Rana pretiosa Baird and Girard, 1853
Oregon spotted frog
Marc P. Hayes1
Christopher A. Pearl2
1. Historical versus Current Distribution.
This recently described species (Green et al., 1997) has an impoverished historic record (about 70 localities verifiable from museum collections prior to 1980; McAllister et al. 1993, Hayes 1994a, unpubl, data) that complicates interpretation of its historic distribution. McAllister et al. (1983) suggested that Oregon spotted frogs were more widespread than the historic record indicates, a view held by others (Nussbaum et al., 1983; R. Storm, pers. comm.). That the available record underestimates the species’ historic distribution at a geographic scale is unquestioned; at finer scale, however, recent investigation indicates that Oregon spotted frogs were generally associated with larger emergent wetlands, i.e., ³ 4 hectares (Hayes, 1994a, 1994b, 1997). Hence, Oregon spotted frogs were likely habitat-constrained in those portions of their geographic range where larger emergent wetlands were fewer and more dispersed, such as higher elevations of the Cascades and in lowlands in their northern range in Washington and British Columbia (Hayes, 1997).
Assuming that the few sites at which Oregon spotted frogs have been discovered over the last 15 years were historically occupied (see Hayes, 1994a, 1994b, 1997; Pearl, 1999; M. Blouin, R. Haycock, K. McAllister, pers. comm.), the species historically ranged from the Pit River system (northeastern California) to the Fraser River system (southwestern British Columbia), occurring west of the Cascade crest in British Columbia, Washington, and the Willamette hydrographic basin of Oregon; and east of the crest in the Deschutes, Klamath, and Pit River basins of Oregon and California (Slipp, 1940, Nussbaum et al., 1983; McAllister, 1995; Green et al., 1996; Hayes, 1997, Pearl, 1999). Over this range, Oregon spotted frogs occurred from near sea level to 1661 meters (5450 feet) with maximum elevations generally increasing with increasing southerly position (Nussbaum et al., 1983; Hayes, 1994b, 1997).
Based on information obtained between 1995 and 2005, Oregon spotted frogs were extant at 40 localities within this range: 31 in Oregon, 6 in Washington and 3 in British Columbia (Hayes et al., 1997; McAllister and Leonard, 1997; Pearl, 1999; Haycock, 2000; M. Blouin, R. Haycock, pers. comm.; C. Pearl, unpubl. data). Surveys across the geographic range are approaching being comprehensive, and potential habitat remaining in unexamined areas is limited. Larger sites are associated with larger breeding populations (Pearl and Hayes 2004), so few additional large populations are likely to exist. Resurveys of historically occupied sites suggest that Oregon spotted frogs are extirpated from 70-90% of their geographic range: range reduction estimates differ based on whether using a site count (70%) or area-based estimate (90%; McAllister et al., 1993; Hayes et al., 1997). The species is thought to be extirpated over its California range, the lowland Willamette Valley of Oregon, and the lower Columbia River of Oregon and Washington. Populations west of the Cascade axis and at lower elevations in most drainages across its historic range appear to have been disproportionately lost: no Oregon spotted frog populations are known to persist across their historical lowland range in western Oregon, and only 7 extant lowland sites exist in western Washington and British Columbia (McAllister et al., 1993; Hayes, 1994; R. Haycock, K. McAllister, pers. comm.).
2. Historical versus Current Abundance.
Sources of historic data imply that lowland populations of Oregon spotted frogs were abundant. The journal of Tracy Irwin Storer from 30 June 1930 reveals that spotted frogs in the Willamette Valley were abundant enough to lead at least one “farmer” of American bullfrogs (Rana catesbeiana) to use spotted frogs as food for his bullfrog stock. Discussing the “past few years”, Stanley Jewett, Jr. (1936) described spotted frogs as “common along…sloughs of the Williamette and Columbia Rivers”; Storer verified Jewett’s amphibian collections (Jewett, 1936), and during this time, Jewett sent specimens that were later identified as Oregon spotted frogs to five collections (i.e., Portland State University, Oregon State University, the University of Puget Sound, the University of California at Davis, and the United Stated National Museum in Washington, D.C.). Slipp (1940) also noted the species as “common” in the Tacoma area of Puget Sound during the late 1930s.
