Oregon spotted frog (Rana pretiosa): COSEWIC assessment and status report 2011
Endangered – 2011
Table of Contents
- COSEWIC Assessment Summary
- COSEWIC Executive Summary
- Technical Summary
- Wildlife Species Description and Significance
- Population Sizes and Trends
- Threats and Limiting Factors
- Protection, Status, and Ranks
- Acknowledgements and Authorities Contacted
- Information Sources
- Biographical Summary of Report Writer
List of Figures
- Figure 1. Rana pretiosa adult (top; April 2003), feet showing webbing to end of toes (middle); juvenile (bottom left; Mountain Slough, 1997), and developing embryos in an egg mass (bottom right; Morris Valley, spring 2008). Denis Knopp photographs.
- Figure 2. Bootstrapped neighbour-joining tree depicting genetic relationships among populations of R. pretiosa in Canada and the closest U.S. populations in Washington State. Genetic distances are based on data from 13 microsatellite loci and calculated using Nei’s standard genetic distance. Illustration provided by I. Phillipsen and M. Blouin.
- Figure 3. The historical global range of Rana pretiosa is restricted to the Fraser River Lowlands in British Columbia, disjunct areas in Washington and Oregon, and small areas of California. Adapted from IUCNet al. (2009).
- Figure 4. Location of Rana pretiosa populations in Canada and their status. See Table 3 for additional details about the populations.
- Figure 5. Sites searched for Rana pretiosa in Canada to 2009. See Table 2 for additional details.
List of Tables
- Table 1. Mean body length and mass of adult male and female R. pretiosa in Canada. Data were obtained from Haycock (2001), which presents average body sizes for frogs at Aldergrove, Mountain Slough, and Maria Slough; and from Haycock (2005), which reports body sizes for frogs captured in 2001–2005 at MD Aldergrove.
- Table 2. Areas searched during targeted surveys for Rana pretiosa from
1996 to 2010.
- Table 3. Estimated number of breeding adults of Rana pretiosa at occupied sites in Canada, search effort, and information on egg collection for captive rearing and release of captive-reared frogs. Estimates are provided for historical populations where available. Adapted and updated from Haycock 2000a.
COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows:
COSEWIC. 2011. COSEWIC assessment and status report on the Oregon Spotted FrogRana pretiosa in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. xi + 47 pp.
COSEWIC. 2000. COSEWIC assessment and status report on the Oregon Spotted Frog Rana pretiosa in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 22 pp.
Haycock, R.D. 2000. COSEWIC status report on the Oregon Spotted Frog Rana pretiosa in Canada, in COSEWIC assessment and status report on the Oregon Spotted Frog Rana pretiosa in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 1-22 pp.
COSEWIC would like to acknowledge Vanessa Craig for writing the status report on the Oregon Spotted Frog (Rana pretiosa) in Canada, prepared under contract with Environment Canada. This report was overseen and edited by Kristiina Ovaska, Co-chair of the COSEWIC Amphibians and Reptiles Specialist Subcommittee.
For additional copies contact:
c/o Canadian Wildlife Service
Oregon Spotted Frog -- Denis Knopp photographs.
© Her Majesty the Queen in Right of Canada, 2011.
Catalogue No CW69-14/94-2011E-PDF
Assessment Summary – May 2011
Oregon Spotted Frog
Reason for designation
This highly aquatic frog has a small Canadian distribution within the populated and highly modified Fraser River Basin in southwestern British Columbia. It currently occurs at four sites, isolated from one another, and has been extirpated from an additional three sites. One extant population is near extinction, and the remaining populations are small and vulnerable to disturbance and stochastic events. Habitat loss and fragmentation, hydrological alteration, disease, introduced predators, and poor water quality continue to threaten remnant populations.
Designated Endangered in an emergency assessment on 13 September 1999. Status re-examined and confirmed in May 2000 and in May 2011.
Wildlife species description and significance
The Oregon Spotted Frog, Rana pretiosa, is a member of the family Ranidae, or true frogs. Prior to 1997, the name Rana pretiosa was applied to both R. pretiosa and R. luteiventris (Columbia Spotted Frog) as currently known. Therefore, when interpreting research conducted prior to 1997, a reader must note the geographic location of the study, which will indicate the species in question.
The Oregon Spotted Frog is a medium-sized frog with a body length in adults of 60 to 80 mm. The background colour is brown or reddish and becomes more reddish as the frogs age. The common name of the species is in reference to the dark spots with light centres that are present over the head, back, and legs. The underside of the legs and belly of juvenile frogs is white or cream-coloured, changing to orange or red in adults.
The distribution of the Oregon Spotted Frogoverlaps with that of the Northern Red-legged Frog (R. aurora), with which it may be confused. The two species can be distinguished by subtle differences in appearance, including lack of green mottling in the groin, shorter legs, and more upturned eyes in the Oregon Spotted Frog. The Oregon and Columbia Spotted Frogs are very similar in appearance but are not found in the same areas.
This species is sensitive to contaminants in its environment including nitrates and nitrites, prevalent in run-off from agricultural areas. The Oregon Spotted Frog may serve as a bioindicator of the condition of shallow wetlands that it occupies.
The historical range of the Oregon Spotted Frogextends from the Pit River drainage in California northward to southwestern British Columbia. The species has disappeared from many areas throughout its range, including three of seven known sites in British Columbia, all three known sites in California, 44 sites in Oregon, and 11 sites in Washington State. Its current range extends from extreme southwestern British Columbia southward to the Klamath Basin in Oregon. In Canada, the species is extant at four sites all within the Fraser River Basin of British Columbia: Aldergrove, Maria Slough, Mountain Slough, and Morris Valley.
The Oregon Spotted Frogis usually associated with large (> 4 ha) wetlands with emergent or floating vegetation within forested landscapes. The frogs are highly aquatic and almost always found in or beside water. In spring, egg-laying occurs in shallow warm water in seasonally inundated areas. In summer, the frogs forage in shallow wetlands. In winter, they often over-winter in springs and seepages that do not freeze completely or in low-flow channels or ditches, or they may bury into silty soil or vegetation at such sites.
The frogs become active and begin breeding in spring after air temperatures reach approximately 5°C. Males produce a characteristic advertisement call consisting of a rapid series of short, low-pitched clucks, and usually call under water. Egg-laying is temperature-dependent and typically begins in March and continues for 2 to 4 weeks in British Columbia. Egg masses, each with up to 1500 eggs per mass, are laid in communal clusters with the tops of egg masses often exposed to the air. The placement of egg masses in shallows makes them vulnerable to freezing and desiccation caused by wind or receding water levels. In some years, embryonic survivorship can be zero. Survival of tadpoles can also be extremely low due to depredation. Most movements of individual frogs between breeding and wintering habitat are localized, but the frogs are capable of longer movements of up to about 3 km along water courses.
Population sizes and trends
Egg-mass counts at the four known extant Canadian sites suggest a total population size of fewer than 500 adult frogs in 2010. One population (Aldergrove) has declined precipitously since monitoring began in 1997 and is nearing extirpation. One population (Mountain Slough) appears stable, and one (Maria Slough) has fluctuated and may be declining. A new population (Morris Valley) was discovered in 2008 and has limited data.
Threats and limiting factors
The greatest threat to the species in Canada is continuing loss of suitable wetlands and associated terrestrial habitat and accompanying habitat fragmentation and population isolation. Additional threats include alteration of site hydrology, which can adversely affect egg-laying habitat and increase mortality of eggs; pollution affecting embryonic or tadpole survival; diseases such as chytridiomycosis and iridoviruses; and predators or competitors such as introduced American Bullfrogs, Green Frogs, and fish.
Protection, status, and ranks
The Oregon Spotted Frogwas assessed in 2000 by COSEWIC as Endangered in Canada and is listed under Schedule 1 of the Species at Risk Act. The speciesis protected under the British Columbia Wildlife Act from being killed, wounded, transported, or collected without a permit. NatureServe (2010) lists the species as Imperiled (G2) globally; Critically Imperiled (N1) in Canada, and Imperiled (N2) in the U.S. In British Columbia, the species is Critically Imperiled (S1) and is on the provincial Red List of species at risk. The species is on the IUCN Red List as Vulnerable.
Two of four currently occupied sites (Mountain Slough and Morris Valley) are privately owned. The water body of one site (Maria Slough) is on Provincial Crown Land; however, the surrounding land is partially on First Nations Reserve land and partially on privately owned land. The remaining site (Aldergrove) is on federal land, managed by the Department of National Defence, which controls site access and limits activities in the surrounding area.
Oregon Spotted Frog Grenouille maculée de l’Oregon
Range of occurrence in Canada: British Columbia
|Generation time Age of maturity + (1/annual mortality rate). See Biology: Life Cycle and Reproduction for details||4.7 – 5.5 years|
|Is there a continuing decline in number of mature individuals? Population at Aldergrove has had a 100% decline in the effective breeding population since monitoring began in 1997. Maria Slough population has fluctuated and might be declining.||Yes|
|Estimated percent of continuing decline in total number of mature individuals within 5 years or 2 generations. Extrapolation from annual egg mass counts suggest a decline of approximately 35% from 2000 to 2010 within continuously monitored areas at three sites (Aldergrove; Mountain Slough; Maria Slough); the decline is 19% if a new expanded search in 2010 at one of the sites (Mountain Slough) is included (see Fluctuations and Trends); there has been loss of one population (Aldergrove) within this period.||Decrease of approximately 35%|
|[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last [10 years, or 3 generations]. Loss of one of four populations (Aldergrove) over the past 3-generation period, representing an inferred decline in mature individuals of approximately 25%.||Decrease of uncertain magnitude|
|Projected or suspected percent reduction or increase in total number of mature individuals over the next 10 years or 3-generation period||Unknown|
|Observed, estimated, inferred, or suspected percent reduction or increase in total number of mature individuals over any 10 years or 3 generations period, over a time period including both the past and the future.||Unknown|
|Are the causes of the decline clearly reversible and understood and ceased? Potential causes of the decline are partially understood but have not ceased.||No|
|Are there extreme fluctuations in number of mature individuals?||Unknown|
Extent and Occupancy Information
|Estimated extent of occurrence||303 km²|
|Index of area of occupancy (IAO) – 2x2 km² grid cell||40 km²|
|Is the total population severely fragmented?
Spatial separation between sites/populations is significant, and the viability of the populations is in doubt for at least two of the four populations (Aldergrove & Maria Slough); none may be viable over the long term.
|Number of locations
Threats that can rapidly affect the population at each of the four extant sites include run-off of pollutants from adjacent agricultural areas, grazing by livestock, and deterioration of breeding sites through vegetation succession. Each site is considered a separate location because the severity and type of threats is variable, depending on land ownership and activities in the surrounding area. However, if the invasion and spread by the non-native American Bullfrog is considered the most significant threat across the sites, then the number of locations is only one.
|1 - 4|
|Is there a projected continuing decline in extent of occurrence?
The Aldergrove population will likely be extirpated resulting in a reduction of 79% in EO to 63 km².
|Is there a projected continuing decline in index of area of occupancy?
The Aldergrove population will likely be extirpated, resulting in a reduction of 20% in IAO to 32 km².
|Is there a projected continuing decline in number of populations?
The Aldergrove population will likely be extirpated.
|Is there a projected continuing decline in number of locations?
