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BC Conservation Data Centre: Conservation Status Report

Rana aurora
Northern Red-legged Frog


 
Scientific Name: Rana aurora
English Name: Northern Red-legged Frog
English Name Synonyms: Red-legged Frog
   
Provincial Status Summary
Status: S3
Date Status Assigned: December 31, 2016
Date Last Reviewed: March 23, 2022
Reasons: Although the species is still common in parts of its range, populations are isolated within and restricted to the most populace region of the province. Over the past ten years, local declines and disappearances have been documented, but the species has persisted across its known historical range. Threats are considered high from introduced species such as American Bullfrog and illegally stocked sport fish, road mortality, urban development, logging, dams and water management, and the pollution of breeding sites (COSEWIC 2015c).
 
Range
Range Extent: F = 20,000-200,000 square km
Range Extent Estimate (km2): 81,901
Range Extent Comments: Northern Red-legged Frogs occur over much of Vancouver Island usually below 500 m, through the lower mainland and north along the Sunshine Coast to Smith Sound. They have been found inland to Whistler and further up valleys surrounding the Fraser Lowlands (COSEWIC 2015c, iNaturalist. https://www.inaturalist.org/. Accessed 2021-12-05). The global range of the Northern Red-legged Frog extends from southwestern British Columbia, south along the Pacific coast, west of the Cascade Mountains, to northwestern California.
There is a population of unknown origin that occurs on Graham Island, Haida Gwaii. It was first documented in 2001 and is presumed to have been introduced (COSEWIC 2015c). This population is not used in the Range Extent calculation, however, if it were, it would be approximately 154,291 sq km based on CDC records and iNaturalist observations, still within "F", 20,000 to 200,000 km2.

New records extend the species? range throughout the Sunshine Coast (southern mainland coast northwest of Greater Vancouver) and the Sea-to-Sky Corridor (Highway 99 from Vancouver north to Whistler) and farther up valleys surrounding the Fraser Lowlands.
Area of Occupancy (km2): G = 501-2,500
Area of Occupancy Estimate (km2): 3404
Area of Occupancy Comments: An AOO of 3404 km2 (851 4km2 grid cells) was calculated just using iNaturalist records (Accessed 2021-12-03) and the occurrences from the CDC and the 2015 COSEWIC report. The Index of Area of Occupancy was estimated to be 2,588 km2 for the 2015 COSEWIC report.This is at the lower end of the range and an underestimate, as survey effort is incomplete. The increase in the value between the 2015 report and the current number can be attributed to the increase in use of the iNaturalist application with the resulting estimate providing better coverage than what would have been available previously. Only "Research Grade" iNaturalist observations were included.

Adult Red-legged Frogs were the most common species encountered during small wetland surveys on Vancouver Island, however breeding sites were rare so populations may be patchily distributed in response to the location of these sites (E. Wind, pers. comm. 2005).
 
Occurrences & Population
Number of Occurrences: D = 81 - 300
Comments: Currently (2021) the B.C. Conservation Data Centre has 141 mapped occurrences with a total of 335 source features, however, this is likely an underestimate. iNaturalist (accessed 2021-12-06) had 1022 observations that were research grade and seen more recently than 2017. A conservative estimate of additional EOs based on these observations would be 20-40.

Populations remain in some urbanized areas within the Lower Mainland and Fraser Valley, but failed searches over the past 5 years suggest they may be extirpated from parts of the cities of Vancouver, Richmond, and Delta, where they occurred historically. Intensive search effort and no detections at Stanley Park since the 1970s indicate that the species is extirpated there (COSEWIC 2015c).
Number of Occurrences with Good Viability / Ecological Integrity: U = Unknown
Comments: Many populations probably have good viability currently, but they are easily isolated so that their long-term status is difficult to predict. Populations are isolated with little chance of rescue effect, and human population growth trends in the southwest suggest continued habitat loss and fragmentation (Ovaska and Sopuck 2004).
Number of Occurrences Appropriately Protected & Managed: DE = 13 to >40
Comments: Many of the known sites are in provincial parks or reserevs such as Strathcona, Goldstream, Chilliwack Lake, Mount Maxwell and Clayoquot Sound just to name a few. Although the wetland may be protected and not developed within the protected area, there still may be fish stocking, American Bullfrog introductions, fungal infections and limited rescue effects all of which could negatively impact Red-legged Frog populations.

