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
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.
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,
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.
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
3. Life History Features.
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
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,
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
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
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,
F. Home range
Territories. Not reported to be territorial.
Aestivation. None documented.
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).
(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.
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,
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.
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).
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.
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).
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.
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.).
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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