The Aldergrove population will likely be extirpate.
|Is there an observed and projected continuing decline in quality of habitat?||Yes|
|Are there extreme fluctuations in number of populations?||No|
|Are there extreme fluctuations in number of locations?||No|
|Are there extreme fluctuations in extent of occurrence?||No|
|Are there extreme fluctuations in index of area of occupancy?||No|
Number of Mature Individuals (in each population)
(2010 estimate from egg-mass counts, assuming 1 adult female and 1 or 2 adult males to each female; see Table 3)
|N Mature Individuals|
|1. Aldergrove: No egg masses were discovered at the site from 2007 to 2011, but one adult male was seen there in 2009 and again in 2011.||~0 (nearing extirpation)|
|2. Mountain Slough: 29 egg masses were found in 2010 within the traditional search area, providing a population estimate of 58–87 adults. An additional 23 egg masses were discovered in an expanded search area, providing a population estimate of 104–156 adults.||104–156|
|3. Maria Slough (based on egg mass counts in 2010)||134–201|
|4. Morris Valley (based on egg mass counts in 2010)||78–117|
|No PVA has been done.||Not available|
Threats (actual or imminent, to populations or habitats)
Habitat loss, habitat fragmentation and genetic isolation, hydrological alteration, disease, introduced predators/competitors, water quality
Rescue Effect (immigration from outside Canada)
|Status of outside population(s): United Stated: N2 (Imperiled); Washington: S1 (Critically Imperiled), listed as a State Endangered species; Oregon: S2 (Imperiled), listed as Critically Sensitive; California: S1 (Critically Imperiled), listed as a Species of Special Concern (potentially extirpated).|
|Is immigration known or possible?||No|
|Would immigrants be adapted to survive in Canada?||Probably|
|Is there sufficient habitat for immigrants in Canada?
Twelve sites have been identified as potential candidates for introduction of R. pretiosa; additional habitat assessments are required, but initial assessments are favourable (Pearson 2010b).
|Is rescue from outside populations likely?||No|
COSEWIC: Endangered (2011)
British Columbia: Provincial Red List; S1
Status and Reasons for Designation
|Reasons for designation:
This highly aquatic frog has a small Canadian distribution within the populated and highly modified Fraser River Basin in southwestern British Columbia. It currently occurs at four sites, isolated from one another, and has been extirpated from an additional three sites. One extant population is near extinction, and the remaining populations are small and vulnerable to disturbance and stochastic events. Habitat loss and fragmentation, hydrological alteration, disease, introduced predators, and poor water quality continue to threaten remnant populations.
Applicability of Criteria
Since the preparation of the previous COSEWIC status report on Rana pretiosa in 2000 (Haycock 2000a), the species has been discovered at one additional site, bringing the total number of extant populations in Canada to four. This discovery resulted in an expansion of the extent of occurrence from 115 km² to 303 km², an increase of approximately 260%. During the same time period, one of the populations has declined > 98% and is nearing extirpation, even though a captive rearing program has released > 2,000 frogs and tadpoles, reared from eggs collected at the site; another population may be declining. The discovery of the additional population in 2008 resulted in a slight overall increase in the estimated number of mature adults in Canada since the previous estimate in 2000. However, egg mass counts at known sites suggest a continuing decline in the total population, and the number of mature adults in Canada remains small (less than 500 adults).
The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act(SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.
The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.
COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.
A wildlife species that no longer exists.
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.
A wildlife species facing imminent extirpation or extinction.
A wildlife species likely to become endangered if limiting factors are not reversed.
Special Concern (SC)*
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.
Not at Risk (NAR)**
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.
Data Deficient (DD)***
A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.
* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
** Formerly described as “Not In Any Category”, or “No Designation Required.”
*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.
The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.
COSEWIC Assessment and Status Report on the Oregon Spotted Frog Rana pretiosa in Canada – 2011
Rana pretiosa, Baird and Girard, 1853, belongs to the family Ranidae, or true frogs. A single species was previously considered to encompass all populations of Spotted Frogs in western North America, including areas of Nevada, Utah, British Columbia, Washington, Oregon, Montana, Alberta, and Yukon. Thompson (1913) designated two subspecies, R. p. pretiosa, and R. p. luteiventris, based on colour and the presence or absence of tubercles. The inconsistency of these defining characteristics meant that the subspecific grouping by Thompson (1913) was generally not recognized (Nussbaum et al. 1983; Stebbins 1985). An analysis by Green et al. (1996) used variations in allozyme frequencies and morphometry to determine that the group included at least two distinct species. With further analysis, including additional samples, Green et al. (1997) reported that two groups (species) could be distinguished at the molecular level based on six diagnostic loci. Based on the samples examined, one species occurred in southwestern Washington and the Oregon Cascades, and Spotted Frogs in the remaining areas of North America belonged to the second species. Green et al. (1997) assigned Spotted Frogs in western British Columbia, Puget Sound, south-central Washington, and Oregon Cascades, to the Oregon Spotted Frog, R. pretiosa. Spotted Frogs in southwestern Yukon, southeastern British Columbia, Alberta, the Great Basin and Rocky Mountains, with isolated localities in Nevada and Utah, were assigned to the Columbia Spotted Frog, R. luteiventris Thompson, 1913 (Green et al. 1997). No subspecies are recognized for either R. pretiosa or R. luteiventris.
The common name of R. pretiosa reflects the dark spots with light centres that are present over the head, back, and legs (Figure 1). These spots become larger and darker, and the edges more ragged, as the frogs age (McAllister and Leonard 1997). Small tubercles are also scattered over the back. Juveniles are typically brown, or occasionally olive-green (McAllister and Leonard 1997). Adults are brown or reddish, and become more reddish with age. Dorsolateral folds, which extend from behind the eye to the lower back, are typically light brown to orange. The underside is white or cream-coloured in juveniles, changing to orange or red in adults. A dark mottled pattern on the underside is prominent in adults but absent from newly metamorphosed frogs (Hayes 1997 cited in U.S. Fish and Wildlife Service 2010).
Figure 1. Rana pretiosa adult (top; April 2003), feet showing webbing to end of toes (middle); juvenile (bottom left; Mountain Slough, 1997), and developing embryos in an egg mass (bottom right; Morris Valley, spring 2008). Denis Knopp photographs.
Rana pretiosa is a medium-sized frog. The mean snout-urostyle length (SUL) of adult frogs captured in British Columbia from 1997 to 2001 was 58.1 mm (range: 38.5–80.2 mm; N = 727), and the mean mass of adult frogs was 21.0 g (range:
5.9–55.4 g; N = 733; Haycock 2001). Females are larger than males (Table 1; Haycock 2001, 2005). These values are similar to measurements from frogs in Thurston County, Washington, where males averaged 56 mm snout-vent length (SVL; approximately equivalent to SUL), and females averaged 66 mm SVL; and from two populations in the south Cascades of Washington, where males averaged 57 mm and females 75 mm SVL (McAllister and Leonard 1997). Egg masses are characteristically clumped, usually laid communally, although single egg masses are sometimes found. The number of eggs per mass has been reported to range from 643 to 940 at Canadian sites (Licht 1974; Haycock 2005; see Biology: Life cycle and reproduction), and up to 1,500 in Oregon and Washington (Cushman and Pearl 2007). Egg masses are never attached to vegetation (Licht 1971) and are laid in shallow water, with the result that much of the upper egg mass is exposed (McAllister and Leonard 1997). Average egg diameter is 2.31 mm ±0.18 (1 SD); N = 292 (Licht 1971). Tadpoles are dark above with a light belly. Older tadpoles have metallic flecks on the head, body, and tail.
Rana pretiosa is sympatric with the Northern Red-legged Frog (R. aurora), which is similar in body size and colouration. Adult R. pretiosa can be distinguished from R. aurora by the following features: eyes that are angled upwards instead of outwards; shorter legs; markings that resemble spots rather than freckles; the undersides of the legs and abdomen are less brightly coloured; mottling is present on the abdomen; groin mottling is absent or is black or grey on a light background, compared to brightly coloured groin mottling of black, green, yellow, or red on R. aurora; and toes that are fully webbed rather than semi-webbed (see Matsuda et al. 2006 for details). Rana pretiosa is not sympatric with R. luteiventris and therefore can be distinguished based solely on geographic distribution. Morphologically, these two spotted frogs are very similar (Green et al. 1997), but adult R. pretiosa can be distinguished from adult R. luteiventris by the presence of mottling on the abdomen (Hayes 1994 cited in U.S. Fish and Wildlife Service 2010; Hayes 1997 cited in McAllister and Leonard 1997).
There are four extant populations of R. pretiosa in British Columbia, Maintenance Detachment (MD) Aldergove, Maria Slough, Mountain Slough, and Morris Valley, which are isolated from each other (Canadian Oregon Spotted Frog Recovery Team 2009a; see Canadian Distribution). In the U.S., known movements of individuals are almost entirely along aquatic corridors (Watson et al. 2003; Pearl and Hayes 2004; U.S. Fish and Wildlife Service 2010) and are less than 3 km (Cushman and Pearl 2007). Aldergrove is 50 to 60 km away from other occupied sites and is not linked to them by aquatic habitat. The Morris Valley population also does not have an aquatic connection to other populations; it is approximately 6.5 km in straight-line distance from the closest population at Mountain Slough and is separated from it by a mountain >800 m in elevation. Although Maria Slough and Mountain Slough have an aquatic connection along the Fraser River, the populations are >15 km apart, which is much greater than the yearly maximum known movement of the frogs. Movements of frogs between these populations would likely require the establishment of at least temporary populations to enable “stepping stone migration” between them (Blouin et al. 2010). The composition of habitat between the populations has not been formally described, but agricultural and urban development and roads have fragmented the terrestrial and remaining wetland habitats in the area (Canadian Oregon Spotted Frog Recovery Team 2009a). It is unlikely that sufficient suitable habitat remains to permit movements of individuals among populations (Knopp pers. comm. 2010).
Blouin (2002) conducted a genetic study of R. pretiosa based on samples from 20 populations in Washington and Oregon (toe snips), and from the Canadian Aldergrove population (adults and eggs). The U.S. populations were analyzed for variation at 13 microsatellite loci and 15 of the U.S. populations and the Aldergrove population for variation in mtDNA. The mtDNA results indicated that the Aldergrove population (N = 13 individuals tested) has a unique allele, but the sample size was inadequate for estimating allelic frequencies for the 13 microsatellite loci. Relatively high variation in allelic frequency for microsatellites among neighbouring U.S. populations (e.g., Fst = 0.23 for the Cascades Lakes population and 0.31 for the Klamath Basin population) suggested that there was little movement of frogs among sites and/or a high rate of genetic drift, even in relatively undisturbed habitat. This same degree of variation was not apparent in mtDNA. Overall, the results suggested that the populations have been isolated long enough for genetic drift to occur but not long enough for mutations to appear in mtDNA.
A subsequent study focused on examining 13 microsatellite loci from egg samples from the Aldergrove, Maria Slough, and Mountain Slough populations (N = 7 egg masses from each site; Blouin pers. comm. 2009). Data were compared to those from the same loci from 20 populations in Washington and Oregon. Canadian populations had relatively low within-population genetic diversity, as measured by expected heterozygosity (HE), compared to populations in Washington and Oregon. HE was 0.18 for Mountain Slough, 0.17 for Maria Slough, and 0.36 for Aldergrove populations, suggesting that at least the Mountain and Maria Slough populations have low effective sizes. A more recent analysis in 2009 that included a larger sample (N=30) of the Morris Valley, Mountain Slough, and Maria Slough populations found that HE for the populations was 0.27, 0.36, and 0.22, respectively (Phillipsen and Blouin pers. comm. 2010). The genetic diversity of Maria Slough and Morris Valley populations was relatively low compared to populations in Washington (Dempsey Creek HE = 0.46; Beaver Creek HE = 0.47; Phillipsen and Blouin pers. comm. 2010). The analyses indicated that the populations at Morris Valley and Maria Slough are more similar to each other than either is to the population at Aldergrove (using data from the previous study) or Mountain Slough, and that all British Columbia populations are distinct from the remaining U.S. populations (Figure 2). Estimates of genetic diversity within the Canadian populations may be biased downwards, and estimates of genetic distances among them biased upwards, because of small sample sizes and because eggs were sampled (gametes from a single season’s breeders), rather than the entire age distribution of adults (Blouin et al. 2010). The current Canadian populations appear to have small effective population sizes, and inbreeding is probably occurring, although its extent and effects on viability are unknown (Blouin pers. comm. 2009).