Those among the last to observe Oregon spotted frogs on the Willamette Valley floor and Puget Lowlands during the 1950s did not perceive their decline until they had almost vanished (D. Marshall, J. Slipp, R. Storm, pers. comm.). Concern about Oregon spotted frog declines in the lowlands of the Pacific Northwest was voiced from the 1960s onward (Storm, 1966; Dumas, 1966; Shay, 1973; Licht, 1974; Nussbaum et al., 1983; Hayes and Jennings, 1986; St. John, 1987; Marshall, 1988). Licht (1969) reported relatively small numbers of egg masses (30 and 54) in one population in British Columbia in 1968 and 1969; Oregon spotted frogs were not detected and presumed extinct at this site when it was resurveyed during the 1980s (Orchard 1984). Today, many extant populations are small and isolated. Based on egg mass surveys since 1995, at least 15 of the 40 sites with extant populations appear to have < 200 breeding females (McAllister and Leonard, 1997; Pearl, 1999; R. Haycock, pers. comm.). Further, abundance surveys for adults imply small populations at 7 additional sites in Oregon and 2 in Washington (Hayes, 1997; K. McAllister, pers. comm.). As of 2005, all three breeding sites in British Columbia were estimated to contain < 150 breeding females, and one site was estimated at < 30 breeding females: the latter site appears to have declined steadily from ~ 90 breeding females in 1997 (R. Haycock, pers. comm.). Breeding surveys across the species’ range since 1999 suggest that less than 10, and probably less than 6 Oregon spotted frog sites currently support > 500 breeding females (J. Engler, J. Bowerman, L. Hallock, M. Hayes, C. Pearl, C. Rombough, unpubl. data). Severe declines in egg mass counts were recorded in two of these large populations, Conboy and Trout Lakes in 2006 (L. Hallock and M. Hayes, unpubl. data).
Loss and modification of suitable marsh habitat to agriculture, hydrological alterations, and urbanization have been severe throughout large areas of the species’ range. Such changes are especially acute in the Klamath basin of south-central Oregon and areas west of the Cascade range (Daggett et al., 1998; Collins, 2000; Taft and Haig, 2003). Introduction and expansion of non-native predators is likely to have reduced abundance of Oregon spotted frogs (Hayes, 1997; Hayes et al., 1997). Non-native bullfrogs and fishes are widely distributed over much of the range of Oregon spotted frogs, and both have been posited as detrimental to spotted frog populations (Dumas, 1966; Nussbaum et al., 1983; St. John, 1987; Hayes and Jennings 1986, Pearl et al. 2004). Potentially damaging nonnative species such as the Louisiana red swamp crayfish (Procambarus clarkii; see Gamradt and Kats, 1996), are present in parts of the historic range of Oregon spotted frogs (Hayes 1997, Pearl et al. 2005), but their effects on OSF are currently unknown. Separating the relative importance of introduced predators from widespread habitat modifications remains difficult (Hayes and Jennings, 1986; also see Adams, 1999). Compromised water quality as a result of agricultural and urban land use is also a stressor (Nussbaum et al., 1983), and laboratory trials suggest that embryonic and larval Oregon spotted frogs may be more sensitive to nitrogen compounds associated with fertilizers than other northwestern wetland-breeding amphibians (Marco et al., 1999). In contrast, field trials and assessment of DNA repair potential imply that embryos of Oregon spotted frogs are resistant to ambient levels of ultraviolet radiation (Blaustein et al., 1999). The chytrid fungus, Batrachochytridium dendrobatidis, has recently been found at 1 site each in Oregon (J. Bowerman, pers. comm.) and Washington (L. Hallock, pers. comm.).
Oregon spotted frogs are considered a candidate for federal listing by the U.S. Fish and Wildlife Service (1997). They have some designation indicating concern about their status in every political entity that encompasses their historic range. They are considered 'Endangered' by the State of Washington (WDFW, 2006), ‘Sensitive-Critical' in Oregon (Oregon Natural Heritage Program, 1995), and a ‘Species of Special Concern’ in California, where they have been recommended for ‘Endangered’ status (Jennings and Hayes, 1994). Oregon spotted frogs are considered 'Red-listed' (COSEWIC Endangered; Seburn and Seburn, 2000) in British Columbia.
