms had enhanced growth, development, and survival to metamorphosis (Kupferberg et al., 1994). Pacific treefrog tadpoles may be good at extracting low quality or low availability resources. Kupferberg (1997a) observed tadpoles swimming on their backs at the surface, grazing on epineustic films of diatoms.
b. Cover. Brattstrom (1962b) noted that tadpoles will aggregate and speculated that by doing so they were thermoregulating. Aggregation is also an anti-predator mechanism (De Vito et al., 1999). Pearl et al. (2003) found that Pacific treefrog tadpoles will not seek cover in the presence of either fish or invertebrate predators.
iii. Larval polymorphisms. Do not occur.
iv. Features of metamorphosis. Tadpoles attain a total length of 45–55 mm prior to metamorphosis (Nussbaum et al., 1983). Newly metamorphosed frogs are < 10–17.0 mm SVL (Wright and Wright, 1949; G. Fellers, personal communication). In one western Oregon study, recently metamorphosed frogs averaged 13.8 mm (Nussbaum et al., 1983). Metamorphosis is prolonged, for example from mid July to mid September in northern Idaho (Schaub and Larsen, 1978), and May–October in California (G. Fellers, personal communication).
v. Post-metamorphic migrations. Metamorphosed frogs leave the natal ponds soon after transformation and move to upland habitats or overwintering sites in mid summer to early fall (Schaub and Larsen, 1978), similar to the adults (see “Breeding migrations" above and “Hibernation" below). In Nevada, all frogs had exited the natal ponds by early October (Weitzel and Panik, 1993). In Montana, metamorphosed frogs disperse by late August and return in the spring as adults. After leaving the ponds, dispersing individuals were found perched 1–1.5 m above the ground in broad leaf shrubs (Marnell, 1997). Newly metamorphosed animals disperse, but apparently not far enough in some populations to form strong metapopulation links (Jameson, 1956b, 1957; Schaub and Larsen, 1978). Juveniles have been recaptured as far as 238 m from their home pond (Jameson, 1956b).
D. Juvenile Habitat. Similar to adult habitats. In Glacier National Park, Montana, juveniles were found in shallow waters among shoreline vegetation at the edges of ponds (Marnell, 1997).
E. Adult Habitat. Adults use a variety of aquatic habitats for breeding, then move upland where they move about in low shrubbery during moist weather. In dry periods or habitats, Pacific treefrogs tend to be more nocturnal (Leonard et al., 1993), and will seek moist, cool retreats for aestivation and for hibernation in the fall (Leonard et al., 1993; Weitzel and Panik, 1993). No differences between male and female habitat characteristics are known.
The name “treefrog” is a misnomer because, although Pacific treefrogs sometimes climb short distances into shrubs or trees, they are usually found on the ground (Dickerson, 1906; Stebbins, 1951; Badaracco, 1962). They are found especially near streams, springs, ponds, wetlands, irrigation ditches, and other moist places (Wright and Wright, 1949; Stebbins, 1985). In these habitats, Pacific treefrogs are found in low plant growth, damp recesses among rocks and logs, under tree bark, in trees in damp forests, and in animal burrows in open country (Wright and Wright, 1949). They can be found far from water outside of the spring–summer breeding season (Stebbins, 1951, Badaracco, 1962; Nussbaum et al., 1983; Leonard et al., 1993). Svihla and Svihla (1933b) collected adults in irrigation ditches in "sagebrush country along the Snake River Canyon." Schaub and Larsen (1978) note that few North American anurans exploit such a variety of habitats, including deserts, grasslands, mountains, and the rain forests of the Pacific Northwest. In western Washington, Pacific treefrogs are associated with open wetland habitats in forests of saplings and in clearcuts (Bosakowski, 1999). Raphael (1988) found Pacific treefrogs to be associated with early successional Douglas-fir forests of northwestern California. However, Welsh and Lind (1988, 1991) found that captures increased from young to older growth forests and were higher in mesic versus wet forests in northwestern California and southwestern Oregon.