A large proportion of parks within Greater Vancouver are small, and habitat degradation and fragmentation are a concern (Waye 1999).
Population Size: G = 100,000 - 1,000,000 individuals
Comments: "The total Canadian population of the Northern Red-legged Frog can be roughly estimated to be well over 100,000 adults based on the known abundance of 12,603 egg masses counted at 197 wetlands in the past ten years, which comprise less than half of the known occurrences." (COSEWIC 2015c).
 
Threats (to population, occurrences, or area affected)
Degree of Threat: B = High
Comments: From COSEWIC (2015c): "Threats known to impact the Northern Red-legged Frog are urban development, road mortality, logging, dams and water management, invasive species, introduced fish, disease, and pollution. Within the Lower Fraser Valley, human population growth is predicted to double every 20 ? 30 years. Growth is also expected along the southeastern side of Vancouver Island, parts of the Sea-to-Sky Corridor, and the Sunshine Coast. Some of the existing habitat in urban and rural agricultural areas will be converted to housing and thereby undergo further fragmentation and exposure to pollution. American Bullfrog populations are predicted to grow and spread, increasing competition and predation pressure and augmenting damage to breeding habitats caused by dams and water management, introduced fish, and disease. Logging has the potential to alter habitat throughout the vast remote parts of the species? range. Temperature extremes, storms, and flooding events associated with climate change will likely exacerbate habitat loss and degradation caused by other factors."

As swritten in 2010:Habitat degradation and loss are the primary threats, with introduced species a secondary threat (Waye 1999; Ovaska and Sopuck 2004). This species appears to be most common at elevations below 500 m (Ovaska and Sopuck 2004), which coincides with areas of intense urban and agricultural development in the Lower Mainland and on south-eastern Vancouver Island, the core of this species' provincial range. Forestry activities occur throughout the remainder of the range, especially at low elevations, and affect the movements of Red-legged Frogs (Chan-McLeod 2003; Ovaska and Sopuck 2004). These activities create barriers to movement, resulting in habitat fragmentation and the isolation of populations. Surveys on Vancouver Island demonstrate that Red-legged frogs frequently breed in small wetlands that are not afforded protection within BC (Wind 2003). Studies have demonstrated that Bullfrogs and introduced fish have a negative impact on native frogs, including Red-legged Frogs (Kiesecker and Blaustein 1998; Adams 2000; Govindarajulu 2004). The introduction and spread of Bullfrogs and non-native fish are facilitated by the loss and replacement of ephemeral wetlands for more permanent ponds in agricultural and urban areas (e.g., ornamental/backyard, stock, and golf course ponds) (Adams 1999, 2000). Isolation of populations makes the species particularly vulnerable to, for example, disease, an increasing issue with native amphibians that can rapidly destroy populations (e.g. Ranids in the Pacific Northwest; Ovaska and Sopuck 2004). In 2008, samples of Red-legged Frog post-metamorphic frogs and tadpoles in B.C. tested positive for Batrachochytrium dendrobatidis (Bd), an aquatic fungus that causes the disease chytridiomycosis in amphibians. Mortality, however, has not been recorded (P. Govindarajulu, unpublished data 2010). Populations of Red-legged Frogs are naturally fragmented, with little potential for rescue effect (Ovaska and Sopuck 2004).
 
Trend (in population, range, area occupied, and/or condition of occurrences)
Short-Term Trend: F = Decline of 10-30%
Comments: There have been no long term inventories or monitoring, however there have been local declines and disappearances, both documented and anecdotal.

