Hotwater physa (Physella wrighti) COSEWIC assessment and upate status report: chapter 4

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Distribution

Global range

Physella wrighti is known only from the Liard River hotsprings complex located within Liard River Hotsprings Provincial Park (LRHPP) in northern British Columbia (B.C.), Canada (Figure 2). LRHPP is located at Mile 496 of the Alaska Highway, about 65 km south of the BC/Yukon border (59°25’35”N, 126°06’18”W; NAD27) and lies within the Boreal White and Black Spruce biogeoclimatic zone (Meidinger and Pojar 1991). In comparison, P. johnsoni is restricted to the hotsprings complex at Banff, Alberta, and P. gyrina is the most widely distributed physid in Canada. Clarke (1981) maps the northern limit of P. gyrina from the St. Lawrence River, Quebec, northwest to the Yukon River delta (Clarke 1981). In the United States is it distributed across all of the continental United States, including Alaska (NatureServe 2007). The population of the Hotwater Physa is therefore surrounded by P. gyrina.

Canadian range

The population within LRHPP was first discovered in 1973 in Alpha Stream and was further observed in the stream in 1979, 1997, 2000, 2001 and 2006 (Table 1). Recent surveys have identified additional areas inhabited by P. wrighti within LRHPP but the Canadian range of this species has not changed from 1973 to 2006 (33 years). The species is known from Alpha Pool, Beta Pool, and Alpha Stream for 200 mdownstream from Alpha Pool.

Searches at other nearby hotsprings, including Grayling Springs, Deer River Hotsprings and Toad Springs in northern B.C., and Coal River Hotsprings in the Yukon Territory have not located additional populations of P. wrighti (Te and Clarke 1985; Salter 2003). Deer River Hotsprings, approximately 50 km from LRHPP, hosts another species of Physella (P. virginea), but a genetic comparison between the two populations has not been made (Heron 2007). Physella virginea is distributed from B.C. south to California, and species of Physella living in hotsprings may not all be narrow endemics. The case for this is strengthened if the phylogeny proposed by Wethington and Lydeard (2007), who lump into P. gyrina 11 different hot- and cool-water physid taxa, is correct.

Georeferenced maps at a suitable scale of the LRHPP area are currently unavailable and the 2006 extent of occurrence (EO) for P. wrighti was estimated from the park map available at the B.C. Parks website (B.C. Parks 2006; Figure 3). The 2006 EO is an irregular polygon enclosing approximately 16,310 m² (0.02 km²; see Figure 3 and Appendix A). The area of occupancy (AO) is calculated to be 1 km², based on a 1 × 1 km grid (A. Filion pers. comm. 2008). A more biologically defensible area of occupancy, which includes only the narrow strip along the margins of water bodies occupied by snails, was calculated to be approximately 4.6 m² (4.6 × 10^-6km²; see Appendix A).