Cunningham and Mullally (1956) note that Pacific treefrog adults are active to 4 ˚C and detail other thermal relations using a combination of field observations and experimental trials. Mean critical thermal maxima for Pacific treefrogs from near Clinton, Montana, ranged from 34.8–35.2 ˚C. Pacific treefrogs prefer lower temperatures at night than during the day (Claussen, 1973). Brattstrom (1963) reports 3.8 and 24.0 ˚C as the minimum and maximum temperatures voluntarily tolerated by Pacific treefrogs. Croes and Thomas (2000) demonstrated freeze tolerance in Pacific treefrogs from northern California. In response to freezing, plasma glucose increased 5–14-fold. The liver is the organ responsible for cryoprotectant synthesis.
Adults are polymorphic in their dorsal body color. Individual frogs become lighter or darker and can lose their spots in response to environmental conditions, but the green and brown color phases and the black eye stripe are genetically determined and do not change (Brattstrom and Warren, 1955; Weitzel and Panik, 1993). The green color phase is more absorptive of solar radiation and may be favored in aquatic habitats. Green may, however, be a disadvantage in hot, dry, terrestrial conditions, where the brown phase may be favored (Jameson and Pequegnat, 1971; Stebbins and Cohen, 1995).
F. Home Range Size. Home ranges include upland activity, overwintering and aestivation sites, breeding ponds, and migratory corridors between these habitats. However, activities and locations of Pacific treefrogs when not at the breeding ponds are poorly documented. Apparent overwintering sites were about 60 m above and 150–300 m from breeding habitats in southern California (Brattstrom and Warren, 1955). Leonard et al. (1993) report active frogs on the ground and in low shrubbery during moist weather outside of the breeding season. During the breeding season, adults usually remain at the same breeding pond; however, Schaub and Larsen (1978) documented movements by breeding males of up to 400 m.
G. Territories. Male Pacific treefrogs produce two types of advertisement calls (monophasic and diphasic) and an aggressive encounter or staccato/trill call (Allan, 1973). The encounter call is given at the beginning of chorusing each evening, possibly to establish spacing between males (Allan, 1973; but see Whitney, 1981). Encounter calls may also be given through the night if another male approaches too closely (Awbry, 1978). Brenowitz and Rose (1999) found that the amplitude of either the encounter or advertisement call perceived by a male is an important trigger in eliciting the encounter call. Switching to the encounter call also occurs in response to movement by an intruder (Snyder and Jameson, 1965; Allan, 1973; Whitney, 1981). Typical spacing among males is about 75 cm (Whitney and Krebs, 1975; Awbrey, 1978). Males switch from the advertisement call to the encounter call when another frog approaches within about 20–50 cm (Awbrey, 1978; Whitney, 1980) or the amplitude of the call exceeds 87 dB (Brenowtiz, 1989). Some authors have found that calling males have a strong connection to a particular location, as evidenced by displacement experiments (Perrill, 1984) and mark-recapture data (Jameson, 1957). However, Whitney and Krebs (1975) found that calling males usually occupied calling sites for only ≤ 1 night. As a result, they concluded that the frogs established spacing or individual distances, rather than territories. However, the frogs defend these “spaces” with territorial behavior (Fellers, 1979a).
Whitney (1980) suggests calling males have three lines of defense: (1) the advertisement call that establishes the male’s presence, (2) the encounter call when another male approaches, and (3) fighting. In response to the encounter call, males may submit by retreating or ceasing to call, they may perform a “bouncing” behavior or may resort to physical encounters that involve butting and wrestling (Fellers, 1979a; Whitney, 1980; Perrill, 1984). Fights typically end with one frog clasping the other anterior to the front limbs, causing the clasped frog’s vocal sacs to deflate. The deflated frog then moves away (Whitney and Krebs, 1975) or becomes subordinate and may sit silently close by the victorious, calling male (Fellers, 1979a). Some males are silent (satellite males) and intercept and mate with females that are attracted to calling, territorial males (Perrill, 1984). Territorial behavior of Pacific treefrogs is similar to several other North American Hylidae (Fellers, 1979a).
H. Aestivation/Avoiding Dessication. Habitat use of Pacific treefrogs is poorly known outside of the breeding season. However, when away from breeding areas, particularly during dry periods or in dry areas, the species may be found in cool, moist retreats such as piles of debris, dense vegetation, rock or log crevices, mammal burrows, artificial drains, basements of homes and buildings, spring boxes, housing units for sprinkler system valves, and other protected places (Brattstrom and Warren, 1955; Nussbaum et al., 1983; Leonard et al., 1993; Weitzel and Panik, 1993). Brattstrom and Warren (1955) found that in California, Pacific treefrogs must seek hiding places during the hot, dry months of July–October. Pacific treefrogs were one of the few vertebrates to survive in the 150,000 acre-blast zone during the eruption of Mount St. Helens. Apparently, individuals that were underground were spared (Weyerhaeuser, 1999).