Figure 2. Bootstrapped neighbour-joining tree depicting genetic relationships among populations of R. pretiosa in Canada and the closest U.S. populations in Washington State. Genetic distances are based on data from 13 microsatellite loci and calculated using Nei’s standard genetic distance.
The entire range of the species in Canada is confined to the Fraser River Lowlands in British Columbia. Although the four extant populations are currently isolated, they are likely remnants of a larger population with a wider geographic distribution that has only relatively recently become fragmented (see Population spatial structure and variability). The four Canadian populations are closely related, compared to the genetic distances within the entire species (Blouin pers. comm. 2009; Phillipsen and Blouin pers. comm. 2010). These data suggest that the Canadian population is a single designatable unit.
In Canada, R. pretiosa is found only in the Fraser Lowlands of British Columbia. Similar to many other amphibians, this species is sensitive to contaminants in the environment, which can cause sublethal effects at low doses (de Jong Westman et al. 2010). The highly aquatic lifestyle of this species increases its exposure to aquatic contaminants. It is particularly sensitive to nitrates and nitrites, prevalent in agricultural run-off (Hecnar 1995; Marco et al. 1999; Rouse et al. 1999). The species is a habitat specialist that requires wetlands with shallow areas and abundant emergent or floating vegetation. Presence of R. pretiosa might serve as a bioindicator of the condition of the shallow, warm water wetlands that it occupies (Canadian Oregon Spotted Frog Recovery Team 2009a).
The historical range of R. pretiosa is from the Pit River drainage in California, northward to southwestern British Columbia (Figure 3). The species has undergone severe declines and extirpations across its range. It has disappeared from three historical sites in British Columbia, all sites where it was known to occur in California, 44 sites in Oregon, and 11 sites in Washington (U.S. Fish and Wildlife Service 2010). The current geographic distribution of R. pretiosa extends from extreme southwestern British Columbia southwards through the Puget Sound / Willamette Valley Trough in Washington and Oregon and the Cascades range from south-central Washington to the Klamath Basin in Oregon (U.S. Fish and Wildlife Service 2010). There are four known extant sites in British Columbia and 38 in the U.S., including 8 in Washington, and 30 in Oregon. Although the species has disappeared from all known sites in California, surveys have been inadequate to determine whether it is extirpated throughout the state (U.S. Fish and Wildlife Service 2010). Range-wide, the species has disappeared from 70% of the known sites and from approximately 90% of its extrapolated historical range (Pearl and Hayes 2005; Pearl et al. 2005).
Figure 3. The historical global range of Rana pretiosa is restricted to the Fraser River Lowlands in British Columbia, disjunct areas in Washington and Oregon, and small areas of California. Adapted from IUCN et al. (2009).
In Canada, R. pretiosa is known from only seven sites, historical and recent, in the Fraser River Basin in the extreme southwest of British Columbia within the Pacific biogeographic zone (Figure 4). Three historical populations are now extirpated. A population was reported from the Sumas Prairie in 1932 (Logier 1932 cited in Carl and McTaggart-Cowan 1945), but the area has since been extensively modified, and no sign of the species was discovered during surveys in 1996–1997 (Haycock 1998) or in 2010 (Pearson 2010a). A population was reported from Nicomen Island (Carl and McTaggart-Cowan 1945), but the species was not found there during wetland surveys in 1997, 2000 (Haycock 2000a), or 2010 (Pearson 2010a). The third extirpated population was in the Campbell Valley Regional Park in Langley, where it was studied in the 1960s and 1970s (Licht 1969, 1971, 1974). The species was last seen at this site in 1981 (Green et al. 1997) and was not found during searches in 1996, 1997, 1999, or 2000 (Haycock 1999; Haycock unpubl. data in B.C. Ministry of Environment files examined by V. Craig in 2009).
Three previously unknown populations were discovered by Denis Knopp in
1996– 1997 during an extensive survey of wetlands in the Fraser River Lowlands (Knopp 1996, 1997; Haycock 1998). The locations are known as Maria Slough, Mountain Slough, and Maintenance Detachment (MD) Aldergrove (here referred to as Aldergrove). The fourth extant population, known as Morris Valley, in the Harrison Lake area was also discovered by Denis Knopp in 2008. The Maria Slough, Mountain Slough, and Morris Valley populations are all in the Harrison Lake area, where an adult R. pretiosa female was captured in the 1940s or 1950s; the specimen was recently discovered in a collection by G.D. Alcorn and J.R. Slater at the University of Puget Sound (B.C. Conservation Data Centre pers. comm. 2009). These four populations are probably currently isolated from each other.
Translocations have been conducted near the Maria Slough site in an attempt to establish a new subpopulation at restored habitat. Although a few egg masses have been found at the restored site indicating successful breeding (2–3 egg masses in 2008 and 2009; V. Craig pers. comm. 2011), this manipulated population is not regarded self-sustaining at this time (P. Govindarajulu pers. comm. 2011). Monitoring of the success of the translocation efforts is in progress (P. Govindarajulu pers. comm. 2011).
An experimental release in 2000 of approximately 700 metamorphs of R. pretiosa into Mirror Lake in the University of British Columbia Malcolm Knapp Research Forest, with the goal of establishing a new population, failed. The young-of-year frogs were raised in captivity from eggs collected from a population in Washington (Barnett and Richardson 2002). Although one frog was found at the site in 2003 (Hawkes 2009), no frogs or egg masses were found during a survey in 2009 (Knopp pers. comm. 2009).
Estimates of the species’ range in Canada are based primarily on known oviposition sites at Maria Slough, Mountain Slough, and Aldergrove, with limited information from capture locations at Maria Slough in 2009 and 2010, and radiotelemetry data obtained in September to October 2009 from captive-reared frogs released at Maria Slough. The current extent of occurrence (EO) of R. pretiosa is 303 km². The discovery of the Morris Valley population in 2008 expanded the previously reported EO from 115 km² by approximately 260%. When historical populations are included in the estimate, the entire EO is 606 km²; therefore, the current EO is approximately half of the known historical range of the species in Canada. The Canadian EO represents less than 5% of the species’ global range, both current and historical. Based primarily on oviposition sites, with limited additional data from trapping and radiotelemetry of frogs, the known area of occupancy (AO) of the species is only approximately 1 km². The actual AO may be larger, because basing the estimate primarily on oviposition locations does not take into account use during other seasons, for which accurate data are not available. Radiotelemetry of one frog at Maria Slough in 2009 indicated that it over-wintered within the range identified by oviposition locations (Pearson 2010c). The Index of area of occupancy (IAO) is 40 km², based on a grid with 2 x 2 km² cells.
The number of locations, based on threats, ranges from one to four. Threats that can rapidly affect frogs at the four extant sites include run-off of pollutants from adjacent agricultural areas, grazing by livestock, and deterioration of breeding sites through vegetation succession. Each site is considered a separate location because the severity and type of threats is variable, depending on land ownership and activities in the surrounding area. However, if the invasion and spread by the non-native American Bullfrog is considered the most significant threat across the sites, then the number of locations is only one.
Since 1996, there have been numerous surveys of potentially suitable habitat for the species in the Fraser River Lowlands (Figure 5, Table 2). Surveys include searching for egg masses during the breeding period (usually February through April), listening for calling frogs, or searching for tadpoles or frogs. Three historical sites (Sumas Prairie, Nicomen Island, Campbell Valley Regional Park) have been searched repeatedly without finding the species (see Canadian Distribution). Knopp and Haycock surveyed 45 sites with appropriate habitat in 1996 and 16 sites in 1997, six of which were resurveys of sites from 1996 (Haycock 1998; Knopp pers. comm. 2009). Note that 22 of the 45 sites surveyed in 1996 were surveyed in June or July, which is not the optimal time to detect the species (Bishop pers. comm. 2009; Knopp pers. comm. 2009). Eight additional sites were searched in 2009 (Pearson pers. comm. 2009) and 30 sites in 2010 (Pearson 2010a) without finding the species. Numerous general surveys for other wetland species have been conducted throughout the Fraser River Lowlands by researchers qualified to recognize R. pretiosa, without finding evidence of new populations (Albrecht pers. comm. 2009; Bishop pers. comm. 2009; Knopp pers. comm. 2009; Pearson pers. comm. 2009; Table 2). Surveyors conducting targeted searches for R. pretiosa spent more than 570 hours at the 80 survey sites for which search effort data are available (Table 2).
Figure 5. Sites searched for Rana pretiosa in Canada to 2009. See Table 2 for additional details.
|Year||Typea||Search hoursb||Location||Monthc||Surveyor||Sp. Det.d|
|1996||Visual||NA||24 Ave Subdivision||May||R. Haycock||N|
|1997||Visual||3.5||264 St & 8 Ave Marsh||March||D. Knopp||N|
|1996||Visual||NA||27 Ave and 196 St||April||R. Haycock||N|
|1996||Visual||NA||32nd Ave and 206th St||May||R. Haycock||N|
|1996||Visual||NA||6 Ave and 216 St||April||R. Haycock||N|
|1996||Visual||NA||7Ave. And 216 St||April||R. Haycock||N|
|1996||Visual||NA||88th Ave and 223rd St||May||R. Haycock||N|
|1996||Visual||NA||Addington Marsh||April||R. Haycock||N|
|1997||Visual||8||Agassiz Slough and Cheam Slough||April||D. Knopp||N|
|2010e||Visual||NA||Aldergrove mushroom farm||April||M.M. Pearson, A. Jonsson||N|
|1997||Visual||1.5||Belrose Road Ditch||March||D. Knopp||N|
|1996||Visual||NA||Blaney Creek||June||R. Haycock||N|
|2010e||Visual||2.2||Brae Slough||March||S. Scotton, S. Gabriel||N|
|1996||Visual||NA||Bridal Falls Golf and Country Club||May||D. Knopp||N|
|2010e||Visual||3.5||Brunette River||March||K. Scotton, G. Geisbrecht||N|
|2009||Visual||12||Camp Slough||April||M. Pearson, M.P. Pearson||N|
|2010||Visual||9||Camp Slough||March||M. Pearson, V. Kilburn||N|
|1997||Trapping||17 day trapping period||Campbell Valley Regional Park||February, March||R. Haycock||N|
|1997||Visual||40||Campbell Valley Regional Park||NA||R. Haycock||N|
|1999||Visual||NA||Campbell Valley Regional Park||NA||R. Haycock||N|
|2000||Visual||60||Campbell Valley Regional Park||NA||R. Haycock||N|
|1996||Visual||2||Chadsey Lake (Pond)||July||D. Knopp||N|
|1997||Visual||1||Chapman Marsh||March||D. Knopp||N|
|1996||Visual||1||Cheam Lake Wetlands Regional Park||May||D. Knopp||N|
|2009||Visual||16||Cheam/Agassiz Slough||April||M. Pearson, M.P. Pearson||N|
|1996||Visual||0.5||Chilliwack River (pond)||July||D. Knopp||N|
|1996||Visual||3.5||Cook’s Marsh||June||D. Knopp||N|
|1996||Visual||1.5||Deer Lake||July||D. Knopp||N|
|1996||Visual||3||Elbow Lake||June||D. Knopp||N|
|1996||Visual||NA||Essondale Lands||May||R. Haycock||N|
