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Northern Leopard Frog (Rana Pipiens)

Population Size and Trend

Population size

Because amphibian populations fluctuate dramatically from one year to the next, based on stochastic factors such as the weather, the number of populations is a more important criterion for stability than population size (Green, 1997). The number of extant populations is unknown. Although R. pipiens is frequently reported at ponds across western Canada many of these sites are not breeding ponds, but sinks for dispersing young of the year (Seburn et al., 1997).

Although data on the number of extant populations in Manitoba is not available, R. pipiens were heard calling at between 27-40% of monitored sites from 1993 to 1997 (R. Larche, pers. comm.). These sites were monitored as part of an annual volunteer "backdoor" survey of calling frogs. Results to date: 1993, 25 of 63 (40%) sites reported calling R. pipiens; 1994, 20 of 51 (39%) sites; 1995, 13 of 49 (27%) sites; 1996, 30 of 109 (28%) sites (R. Larche, pers. comm.). Unfortunately geographic distribution of these records is not currently available.

The number of remaining Saskatchewan populations is unknown. Based on a lack of recent sightings, populations have greatly declined in number since the late 1970s (Didiuk, 1997). Reports to the Saskatchewan atlassing program indicate R. pipiens is still widespread (A. Didiuk, pers. comm.; Fig. 4) although there is no information on the size of any of these populations. Given the preliminary nature of the atlassing program it is quite probable that additional populations will be recorded. Volunteer amphibian monitoring in Saskatchewan began in 1993 (SAMP, 1994). Nine of 35 (26%) survey routes reported R. pipiens calling that year. Rana pipiens was reported from Saskatoon (three routes), Regina, Alvena, Canwood, Conquest, Paddockwood and Perdue. Only two of the routes reported R. pipiens from more than one pond per route. Rana pipiens may be under-represented in the surveys as their calling is not always easy to detect. The current distribution map (Fig. 4) implies a wide range, although populations tend to be in isolated drainages or waterbodies (Didiuk, 1997).

Figure 4.  Current distribution of Rana pipiens in western Canada.

Figure 4.  Current distribution of Rana pipiens in western Canada. Data are from various sources --  Saskatchewan: Andrew Didiuk (pers. comm.) and Saskatchewan Herpetofaunal Atlas Project,  Alberta: Wagner (1997), British Columbia: Ohanjanian and Teske (1996), Northwest Territories: M. Fournier (pers. comm.).Current distributional data are not available for Manitoba (R. Larche, pers. comm.). Data for  Saskatchewan include both historic and current records, some of which have not recently been confirmed.

Of 74 historically known breeding populations in Alberta only 26 remain, with breeding confirmed at only 12 (Fig. 4; Seburn, 1992c). With the exception of one population in the extreme northeast of Alberta (Boquene Lake), all known extant populations are south of the Battle River and all known populations with evidence of successful breeding are south of Drumheller, the majority in the extreme southeastern corner of the province. The maximum number of adults observed at any breeding pond is 40 (Seburn, 1992c). There are seven known major populations. Other sites with confirmed breeding are Milk River Natural Area, Little Bow River, Bow City (Bow River), South Gleichen (Bow River), Coaldale, and Kennedy's Coulee. Recently, R. pipiens has been reported to be abundant in Kin Coulee Park in Medicine Hat (Powell et al., 1996). Alberta Fish and Wildlife has been monitoring one population of R. pipiens at Prince's Spring since 1990 (E. Hofman, pers. comm.) and large numbers of young of the year have been seen each year.

Rana pipiens is virtually extirpated in British Columbia. During the mid-1970s, R. pipiens at the Creston area in southeastern British Columbia was described as being "numerous" (Ohanjanian, 1996). It was considered uncommon by 1981 and staff at the Creston Valley Visitor's Centre have not seen R. pipiens since the mid-1980s. Surveys for R. pipiens from 1988-1990 were unsuccessful (Orchard, 1992). In 1991 four R. pipiens were located north of Creston at the Duck Lake Nesting Area within the Creston Valley Wildlife Management Area, near the U.S. border. None were located in surveys conducted in 1995, in the Southern Interior Mountains Ecoprovince from Bush Arm, north of Golden, south to the U.S. border at Grasmere and west to Creston (Ohanjanian and Teske, 1996). Neither were they located at Waneta and along Highway 3 west of Creston (Ibid.). Calling surveys in 1996 detected a total of only 3‑4 males at only one location in the Creston Valley Wildlife Management Area (Ohanjanian, 1996). No egg masses or tadpoles were observed. Summer surveys found no young of the year R. pipiens. However, the presence of one sub-adult indicates recent successful breeding. Rana pipiens was found at Creston again in 1997 (L. Friis, pers. comm.).

Rana pipiens in the NWT is limited to a small area of "boreal plain" between the Alberta border and Great Slave Lake (Fournier, 1997). There are few observations and no estimates of population sizes. There are reports from this area as recently as 1995.

 

Population distribution and persistence

Throughout western Canada, the distribution of Rana pipiens appears to be more closely tied to major river drainages than is the case in eastern Canada. This may reflect the distribution of suitable breeding and especially hibernation sites as well as the relatively drier climate of the west.

