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Northern Leopard Frog (Rana Pipiens)

Limiting Factors

Habitat modification

Rana pipiens requires a minimum of three kinds of distinct habitats: temporary ponds for spring breeding, terrestrial summer foraging habitat, and overwintering ponds that do not freeze solid or become anoxic. The removal of any of these habitats can eliminate a given population. Isolating any of the habitats from the other two -- for example, through road development -- can also eliminate a population. Calling intensity at breeding ponds is inversely related to proximity to paved roads, in Ontario (Pope, 1996).

Habitat studies indicate that proximity of summer forage areas to breeding sites is the strongest factor in explaining variation in chorus intensity in the Ottawa area (Pope, 1996). Possibly mortality rises dramatically if adults must wander large distances from the breeding ponds. Alternatively, breeding ponds with nearby summer ranges may simply be optimum habitat and therefore have higher densities of individuals.

It has been suggested that the modification and linking of wetlands for game fish introduction can be detrimental to R. pipiens (Orchard, 1992). In addition, some fish, such as Cyprinus carpio (Common Carp), can displace R. pipiens by habitat modification. Carp feeding can destroy emergent vegetation, increase turbidity and eliminate or greatly reduce algal and invertebrate populations (Leonard and McAllister, 1996). Alien plant invasions (e.g. Lythrum salicaria, or Purple Loosestrife) may also alter the structure of wetland environments (Ibid.).

Habitat modification could influence the thermal characteristics of the breeding pond. Hayes and Jennings (1986) speculated that warming of breeding ponds during critical periods could eliminate some ranid species. Many ranids are most vulnerable to temperature during the embryo stage. Rana pipiens embryos at breeding ponds in southern Alberta generally experience maximum temperatures of less than 20° C (C Seburn, pers. obs.), well below the thermal maximum of approximately 28° C (Moore, 1939).

Increased irrigation because of drought in some areas of the prairies may result in disturbances to the groundwater and lowering of the water table (Seburn, 1992c). This in turn may accelerate the drying of breeding habitats or degrade overwintering sites. In Alberta, the remaining known breeding sites are all spring-fed ponds and most are in areas that are not irrigated.

 

Predation

Rana catesbeiana (Bullfrog) introductions in Colorado and Washington have been implicated in R. pipiens declines (Hammerson, 1982; Leonard and McAllister, 1996), but an evaluation of R. catesbeiana responsibility for ranid declines in western North America failed to find unequivocal evidence (Hayes and Jennings, 1986). Although R. catesbeiana is a predator of R. pipiens tadpoles, juveniles and possibly even adults, it does not naturally occur west of Ontario. It has been introduced into British Columbia and its range is expanding, but it does not overlap with R. pipiens. It is not involved in the decline of R. pipiens in western Canada.

Fish introductions are more suspect because of predation on unprotected embryos (Hayes and Jennings, 1986). Fish may be responsible for declines in species of western ranids that have evolved in relatively fish-free environments. Rana catesbeiana embryos and larvae are adapted to coping with fish, hence their expansion may even be facilitated by fish removing other ranids. The effect of fish is complicated, because they also eat major tadpole predators such as odonate larvae (P. Gregory, pers. comm.). However, R. pipiens normally breed in fishless ponds (Merrell, 1968).

 

Collecting

Rana pipiens has been commercially harvested in Manitoba since at least 1920 (R. Larche, pers. comm.). Harvest records represent a minimum amount as sales go unrecorded annually. Records from dealers indicate that up to 49 907 kg of R. pipiens were collected annually during the early 1970s (Koonz, 1992). Given 20-26 R. pipiens per kg then the annual harvest removed over one million frogs per year. By 1974, the harvest had declined to 5 900 kg despite no apparent change in the market. There was no commercial harvesting in 1993 or 1994, but in 1995 5,800 kg of R. pipiens were collected (R. Larche, pers. comm.). Rana pipiens is not commercially exploited in Saskatchewan (Seburn, 1992a) Alberta (Seburn, 1992c) or in British Columbia (L. Friis, pers. comm.). However, juvenile R. pipiens are used as bait by anglers in Manitoba (R. Larche, pers. comm.), Saskatchewan (K. Roney, pers. comm.) and Alberta (Roberts, 1991).

Eggs and tadpoles are commonly collected in all parts of Canada by children and adults who keep them to watch the tadpoles transform. Although surviving young are usually released to the wild, they may not be taken back to their place of origin and the effect of this activity is unclear.

 

Other factors

The prevalence of renal carcinomas in R. pipiens from Minnesota was as high as 10.5% during the 1960s (Hunter et al., 1989). During the late 1970s, no renal carcinomas were found in an examination of 2151 frogs. From 1986-88 renal carcinomas were found in 4 R. pipiens; however, they were from a commercial dealer and it is unclear if they were raised in captivity or were wild-caught.

High levels of hind-limb deformities in R. pipiens as well as R. clamitans, R. catesbeiana and Bufo americanus have been reported from areas in the St. Lawrence Valley, Quebec which are exposed to high levels of pesticide runoff (Ouellet et al., 1997). A wide variety of factors including parasites, disease and toxins can cause deformities and the cause of the deformities in this case has not been determined. High levels of deformities have not been reported from western Canada and therefore are unlikely to be a significant factor in the decline.