Red-legged frog (Rana aurora) COSEWIC status appraisal summary: chapter 8
Population Sizes and Trends
The number of adult frogs may vary from hundreds to thousands at breeding sites depending on the size and characteristics of the wetland, but little quantitative information is available. Although population estimates for this species are largely unavailable, there is every indication that there are more breeding adults in Canada than would trigger any of the COSEWIC numerical criteria. Waye (1999) summarized what is known of population sizes of the Red-legged Frog in British Columbia and United States. Two mark-recapture studies at breeding sites were carried out in 1968 – 1970 in southern British Columbia (Licht 1969, 1971, 1974, Calef 1973a, b). At Marion Lake, near Squamish, the number of egg-masses, an index of the number of breeding females, was 618 and 620 in two years, respectively; the average number of adult males was estimated to be 1770 (SE = 280) and 3600 (SE = 775) in the two years (Calef 1973a, b). In marshes near Vancouver, the number of egg-masses was 6 and 33 in two years, respectively; the average estimated number of adults of both sexes was 531 frogs (SE = 19; Licht 1969, 1974). No information exists on current population sizes in these or other breeding sites in the province, or on densities in terrestrial habitats.
Populations of the Red-legged Frog are likely to undergo high year-to-year or longer-term fluctuations in size, similar to those reported for many other species of aquatic-breeding anurans (Pechmann and Wilbur 1994). However, populations have not been monitored over multiple years, and the magnitude of multi-year fluctuations is unknown.
On Vancouver Island, the species is relatively widely distributed, based on historical and recent records (Figure 3). The island appears to remain a stronghold of the species’ Canadian distribution, although habitats continue to be modified by forestry, urban developments, agriculture, and other human activities, and by the spread of introduced species. Recent (since 1998) systematic surveys of wetlands for aquatic-breeding amphibians, including the Red-legged Frog, have been conducted in Clayoquot Sound on the west coast of the island (Beasley et al. 2000), on forestry lands on southeastern and northern parts of the island (Wind 2003), and near Victoria (Govindarajulu 2003 and unpublished data). A research project on interactions of this species with forestry activities was carried out on northern Vancouver Island (Chan-McLeod 2003a). Additional locality records are available from researchers working on other amphibian species (Western Toad: Davis, pers. comm.), from the public through the provincial Frog Watch Program, and from serendipitous encounters by various field workers.
In Clayoquot Sound, the Red-legged Frog was located in 26% of the 148 wetlands surveyed in 1998 – 1999 (Beasley et al. 2000). Its frequency of occurrence was similar to that of the Pacific Treefrog (33%) and Rough-skinned Newt (22%). The Northwestern Salamander was the most frequently encountered amphibian (found in 61% of the wetlands), whereas the Western Toad was rare (found in 1% of the wetlands). The Red-legged Frog was unevenly distributed among the four watersheds surveyed; it was not found in one watershed, and its frequency of occurrence ranged from 18 – 50% in the remaining three watersheds.
Wind (2003) surveyed 116 wetlands on northern and 97 wetlands on southeastern Vancouver Island; these wetlands were located both in forested and recently logged areas. Amphibians were detected in 41% of all wetlands surveyed. Overall, the Red-legged Frog was found at 21% of the wetlands, including sites in both recently logged (0-5 years) and older forest (>150 years). It was the most frequently encountered amphibian in the southeast, followed by the Pacific Treefrog and Rough-skinned Newt. In the north, however, the Northwestern Salamander was by far the most frequently encountered amphibian, followed by the Red-legged Frog and Pacific Treefrog; the difference between the north and southeast possibly reflected higher elevations of the sites surveyed in the north.
Govindarajulu (2003 and unpublished data) surveyed wetlands on the Saanich Peninsula near Victoria on southern Vancouver Island as a part of her dissertation study on the effects of the Bullfrog on native amphibians. She located the Red-legged Frog at 15 lakes and ponds, most of which were on the west side of the peninsula and lacked Bullfrogs. Several other wetlands, especially on the east side of the peninsula, supported Bullfrogs only. This distribution pattern, together with experimental studies of interactions between the two species, suggest that the Red-legged Frog is in the process of being displaced from this area by the Bullfrog (see section “Limiting Factors and Threats”). The Bullfrog may be influencing the distribution of the Red-legged Frog over a wider area along the east coast of Vancouver Island, as well as on the Lower Mainland.
Only sporadic distribution data are available for other areas of Vancouver Island. For example, the species continues to be found in Jordan Meadows, a large wetland complex on southwestern Vancouver Island from where the Western Toad, for unknown reasons, has disappeared over the past decade (Davis and Gregory 2003). However, no quantitative data on abundance are available for the Red-legged Frog at this site, and the population should be monitored for epidemic disease, which can affect a multitude of amphibian species, spread rapidly, and decimate populations. Many parts of Vancouver Island, such as most of the west coast, mountains in the central part of the island, and the far north have not been surveyed systematically for amphibians, and the present status of populations remain unknown.