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Initiation of breeding activity occurs soon after winter thaw and adults are thought to move into breeding sites as early as 1 - 2 weeks prior to oviposition (Licht, 1969, 1971). Adults have been observed active under ice (Leonard et al., 1997), and underwater migration to breeding sites when snow is still on the ground have been identified from an inland location in each of Oregon and Washington (J. Engler, M. Hayes, J. Bowerman, unpubl. data.). Emergence of adults has been documented in early February in Oregon’s Willamette Valley (Nussbaum et al., 1983) and in late February-early March in southwestern British Columbia and western and west-central Washington (Licht, 1969; Leonard, 1997; Watson et al., 1998; Hayes and Engler, 2000).
ii. Breeding habitat. Oregon spotted frogs breed in warm, vegetated shallows of open, freshwater marshes and lake margins with little flow (Licht, 1971). Breeding micro-environments are often located in seasonally inundated shallows, and are usually hydrologically connected to permanent water (Licht, 1971). Breeding is usually concentrated within a 1-2 week period (rarely can extend to 4 wks, especially where population sizes are large or suboptimal temperatures alter breeding phenology). Oviposition at selected sites is initiated when water temperatures reach 8˚C (Licht, 1969, 1971; McAllister and Leonard, 1997; Hayes and Engler, 2000), which is 2˚C above the critical thermal minimum identified for early stage embryos in British Columbia (Licht 1969, 1971). However, other cues are likely to play some role in timing of oviposition, particularly at higher elevations (C. Pearl, J. Bowerman, unpubl. data). Based on observations at Conboy Lake NWR, initiation of oviposition seems to advance in drought years, whereas in wet years it is delayed (J. Engler, M. Hayes, pers. obs.). Timing of oviposition varies from late-February-early March at lowland sites (McAllister and Leonard, 1997) to late-May-late-June at montane sites in Oregon (C. Pearl, pers. obs.).
i. Egg deposition sites. Egg masses are often deposited communally, and aggregations can contain > 75 individual masses, especially where the population size is large (McAllister and Leonard, 1997; Hayes and Engler, 2000). At a lowland Washington site, mean water depth near time of oviposition ranged between 5.8 and 11.5 cm (McAllister and Leonard, 1997); at a mid-elevation site in west-central Washington, water depths at oviposition range from 7.0 to 35 cm, but over 99% of thousands of egg masses laid were at water depths < 15 cm (Hayes and Engler, 2000). Desiccation of shallowly deposited egg masses can be a major source of embryonic mortality (Licht, 1974; McAllister and Leonard, 1997; Hayes and Engler, 2000).
ii. Clutch size. Egg masses (N = 18) averaged 643 eggs per mass (Licht, 1971) in lowland British Columbia, and 598 eggs (N = 6) in western Washington lowlands (McAllister and Leonard, 1997). Breeding females are thought to deposit one egg mass with their full complement of eggs in a breeding year (see Licht 1975).
i. Length of larval stage. Larvae hatch in 18 - 30 days (McAllister and Leonard, 1997). Licht (1975) reports tadpoles reaching metamorphosis between 110 and 130 days after hatching in British Columbia, although development can occur in as short as 95 days in Oregon (C. Pearl, pers. obs.).
ii. Larval requirements
a. Food. Larvae are thought to consume bacteria, algae and organic detritus (Licht, 1974).
b. Cover. Larvae utilize warm shallows with relatively dense aquatic vegetation and can be extraordinarily cryptic, particularly where predatory fish are present (M. Hayes and C. Pearl, pers. obs.).
iii. Larval polymorphisms. None reported.
iv. Neoteny. Unreported and unlikely.
v. Features of metamorphosis. Size at metamorphosis varies with elevation and growing season. In lowland British Columbia, metamorphosis began first week of August, and transforming Oregon spotted frogs averaged 33 mm (Licht, 1974). At one site at about 1500 m (4950 ft) elevation in the Oregon Cascade Range, 24 transforming frogs averaged 26.1 mm in late-September during a particularly large snow year (C. Pearl, pers. obs.). Survivorship to metamorphosis can be extremely low (as low as 1%) due to desiccation of egg masses and high predation rates (Licht, 1974).
vi. Post-metamorphic migrations. Juveniles may remain around breeding ponds after transformation. Emigration patterns are unknown.