I. Seasonal Migrations. Most Pacific treefrogs move from overwintering sites to breeding sites in winter or spring and then leave the breeding sites by early fall. Habitat use and movements outside of the breeding season are poorly known. See “Breeding migrations” and “Aestivation/Avoiding Dessication" above and “Hibernation" below for further details.
J. Torpor (Hibernation). In some localities at lower elevations, Pacific treefrogs are active throughout the year (Stebbins, 1951); elsewhere, Pacific treefrogs must avoid cold temperatures. In western Montana, Pacific treefrogs hibernate in subterranean shelters (Cunningham and Mullally, 1956). At elevations above 2,500 m in the Sierra Nevada, California, frogs overwinter in terrestrial shelters (Bradford, 1989). A drought in the winter of 1976–'77 in northern Idaho severely reduced snowpacks, which likely resulted in ground freeze at greater depths than normal. Ground freeze probably increased overwintering mortality of Pacific treefrogs, which was reflected in reduced populations in the spring and summer of 1977 (Schaub and Larsen, 1978). Near Gorman, California, Pacific treefrogs were found calling in early January from 2.5 cm diameter holes about 60 m above and 150–300 m from breeding habitat. Holes contained 1–5 frogs each (Brattstrom and Warren, 1955).
K. Interspecific Associations/Exclusions. Pacific treefrogs will breed in association with many species of western amphibians. At Fort Lewis Military Reservation, Washington, they are found in the same wetlands as northwestern salamanders (Ambystoma gracile), long-toed salamanders (A. macrodactylum), rough-skinned newts (Taricha granulose), western toads (Bufo boreas), northern red-legged frogs (Rana aurora), and American bullfrogs (Rana catesbeiana; Adams et al., 1998). In the Sierra Nevada, Pacific treefrogs occur with mountain yellow-legged frogs (Pope, 1999). On the lower Colorado River, Arizona–Nevada–California, Pacific treefrogs breed in the same habitats as Great Plains toads (B. cognatus), Woodhouse’s toad (B. woodhousei), and bullfrogs (J.C.R., personal observations). In August 1925 near Las Vegas, Nevada, Wright and Wright (1949) found Pacific treefrogs in association with Vegas Valley leopard frogs (R. fisheri, now extinct) and Great Plains toads. Pacific treefrogs occur, or occurred until recently, with relict leopard frogs (R. onca) and red-spotted toads (B. punctatus) near Hoover Dam, Nevada–Utah (R.D. Jennings, 1995b), and occurred historically with lowland leopard frogs (R. yavapaiensis) at San Felipe Creek, California (Ruibal, 1959), before lowland leopard frogs were extirpated from that site. Brattstrom and Warren (1955) observed breeding in association with California toads (B. boreas halophilus) and western spadefoot toads (Spea hammondi). In southern Nevada, Pacific treefrogs occur with Amargosa toads (B. nelsoni; Wright and Wright, 1949). Wright and Wright (1949) note an observation of a male western spadefoot toad in amplexus with a female Pacific treefrog. The latter died soon after, apparently from a rupture of the abdominal wall. In the Central Valley and in southern California, Pacific treefrogs show less variation in recruitment across years, and presence of tadpoles is less reliant on rainfall compared to California toads or western spadefoot toads (Fisher and Shafer, 1996).
Brattstrom and Warren (1955) suggest that Pacific treefrogs compete with California treefrogs (Hyla cadaverina), noting that Pacific treefrogs are less abundant, even absent, where California treefrogs occur. These authors collected an apparent hybrid of these two species in San Diego County, California. Littlejohn (1971) observed mixed choruses of Pacific treefrogs and California treefrogs in Whitewater Canyon, southern California.