In Manitoba, R. pipiens began dying off in 1975 and by the next year was "virtually gone from the major centres of population" (Koonz, 1992). Unlike in some areas, large numbers of dead frogs were found. "Piles of dead and dying frogs were reported from many Lake Manitoba shorelines, whereas heaps nearly a metre high were recorded from the major frog hole area" (Koonz, 1992). Virtually all frogs were wiped out in the densest areas. "Large marshes along Lake Manitoba were silent of Northern Leopard Frog calls, and egg masses were absent" (Koonz, 1992). Small, isolated populations survived. Some recovery was noticed in 1983 and currently R. pipiens has reoccupied much of its historic range, and although densities are far below previous levels, at some sites recovery has been "dramatic" (R. Larch, pers. comm.).

There is little information about R. pipiens in Saskatchewan. Populations appear to be associated with major river drainages, particularly the North Saskatchewan, South Saskatchewan, Qu'Appelle, Frenchman and Souris Rivers. Anecdotal information suggests that populations reached a low in the early to mid-1970s but are recovering (Seburn, 1992a; Weller et al., 1994). Rana pipiens has been tentatively labelled "secure" in the province (Secoy, 1987). Although populations were acknowledged to fluctuate greatly in size their wide geographic distribution is considered sufficient to classify them as secure. Secoy highlighted two possible factors affecting R. pipiens: a decade-long drought and the modification of wetlands in the southern portion of the province. Sloughs and other wetlands are being or have been drained and many streams and rivers are being channelized for irrigation. Because of these factors Secoy argued that R. pipiens and other "secure" species may in fact be threatened in the long term.

Rana pipiens vanished from much of its range in Alberta by 1979 (Roberts, 1981, 1992). Subsequently, some additional populations have disappeared (Roberts, 1991). Loss of spawning and overwintering sites does not appear to be a factor (Roberts, 1992). High mortality (attributed to "red leg" disease) was locally evident in 1976. No populations are believed to have been wiped out as a result of this, but it may have been a factor in the decline.

In British Columbia, R. pipiens is now quite rare (Orchard, 1992). Alteration of waterways and introduction of predatory fish are two possible causes of the decline. Fish introductions have been implicated in anuran declines in the Sierra Nevada of California (Drost and Fellers, 1996) and in the southern portion of the province "game fish have been introduced into virtually every water body that can support them" (Orchard, 1991).

Rana pipiens declines are not confined to western Canada. In eastern Canada, there is some evidence to indicate that R. pipiens is not as common in northern Ontario as it was historically (Weller et al., 1994). A recent survey of areas from Sudbury to Geraldton failed to locate any R. pipiens north of Sault Ste. Marie (Seburn and Seburn, 1997). In Washington state, R. pipiens has "been extirpated from most of its historic range" (Leonard and McAllister, 1996). Rana pipiens was found in only two of 15 known historic locations surveyed between 1992 and 1995. The cause of the decline is unknown, although Rana catesbeiana (Bullfrog) introductions (R. catesbeiana was not found in the two remaining R. pipiens sites), game fish introductions, wetland modification and pollution have been suggested. In western Montana, R. pipiens "has nearly disappeared" from where it was once common (J. Reichel, pers. comm.). Declines may have begun as early as 1970. The current distribution in eastern Montana is poorly known, although R. pipiens appears to be declining. Declines have been reported from southern Idaho and eastern Oregon (Koch et al., 1996). There has been some evidence of increasing numbers of R. pipiens in eastern Idaho in recent years. Nine populations studied in Colorado from 1973-1982 all went extinct (Corn and Fogleman, 1984). Five populations were eliminated because of the ponds drying up in the mid-1970s as a result of drought. It is unknown why the others were wiped out. Examination of 13 historic sites in Arizona from 1983-1987 failed to locate any R. pipiens (Clarkson and Rorabaugh, 1989). One previously unreported population was found. R. catesbeiana was present at only one of the 13 sites.

 

Trends

There has been a significant and widespread decline of Rana pipiens throughout the western portion of its range in Canada and the United States. While declines in Manitoba and Wisconsin were accompanied by evidence of dead frogs this was not generally so. In British Columbia and Alberta the decline has resulted in a significant range contraction with more southern populations faring better than those at the northern limit (with the exception of the N.W.T. and extreme northeastern Alberta). In Saskatchewan and Manitoba, however, the historic range appears to be intact although the number of populations was reduced. There is some evidence that numbers have rebounded moderately within the extant range but there is no evidence of recolonization in the broader areas where the species completely disappeared. Although precise dating of the decline is not possible, except in Manitoba, the data are consistent with the decline having occurred during the mid-late 1970s.

Further detail on historic events is hampered by a lack of data. Volunteer monitoring programs may eventually determine the number of extant populations in Saskatchewan and Manitoba. Thus, while it is clear that a dramatic decline occurred it is unclear if R. pipiens continues to decline, is currently stable or increasing. For example, the Manitoba Conservation Data Centre lists the population trend for R. pipiens as either Stable or Declining (Duncan et al., 1994).