Great Basin spadefoot (Spea intermontana) COSEWIC assessment and status report: chapter 4

Distribution

Global range

Spea intermontana is widely distributed in dry grasslands in western North America and occupies the intermontane region between the Rocky Mountains and coastal ranges (Hallock 2005, Matsuda et al. 2006). Its range extends north from the Colorado River to southern British Columbia, west to the Sierra Nevada and Cascade ranges and east to the Rocky Mountain divide (Fig. 2). In Arizona, it occurs up to 2,800 m elevation (Stebbins 1985).

Figure 2. Global distribution of Spea intermontana. Map produced by Ophiuchus Consulting and printed courtesy of Mike Sarell.

Figure 2. Global distribution of Speaintermontana.Map produced by Ophiuchus Consulting and printed courtesy of Mike Sarell.

Canadian range

In Canada, S. intermontana occurs in arid areas of south-central British Columbia (Fig. 3). The species is found in the Okanagan Valley, in the Similkameen, Kettle-Granby, Thompson and Nicola river valleys, and the South Cariboo region, where the species reaches the northernmost limit of its distribution (Sarell 2004, Matsuda et al. 2006; records collected for this report). It occurs primarily in the Bunchgrass, Ponderosa Pine, and Interior Douglas-fir biogeoclimatic zones (see Meidinger and Pojar 1991 for descriptions). Additionally, there are limited records from the Engelmann Spruce-Subalpine-fir zone (tadpoles in three small lakes) and the Montane Spruce zone (Leupin et al. 1994, D. Low, pers. comm.). Canada has less than 5% of the species’ global distribution.

Figure 3. Canadian distribution of Spea intermontana in south-central British Columbia. Map prepared by Jenny Wu and Alain Filion (COSEWIC Secretariat), based on records compiled for this report.

Figure 3. Canadian distribution of Spea intermontana in south-central British Columbia.Map prepared by Jenny Wu and Alain Filion (COSEWIC Secretariat), based on records compiled for this report.

The Canadian range of the species is discontinuous. The northern portion is centred in the Thompson River Valley and extends east from 70 Mile House to Barrière along the North Thompson River and west to Big Bar Creek along the Fraser River. The northernmost records, in the South Cariboo region, are about 100 km northwest from the nearest records in the Thompson River Valley. In the Nicola Valley, records exist from Quilchena to Douglas Lake; it is unclear whether this population is disjunct from the Thompson Valley population or part of the northern population. The southern portion includes the Okanagan, Similkameen, and Kettle-Granby River valleys, extending north to past Vernon, west to Princeton, and east to the Grand Forks area.

Within the past 10 years (1996 to spring 2006), the species continues to be found within all portions its range in British Columbia. However, systematic surveys of historic sites have not been conducted in any areas, and local extinctions or range contractions cannot be determined from the available data.

There are 12 new records (1 from 2005 and 11 from 2006) from the extreme northwest of the species’ range in the South Cariboo region, where the species was previously known from only two old records. During surveys in 2006 near Alberta and Meadow Lakes, west of 70 Mile House, the species was found at 11 of 17 sites surveyed (Verkerk et al. 2006). Verkerk et al. (2006) noted that numerous similar wetlands and lakes that might support spadefoots exist in the 100 Mile Forest District over an area of about 1,300 km².

There are a few older records from the Cache Creek-Ashcroft area. Anecdotal observations by local residents suggest that the species continues to persist in this area (M. Sarell, pers. comm.). There are clusters of records, both historical and recent, from around Kamloops. In contrast, no recent records exist from the North Thompson River Valley, where historical records are available from the McLure and Barrière areas. Lack of recent records might reflect lack of search effort. In the Nicola River valley, there are a number of new records, and the species appears to be well distributed in the lowlands within the valley.

Most records, historical and recent, are from the southern portion of the species’ range in the Okanagan and Similkameen valleys. The Okanagan Valley, in particular, appears to harbour the bulk of the Canadian population. Although there are only historical records from the Princeton area, anecdotal observations suggest that the species is locally common in the grasslands around Princeton in the Similkameen Valley (Jerry Herzig, pers. comm.). Bothrecent and old records are common from the southern Similkameen, from about 40 km south of Princeton to Keremeos and to the Canada-USA border. Numerous historical and recent records exist from the South Okanagan, especially from Okanagan Falls south to Osoyoos and the USA border. Recent surveys in the North Okanagan have established that the species is relatively widespread in the Vernon area; numerous new records exist from the Lumby/Blue Springs area west to Kalamalka Lake and to north of Okanagan Lake. There are three older (1994) records from small lakes just west of the Salmon River valley, which are isolated from the cluster of records in the North Okanagan. Surveys within the past 10 years have established that the species is relatively widespread within the southern part of the Kettle-Granby drainage. There are numerous new localities from the Grand Forks area, extending east to the south end of Christina Lake. In the Kettle River valley, there are new records from Zomora south and east through Midway to the Canada-USA border.

Based on records from 1985 to 2006, the species is known from about 235 sites, if localities closer than 500 m are considered the same site and each distribution record, whether a breeding chorus or a single animal, is treated equally. Sites vary from single observations to strings of many nearby localities separated from their nearest neighbours by less than 500 m. The extent of occurrence is about 30,770 km². If the 235 sites are surrounded, conceptually, by a 1 km buffer zone to account for spadefoot dispersal (Hammerson 1995), the area of occupancyis about 619 km². If a 2 km ×2 km grid is overlaid on the map, then the area of occupancy is 864 km², according to the IUCN methodology.

Leupin et al. (1994) searched for the species on the Cariboo plateau north to Williams Lake but failed to detect the species in the northern part of this area. Anna Roberts (pers. comm.) has not come across the species in the Williams Lake or Riske Creek areas despite many hours of nocturnal surveys for bats and owls. Although suitable habitat exists in the East Kootenay region of southeastern British Columbia, that area is separated from the Thompson and Okanagan valleys by a broad swath of moist forest and high mountains.

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