Tadpoles of Cascade frogs (Rana cascadae) and Pacific treefrogs have similar diets and larval periods and frequently breed in the same ponds in the Oregon Cascades (Nussbaum et al., 1983). Keisecker and Blaustein (1999) demonstrated that Cascade frog tadpoles had strong negative effects on the growth, development, and survival of Pacific treefrog tadpoles. However, in the presence of the water mold (Saprolegnia ferax), competitive interactions were reversed. Pacific treefrog tadpoles had higher survival, faster development, and were larger at metamorphosis when exposed to both Saprolegnia and Cascade frog tadpoles, as compared to exposure to Cascade frog tadpoles alone.
Pacific treefrogs are also found in ponds with introduced American bullfrogs, although Brattstrom and Warren (1955) never found them closer than 1.2 m to the bullfrogs. Average daily survival rate for Pacific treefrog tadpoles did not differ between ponds that contained or did not contain bullfrog tadpoles, although in the presence of bullfrogs, later development was slowed (Govindarajulu, 2000). In enclosure experiments, Pacific treefrog larval survival was not affected by the presence of American bullfrog larvae (Adams, 2000). However, in northern California, exploitative competition from large overwintering bullfrog tadpoles reduced survivorship and growth of Pacific treefrog tadpoles. Competition from recently hatched bullfrog tadpoles also reduced survivorship of Pacific treefrog tadpoles. Nevertheless, Pacific treefrogs are tolerant of bullfrog invasions because their tadpoles are good competitors, and they have a broad range of physical tolerances and can reproduce in shallow, ephemeral habitats unsuitable for bullfrogs (Kupferberg, 1997a).
In the presence of Oregon garter snakes (Thamnophis atratus hydrophilus), Pacific treefrog tadpoles moved less and spent more time in lower quality food patches. At these sites, reduced activity resulted in the tadpoles sinking away from floating Cladophora algal mats, which are high quality foods for tadpoles (Kupferberg, 1997b). In the Sierra Nevada above 2,440 m, the presence of amphibians is a prerequisite for the presence of western terrestrial garter snakes (T. elegans). As the most common amphibian in the region, if Pacific treefrogs declined in the Sierra Nevada, sympatric populations of garter snakes could also disappear (Jennings et al., 1992; Matthews et al., 2002). Jameson (1956b) attributed a rapid increase in a Pacific treefrog population to removal of garter snakes and American bullfrogs.
In high elevation lakes of the Sierra Nevada, Pacific treefrog tadpoles are not found in lakes that support salmonid fish (rainbow trout [Oncorhynchus mykiss] and/or brook char [Salvelinus fontinalis]). However, Pacific treefrogs often breed in lakes that are shallow and ephemeral, those which are not suitable for fishes (Bradford, 1989). At Glacier National Park and ponds in northwestern Idaho, Pacific treefrogs were only found in waters that lacked fish (Marnell, 1977; Monello and Wright, 1999). Monello and Wright (1999) suggested egg predation by goldfish (Carassius auratus) eliminated Pacific treefrogs and other amphibians from northwestern Idaho ponds. In enclosure experiments, presence of sunfish (Centrarchidae) reduced survival of Pacific treefrog tadpoles to near zero (Adams, 2000).
Overwintering Pacific treefrogs were found in holes near Gorman, California, with side-blotched lizards (Uta stansburiana; Brattstrom and Warren, 1955).
L. Age/Size at Reproductive Maturity. Males at 25.5–48.0 mm SUL; females at 25.0–47.0 mm SUL. Schaub and Larsen (1978) and Leonard et al. (1993) note that females are larger than males. Sexual maturity probably occurs in < 1 yr (Jameson, 1956b, 1957; Nussbaum et al., 1983; Weitzel and Panik, 1993), although Cochran and Goin (1970) and Pickwell (1947) state that Pacific treefrogs take at least 2 yr to reach sexual maturity.
M. Longevity. Unknown. Of 65 Pacific treefrogs captured in 1977 at a series of wetlands in northern Idaho, 9 (13.8%) had been marked the year before (Schaub and Larsen, 1978).
N. Feeding Behavior. Pacific treefrogs are primarily nocturnal, terrestrial foragers (Brattstrom and Warren, 1955; Johnson and Bury, 1965). However, during the breeding season, male Pacific treefrogs apparently feed during the day (Whitney and Krebs, 1975). Food habits of Pacific treefrogs include isopods, spiders, snails, and a variety of insects first reported by Needham (1924, in Brattstrom and Warren, 1955), summarized in Brattstrom and Warren (1955), and listed in Johnson and Bury (1965). Insects constituted 73.5% of the winter diet of 135 Pacific treefrogs collected from northern California. More adult insects were eaten than larvae, indicating frogs primarily catch flying insects (Johnson and Bury, 1965). Pacific treefrogs typically feed above water, either at the surface or in vegetation above the water surface (Brattstrom and Warren, 1955).