|1996||Visual||NA||Fern Ridge Lake||May||R. Haycock||N|
|1997||Visual||1||Gloucester Industrial||March||D. Knopp||N|
|2010e||Visual||1||Gordon’s Brook||March||M.M. Pearson||N|
|2009||Visual||5.5||Grace Lake||NA||D. Knopp||N|
|2009||Visual||13||Great Blue Heron Reserve||April||M. Pearson, M.P. Pearson||N|
|2010e||Visual||6.5||Great Blue Heron Reserve||March||V. Kilburn||N|
|2008||Visual||2||Harrison area||March||D. Knopp, C. Albrecht||N|
|2009||Visual||69.09||Harrison area||March, April||M. Firman||N|
|2010e||Visual||3.5||Harrison River/Chehalis Delta||March, April||B. Johnson, A. Lentini, D. Knopp||N|
|2010e||Visual||NA||Harrison River (Harrison Bay marsh)||April||D. Knopp||N|
|1996||Visual||3||Hatzic Lake area||June||D. Knopp||N|
|2008||Visual||0.5||Hicks Creek||March||D. Knopp, Albrecht||N|
|1996||Visual||2.5||Hicks Lake (Beaver Pond)||July||D. Knopp||N|
|2010e||Visual||1||Hicks Lake (Beaver Pond)||July||D. Knopp||N|
|2009||Visual||1.25||Hogg Slough||March, April||M. Pearson, M.P. Pearson||N|
|2010e||Visual||1||Hogg Slough (upper)||March||M. Pearson||N|
|2010||Visual||>0.33||Hogg Slough (lower)||March||D. Knopp||N|
|2010||Visual||4.25||Hope Slough||March||V. Kilburn||N|
|1997||Visual||2||Hornby Lake||March||D. Knopp||N|
|1996||Visual||5.5||Hornby Lake (Ryder Lake Area)||July||D. Knopp||N|
|2010e||Visual||0.5||Johnson Slough||March||S. Knopp||N|
|1996||Visual||4||Judson Lake||July||D. Knopp||N|
|1996||Visual||NA||Knopp Ponds (Ryder Lake Area)||July||D. Knopp||N|
|1996||Visual||NA||Lafarge Lake - Coquitlam||May||R. Haycock||N|
|1996||Visual||NA||Latimer Pond - Surrey||May||R. Haycock||N|
|1996||Visual||NA||Majuba Hill (Jenson Creek)||May, June, July||D. Knopp||N|
|2010e||Visual||11||McGillavray/Bert Brink WMA||March, April||A. Gielens, D. Knopp, M. Pearson||N|
|2009||Visual||7||Mirror Lake||NA||D. Knopp||N|
|1996||Visual||6||Maria Slough area||June||D. Knopp||Y|
|1997||Visual||8||Maria Slough area||April||D. Knopp||N|
|2010||Visual||2||McLure wetland||March||M.M. Pearson, N. Cox||N|
|1996||Visual||5||McGillivray Creek (Beaver Pond)||June||D. Knopp||N|
|1997||Visual||3.5||McGillivray Creek Game Reserve||March||D. Knopp||N|
|1996||Visual||NA||McLean Pond - Langley||April||R. Haycock||N|
|1996||Visual||11.5||MD Aldergrove||May||D. Knopp||N|
|1997||Visual||55||MD Aldergrove||March, April||D. Knopp, R. Haycock||Y|
|1996||Visual||2.17||Miami Creek||July||D. Knopp||N|
|1997||Visual||8||Miami Creek and Hotsprings Slough||April||D. Knopp||N|
|1996||Visual||NA||Minnekhada Regional Park||June||R. Haycock||N|
|2010||Visual||4.5||Morris Rd/Chehalis estuary||March||M. Pearson, B. Johnson, A. Lentini||N|
|2008||Visual||22.5||Morris Valley||March||D. Knopp, C. Albrecht, K. McNeil||Y|
|1997||Visual||9||Mountain Slough||April||D. Knopp||Y|
|1996||Visual||NA||Murchie Pond - Langley||April||R. Haycock||N|
|1997||Visual||6||Nicomen Slough||NA||R. Haycock||N|
|2000||Visual||40||Nicomen Slough||NA||R. Haycock||N|
|2010e||Visual||2.25||Nicomen Slough||March, April||M. Pearson, M.P. Pearson, D. Knopp||N|
|1996||Visual||NA||Pitt Marsh||June||R. Haycock||N|
|2010e||Visual||5.5||Pitt Lake marsh||April||D. Knopp||N|
|1996||Visual||4||Ross Lake area||June||D. Knopp||N|
|1997||Visual||0.25||Ross Road & Railroad Crossing||March||D. Knopp||N|
|1996||Visual||NA||Rowlatt Pond - Langley||April, June||R. Haycock||N|
|1996||Visual||NA||Salmon River Marsh - Langley||May||R. Haycock||N|
|2010e||Visual||NA||Sechelt Inlet Rd (small beaver pond)||March||V. Kilburn, A. Mitchell||N|
|2010e||Visual||2.5||Shrew pond, Harrison West||March||D. Knopp||N|
|1997||Visual||2||Silverdale Creek||April||D. Knopp||N|
|1996||Visual||4||Smith Falls Creek (Beaver Pond)||July||D. Knopp||N|
|2010e||Visual||13||Smugglers Cove||April||V. Kilburn, A. Mitchell||N|
|2010e||Visual||3.5||Sterline Rd – N side of Fraser||April||D. Knopp||N|
|1996||Visual||NA||Straiton Bowl||May||D. Knopp||N|
|1996||Visual||NA||Sturgeon Slough||June||R. Haycock||N|
|1996||Visual||1||Sumas Mountain above Chadsey (Pond)||July||D. Knopp||N|
|1996||Visual||NA||Tall Timbers - Langley||May||R. Haycock||N|
|2010e||Visual||2.5||Town Rd., N side of Vedder Mtn||April||D. Knopp||N|
|1996||Visual||2.5||Trout Lake||July||D. Knopp||N|
|1997||Visual||6||Trout Lake||April||D. Knopp||N|
|2010e||Visual||9||UBC Farm 2||March||M. Pearson, D. Knopp||N|
|1996||Visual||4||Unnamed Cr. (East of Popkum)||July||D. Knopp||N|
|2010e||Visual||12.5||West Creek wetland||March||A. Gielens, M. Pearson, M. Cruickshanks||N|
|1996||Visual||NA||Widgeon Creek||June||R. Haycock||N|
|2010e||Visual||6.5||Wilson Farms||March||K. Scotton||N|
|2009||Visual||8||Wolf Lake||NA||D. Knopp||N|
a Type of search: Visual – search for egg masses/frogs; Trapping – trapping program
b Search hours: Number of person hours in targeted search for the species. NA = data not available
c Month: The month the search took place
d Sp. Det.: Whether or not the species was detected. Y = Yes, N = No
e Search effort for 2010 are minimum estimates, based on Pearson 2010a.
Rana pretiosa is a habitat specialist associated with water bodies that have seasonally warm shallow areas with emergent or floating vegetation (Licht 1969, 1986a,b; McAllister and Leonard 1997). In Washington, the species prefers habitat with a large amount of open water and low to moderate amounts (25–50%) of cover by emergent vegetation (Watson et al. 2003). In Canada, populations are associated with low-elevation water bodies. Maria Slough, Mountain Slough, and Morris Valley sites are all <25 m in elevation; Aldergrove is at approximately 100 m, and the extirpated population in Campbell Valley Regional Park was at 70 m in elevation (Licht 1986b). In Washington State, sites up to 850 m in elevation provide suitable habitat for the species (Germaine and Cosentino 2004), but based on a relationship between elevation and latitude for 73 known populations in the U.S. and Canada, Pearl and Hayes (2004) suggested that the species is unlikely to be found above 200 m in Canada. A screening model developed to identify potential habitat for the species in Washington specified that soils characteristic of occupied sites are loams, mucks, loamy sands, or other poorly drained fibrisols, mesisols, organic cryosols, gleysols, and humisols (Germaine and Cosentino 2004).
The species is usually associated with large (>4 ha) marshes within forested landscapes (Hayes 1994 cited in U.S. Fish and Wildlife Service 2010). Smaller wetlands may not have extensive shallows with seasonally warm water and may not provide sufficient space for the population to persist under high larval predation rates (Hayes 1994 cited in U.S. Fish and Wildlife Service 2010). Although R. pretiosa has been reported from sites as small as 1 ha in the United States, Pearl and Hayes (2004) suggested that these are remnant populations at sites that were previously connected to larger wetlands. All known populations in Canada are associated with marshes >4 ha in size.
Rana pretiosa has three distinct activity seasons: breeding, summer foraging, and over-wintering periods. Oviposition occurs in shallow warm water in seasonally inundated areas (Licht 1969; McAllister and Leonard 1997; Watson et al. 2003). These areas of slow-moving or still water are near to or seasonally connected with larger bodies of water. Oviposition sites usually have abundant aquatic vegetation. Eggs are rarely laid over open soil or rock substrates (Pearl and Hayes 2004), although at Mountain Slough, egg masses are often laid on top of soft mud (Knopp pers. comm. 2009). Across the species’ range, oviposition sites have been found in water 5 – 30 cm deep (Licht 1969; Pearl and Hayes 2004; Haycock 2000a). The shallow margins of wetlands, ponds and rivers used for oviposition become seasonally warm (Licht 1971). Water temperatures recorded at Canadian oviposition locations range from 4 °C to 14 °C, with an average daytime temperature of 9 °C (Canadian Oregon Spotted Frog Recovery Team 2009a).
In summer, the frogs stay close to shallow wetlands and are usually found in water or along the shoreline, close to water (Licht 1986a). The preferred habitat has abundant floating emergent vegetation within warm, shallow wetlands (Licht 1971; Hayes 1997 cited in U.S. Fish and Wildlife Service 2010). In British Columbia, frogs remained within densely vegetated portions of the wetland, dominated by floating Potamogeton spp., with some submerged Potamogeton species present as well (Canadian Oregon Spotted Frog Recovery Team 2009a). At a site in Washington, frogs selected Hardhack (Spiraea douglasii)-dominated areas in summer (Watson et al. 2003). Watson et al. (2003) reported that summer locations in Washington were in deeper water (average 23.6 cm ± 1.0) than water depths at random locations (average 16.5 cm ± 1.0). In Canada, summer locations were in water 42–112 cm deep (Canadian Oregon Spotted Frog Recovery Team 2009a).
Areas used in winter are in deeper water. Watson et al. (2003) reported that water depth at winter locations of frogs in Washington was on average 17.4 ±0.8 cm. Pearl and Hayes (2004) provided data from other studies in Washington that reported mean water depth at winter locations as follows: 0–120 cm (average 22 cm) (Risenhoover et al. 2001 cited in Pearl and Hayes 2004); 1–88 cm (26.2 cm average) (Hallock and Pearson 2001 cited in Cushman and Pearl 2007); 6–111 cm (averages of 62, 49, 34, and 29 cm) (Hayes et al. 2001 cited in Pearl and Hayes 2004). Frogs will often use springs and seeps that do not freeze or low-flow channels (Hayes et al. 2001 cited in Pearl and Hayes 2004). Data from Oregon indicated that in winter R. pretiosa used a small natural spring that was rarely used during the rest of the year (Chelgren et al. 2007). At a low elevation site in Washington, over-wintering frogs selected open water with submerged vegetation and some emergent vegetation (Risenhoover et al. 2001 cited in Pearl and Hayes 2004). Over-wintering frogs may also bury themselves in silty bottom substrate or in aquatic vegetation (McAllister and Leonard 1997). Frogs in Washington have also been reported using ditches in late autumn or winter (Watson et al. 2003; Hayes et al. 2001 cited in Pearl and Hayes 2004). Watson et al. (2003) reported that frogs at a site in Washington buried themselves at the base of clumps of Soft Rush (Juncus effusus), and remained immobile from mid-December through January under ice <5 cm thick.
Data on winter habitat use by R. pretiosa in Canada are limited. For one frog, which was captive-reared in 2008, allowed to over-winter in captivity, and subsequently released at Maria Slough, telemetry data were collected from September 2009 to February 2010. This frog spent most of December and January in a Cattail (Typhus sp.) marsh at the perimeter of the wetland, before moving back to the main wetland in late January (Pearson 2010c). The researchers recorded the frog moving underneath 2 to 10 cm of ice. Four captive-reared and over-wintered frogs at Aldergrove were also tracked to their over-wintering sites in 2009 – 2010: one frog used an active beaver dam, one used a submerged island of Hardhack vegetation, one used a small island of Hardhack vegetation connected to an inactive beaver dam, and one was in an area of thick Hardhack roots and debris (Govindarajulu 2009). An earlier radiotelemetry study by R. Haycock of Canadian frogs suggested a possible association between winter habitat and areas impacted by Beaver (Castor canadensis) activity, which resulted in deeper water (Canadian Oregon Spotted Frog Recovery Team 2009a). This relationship between Beaver activity and winter habitat has also been noted in the United States (Hayes pers comm. 2008).
