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Northern Leopard Frog (Rana Pipiens)

Evaluation and Proposed Status

Decline of R. pipiens populations in western Canada began approximately 20 years ago and it may no longer be possible to reconstruct the causal conditions. Potential causes include wetland drainage, drought, habitat modification, game fish introductions, pesticide use, disease, wetland eutrophication and/or ultraviolet radiation. As much as 70% of prairie wetlands have been lost during this century (Biodiversity Science Assessment Team, 1994). Not only does wetland drainage eliminate populations, it increases isolation of remaining populations and may weaken metapopulation structures. Reduced water tables can cause the reduction and elimination of many temporary ponds (Corn and Fogleman, 1984). Drawdown can result in more ponds freezing solid over the winter, increasing mortality. Waterways have been modified in a number of ways. Linkages of ponds have occurred in British Columbia (Orchard, 1992) altering hydrology and often opening up ponds to fish. Channelization of streams in Alberta may reduce ability of young of the year frogs to disperse along riparian corridors (Seburn et al., 1997). The introduction of predatory fish into many modified wetlands has occurred in British Columbia (Orchard, 1992), Alberta (Seburn, 1992c) and Saskatchewan (Didiuk, 1997). No information is available on Manitoba but fish introductions likely have occurred. In the U.S., fish introductions have been implicated in anuran declines in Nevada (Drost and Fellers, 1996) and Washington (Leonard and McAllister, 1996). Chemical pesticides have a number of direct and indirect effects on amphibians (Bishop, 1992). Reduction of food, behavioural effects and mortality of tadpoles have all been observed. The most common disease of R. pipiens is red leg, caused by a bacterium, Aeromonas hydrophila. The bacterium is naturally widespread but generally is not lethal unless individuals are already under stress. Eddy (1976) witnessed tadpole die-offs when algal blooms occurred in her study site in Manitoba. High levels of algal production resulted in anoxic water. Fertilizer run-offs from agricultural fields could make this phenomenon widespread. Finally, ambient levels of UV radiation have caused embryo mortality of Rana cascadae (Cascades Frog) in Oregon (Blaustein et al., 1994). It is unclear if R. pipiens is susceptible to current levels of UV.

Regardless of the cause or causes of its decline, the magnitude and rate at which R. pipiens collapsed in both Manitoba and Alberta dispell any doubt that it is vulnerable to catastrophic declines. As the cause or causes of these declines remain elusive, it is unwarranted to assume that populations cannot collapse again. The reduced distribution may make R. pipiens more susceptible to future regional collapses. A complete evaluation of the current status of R. pipiens is hampered by a lack of data for Saskatchewan, Manitoba, or the N.W.T.  Neither the extent of the decline nor the degree of recovery can be described by any more than anecdotal information. To put the matter into context, "no detailed natural history studies or focused research projects have addressed any species of amphibian in Saskatchewan" (Didiuk, 1997). In Manitoba, despite the highly visible and dramatic collapse of this commercially harvested species, no detailed research projects have been conducted in the more than 20 years since the decline. In British Columbia and Alberta, where post-decline research has been conducted, both jurisdictions have found significant range and population reductions.

It is logical to consider separately the status of R. pipiens in two regions. In western Canada east of the Rocky Mountains (Alberta, Saskatchewan, Manitoba, and N.W.T.), the climate, ecosystems, and manner of human disturbance differ substantially from conditions west of the Rocky Mountains (British Columbia). The distribution of R. pipiens is continuous across the prairies and into the N.W.T. but populations in British Columbia are distinct both geographically and ecologically, existing in very different habitats.

A designation of Special Concern for populations on the prairies is recommended. Rana pipiens underwent a dramatic range contraction in both the eastern and western prairies, the cause of which is still unknown. There is no sign of recolonization in western areas and little evidence from the east. Overall, R. pipiens seems limited to major drainage systems and high quality habitat. These isolated populations are more vulnerable to extirpation in view of a proven lack of recolonization.

For R. pipiens in British Columbia, a status of Endangered is recommended. There is strong evidence that only one population remains and breeding was not confirmed there in 1996. It is isolated from other Canadian populations, precluding recolonization from Alberta. Immigration from Idaho, directly to the south of the remaining population, is possible but cannot be evaluated. If immediate action is not taken to ensure the survival of the British Columbia population, in all likelihood it will soon be extirpated.