O. Predators. Predators on Pacific treefrogs include mountain garter snakes (T. e. elegans), common garter snakes (T. sirtalis), Oregon garter snakes (Livezey, 1953; Schaub and Larsen, 1978; Kupferberg, 1998; Mathews et al., 2002), northern red-legged frogs (Arnold and Halliday, 1986), mountain yellow-legged frogs (Pope, 1999), American bullfrogs (Cook and Jennings, 2001), northwestern salamanders, egrets and herons (Ardeidae), belted kingfishers (Megaceryle alcyon), various species of fish (Bradford, 1989; Goodsell and Kats, 1999; Monello and Wright, 1999), and mammals such as raccoons (Procyon lotor), skunks (Mustelidae), feral cats (Felix domestica), and opossums (Didelphis marsupialis; Storer, 1925; Brattstrom and Warren, 1955; Peterson and Blaustein, 1991; Weitzel and Panik, 1993). In the Oregon Cascade Mountains, some populations of Pacific treefrogs undergo intense predation of eggs by predatory leeches (Glossiphonidae and Erpobdellidae; Chivers et al., 2001). Pacific treefrog populations remain robust in the Santa Monica Mountains, California, despite heavy predation by mosquito fish (Gambusia affinis; Goodsell and Kats, 1999).
P. Anti-Predator Mechanisms. Tevis (in Wright and Wright, 1949) observed that in response to disturbance, Pacific treefrogs swim to and hide in masses of filamentous algae or hop upslope to dry brush cover, rather than into water. The latter observation is considered unreliable by G. Fellers (personal communication). Brattstrom and Warren (1955) found that if flushed, frogs jump into water. In a laboratory experiment, frogs presented with a model predator typically jumped at a mean angle of 70˚ from the frog’s initial bearing. Most frogs have longer right limbs and generally jump to the right, suggesting “handedness” (Dill, 1977).
Movement is not always an advantageous predator defense, however. As mentioned above (see "Adult Habitat"), Pacific treefrogs are polymorphic for dorsal body color, being either green or brown. When stationary on a matching substrate, laboratory studies have shown they are able to avoid predation by terrestrial garter snakes (T. elegans) by remaining motionless (Morey, 1990).
In the laboratory, Pacific treefrog tadpoles aggregate as an anti-predator mechanism (De Vito et al., 1999). Tadpoles also exhibit burst swimming in response to predators, and the faster tadpoles are more likely to avoid predation from common garter snakes (Watkins, 1996). However, on the Eel River in northern California, tadpoles reduced their activity in the presence of Oregon garter snakes and tended to sink toward the bottom, away from high quality food resources—floating algal mats. Garter snake predation did not substantially reduce the number of tadpoles on the Eel River, but reduced tadpole growth by 28%, because tadpoles spent less time feeding in floating algal mats (Kupferberg, 1998).
In experimental trials, as the relative size of tadpoles increased, the anti-predator response decreased in the presence of larval northwestern salamanders. The results provide evidence that Pacific treefrog tadpoles are able to assess their individual vulnerability to northwestern salamander larvae and adjust their anti-predator response according to their level of risk (Puttlitz et al., 1999).
Pacific treefrog eggs show plasticity in timing of hatching in response to the threat of predation. Eggs hatch sooner and at an earlier developmental stage when eggs come in contact with predatory leeches, chemical cues of leeches, or chemicals released from injured eggs (Chivers et al., 2001).
Dickerson (1906) found that when Pacific treefrogs were "greatly annoyed" or injured, they would exude a milky secretion from the skin of the dorsum. However, this behavior has not been noted by other authors.
Q. Diseases. A pathogenic fungus (Saprolegnia ferax) infects egg masses of Pacific treefrogs. Eggs in communal masses and eggs laid later in the season are most susceptible to infection (Kiesecker and Blaustein, 1997b). Chytridiomycosis, an amphibian fungal disease of global distribution, has been found in a California population of Pacific treefrogs (Fellers et al., 2001). Brattstrom and Warren (1955) mention a haemorrhagic condition of the gut that develops in captive tadpoles reared without access to natural (mud and sand) substrates.