Wetland habitats required by R. pretiosa are declining in the Fraser River Lowlands (Boyle et al. 1997; Moore pers. comm. 2009). Historically, extensive dyking of river ways, starting in the 1860s and completed in the 1930s, reduced flooding and eliminated suitable habitat. In particular, the draining of the large, shallow Sumas Lake in the 1920s significantly modified drainage patterns and resulted in loss of associated wetlands. This lake and associated wetlands were comprised of 8000 ha of marshlands and slough and 3600 ha of open water, and tripled in area during freshet flooding (Chilliwack Museum 2009). Boyle et al. (1997) estimated that the activities of draining and conversion of land to agriculture reduced fen and swamp/bog/marsh habitat, which had formed 10% of the area of the Lower Fraser Basin prior to 1820, by 21% by 1930, and by 85% by 1990. A more recent study by Moore et al. (2003) assessed the changes to 320 freshwater wetlands in the Fraser Basin between 1989 and 1999 by comparing orthophotos taken during the 2 years. They found that 71 of the 320 wetlands, approximately 20%, had lost habitat due to encroachment from development. Approximately 41% of habitat loss was due to agricultural development, the rest to urban development including golf courses, or habitat removal while the land was in transition to some future land use. Of the affected wetlands, two wetlands lost >50% of their area, 36 lost <5% of the area, and 33 lost 5–50% of the area. These changes resulted in a total loss of 965 ha of wetlands during the decade. The extensive creation of dykes, ditches, and channels fragmented the remaining wetlands.
Habitat rehabilitation and creation have occurred at three of the extant locations of R. pretiosa in Canada. At Aldergrove, 1,300 m2 of wetland habitat was constructed in 2001, an additional area of 18,000 m2 was constructed in 2004, and large areas of invasive Reed Canarygrass (Phalaris arundinacea) were removed. Beaver dams continue to increase habitat through flooding (Knopp pers. comm. 2009). At Maria Slough, 1,500 m2 of habitat was created approximately 4 km from the known oviposition location, an additional 1,000 m2 was rehabilitated in 2000, and Reed Canary Grass was removed in 2003. In 2009 the created habitat was expanded by 5,000 m2, and it was expanded by another 3,000 m2 in 2010. At Mountain Slough, 1,800 m2 of habitat was constructed in 2005, and remaining habitat has also been rehabilitated.
Potentially suitable habitat for R. pretiosa is declining in Washington and Oregon. Throughout Washington, at least 33% of wetlands were drained, dyked, or filled in by the 1980s (Canning and Stevens 1990 cited in U.S. Fish and Wildlife Service 2010), and the rate of loss is even higher within the historical range of R. pretiosa (McAllister and Leonard 1997). From 1780s to 1980s, 38% of wetland habitat was lost In Oregon and 91% in California (Dahl 1990).
Prior to 1997, the name Rana pretiosa was applied to both R. pretiosa and R. luteiventris, as it is now known. Therefore, when interpreting results of research prior to 1997 the reader must note the geographic location of the research, which will indicate which species was studied. The majority of research on R. pretiosa in Canada was conducted by L. Licht, whose series of papers focused on a now-extirpated population in Campbell Valley Regional Park (Licht 1969, 1971, 1974, 1975, 1986a,b). D. Knopp, R. Haycock, and C. Bishop have studied the extant populations in Canada since the 1990s, and their data are available primarily in unpublished reports to the B.C. Ministry of Environment, the Canadian Wildlife Service, the Canadian Oregon Spotted Frog Recovery Team, and the Department of National Defence (Bishop 2007; Haycock 1999, 2000b, 2001, 2005; Knopp 1996, 1997; McKibbin et al. 2008). Rana pretiosa has been well-studied in the United States (Chelgren et al. 2008; McAllister and Leonard 1997; Pearl and Hayes 2004; U.S. Fish and Wildlife Service 2010; Watson et al. 2003).
The frogs become active and begin breeding early in the spring, after the air temperature reaches approximately 5°C (Licht 1969). In southwestern B.C., breeding activity begins in February or March when males arrive at traditional breeding sites and begin calling (Licht 1969). The advertisement call is species-specific and consists of rapid series of low-pitched clucks; it has little carrying power in the air as males usually call under water (Matsuda et al. 2006). Males are not territorial and will form groups in small areas (Licht 1969). Egg-laying is temperature-dependent; females begin to lay eggs when water temperatures reach approximately 6°C (Licht 1971). Egg-laying usually begins in March, and continues for 2 to 4 weeks (Licht 1969; Bishop pers. comm. 2009; Pearson pers. comm. 2009). Egg-laying occurs at traditional sites that are used in successive years, which suggests that these sites offer specific, required features (Licht 1969; Knopp pers. comm. 2009).
In Canada, R. pretiosa probably become sexually mature by the age of 3 years (Licht 1974). Based on measurements of male and female frogs in amplexus at Aldergrove, Haycock (2005) estimated that males begin to breed in their third year, and females in their third or fourth year. In Washington, males begin to breed at 2 years and females at 3 years of age (McAllister and Leonard 1997). In the Klamath Basin, Oregon, both males and females begin breeding when 2 years old (Haycock 2000a). Females breed every year (Licht 1974), lay one egg mass per year, and mate with one male (Phillipsen et al. 2009).
Licht (1974) reported that the average number of eggs per egg mass was 643 in Campbell Valley Regional Park (N = 9); Haycock (2005) reported an average (± 1SE) of 861 ± 247, 940 ±296, and 649 ±67 eggs/mass in 2003, 2004, and 2005, respectively at Aldergrove. At one site in Washington, there was an average of 598 eggs/cluster (McAllister and Leonard 1997). Cushman and Pearl (2007) reported that clusters in Washington and Oregon could contain up to 1,500 eggs. Development of R. pretiosa from hatching through to metamorphosis takes approximately 3 to 4 months (Licht 1974).
Survival rates of R. pretiosa vary with life stage and can vary substantially between areas and years. For the population at Campbell Valley Regional Park, Licht (1974) estimated that embryonic survivorship varied from 68% to 74% (N = 22) in 1968 but probably would have been 0% in 1969 had he not moved egg masses four times to compensate for receding water levels. The potential stranding of egg masses has also been reported for Aldergrove (Haycock 2000a) and for two locations in Washington (McAllister and Leonard 1997). Monitoring of embryonic survivorship in partial egg masses placed in floating cages to protect them from predators at Maria Slough and Aldergrove between 2002 and 2009 indicated that survivorship at Maria Slough ranged from an average of 76.6% ±34.5% in 2004 to 96.0% ± 8.9% in 2007 (Potvin 2009). Embryonic survivorship was much lower at Aldergrove, where it ranged from 40.4% ±43.2% in 2004 to 60.6% ±34.5% in 2002 (Potvin 2009). In 2005, survivorship at one subsite of Aldergrove was only 9% (McKibbin et al. 2008). At both Maria Slough and Aldergrove, egg masses were monitored and moved if necessary to avoid desiccation. The higher survival rates reported by Potvin (2009) compared to those by Licht (1974) probably reflect the fact that the eggs in Potvin’s study were protected from predation and were not threatened by changes in water levels. Shortly after hatching there is a high rate of tadpole mortality, probably from predation. Licht (1974) estimated that only 1% of tadpoles survived to metamorphosis at one site in one year of his study; at another site survival was 7.3%. First-winter minimum survival rate for juveniles was 67.1%, and yearly minimum survival rates for adult frogs were estimated to be 44.9% for males and 66.7% for females (Licht 1974). A mark-recapture study in Oregon found that annual survival was 23% and 68% for small (<53 mm SUL) and large (>53 mm SUL) female frogs respectively, and annual survival was 12% and 57% for small and large male frogs respectively (Chelgren et al. 2008). A separate mark-recapture study focused on over-winter survival instead of annual survival reported a minimum 27% winter survival estimate (U.S. Fish and Wildlife Service 2010).
Little information exists on longevity of R. pretiosa. At Maria Slough, a marked frog captured in 2009 was probably released in 2002, and a marked frog captured at Aldergrove in 2009 was probably released in 2003 (Potvin 2009). In Washington, an adult male was at least 11 years old, and several other frogs from Oregon populations were thought to have been adults for at least 7 or 8 years (U.S. Fish and Wildlife 2010). U.S. studies of lines of arrested bone growth suggested that younger frogs make up the majority of the population (U.S. Fish and Wildlife 2010).
Little information is available on the sex ratio of R. pretiosa. Although males are predominant in trapping samples, the samples are probably biased towards males because they remain at the breeding area for much longer than do females (Haycock 2005). A mark-recapture study of the now-extirpated Campbell Valley Regional Park population showed a male to female sex ratio of 0.6:1 in 1968 (N = 183 frogs) and 0.4:1 in 1969 (n = 117 frogs) (Licht 1974). Trapping and hand-capture at Aldergrove from 2001 to 2005 showed a male to female sex ratio of 3.5:1 (N = 350 frogs) (Haycock 2005). A project in Oregon, which involved intensively trapping and relocating an entire resident population of R. pretiosa, resulted in the capture of 9 females, 11 males, and 21 juveniles, which would give a male to female sex ratio estimate of 1.2:1 (Chelgren et al. 2008); however, this small population may not be representative of larger populations. C. Pearl suggested that, for estimating population size, calculations should assume that one egg mass is equivalent to one mature female and one to two mature males (U.S. Fish and Wildlife 2010). The generation time is
4.7 – 5.5 years, calculated as follows: age of maturity + (1/annual mortality rate), where age at maturity is 3 years (Licht 1974) and annual mortality rate is the average of values from Licht (1974) (mortality rate = 0.4) and from Chegren et al. 2008 (mortality rate = 0.6). Hammerson and Pearl (2004) also reported generation length of about 5 years for this species.
Rana pretiosa shows strong site-fidelity, probably related to its specific habitat requirements, especially for oviposition sites (see Life cycle and reproduction). Embryos of this species have a fairly wide temperature tolerance; at least 50% of embryos will develop normally between 6 °C and 28 °C, and they can survive temperatures as low as 1°C for several hours (Licht 1971).
Females lay eggs communally at the edge of shallow, often temporarily inundated areas of water, with the result that the pile of egg masses is partially exposed to air. Communal oviposition and the placement of egg masses in the shallows increases the rate of embryonic development by maximizing the temperature around eggs, but it also makes eggs extremely vulnerable to freezing and to drying by wind or receding water levels (Licht 1969, 1974; McAllister and Leonard 1997; Haycock 2000a; Watson et al. 2000). In some years, the entire reproductive output at a site can be lost due to water fluctuations (Licht 1974). Females will deposit their egg masses in almost the exact same place year after year (Licht 1969). This reliance on traditional oviposition sites increases the vulnerability of the species to habitat alteration.
