Great Basin spadefoot (Spea intermontana) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

The search effort has been uneven across the species’ Canadian range with much focus on the South Okanagan. Many distribution records resulted from FrogWatch submissions, volunteer efforts, or studies on other organisms, or were serendipitous observations. The total search effort is very difficult to quantify, as negative data (i.e., number of localities where the species was not found) is available for only a portion of the records.

In the South Okanagan, St John (1993) surveyed 86 sites and located the species at 56 sites. Also in the South Okanagan, Ashpole and the Canadian Wildlife Service surveyed 108 ponds and conducted additional surveys of river channels from Osoyoos to Oliver in 2003-2006; 43 ponds contained spadefoots but breeding was observed only in about half of the ponds (Ashpole et al. 2006a, C. Bishop, pers. comm.). In the Central Okanagan, Tarangle and Yelland (2005) surveyed 24 wetlands in the Kelowna area in the spring of 2005 and found the species at three sites. In the North Okanagan, Sarell (2006) summarized surveys for the species conducted in 39 ponds on Vernon Military Camp lands from 1999 – 2005; breeding was noted in 10 of these ponds. Leupin et al. (1994) surveyed 38 sites in the Thompson area and found the species at 24 sites. In 2005, about 50 water bodies were surveyed within Lac Bios Provincial Park, near Kamloops; three were found to contain breeding S. intermontana (Simpson 2005).

After identifying some 800 possible breeding ponds in a 1,300 km² region of the South Cariboo region, Verkerk et al. (2006) surveyed 17 locations, each consisting of one or two water bodies, during four days in June – August, 2006, near Alberta and Meadow Lakes, west of 70 Mile House. They located calling males or tadpoles of S. intermontana at 11 of those 17 localities. It is probable that spadefoots may occupy many more sites, as yet unsurveyed, in the South Cariboo.

Since 1998, many new localities have been found in the Kettle/Granby drainages and in the Nicola River Valley, but there is no information on search effort associated with these records. Several recent surveys have focused on Aboriginal lands in the South Okanagan (Sarell and Alcock 2004, Rebellato 2005) and North Okanagan (2003 – 2006; R. Weir, pers. comm.); search effort and details of the surveys are unavailable at this time.

Abundance

There are no accurate population estimates of the species for any areas. The available information, taken together, indicates that the maximum Canadian population is probably at least 10,000 individuals but much uncertainly is associated with this number and it could be much larger. However, it is almost certain that populations fluctuate greatly in size; the Canadian population at its lowest ebb may well fall below the 10,000 individual threshold. Orchard (1985) stated that the British Columbia population of Spea intermontana is certainly greater than 5,000 individuals. This is confirmed by censuses in the South Okanagan (St. John 1993), Kamloops and Douglas Lake areas (Leupin et al. 1994), and Nicola Valley (surveys by W.C. Weber). These surveys recorded a total of about 4,200 calling males. Important regions not covered in these surveys were the North Okanagan and Kettle/Granby drainages, where surveys conducted within the past 10 years show that the species is widespread where habitat still exists.

The database of distribution records, including submissions to the BC FrogWatch program, contains auditory observations of breeding choruses from 2000 to spring 2006. The number of calling males was estimated as low (individuals can be counted, calls not overlapping), moderate (some individuals can be counted, other calls overlapping), or high (full chorus, calls continuous and overlapping, individual calls not distinguishable). Of these observations, 187 were rated as low, 114 as moderate, and 20 as high. Although many environmental factors, such as date, weather, habitat, and condition and size of water body, can influence the size of a breeding chorus, these data suggest that most choruses are small, possibly reflecting the small size of many breeding ponds.

The Osoyoos Lake area seems to be a very important locality for the species in Canada, with 2,000 calling males detected by St. John (1993) around the oxbows at the north end of the lake and in the Osoyoos sewage lagoons. About 40% of the males detected on major surveys by St. John (1993), Leupin et al. (1994), and W.C. Weber were found in an area roughly 100 km² in size, less than 0.5% of the Canadian range. 

Fluctuations and trends

Lack of baseline data and poor understanding of current population sizes are common problems in assessing population trends for amphibians, and S. intermontana is no exception. Orchard (1985) presumed that the British Columbia population was decreasing, based on the loss of prime breeding, foraging and hibernating habitats, particularly in the Okanagan Valley. There are no historical data to validate this presumed decline although local breeding populations in some areas undoubtedly have been lost, and continue to be lost, due to draining and infilling of water bodies used as breeding sites. Although no changes in overall distribution can be discerned from the available data (see “Canadian distribution”), even large changes in abundance at particular localities cannot be determined from distribution data alone.

Spea intermontanacontinues to persist in the Osoyoos sewage lagoons where about 1,000 males were heard calling in early 1990s, but numbers may be much reduced. A full breeding chorus (calls continuous and overlapping, individual calls not distinguishable)was recorded there in May 2002 (distribution records collected for this report). In contrast, few spadefoots were found calling in the ponds in 2003 and 2004, and no tadpoles were seen, but the ponds were surveyed only infrequently (C. Bishop and S. Ashpole, pers. comm.).

Breeding populations of S. intermontana can vary substantially from year to year depending on water table levels, temperature and rainfall, although data concerning annual numbers and the precise magnitude of fluctuations are lacking. In the Kamloops area, Thompson River Valley, little breeding was observed in 2003 and 2004, whereas breeding was confirmed at numerous locations in 2005 (Karl Larsen, pers. comm.). Similarly, in the Vernon area, North Okanagan,breeding attempts and success were highly variable from 1999 to 2005, depending on the availability of surface water (Sarell 2006). Differences in recruitment rates can be expected to translate into variance in the size of the breeding population in subsequent years. This aspect of fluctuating population size is consistent with other pond-breeding anurans (Green, 2003) especially those living in areas with sporadic and unpredictable rainfall. Greenberg and Tanner (2005) found considerable temporal and spatial variability across the landscape in breeding ecology of the Eastern Spadefoot (Scaphiopus holbrookii). Most recruitment of young occurred in only four of nine years, and only four of the ponds produced metamorphs, although breeding took place in all but one of the ponds. Only ponds with large numbers (>175) of breeding adults produced substantial recruitment. Numbers of breeding adults showed dramatic fluctuations from year to year. Like S. intermontana, adult Scaphiopus holbrookii breed in ephemeral ponds and occupy surrounding upland habitat in arid environments.

Rescue effect

In Washington State, there are a handful of records of the species from Okanagan, Ferry, and Stevens counties that abut the Canadian border, but no records exist from the immediate vicinity of the border (Washington Herp Atlas 2005). The majority of records from the state are from extensive shrub and grasslands farther south. The northern populations in Washington State are potentially contiguous with the Okanagan-Similkameen and Kettle-Granby populations in British Columbia, and some movement northward is possible. However, modification and fragmentation of the habitat in the valley bottom areas probably restricts dispersal to trivial levels.

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