Chlorpyrifos and diazinon, potent organophosphorus cholinesterase inhibitors and several other pesticides are carried via prevailing summer winds from the agricultural areas of the Central Valley to the Sierra Nevada, California. Chlorpyrifos and diazinon bind with cholinesterase and disrupt neural function. Cholinesterase activity in Pacific treefrog tadpoles from the Sierra Nevada downwind of the Central Valley was lower than at sites on the coast or to the north, and was also lower in areas where ranid population status was poor or moderate compared to sites with good ranid populations. In affected areas, up to 50% of sampled tadpoles had detectable levels of organophosphorus residues. Endosulfan, 4,4’-dichlorodiphenyldichloroethylene, 4,4’-DDT, and 2,4’-DDT residues were also commonly found (Sparling et al., 2001). Presence of polychlorinated biphenyls (PCBs) and toxaphene in tadpoles from the Sierra Nevada showed a trend of increasing concentrations from high to low elevation and from east to west; the latter suggests a rain shadow effect (Angermann et al., 2002). Greater survivorship of Pacific treefrog tadpoles was noted at Lassen National Park than in the affected areas of Yosemite and Sequoia National Parks in the Sierra Nevada. Relatively high (25%) deformity rates were also detected at Yosemite (Cowman et al., 2002). Pacific treefrogs may be less affected by pesticide drift than ranid frogs because they are less dependent on aquatic sites (e.g. the tadpoles metamorphose in the same year they are laid and adults often spend considerable time in the uplands; Sparling et al., 2001).
R. Parasites. Lehman (1964) reported nematodes and trematodes in Pacific treefrogs from eastern Washington and Oregon. In central California, Lehman (1960) reported only Opalina sp., a protozoan. Waitz (1961) found no parasitic worms in Pacific treefrogs in Idaho. Frogs from two localities in southern California were infected with the helminths Rhabdias, Oswaldocruzia, Cosmocercoides, and Distoichometra. Mean intensity of infection was 10.3 helminths at Malibu and 8.4 helminths at Big Tujunga (Koller and Gaudin, 1977).
Pacific treefrogs with a variety of morphological abnormalities, particularly malformed hindlimbs, have been found in western Montana (Hebard and Brunson, 1963; Van Valen, 1974), Spokane, Washington (Miller, 1968, in Reynolds and Stephens, 1984), Boise, Idaho (Reynolds and Stephens, 1984), and northern California (Johnson et al., 1999, 2001b). At two ponds in northern California, 10–25% of larval and post-metamorphic Pacific treefrogs exhibited abnormalities; of those, > 60% were severe malformations involving extra hindlimbs, femoral projections, and skin webbings that probably reduced survivorship (Johnson et al., 2001b). Potential causes include UV-B radiation, retinoid exposure, genetic mutation, pesticide contamination, predation, microbes, and trematode parasites (see Van Valen, 1974; Sessions et al., 1999; Johnson et al., 2001b). Abnormalities observed in northern California are likely caused by a cathaemasiid trematode, Ribeiroia sp. Refer to Sessions and Ruth (1990), Johnson et al. (1999, 2001b), Sessions et al. (1999), and Souder (2000) for discussions of trematode parasites and their role in producing limb malformations.
4. Conservation. Pacific treefrogs have no status under C.I.T.E.S., the U.S. Endangered Species Act, with the Canadian government (COSEWIC, Committee on the Status of Endangered Wildlife in Canada, 2002), or with the government of Mexico (Secretaria de Desarrollo Social, 1994). Where they occur, Pacific treefrogs are typically one of the most common amphibians, often exhibiting robust populations. With a few exceptions, noted in "Historical versus Current Distribution" and "Historical versus Current Abundance," the species is not declining and has not been targeted for conservation or recovery actions.
Acknowledgments. We thank Gary Fellers for valuable comments reflecting his experience with Pacific treefrogs.
1James C. Rorabaugh
U.S. Fish and Wildlife Service
2321 West Royal Palm Road, Suite 103
Phoenix, Arizona 85021
2Michael J. Lannoo
Muncie Center for Medical Education
Indiana University School of Medicine
Ball State University
Muncie, Indiana 47306
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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