Egg masses collected from R. pretiosa populations in Canada have been successfully reared in captivity. Early attempts resulted in high mortality from disease, including iridovirus, Aeromonas hydrophilia, Pseudomonas species, and associated bacterial infections (Hawkes 2009). Recent efforts at captive rearing at Mountain View Conservation Society and at the Greater Vancouver Zoo have been more successful, achieving embryonic survivorship similar to that seen for egg masses in situ that were kept in protected net cages (Bishop pers. comm. 2009). Survivorship of captive-reared tadpoles through metamorphosis averaged approximately 23% in 2007 and ranged from 28 to 41% in 2008, which is much higher than the estimated 1% to 7.3% tadpole survivorship reported by Licht (1974). The resulting captive-reared metamorphs have been either released in autumn, or over-wintered in captivity for release the following summer. Until recently, frogs were released only at the site where the eggs were collected; however, in 2009, some captive-reared Maria Slough frogs were released at Aldergrove as part of a radio-telemetry project (Govindarajulu pers. comm. 2010). There is some evidence that captive-reared frogs survive in the wild. In 2009, three frogs captured at Maria Slough had been captive-reared and released in 2006, and another frog had been released in 2007 (Potvin 2009). In addition, egg masses have been discovered at Maria Slough in habitat created 4 km from the original population, where captive-reared frogs were released. Because the longest recorded movement of R. pretiosa is 2.8 km (Cushman and Pearl 2007), the presence of egg masses could indicate that the released frogs were breeding. An attempt to establish a population of R. pretiosa at Mirror Lake in the University of British Columbia’s Malcolm Knapp Research Forest failed, with no signs of oviposition detected at the site (Knopp pers. comm. 2009). However, this site was at a higher elevation (260 m) and was smaller (2 ha) than would be considered optimal (Hayes 1994; Pearl and Hayes 2004). In Oregon, a population of R. pretiosa was relocated in 2001 to a newly created habitat (Chelgren et al. 2008). The population increased in size and continued to persist in 2009 (U.S. Fish and Wildlife Service 2010). Chelgren et al. (2008) reported that the initial survival rate for relocated frogs was lower than for frogs reared from eggs at the site. The Canadian Oregon Spotted Frog Recovery Team has identified 12 candidate areas for release of R. pretiosa to establish new populations. Additional habitat surveys and disease testing are required to determine the suitability of these areas (Pearson 2010b).
Frogs are being held for a captive breeding program at the Vancouver Aquarium and the Toronto Zoo. Captive females held at the Aquarium since 2002 were gravid in 2009, but no males were available for breeding (Thoney pers. comm. 2009). In 2010, for the first time, two pairs of captive R. pretiosa bred, producing 600–900 eggs.
Rana pretiosa is highly aquatic. Aquatic connections between over-wintering and breeding habitat may be essential (Watson et al. 2003; Pearl and Hayes 2004; Canadian Oregon Spotted Frog Recovery Team 2009a). A radio-telemetry study of 18 captive-reared frogs in 2009 at Maria Slough revealed that the frogs were almost always at the water’s edge on or in islands of vegetation (Pearson 2010c). Only one frog frequently used terrestrial habitat, where it was found in tunnels created by tree roots and vegetation (Pearson 2010c). A similar telemetry study on habitat selection of 11 captive-reared frogs released at Aldergrove in 2009 revealed that the frogs preferred habitats with complex emergent vegetation. These sites were near open water with submergent vegetation and deep sediment; some frogs also temporarily resided in beaver dams (Govindarajulu 2009). In Washington, 99% of locations of radio-tagged frogs (N = 654) were in at least 1 cm of water (Watson et al. 2003). A road blocked the access to a major breeding pond in this study, but the authors found no road-killed R. pretiosa, although there were mortalitiesof R. aurora and Pseudacris regilla. They suggested that R. pretiosa accessed the pond via a culvert. Watson et al. (2003) reported an overland movement through marshy habitat.
Movement distances of R. pretiosa vary by season. Radio-telemetry projects in Washington and Oregon revealed that individual frogs moved substantially more during the spring breeding and autumn periods than during the dry summer season (Watson et al. 2003; Chelgren et al. 2008). Home ranges during the breeding and autumn seasons averaged 1.8–1.9 ha, whereas the dry-season home range averaged only 0.9 ha (Watson et al. 2003). Individual frogs can move 100s of metres between breeding and winter habitats. In Washington, Watson et al. (2003) reported movements of 32 – 111 m/day for 2 to 18 days, which suggests that the frogs are capable of longer-distance dispersal. One telemetry project indicated that the frogs did not usually move more than 400 m from the original capture location (Hallock and Pearson 2001 cited in Cushman and Pearl 2007); another study found that the frogs usually moved less than 100 m between years (U.S. Fish and Wildlife Service 2010). However, movements of >1 km have been recorded within wetland complexes and along linear riparian systems (Watson et al. 2003; Pearl and Hayes 2004). The longest reported movement is of an adult female frog along Jack Creek in Oregon, which moved 2,799 m (stream distance) from her original capture location (Cushman and Pearl 2007). In the same study, two juvenile frogs were recorded moving 1,245 m and 1,375 m downstream from their initial capture location. In Washington, three frogs moved 2.4 km along a creek (McAllister and Walker 2003 cited in U.S. Fish and Wildlife Service 2010).
The four extant populations of R. pretiosa in Canada are most likely isolated from each other. Suitable connecting habitat that would allow movement of individuals between populations is not available (see Population spatial structure and variability).
This species is preyed upon by a large number of aquatic and terrestrial vertebrates including mammals, birds, reptiles, and other amphibians (Licht 1974; Watson et al. 2000, Watson et al. 2003; Hayes et al. 2005; Pearl et al. 2005; Pearl and Hayes 2005; U.S. Fish and Wildlife Service 2010). Both introduced American Bullfrogs (Lithobates catesbeianus) and introduced fish, such as Brook Trout (Salvelinus fontinalis) and centrarchids, have been suggested as contributors to the decline of the species across its range (U.S. Fish and Wildlife Service 2010), including the Aldergrove and Campbell Valley populations in Canada (Govindarajulu pers. comm. 2010; see Threats and Limiting Factors: Introduced predators). The American Bullfrog is a predator of both tadpoles and adults of R. pretiosa (McAllister and Leonard 1997) and was documented to prey on tadpoles at Aldergrove (recorded on video by R. Haycock and reported to the Recovery Team). It is also a carrier of the fungus Batrachochytrium dendrobatidis, which causes chytridiomycosis; testing of Bullfrogs at Aldergrove and Maria Slough indicated that they carry the pathogen (Potvin 2009; Govindarajulu pers. comm. 2010). Introduced predaceous fish probably consume tadpoles and over-wintering frogs (U.S. Fish and Wildlife Service 2010; see Threats and Limiting Factors: Introduced Predators).
The River Otter (Lontra canadensis) is confirmed as a predator of adult R. pretiosa at Aldergrove and Maria Slough (Govindarajulu 2009; Pearson 2010c); Great Blue Heron(Ardea herodias) preys on adults (Licht 1974); Gartersnakes (Thamnophis spp.) prey on larvae and adults (Licht 1974; Watson et al. 2000, 2003; Pearl and Hayes 2005). Other potential major predators include the Belted Kingfisher (Megaceryle alcyon) (Licht 1974; U.S. Fish and Wildlife Service 2010) and Raccoon (Procyon lotor) (U.S. Fish and Wildlife Service 2010). Licht (1974) lists the following important predators of tadpoles and/or eggs: larval Northwestern Salamander (Ambystoma gracile), Rough-skinned Newt (Taricha granulosa), Giant Water Bug (Lethocerus americanus), larval backswimmers (Notonecta spp.), leeches (Batrachobdella picta), and dragonfly nymphs.
Rana pretiosa has a broad diet. Tadpoles are grazers, feeding on plant tissue, algae, detritus, and rotting organic matter (Licht 1974; McAllister and Leonard 1997). Newly transformed frogs eat spiders (Arachnida), long-legged flies (Dolichopdidae), hover flies (adult Syrphidae), spittlebugs (Cercopidae), ants (Formicidae), and aphids (Aphididae; Licht 1986b). Adults consume a variety of insects such as ground beetles (Carabidae), rove beetles (Staphylinidae) and long-legged flies, spiders, and small vertebrates such as newly metamorphosed R. aurora, juvenile Anaxyrus (Bufo) boreas, juveniles of their own species, and adult Pseudocris regilla (Licht 1986b; Pearl and Hayes 2002; Pearl et al. 2005).
Extensive surveys have been carried out, usually annually, at each of the known extant locations of R. pretiosa in Canada since their discovery (Table 3). Additional surveys have been conducted at historical and other potential sites (Table 2, Table 3). The most efficient method of sampling to estimate the size of the breeding population is by searching for egg masses in spring (Knopp pers. comm. 2009). The surveys involve repeated visits to a site and visually searching for egg masses in shallow water. The amount of survey effort has been highly variable across sites and years (Table 2), but recent survey effort has been more systematic and has hence resulted in more accurate counts of egg masses. In 2009, surveyors spent more than 400 hours looking for egg masses at the four known extant locations of R. pretiosa in Canada.
|Site & year||Total number of egg masses||Number of communal oviposition sites||Estimated number of breeding adultsa||Comments||Activity typeb||Survey effortc||No. of
|Collected 300 eggs||EC|
|Collected 1,860 eggs||EC|
|Collected 1,800 eggs||EC|
|Collected 900 eggs||EC|
|Released 317 young-of-year frogs||R|
|Collected a total of 750 eggs at Maria Slough and MD Aldergrove||EC|
|Released 115 young-of-year frogs||R|
|2009h||0||0||1i||1 adult male seen||V||~144||1|
|1997||16||2||32–48||1 adult seen||V||9h||1|
|Collected ~ 160 eggs|
|~140 metamorphs released||R||1|
|Collected 2,500 eggs||EC|
|2008||50||5||100–150||1 juvenile seen||V||27.5||1|
|2010j,k||52||13||104–156||4 oviposition sites had 1 egg mass||V||NAe||3|
|1996||0||0||NAe||~350 larvae found||V||6||1|
|Released 400 juveniles||R|
|~7,000 embryos translocated to new habitat created in 2000||EC|
|20 egg masses (about 7,500 embryos) translocated to new habitat created in 2000||EC|
|Collected 2,000 eggs||EC|
|Collected 159–265 eggs for genetic analysis||EC|
|Released 461 young-of-year frogs||R|
|Released 100 metamorphs + 25 tadpoles||R|
|Released 34 over-wintered frogs||R|
|Released 381 young-of-year frogs|
|~10,500 embryos translocated to new habitat created in 2000||EC|
|Collected >1,300 embryos||EC|
|Released 836 young-of-year frogs/larvae||R|
|~10,000 embryos translocated to new habitat created in 2000||EC|
|Collected 480 eggs||EC|
|Collected a total of 750 eggs at Maria and MD Aldergrove||EC|
|Released 308 young-of-year frogs||R|
|Collected 4,250 eggs|
|Released 846 young-of-year frogs||R|
|Collected 3,800 eggs||EC|
|Released 1,012 young-of-year frogs||R|
|Collected 2,500 eggs||EC|
|Released 357 over-wintered frogs in April||R|
|Released 1257 young-of-year-frogs||R|
|2010j||39||7||78–117||2 oviposition locations had 1 egg mass||V||NAe||6|
|Campbell Valley Regional Park|
|Mirror Lake - introduced population|
|2000||~700 young-of-the-year frogs released||R|
|2002||0||0||0||1 adult/juvenile trapped||V||NAe||1|
a Assume female frogs lay one annual clutch of eggs. Assume egg mass = 1 adult female + 1 to 2 adult males (range)
b Activity type: V = visual search; EC = egg collection; R = release
c Survey effort: total number of person-hours
d No. surveyors: number of people who completed survey. Number hours per surveyor not known
e Data not available
f Incomplete survey
g Includes a count of one pair observed in amplexus. Egg mass not located
h minimum # egg masses; visits by multiple researchers confused count
i no egg masses discovered; estimate of breeding population from the 1 frog trapped
j one site had multiple single egg masses, which were moved due to water fluctuations
k Only 29 egg masses were found in the traditional search area. After expansion of the search area by up to 2 km, 23 additional egg masses at 5 sites were discovered.
Licht (1974) conducted a mark-recapture study of the now-extirpated Campbell Valley Regional Park population in 1968 and 1969 and estimated population sizes for those years (Table 3). He captured frogs by hand two to three times per week from February to November and marked them for individual recognition. Trapping and marking has occurred at Aldergrove from 2001 to 2009, and at Maria Slough in 2008-2009 (Bishop pers. comm. 2009; Haycock unpubl. data). Trapping effort has been variable and, until recently, poorly documented. The trapping program provided information on body size of frogs, and more recently on survival of wild, marked and/or captive-released frogs (Bishop pers. comm. 2009). However, the limited amount of mark-recapture trapping that has occurred is insufficient to generate estimates of population size, or to determine population age structure.