D. Juvenile Habitat Requirements. Thought to be similar to adult habitat requirements, but poorly known.
E. Adult Habitat Requirements. Adult Oregon spotted frogs are highly aquatic, and are rarely found more than 2 m from surface water (Licht, 1986b). In one shallow wetland in western Washington, adults were associated with microhabitats characterized by relatively deeper water and more open canopy than randomly selected points (Watson et al., 1998). Adults remained associated with standing water as inundation limits shrank through the summer season (Watson et al., 1998). Adults may also seasonally utilize temporary pools within 300 m of the breeding site (McAllister and Leonard, 1997; Pearl, 1999).
F. Home range size. Unknown.
G. Territories. Not reported to be territorial.
H. Aestivation. None documented.
I. Seasonal migrations. See A and C above. Watson et al. (1998) reported that adults were found increasing distances from the breeding site with onset of substantial fall rains, but whether this represents movement toward overwintering sites is unclear. An aggregation of Oregon spotted frogs found in atypical habitat (beneath a boulder) during late fall was hypothesized to represent staging entry to potential overwintering areas (inflow stream; Rombough and Pearl 2005). Movements observed between aquatic active-season habitat and aquatic overwintering sites (see Torpor) may be a seasonal pattern (Hallock and Pearson, 2001, Hayes et al., 2001, Risenhoover et al., 2001).
J. Torpor (Hibernation). Overwintering habits are incompletely known, but adults appear to spend winter hibernating in freeze-free seeps, springs, and channels that are hydrologically linked to breeding sites (M. Hayes, J. Engler and C. Pearl, pers. obs.). Dickerson (1907) reported an overwintering site in a mud bottom of a marshy lake fringe that was at least one foot deep. Three recent studies of overwintering in Washington have confirmed aquatic overwintering (Hallock and Pearson, 2001, Hayes et al., 2001, Risenhoover et al., 2001). This work also revealed that frogs can actively move within the aquatic habitat throughout the overwintering period.
K. Interspecific Associations/Exclusions. Oregon spotted frogs may share breeding sites with northwestern salamander (Ambystoma gracile), long-toed salamander (A. macrodactylum), Pacific chorus frogs (Pseudacris regilla), roughskin newts (Taricha granulosa), and western toads (Bufo boreas). At higher elevations in Oregon, Oregon spotted frogs co-occur with the Cascades frog (Rana cascadae), and the two have been observed to hybridize in laboratory and field (Haertel and Storm, 1970; Green, 1985). Oregon spotted frogs historically co-occurred with the northern red-legged frog (Rana aurora) in lowland western Washington and southwestern British Columbia (Licht, 1971, 1974, 1986; McAllister and Leonard, 1997), but areas of sympatry with are now limited due to loss of lowland spotted frog populations. Occurrence of the non-native bullfrog at low and mid-elevation historic Oregon spotted frog sites now lacking spotted frogs may suggest a negative interaction (Dumas, 1966; Licht 1974; Nussbaum et al., 1983; Hayes, 1997; Pearl et al. 2004). Based on a more limited escape mobility via jumping, Pearl et al. (2004) suggested that Oregon spotted frogs would be less able to evade predation from introduced bullfrogs than northern red-legged frog. Negative interactions have been suggested between Oregon spotted frogs and introduced salmonids and centrarchids, both of which are widely established within Oregon spotted frog range (Hayes and Jennings, 1986; Hayes, 1997).
L. Age/Size at Reproductive Maturity. At its northern range limit in lowland British Columbia, male Oregon spotted frogs are thought to become sexually mature in their second year (mean size 45 mm), and females are thought to become sexually mature in either their second or third year (mean size 62 mm; Licht, 1975). Licht (1974) suggested females at that site may breed each year. In central Washington, most males become sexually mature by the end of their first year at 48-50 mm SVL, whereas most females become sexually mature at 59-61 mm SVL in their second year (M. Hayes, J. Engler, unpubl. data).
M. Longevity. Not well understood, but adults are thought to be relatively short-lived (i.e., generally 2 - 5 yrs) compared to other western North American ranids (see Licht, 1975).
