Cœur d’Alene salamander COSEWIC assessment and status report: chapter 7
Population Sizes and Trends
Search effort
Plethodon idahoensis was the subject of directed surveys from 1995 to 2003, and was included in two amphibian surveys of Glacier and Mt Revelstoke National Parks in 2003 and 2004 (Dykstra 2004, Adama and Ohanjanian 2005). A presence/not detected protocol was used for all surveys, and consisted of non-destructive, visual scanning by two people for individuals out at night in wet areas around waterfalls, seepages and stream banks.
Approximately 280 person hours were spent conducting nocturnal surveys at 189 potential sites. Due to the steep topography and difficulty of access of P. idahoensis habitat, many watercourses with the same surficial habitat features were not surveyed. Many dozens of these were located between known occurrence sites, well within the species’ range. It is highly probable, therefore, that the number of occurrences is an under-estimate. In addition, surveys were carried out upstream or downstream of 8 occurrence sites; P. idahoensis was found either upstream or downstream of a known site on 6 of 8 (75%) watercourses. These data are preliminary, but indicate that it is highly probable that the total number of occurrence sites is an under-estimate.
Abundance
The total number of adult P. idahosesis in Canada cannot be estimated with certainty as most watercourses cannot be completely sampled and over 50 watercourses situated between known occurrence sites cannot be accessed for nocturnal surveys. Nevertheless, an approximate figure can be derived.
In line with the behaviour of other species of terrestrial plethodontid salamanders, an unknown percentage of individuals is hidden and undetectable via surface surveys. Taub (1961) estimated that between 2% and 32% of eastern red-backed salamanders, Plethodon cinereus, were at the soil surface at any one time. Burton andLikens (1975) refined that estimate for P. cinereus in the Hubbard Brook Experimental Forest of New Hampshire to 21%. Davis (1996), working on southern Vancouver Island, estimated that no more than 24% of the estimated number of western red-backed salamanders, Plethodon vehiculum, in enclosed experimental plots were found at the surface at any given time. Considering that a total of 919 P. idahoensis have been found at the 56 known occurrence sites in British Columbia and that juveniles comprise about 37% of encountered individuals (Ohanjanian, unpublished), and assuming the more conservative Burton and Likens estimate of detectability of 21%, there are at least 2,776 mature individuals in the surveyed areas of known sites. This is certainly an underestimate of the total population. Taking in account that no more than about 10% of the Canadian range that has been surveyed, it seems safe to say there are more 10,000 mature individuals in Canada. Distributed as they are along small streams, no site is likely to have more than 1,000 individuals present.
Fluctuations and trends
Population densities, sex ratios and age distributions of other plethodontid species are relatively stable over time (Hairston 1987, Grover 1998) compared to most amphibians. Several measures suggest that it may also be true for P. Idahoensis. Juveniles were observed at over 50% of occurrence sites, indicating that reproduction is occurring at these locations. Frequency histograms of size classes at three monitoring sites revealed that all size classes were present and size distribution was stable over a three year period (Ohanjanian and Beaucher 2001, 2002; Ohanjanian 2001a).Individuals in the 50 – 60 mm SVL size categories (young adults and mature adults) are consistently encountered (Ohanjanian and Beaucher 2002, 2003; Ohanjanian 2001a). These provide indirect evidence that ordinarily the species is largely stable in numbers.
Rescue effect
Given the species’ physiological constraints and low dispersal ability, rescue from the closest US populations is unlikely.
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