The total population size of R. pretiosa in Canada cannot be estimated accurately based on available data. However, based on the number of egg masses, it is possible to estimate the number of breeding individuals per site given the following assumptions: (1) mature females breed every year (Licht 1974); (2) females lay one egg mass per year and mate with one mature male (Phillipsen et al. 2009); and (3) all egg masses are discovered. It can be assumed that one egg mass equals one adult female and one to two adult males (U.S. Fish and Wildlife Service 2010). Using this range in the number of mature males, in 2010 the total adult population of R. pretiosa in Canada was estimated to be 316 to 474 frogs (Table 3). It should be noted that considerable error might be associated with estimates from egg-mass counts. However, the total number of individuals is likely to be <500 and is almost certainly <1,000.
The four populations of R. pretiosa show different patterns of abundance. The Aldergrove population is nearing extirpation, even though >2,000 captive-reared frogs and tadpoles were released into the population from 2000 to 2006 (Table 3). The abundance of mature individuals has declined from an estimated breeding population of 210–315 in 1997, to 0 in 2007 through 2010 (Table 3). Since 1997, search effort has increased more than two-fold. Although there is no evidence of breeding, some frogs still exist at the site; one adult captive-reared male, originally released in 2006, was captured in 2009. Enhancing the population through the release of captive-reared frogs ended in 2006. Previously, only frogs that were reared from eggs collected at the site were eligible for release back to the site; therefore, the lack of egg masses in the past four years prevented captive rearing. Most of the frogs released at the site were expected to have reached breeding age by 2009. In 2009, 11 frogs captive-reared from eggs collected from Maria Slough were released into the wetland as part of an ongoing telemetry study (Govindarajulu 2009). A population trend analysis for R. pretiosa in Canada from 1997 to 2007 was completed by B. Smith, Environment Canada (Bishop 2007; Smith pers. comm. 2009). The analyses by Smith were part of a science assessment for Environment Canada and were presented to the Recovery Team in 2008 (no report was produced). The model estimates the most likely population outcome over time, assuming a simple exponential trend in population size (Bishop 2007). The inter-annual variation in population estimate was assumed to follow a negative binomial distribution. Based on data up to 2007, Smith reported that there was an 80% probability that the Aldergrove population would become extirpated before 2011. The probability of long-term (>2050) persistence of the population at Aldergrove was calculated as <1% (Bishop 2007). Data collected since 2007 do not suggest recovery of the population. The recent increase in the American Bullfrog population size at the site has been suggested as contributing to the dramatic population decline through predation and spread of disease (Govindarajulu 2009).
The overall breeding population at Mountain Slough has been fairly stable since the population was discovered in 1997 (Table 3). However, water levels were very low in 2010, and only 29 egg masses were discovered within the traditional search area, a large decline from previous years. Expanding the search area by up to 2 km along connected waterways resulted in the discovery of 23 egg masses at five previously unknown oviposition sites (Pearson 2010a). A population trend analysis for 1997 and 2009, conducted by B. Smith as described above for Aldergrove, indicated that there was a 17.2% probability of population extinction by 2015, a 32.5% probability of extinction by 2020, and that the population has an approximate 50% probability of continuing past 2050 (Smith pers. comm. 2009).
The breeding population at Maria Slough has fluctuated broadly since monitoring began in 1996. The estimated number of mature adults has declined from highs of over 200–432 frogs in 2002–2006, to an estimated 134–201 frogs in 2010. The estimates of number of breeding adults since 2006 is the lowest recorded since search effort increased starting in 2000. From 2000 to 2009, over 6,000 captive-reared frogs were released into the population (Table 3). Over 2,600 frogs were released in 2008 and 2009; these are expected to be of breeding age in 2010 or 2011. A population trend analysis for data collected from 1997 to 2009 indicated a 46.5% probability that the population will be extirpated by 2015, a 61.7% probability of extinction by 2020, and an approximately 70% chance of extirpation by 2035. There is an approximately 28% probability the species will still inhabit the site past 2050 (Smith pers. comm. 2009).
Only three years of data are available for the Morris Valley population, discovered in 2008. The estimate of number of breeding individuals declined by half from 2008 to 2010, but the time series is too short for meaningful conclusions about trends. Of concern is an apparent reduction in the number of oviposition sites, which could indicate that the habitat has become less suitable (Welstead pers. comm. 2009).
Three populations have been extirpated from Canada. The population in Campbell Valley Regional Park in Langley was estimated to have 6 –90 and 108–162 breeding adults in 1968 and 1969, respectively (Licht 1974). Estimated total population size for those years, based on a mark-recapture study, was 183 (69 males and 123 females) in 1968, and 117 (31 males and 82 females) in 1969 (Licht 1974). The species continued to persist at the site in 1981 (Green et al. 1997), but was extirpated by 1996 (Haycock 2000a). The movement of Bullfrogs into the breeding site in 1970 (Licht 1974), together with significant successional habitat changes that resulted in loss of suitable breeding habitat and habitat connectivity (Haycock 1999), might have contributed to the decline of R. pretiosa at the site. There are no estimates of historical breeding population size from the Nicomen Slough and the Sumas Prairie areas.
Simple extrapolation from egg-mass counts suggests that the size of the total adult population in the three most well-studied populations combined (Aldergrove, Maria Slough, and Mountain Slough) has declined by 34.7% from 2000 to 2010 (only traditional search area for Mountain Slough included). Including the expanded search area at Mountain Slough in 2010 resulted in a corresponding estimated decline of 19% for the three populations combined. The discovery of the Morris Valley population in 2008 increased the overall total known adult population in Canada by 6.9% from 2000 to 2010. This apparent increase is an artifact of incomplete previous knowledge and does not reflect a true population increase. The magnitude of population decline over the past 3 generations (14-17 years) is uncertain.
Populations of R. pretiosa in Canada are >200 km from the closest populations in Washington and embedded within a highly modified environment on the Lower Mainland of British Columbia; therefore populations are isolated both from each other, and from U.S. populations. Given the species’ specific habitat requirements and limited range of dispersal, a rescue effect between Canadian populations or from U.S. populations is highly unlikely. Suitable habitat is available if populations were introduced (Pearson 2010b).
The Canadian Oregon Spotted Frog Recovery Team (2009a) has identified the following main threats to the species: habitat loss, habitat fragmentation and genetic isolation, hydrological alteration, water quality, disease, and exotic predators.
Habitat loss is the largest historical and imminent threat to R. pretiosa in Canada (see Habitat trends). Habitat loss is still occurring at occupied sites. Activities that have been documented at occupied sites include: municipal stream and ditch maintenance that can include clearing and brushing, which reduces available habitat (Mountain Slough); agricultural land use changes including the establishment of new crops, which can involve the placement of drain tiles and removal of riparian vegetation (Mountain Slough); grazing by livestock, which could lead to trampling of egg masses, bank erosion and input of nutrients to the system through input of manure (Morris Valley); and burning for vegetation management, which can cause direct mortality of frogs and removes important habitat for juveniles (Morris Valley) (Canadian Oregon Spotted Frog Recovery Team 2009a).
Natural succession modifies the structure of wetlands and reduces the amount of suitable oviposition habitat for R. pretiosa (Chelgren et al. 2007). Growth of vegetation reduces the available areas of shallow water and also shades the surrounding area, reducing habitat suitability for oviposition. Chelgren et al. (2007) reported reduced larval survival R. pretiosa in Oregon, caused by pond succession. Natural succession is of concern at Maria Slough and Aldergrove.
Habitat loss is also caused by growth of the exotic invasive Reed Canarygrass. Reed Canarygrass changes the structure of the wetland community, creating a dense vegetated area around the perimeter of wetlands that eliminates or reduces the amount of suitable oviposition habitat for R. pretiosa (McAllister and Leonard 1997; U.S. Fish and Wildlife Service 2010). Results of telemetry studies in British Columbia show varying use of Reed Canarygrass by frogs, ranging from avoidance of dense beds at Aldergrove in 2001–2002 (Haycock, unpublished data) to use of floating mats in autumn but not in winter, also at Aldergrove (Govindarajulu 2009), to extensive use in both autumn and winter at Maria Slough (Pearson 2010c). Reed Canarygrass is present at Aldergrove, Maria Slough and Mountain Slough. Attempts to remove it have taken place at all three locations.
Widespread loss of habitat in the Fraser River Basin since the 1860s (Boyle et al. 1997; Moore 1990) has led to fragmentation of the remaining wetlands and population isolation. Research on movements of R. pretiosa indicates that frogs move almost exclusively along connected watercourses, and that movements are usually short (Hallock and Pearson 2001 cited in Cushman and Pearl 2007; U.S. Fish and Wildlife Service 2010). Because of lack of connecting habitat, the four extant populations are now probably isolated from each other (see Population spatial structure and variability). It is unknown how the frogs dispersed across the landscape historically, but it is possible that occasional floods of the Fraser River might have provided dispersal habitat (Weldstead pers. comm. 2009). In addition, yearly flooding of areas around the Fraser River during the spring freshet (Boyle et al. 1997), now prevented from occurring by dykes, would have provided suitable dispersal habitat.
Rana pretiosa oviposits at the edge of shallow wetlands, and its egg masses are very susceptible to mortality caused by changes in water levels (Licht 1974; McAllister and Leonard 1997; U.S. Fish and Wildlife Service 2010). Changes to the hydrology of occupied sites are an imminent threat to oviposition sites. Specific issues include a planned upgrade to the pump station at Mountain Slough, which could potentially require a drawdown of the site (Canadian Oregon Spotted Frog Recovery Team 2009a). Depending on the timing, the event could adversely affect R. pretiosa egg masses. Removal of culvert blockages in spring can cause sudden fluctuations in water level, which can strand egg masses; this has occurred at Maria Slough in the past. Diversion of water, for irrigation or other purposes, can also lower water levels.
Normally, flooding caused by beaver dams helps create suitable habitat by flooding areas or creating stable water levels (Haycock 2000a; Canadian Oregon Spotted Frog Recovery Team 2009a; U.S. Fish and Wildlife Service 2010). Habitat creation by Beavers has been documented at Aldergrove, Mountain Slough, and Morris Valley. Beaver activity can limit habitat in some cases, as documented at Aldergrove (Haycock 2000a,b); flooding behind beaver dams in 1995–1999 removed approximately 300 m of suitable oviposition habitat, and the remaining habitat was either too steep or too vegetated to be useful (Haycock 2000a,b).
Mining or quarrying can require explosives, which can change the groundwater flow and may eliminate the water source to an area (Canadian Oregon Spotted Frog Recovery Team 2009a). This is a potential threat at Mountain Slough, where blasting in 2009 at the adjacent quarry removed a hillside along which flowed a stream that fed into an oviposition site in the slough (pers. obs. by V. Craig 2009; Knopp pers. comm. 2009).
Aquatic habits of the frogs make them vulnerable to contaminants accumulating in water bodies. This species is sensitive to the presence of nitrates, nitrites, and ammonium (Marco et al. 1999; Rouse et al. 1999). Nitrates and nitrites are toxic even at low levels. Marco et al. (1999) found that the median lethal concentration at 15 days of exposure was 0.57 mg/L of nitrite and 16.45 mg/L of nitrate for embryos collected in Oregon. The recommended maximum level of nitrate for drinking water in the U.S. and Canada (10 mg/L) was moderately toxic to R. pretiosa embryos (Marco et al. 1999). Hecnar (1995) suggested that nitrate fertilizers might be an important contributor to amphibian declines, and that ammonium nitrate concentrations in agricultural areas commonly exceeded levels that were toxic to tadpoles of the American Toad (Anaxyrus americanus), Western Chorus Frog (Pseudacris triseriata), Northern Leopard Frog (Lithobates pipiens), and Green Frog (Lithobates clamitans). de Jong Westman et al. (2010) reported that exposure of Northern Pacific Treefrog (Pseudacris regilla) and Great Basin Spadefoot (Spea intermontana) embryos and tadpoles to field-measured concentrations of the pesticide endosulfan resulted in changes in behaviour, and increased mortalities and deformities. The Morris Valley, Maria Slough, and Mountain Slough locations are within largely agricultural areas, and water quality in these areas could be affected by fertilizers or pesticides. At Maria Slough and Aldergrove, water testing in March and April indicated that levels of nitrate and total nitrogen were below levels that would affect R. pretiosa (McKibbin et al. 2008); however, levels would likely be much higher later in the season after fertilizer was applied to the adjacent agricultural fields (Bishop pers. comm. 2009).