N. Feeding Behavior. Oregon spotted frogs consume a variety of invertebrate prey, much of which is captured in aquatic or near-shore environs (Licht, 1986). Adult Oregon spotted frogs will occasionally move from water under moist conditions to feed, and will consume Pacific treefrogs and juvenile western toads, as well as juvenile northern red legged frogs and conspecifics (Licht, 1986; C. Pearl and M. Hayes, pers. obs.). McAllister and Leonard (1997) discounted a report of Dickerson (1907) that the species fed greedily on small fish, but two different observations of adult females Oregon spotted frog feeding on three-spined stickleback (Gasterosteus aculeatus) in the Deschutes River system of Oregon (M. Hayes, pers. obs.) suggests that Dickerson’s report may require re-evaluation.
O. Predators. Documented predators of larval Oregon spotted frogs include common garter snakes (Thamnophis sirtalis concinnus) and larval diving beetles (Dytiscidae; C. Pearl, personal observations). Recently, R. Haycock (pers. comm.) obtained video footage of juvenile bullfrogs consuming hatchling Oregon spotted frogs from the post-hatching aggregations in egg jelly. Suspected predators of larvae include sandhill cranes, a variety of introduced fish, roughskin newts, northwestern salamanders, backswimmers (Notonectidae), giant water bugs (Belostomatidae), and anisopteran odonates (Licht, 1974; M. Hayes, C. Pearl, pers. obs.). Documented predators of post-metamorphic Oregon spotted frogs include bullfrogs, adult Oregon spotted frogs, garter snakes (Thamnophis sirtalis concinnus and T. elegans biscutatus), mink, otter (Lontra canadensis), and greater sandhill cranes (Grus canadensis tabida) (Licht, 1974; Watson et al., 1998; Hayes et al., 2005; Hayes et al., in press). Two of the recent studies on overwintering in Washington suggest high vulnerability to predation, probably by mink, during the overwintering period (Hallock and Pearson 2001, Hayes et al. 2001). Suspected predators of post-metamorphic Oregon spotted frogs include raccoons, great blue herons, kingfisher, red fox and striped skunks (Licht, 1974; Licht, 1986).
P. Anti-predator Mechanisms. Larval Oregon spotted frogs occupy densely vegetated shallows, which may afford them some protection from visually orienting predators (C. Pearl, pers. obs.). Adults usually position themselves within several bounds of water. Once in water, adults may bury themselves in organic sediments and remain still (Licht, 1986b). Immobility appears to be one important anti-predator strategy of post-metamorphic frogs, which allow close approach prior to fleeing (M. Hayes, pers. obs.).
Q. Diseases. Not well known, but in related and sympatric Cascades frogs (Rana cascadae), a similar communal breeding pattern may be associated with increased occurrence of the oomycete fungus Saprolegnia sp. in egg masses (Kiesecker and Blaustein, 1997). The chytrid fungus Batrachochytridium dendrobatidis has recently been detected at single sites in Oregon (L. Hallock, pers. comm.) and Washington (J. Bowerman, pers. comm.).
R. Parasites. Flukes in genus Haematoloechus, which have larval dragonfly and damselfly intermediate hosts, are common and widespread parasites as adults in the lungs of ranids frogs (Kennedy 1979, 1980, 1981; Sutherland 2005; see also Ingles 1936), and are reported from Oregon spotted frogs in British Columbia (Kennedy 1979, 1980). However, these reports predate the partitioning of the two spotted frog species; as geographic detailing was lacking, it is unclear whether Oregon spotted frogs were represented. However, based on dissection of 42 male and 8 female Oregon spotted frogs found dead or moribund from attack by mink during the breeding intervals over 8-years (1998-2005) at Conboy Lake National Wildlife Refuge, Haematoloechus infects almost all adults (M. Hayes, unpubl. data). The few data available suggest that Haematoloechus has little effect on their host (Sutherland 2005). Trematodes of the genus Haplometrana have been reported from spotted frogs in Washington (Lucker, 1931; Pratt and McCauley, 1961), and Clark et al. (1969) reported the blood parasites Trypanosoma and Lankesterella from spotted frogs in Oregon. Both these reports also predate the taxonomic revision of western spotted frogs. Available information on parasites that Oregon spotted frogs may harbor is almost entirely descriptive; ecological data that might indicate negative effects is lacking.
1Marc P. Hayes
Washington Department of Fish and Wildlife
Habitat Program, Science Team
600 Capitol Way North
Olympia, Washington 98501-1091
2Christopher A. Pearl
Forest and Rangeland Ecosystem Science Center
U.S. Geological Survey
3200 SW Jefferson Way
Corvallis, Oregon 97331
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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