Some examples from the COSEWIC (2015c) status report include; at one wetland in the Sea-to-Sky Corridor, there was an estimated decline of 73 to 92% over 3 years as a result of highway construction and road mortality. Also, failed searches over five years (2007-2013) suggest that they may be extirpated from parts of Vancouver, Richmond, and Delta, where they occurred historically. Intensive search effort. Since that time there have been observations entered into the online community science application, iNaturalist with increasing frequency and coverage. This does not mean that there are actual increasing numbers or locations, but that there are naturalists constantly looking and noting what they see and can therefore provide much greater temporal and spatial coverage. There have been 1022 observations entered since 2016 that are "research grade" , and there are observations in Vancouver, Richmond and Delta, however not in Stanley Park and limited areas in Delta and Richmond.

American Bullfrogs are expanding their range in the heart of this species' distribution within BC; there is evidence that Red-legged Frogs may be displaced by Bullfrogs (Govindarajulu 2004). There have been no detections at Stanley Park since the 1970s indicating that the species is extirpated there. There have been American Bullfrogs detected at Stanley Park as far back as 2010 and potentially earlier. The Range Extent and Area of Occupancy of American Bullfrog in BC is approximately one-third of that of Red-legged Frog. There are areas such as the Saanich Peninsula where bullfrogs occur and Red-legged Frogs do not yet anecdotally they did. There also are areas where bullfrogs do not appear to be prevalent such as Galiano, Mayne and Saturna Island and Red-legged Frogs do occur (iNaturalist accessed 2021-12-14).

Prior to 2010: There is little information regarding their abundance and trends outside urban and rural areas, but they appear to be common at sites in the more remote parts of Vancouver Island (Ovaska and Sopuck 2004; Waye 1999; S. Cannings, pers. obs.; Wind, pers. obs.). The species has undoubtedly declined in the Lower Mainland and on south-eastern Vancouver Island due to the extensive loss of wetland habitat. In addition, Bullfrogs are expanding their range in the heart of this species' distribution within BC; there is evidence that Red-legged Frogs may be displaced by Bullfrogs (Govindarajulu 2004). California populations have declined dramatically in recent decades due to habitat loss and perhaps air borne pollutants (Ovaska and Sopuck 2004).
Long-Term Trend: U = Unknown
Comments: "Long-term population monitoring has been initiated but has not been carried out for sufficiently long to assess trends." (COSEWIC 2015c). Although the extent of the native range likely remains the same, populations have probably declined in the Lower Mainland and on south-eastern Vancouver Island as a result of the historic and continued loss of natural habitat from rural and urban development and forestry (Ovaska and Sopuck 2004).
 
Other Factors
Intrinsic Vulnerability: B=Moderately vulnerable
Comments: Ranids appear to be particularly vulnerable to non-native predators and disease (Corn 2000).
Environmental Specificity: C=Moderate. Generalist or community with some key requirements scarce.
Comments: Red-legged Frogs breed in small wetlands that have no riparian buffer protection (Wind 2003). In addition, although adults are commonly encountered, Red-legged frog breeding sites were less common than that of other native aquatic-breeding species during pond surveys on Vancouver Island, suggesting that they may have specific habitat requirements that may be spatially limiting; larvae were found at less than 10% of breeding ponds versus 25-50% of sites for other species (E. Wind, pers. comm. 2005).
Other Rank Considerations: This species has little potential for rescue effect from stochastic events (Ovaska and Sopuck 2004). Approximately 35% of the global distribution of Northern Red-legged Frog is in British Columbia (COSEWIC 2015c).
 
Information Gaps
Research Needs: 1) Are non-native species, such as Bullfrogs, Green Frogs, and fish, excluding Red-legged Frogs from breeding sites and causing the isolation and decline of populations; 2) What habitat requirements are required by Red-legged Frogs for breeding at the stand and landscape level ? (i.e., within pond and terrestrial habitat conditions - e.g., hydroperiod, emergent vegetation, distribution and density of breeding sites, percent forest cover versus urban development, road density).
Inventory Needs: Baseline inventories are needed for this species in both rapidly developing and pristine areas. We need to know whether Red-legged frogs are declining on Vancouver Island and in the Lower Mainland, and what the range of the species is along the mid-coast of the mainland (e.g., is the species common?)
 