Eutrophic conditions can cause algal blooms, high pH, and low dissolved oxygen and have been implicated in the decline of a population of R. pretiosa in Oregon (U.S. Fish and Wildlife Service 2010). Eutrophic conditions may exist in areas of historical R. pretiosa populations. de Solla et al. (2002) suggested that poor water quality associated with low dissolved oxygen levels, high levels of nitrogenous compounds such as ammonia, and organophosphate pesticides, contributed to low hatching success of R. aurora and Ambystoma gracile at sites in the Sumas Prairie.
Acidification of water (low pH) has been linked to decreased embryonic survival in numerous species of amphibians (Boyer and Grue 1995). Leachate from the nearby quarry at Mountain Slough could reduce pH and increase iron content in the slough (Canadian Oregon Spotted Frog Recovery Team 2009a). Comparison of water quality at Aldergrove and Maria Slough indicated that pH and dissolved oxygen levels were not likely to cause problems in embryonic survivorship at these locations; however, low chloride and conductivity levels at the Aldergrove site might have contributed to low embryonic survivorship during the study period (McKibbin et al. 2008). Water quality at Mountain Slough and Morris Valley has not been examined.
Disease, in particular chytridiomycosis and iridoviruses, has been identified as a contributor to amphibian declines around the globe (Daszak et al. 1999). Chytridiomycosis, caused by the chytrid fungus Batrachochytrium dendrobatidis, has been linked to amphibian mass mortalities (Daszak et al. 1999; Lips et al. 2006; Rachowicz et al. 2006; Voyles et al. 2009). The risk to R. pretiosa populations in Canada is unknown (Pearl et al. 2009). Chytrid infection has been reported as common in wild R. pretiosa populations in Washington and Oregon (Pearl et al. 2007, 2009). Twelve R. pretiosa from Maria Slough were tested for the presence of chytrid, and at least 1 frog tested positive; American Bullfrogs at both the Aldergrove and Maria Slough sites also tested positive (Potvin 2009; Govindarajulu pers. comm. 2010). Some R. pretiosa in the captive rearing program tested positive for chytrid and were not released. The potential impact of iridovirus to R. pretiosa is also unclear, but iridovirus has been documented to cause high mortality in some amphibian species (Daszak et al. 1999). Iridovirus outbreaks have been identified as a major cause of mortality during captive rearing of R. pretiosa in Canada (Welstead pers. comm. 2009).
The introduced American Bullfrog has been proposed as a cause for the disappearance of R. pretiosa from areas in Washington (Nussbaum et al. 1983) and may negatively affect R. pretiosa populations across its range (U.S. Fish and Wildlife Service 2010). Bullfrogs may have contributed to the decline of the population at Aldergrove (Govindarajulu 2009) and Campbell Valley Regional Park, where Bullfrogs first appeared at the breeding area in 1970 (Licht 1974). The American Bullfrog uses similar habitats as R. pretiosa and has been shown to outcompete or displace R. aurora (Kiesecker and Blaustein 1998; Kiesecker et al. 2001). Pearl et al. (2004) determined that R. pretiosa was more vulnerable than R. aurora to predation by American Bullfrog. Bullfrogs have been documented as predators of hatchling R. pretiosa at Aldergrove (R. Haycock unpubl. data), and of tadpoles and adults at a population in Washington (McAllister and Leonard 1997). However, a small study of Bullfrog stomach contents at Aldergrove (N = 21) in 2006 did not find evidence of recent consumption of R. pretiosa (Govindarajulu 2006). Another potentially negative effect of Bullfrogs on R. pretiosa is that this species commonly carries the chytrid fungus, and can serve as an asymptomatic vector of disease (Daszak et al. 2004). Testing of Bullfrogs at Aldergrove in 2006 (Govindarajulu 2006) and 2008 (Potvin 2009), and Maria Slough in 2009 (Govindarajulu pers. comm. 2010) confirmed that they are infected with chytrid at these sites.
The Green Frog is another invasive introduced species that occurs at the Maria Slough and Mountain Slough sites. This specieshas been noted to degrade habitat quality for R. aurora by displacing them from preferred habitat (COSEWIC 2004). Adult Green Frogs may prey on young R. pretiosa (Canadian Oregon Spotted Frog Recovery Team 2009a), but predation has not been documented.
Introduced fish could also have a negative impact on R. pretiosa by consuming tadpoles and by preying on frogs at over-wintering sites (Pearl et al. 2009; U.S. Fish and Wildlife Service 2010). The concentration of R. pretiosa in warm water oviposition sites and cold water springs in winter may increase its exposure to these nonnative species, particularly during drought years (U.S. Fish and Wildlife Service 2010). In Washington, sites with significant populations of Brook Trout (Salvelinus fontinalis) or Fathead Minnow (Pimephales promelas) showed evidence of poor recruitment of R. pretiosa and had a disproportionate ratio of older to younger frogs (Hayes 1997 cited in U.S. Fish and Wildlife Service 2010).
There is a proposal to install a second 500 KV transmission line at the Morris Valley site, which would be placed directly over known oviposition sites of R. pretiosa. The level of magnetic field that a frog at the Morris Valley population is expected to experience is 200–250 mG (Canadian Oregon Spotted Frog Recovery Team 2009a). Severini et al. (2003, 2010) reported that amphibian larvae exposed to 25 microteslas (250 milliGauss; mG) of magnetic radiation for 12 h/day during their development experienced significant maturation delays. The Morris Valley frogs will experience this level continuously from at least egg-laying through metamorphosis (Canadian Oregon Spotted Frog Recovery Team 2009a). It is possible that R. pretiosa at Morris Valley may experience maturation delays if the transmission line is installed, which may have detrimental effects on their survival (see Balmori 2006). The currently available evidence of effects of electromagnetic current on amphibians is limited.
In Canada COSEWIC ranked R. pretiosa as Endangered in an emergency listing in 1999 and re-examined and confirmed it as Endangered in 2000. The species is listed under Schedule 1 of the Species at Risk Act (SARA). General prohibitions under SARA currently apply on federal lands and protect individuals of R. pretiosa and their residences.The species is protected under the British Columbia Wildlife Act from being killed, wounded, transported, or collected without a permit.
In the U.S., the Pacific Coast populations of the Spotted Frog complex (now R. pretiosa) have been federal candidates for listing under the U.S. Endangered Species Act since 1993 (McAllister and Leonard 1997). In Washington, R. pretiosa was listed by the state as an endangered species in 1997 (U.S. Fish and Wildlife Service 2010). Listed species are protected from being removed, but their habitat is not protected. Although the Washington State Forest Practices Board is able to designate critical habitat for listed species, critical wildlife habitat has not been proposed to date (U.S. Fish and Wildlife Service 2010). In Oregon, R. pretiosa is on the sensitive species list and is considered critically sensitive; however, this designation provides little protection (U.S. Fish and Wildlife Service 2010). Although Oregon has an Endangered Species Act, R. pretiosa is not listed in the state. In California, the species is listed as a Species of Special Concern (DFG 2011).
According to NatureServe (2009), R. pretiosa is considered Imperiled (G2) globally, Critically Imperiled (N1) in Canada, and Imperiled (N2) in the U.S. It is Critically Imperiled (S1) in California and Washington State, and Imperiled (S2) in Oregon. In British Columbia, the species is considered Critically Imperiled (S1) and is on the provincial Red List of species at risk. This species is on the IUCN Red List as Vulnerable (Hammerson and Pearl 2004).
Habitat protection of the four known locations of R. pretiosa in Canada is limited. There is some protection afforded to the habitat through federal and provincial fisheries legislation. Habitat for R. pretiosa has some protection under the federal Fisheries Act. The Act controls activities that can cause harmful alteration, disruption or destruction of fish habitat; its goal is to ensure no net loss of fish habitat. Additional protection is provided by the British Columbia Water Act and the provincial Riparian Areas Regulation. Both protect the in-stream environment and surrounding habitat, and apply to projects associated with specific activities that can alter fish or wildlife habitat. The Recovery Team has drafted a recovery strategy and a definition of critical habitat for the species (Canadian Oregon Spotted Frog Recovery Team 2009a,b). Five recovery implementation groups have been formed: (1) habitat protection, management and restoration; (2) husbandry, invasive species and disease; (3) recovery planning; (4) science acquisition, information management and inventory/monitoring; and (5) outreach/stewardship (Canadian Oregon Spotted Frog Recovery Team 2009a).
The development potential of the occupied wetlands appears to be limited (Canadian Oregon Spotted Frog Recovery Team 2009a). The Mountain Slough and Morris Valley sites are privately owned. Maria Slough itself is recorded as Provincial Crown Land; however, the land on the southeast side of the Slough is located on First Nations Reserve land, and the north side is private land. The Aldergrove site is federally owned by the Department of National Defence. Access to the site is restricted, which affords some protection to the species. In addition, spraying of pesticides is prohibited at the site (Haycock 2000b). A management plan was developed in 2000 for this population (Haycock 2000b).
Thanks to the Canadian Oregon Spotted Frog Recovery Team for access to unpublished information and participation in team meetings. Monique Goit and Alain Filion from the COSEWIC Secretariat answered questions. Denis Knopp, Kym Welstead, Christine Bishop, Purnima Govindarajulu, Tracy Cornforth, Bob Woods, Barry Smith, Monica Pearson, Marc Hayes, Deanna Lynch, Kathleen Moore, Annette Potvin, Michael Blouin, and Ivan Phillipsen answered questions about the species and/or provided unpublished information for inclusion in the report. This document benefited from comments by Kristiina Ovaska, Jacqueline Litzgus and Cynthia Paszkowski (members of the Amphibians and Reptiles Subcommittee, COSEWIC); David Anthony Kirk; and Christine Bishop. The B.C. Conservation Data Centre provided access to data. Funding for this project was provided by Environment Canada.
Authorities consulted included: Stephen Hureau, David Toews, and Kevin Fort (Canadian Wildlife Service); Patrick Nantel and Gilles Seutin (Parks Canada); Dave Fraser (BC Ministry of Environment); Kristina Stipec (BC Conservation Data Centre), Lynn Gillespie and Michèle Steigerwald (Federal Biodiversity Information Partnership - Canadian Museum of Nature); Sonia Schnobb, Gloria Goulet, Alain Filion, and Monique Goit (COSEWIC Secretariat); Kym Welstead, Bob Woods, Purnima Govindarajulu (Canadian Oregon Spotted Frog recovery team chairs); Christine Bishop, Tracy Cornforth, Andrea Gielens, Sylvia Letay, and Monica Pearson (Recovery Team members); Denis Knopp (Canadian researcher), and Marc Hayes, Deanna Lynch, and Christopher Pearl (U.S. researchers).
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Vanessa Craig is a Director of EcoLogic Research, based on Gabriola Island. She holds a Ph.D. from the Faculty of Forestry at the University of British Columbia (UBC), an M.Sc. from the Centre for Applied Conservation Biology at UBC, and a B.Sc. (Honours) in Biology from Simon Fraser University. Her >20-year career as a biologist in British Columbia has included work on amphibians, small mammals, and bats. Vanessa’s recent work has focused on the Oregon Spotted Frog and Pacific Water Shrew; both are species at risk with ranges constrained to the lower mainland of British Columbia. Vanessa recently co-authored the recovery strategies for Oregon Spotted Frog and Pacific Water Shrew, and authored draft definitions of critical habitat for both species. She is a member of the Pacific Water Shrew recovery team.
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