Little information exists on the current distribution and abundance of the species in most areas of mainland British Columbia. An exception is the Delta area south of Vancouver, where the Corporation of Delta has assessed fish and amphibian distributions and status within watersheds throughout the district based on historical records and new survey data (R. Rithaler, pers. comm.). The Red-legged Frog occurred in 10 of 23 watersheds within Delta in 1990 – 2002; there were no known disappearances from entire watersheds when this pattern was compared to pre-1990 records (Rithaler 2002, 2003). The species is considered to be uncommon within the district, although it is regularly observed within certain areas (R. Rithaler, pers. comm.). The introduced Bullfrog and Green Frog are widespread in Delta and have been recorded from 18 and 17 watersheds, respectively, in 1990 – 2002 (Rithaler 2002, 2003). Habitat modifications, particularly the removal of riparian vegetation and channel deepening, appear to have contributed to the expansion of these introduced species and the disappearance of the Red-legged Frog from particular wetlands in Delta (R. Rithaler, pers. comm.).
De Solla et al. (2002a, b) studied effects of agricultural pollutants on amphibians in the Lower Fraser River Valley. The Red-legged Frog was primarily found at the periphery of the farmlands in the Sumas Prairie area during surveys in mid-1990s, and subsequent studies using experimental enclosures suggested that poor hatching success in contaminated water might contribute to this pattern (De Solla et al. 2002a). During two other surveys in the mid-1990s, the species was found at 14 sites in the Lower Fraser Valley but not at 25 sites where apparently suitable habitat existed (Haycock 1996 and Knopp 1996, cited in Waye 1999). Haycock (1998) carried out additional surveys of wetlands for amphibians throughout the Fraser River Lowlands in and summarized records from Knopp (1996, 1997) for the same area. Whereas the Oregon Spotted frog (Rana pretiosa) has virtually disappeared, the Red-legged Frog was found at 50% of the sites surveyed and was considered a common amphibian species in the Fraser River Lowlands. However, much habitat has been lost or seriously degraded in this area.
Only a few old (1940s) locality records exist from the Sunshine coast and farther north, and the species’ past and current distribution there is virtually unknown.
Over the past several decades, the California Red-legged Frog has undergone precipitous population declines, including local extirpations, over most of its range in California (reviewed in Stebbins and Cohen 1995). While exploitation and habitat loss have undoubtedly contributed to the declines, especially in human-modified landscapes, declines in more remote areas are incompletely understood. Wind-borne transport of agricultural pesticides appears to be an important contributing factor (Davidson et al. 2002).
Whereas population trends of the California Red-legged Frog have received much attention, relatively little is known of population trends of the Northern Red-legged Frog. Since the mid-1970s population declines have occurred in the southern portion of its range in the Willamette Valley, Oregon (Blaustein et al. 1994b), but it appears to remain relatively common in at least some areas of Washington State, including human-modified landscapes (Ostergaard et al. 2003).
In British Columbia, the species’ range overlaps with heavily urbanized and modified landscapes in the Lower Fraser Valley and on southern and southeastern Vancouver Island, where draining of wetlands, clearing of forest cover and riparian vegetation, and pollution have most likely contributed to the loss of breeding sites and local populations over the past century. Habitats continue to be lost and fragmented at an alarming rate in these areas (see section of “Habitat Trends”), and introduced predators and competitors, such as the Bullfrog, often compromise the quality of remaining habitats (Govindarajulu 2003). However, populations and distribution trends have not been monitored systematically, and there is little documentation of population declines or disappearances.
Limited distribution records suggest that the species continues to be found throughout most of its known range in British Columbia (Figure 3). Distribution records from 1997 to present suggest that the species remains relatively widespread within portions of its range on Vancouver Island (see section “Population Distribution and Persistence”, above). However, “presence/absence” data collected for these surveys do not necessarily reflect patterns of abundance, and even large population declines could go undetected using solely this type of data. In the Lower Fraser Valley, there is some evidence that agricultural pollutants might contribute to the paucity of these frogs within agricultural edges in the Sumas Prairie (De Solla et al. 2002a), but the loss of both aquatic and terrestrial habitats is also likely to be a factor. No information exists on recent or historical population trends on the Sunshine Coast or areas north of Powell River.
Waye (1999) suggested that surveys of both historic sites and other localities with suitable habitat should be undertaken in different parts of the species’ range in British Columbia to fill in many uncertainties about distributional trends and the status of this species. Such surveys have not been undertaken to date, although several recent amphibian surveys on Vancouver Island, as a part of various projects, have contributed to our knowledge of its frequency of occurrence within wetlands. Future surveys should target the Sunshine Coast and the Lower Fraser Valley, where relatively little recent information on the species exists. Intensive monitoring of population trends at selected sites, either those with historic data or where populations of other species have undergone declines, would also be desirable.
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