Stewardship
Protection: Protect known breeding sites and adjacent upland forested areas; maintain connectivity among populations; protect all wetlands, regardless of size.
Management: Control and eradicate non-native species (Bullfrogs, Green Frogs and fish). Limit fish stocking on a provincial level and discontinue fish stocking in parks, especially known amphibian breeding sites. Identify and protect known breeding sites and adjacent upland forest areas (maintain connectivity among breeding sites, and between breeding sites and upland forested areas - e.g., buffer riparian corridors). Wetland legislation is needed that protects all wetlands, regardless of size.
 
Version
Author: Wind, E., L. Westereng and L. Gelling and L. Ramsay
Date: December 07, 2021
 
References
Adams, M.J. 1999. Correlated factors in amphibian decline: exotic species and habitat change in western Washington. Journal of Wildlife Management 63:1132-1171.
Adams, M.J. 2000. Pond permanence and the effects of exotic vertebrates on anurans. Ecological Applications 10:559-568.
B.C. Ministry of Environment. 2015j. Recovery plan for the Northern Red-legged Frog (Rana aurora) in British Columbia. B.C. Ministry of Environment, Victoria, BC. 51 pp.
Chan-McLeod, A. C. A. 2003. Factors affecting the permeability of clearcuts to red-legged frogs. Journal of Wildlife Management 67:663-671.
Corn, P.S. 2000. Amphibian declines: review of some current hypotheses. In D.W.Sparling, G. Linder, C.A. Bishop, eds. Ecotoxicology of Amphibians and Reptiles. U.S. Geol. Sur., Midwest Sci. Cent. Columbia MO, 663-696.
Govindarajulu, P. 2004. Introduced bullfrogs (Rana catesbeiana) in British Columbia: impacts on native Pacific treefrogs (Hyla regilla) and red-legged frogs (Rana aurora). PhD. Thesis, University of Victoria, Victoria BC.
Green, D. M. 1978. Northern leopard frogs and bullfrogs on Vancouver Island. Canadian Field-Naturalist 92:78-79.
Haycock, R. 1996. Oregon Spotted Frog project. Unpublished report to B.C. Ministry of Environment, Lands and Parks, Victoria, B.C.
Haycock, R. 1998. Amphibian survey with special emphasis on the Oregon spotted frog, Rana pretiosa. Unpubl. rep. prepared for B.C. Minist. Environ., Lands and Parks, Wildl. Branch, Victoria. 230pp.
Haycock, R. D., and D. Knopp. 1998. Amphibian survey with special emphasis on the Oregon spotted frog (Rana pretiosa). Selected wetland sites: Fraser River Lowlands and corridors to the interior Plateau. Draft Report. Minist. Environ., Lands and Parks.
Kiesecker, J.M., and A.R. Blaustein. 1998. Effects of introduced bullfrogs and smallmouth bass on microhabitat use, growth, and survival of native red-legged frogs (Rana aurora). Conservation Biology 12:776-787.
Knopp, D. 1996. Fraser Valley amphibian survey. Unpublished report to B.C. Ministry of Environment, Lands and Parks, Victoria, B.C.
Licht, L. E. 1974. Survival of embryos, tadpoles, and adults of the frogs RANA AURORA AURORA and RANA PRETIOSA PRETIOSA sympatric in southwestern British Columbia. Canadian Journal of Zoology 52:613-627.
Ovaska, K. and L. Sopuck. 2004. Update COSEWIC Status Report on the Red-legged Frog Rana aurora in Canada. Unpublished revised report prepared for COSEWIC. 63 pp.
Wind, E. 2003. Aquatic-breeding amphibian monitoring program: Analysis of small wetland habitats on Vancouver Island. Annual Progress Report 2002. Unpublished report prepared for Weyerhaeuser Company, Nanaimo, BC.
Wind, E. In progress. No date. Impacts of non-native predators on aquatic ecosystems. Report produced for the BC Ministry of Water, Land and Air Protection. Victoria, BC.
 

Please visit the website Conservation Status Ranks for information on how the CDC determines conservation status ranks. For global conservation status reports and ranks, please visit the NatureServe website http://www.natureserve.org/.

Suggested Citation:

B.C. Conservation Data Centre. 2021. Conservation Status Report: Rana aurora. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Mar 5, 2025).