Red-legged frog (Rana aurora) COSEWIC status appraisal summary: chapter 2
COSEWIC
Executive Summary
Red-legged Frog
Rana Aurora
Species Information
The Red-legged Frog (Rana aurora) is one of six species of ranid or “true” frogs (family Ranidae) native to western North America. Two subspecies are recognized: the Northern Red-legged Frog (R. a. aurora), which occurs in Canada, and the California Red-legged Frog (R. a. draytonii). It is a moderate-sized frog, averaging 50 – 70 mm in length as adults, with relatively long legs and webbed feet.
Distribution
The distribution of the Northern Red-legged Frog extends from southwestern British Columbia to northwestern California. In Canada, the species occurs throughout Vancouver Island, on several of the islands in the Strait of Georgia, and on the adjacent mainland of southwestern British Columbia where its range overlaps with that of the rare Oregon Spotted Frog (R. pretiosa). In 2001 the species was documented from several localities on Graham Island, Queen Charlotte Islands, evidently introduced.
Habitat
The Red-legged Frog is an inhabitant of moist, lower elevation forests and requires both aquatic breeding habitats and terrestrial foraging habitats. The frogs breed in ponds, ditches, springs, marshes, margins of large lakes, and slow-moving portions of rivers, typically where emergent vegetation is abundant. Metamorphosed individuals are largely terrestrial and inhabit a variety of forest types but are most abundant in older, moist stands. Clearcuts are barriers to movement, especially during dry conditions.
Biology
The Red-legged frog breeds during a short period in early spring. Male frogs call mostly from under water and consequently breeding choruses can remain undetected. Clutches contain up to 1300 eggs. Eggs usually hatch in late spring, and tadpoles transform in July – August. The greatest mortality occurs during the tadpole stage.
Adult frogs migrate between aquatic breeding sites and terrestrial foraging habitats, sometimes over many kilometers. Hibernation occurs either under water or on land.
Population Sizes and Trends
Although no recent population estimates are available for any locality, there is no compelling reason to suspect that the number of breeding adults in Canada is small enough to trigger any of COSEWIC’s quantitative criteria. Surveys since 1997 suggest that the species remains widespread within its known range. In the Lower Fraser Valley, there is some evidence that pollutants might contribute to the paucity of these frogs within agricultural areas, but habitat is also likely to be a factor. Only a few records are available from the mainland coast north of Vancouver and the northern limits of the species’ distribution and its patterns of abundance there are unknown. Population trends for the Red-legged Frog are unknown as available presence/absence data do not necessarily reflect patterns of abundance.
Limiting Factors and Threats
The Red-legged Frog reaches the northern limits of its natural distribution in southern British Columbia. Human-induced threats include habitat fragmentation, draining of wetlands, loss and modification of forest habitats, removal of riparian vegetation, pollution of breeding habitats with pesticides, herbicides, and fertilizers, and the introduction non-native sport fish and bullfrogs to aquatic habitats. The distribution of this species overlaps the most populated parts of the province, where the lower elevation areas extensively used by this species have seen the most development. Habitat modification continues. Habitat fragmentation is of particular concern in view of the species’ seasonal migrations between forested areas and wetland breeding sites.
Special Significance of the Species
Declines in amphibian populations worldwide have featured prominently in popular literature and news coverage. Because of its relatively large spatial requirements and close association with moist forests, stream banks, and wetlands, the Red-legged Frog is emblematic of wilderness values, forest ecosystem health and the need to consider landscape-wide habitat connections.
Existing Protection or Other Status Designations
In British Columbia, the Red-legged Frog is on the provincial blue list of species at risk (i.e., species considered particularly sensitive or vulnerable to human activities or natural events). Rana aurora is included in the Identified Wildlife Management Strategy (IWMS), Version 2004, which contains specific guidelines for habitat management. However, much uncertainty exists on how these guidelines will be implemented. The Act does not apply to private lands, including vast tracts of private forestry land on Vancouver Island within the core area of the species’ range. The British Columbia Wildlife Act prohibits the collection, possession, and trade of all native vertebrates, including amphibians. This law has limited effectiveness in protecting frogs, because it is difficult to enforce and does not cover damage to habitats.
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