Great Basin spadefoot (Spea intermontana) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

Spea intermontana inhabits semi-arid grasslands and open forests. In British Columbia, it is commonly found from valley floors to about 800 – 1,200 m (St. John 1993, Leupin et al. 1994) and rarely as high as 1,800 m (Leupin et al. 1994). In the South Okanagan, St. John (1993) found most breeding sites to be below 600 m.

Spadefootsrequire aquatic habitats for breeding and terrestrial habitats for foraging, hibernation, and aestivation. These habitats must be suitably connected to allow for seasonal movements. Breeding habitats of S. intermontana range from ephemeral pools to wetted margins of lakes to shallow water areas of permanent water bodies, but sites that fill with water and dry up each year seem to be preferred (Hallock 2005, Sarell 2004). Wright and Wright (1949) listed the following aquatic habitats for S. intermontana: “canyon pools, desert springs and pools, intermittent and permanent, irrigation ditches, stream edges, rain puddles, water pockets, water depressions made by cattle.”  Breeding sites often, but not always, contain abundant emergent and riparian vegetation (Leupin et al. 1994). Spadefoots readily use human-made spawning sites and have been recorded from dug-out ponds, plastic pools, and ditches in the Okanagan valley (C. Bishop, pers. comm.). Breeding sites must last long enough for larval development to take place; given a period of at least six weeks for breeding and development, a usable breeding site in British Columbia must retain ample water from about mid-April until the end of May.

Spadefoots shelter underground from unfavourable conditions and require terrestrial habitat year-round (Hallock 2005, Matsuda et al. 2006). Loose, deep, and friable (crumbly) soils that allow for burrowing are thought to be important (Sarell 2004) (see General Biology: Terrestrial Ecology). Clustering of over-wintering burrows in areas that contain suitable soils and close to breeding sites has been reported for S. hammondii (Ruibal et al. 1969).

Habitat trends

Grasslands in the arid southern interior of British Columbia are among Canada’s most endangered ecosystems (Scudder 1980) and cover less than 1% of the land area of the province (662,872 ha; MOE 2004a). Pitt and Hooper (1994) emphasize both the rarity and lack of protection for grasslands in British Columbia. These naturally rare habitats have been reduced drastically since European settlement, especially in productive and biologically rich valley bottom habitats and continue to be lost due to expanding urban development, agriculture, and other land conversions (MOE 2004a). In the Okanagan Basin and Boundary District, about one third of grasslands have been lost, whereas in the North Okanagan, about 50% of grasslands have been lost (MOE 2004a). Fire suppression, invasive species, livestock grazing, intensive recreation, and other human-mediated activities further continue to degrade and diminish these habitats.

Wetlands cover a small proportion of the arid southern interior of British Columbia, comprising about 0.3% to 0.7% of grassland and Ponderosa Pine ecosystems (MOE 2004b), but are exceedingly important for S. intermontana. At least half of the wetlands in the area have probably been lost since European settlement (Ted Lea, pers. comm.). Accurate information on historical rates of wetland loss is lacking. Much of the valley bottom habitat was already modified by agriculture in 1938, when air-photos first became available. In the southern Okanagan valley, 85 – 90% of larger marshes have been lost (MOE 2004b). Rates of recent wetland losses are difficult to calculate because small, seasonal wetlands that S. intermontana frequently uses for breeding are often not mapped. Small wetlands and ponds are often the first to be filled in for development or modified for cattle water holes. Many of the remaining wetlands are degraded as a result of invasive plants and animals, livestock use, channelization, contamination with agricultural chemicals and fertilizers, and withdrawal of water for irrigation, domestic, and other uses (MOE 2004b). In the Okanagan, major changes in irrigation practices and water conservation are expected to reduce agricultural ponds used by spadefoots for breeding. For example, many farm operations are switching to drip irrigation, which reduces the need for irrigation ponds (C. Bishop, pers. comm.).

The water table has dropped significantly at many sites in the Canadian range of S. intermontana in the past two to three decades. On the southern Cariboo Plateau, at the northern end of the species’ range, the water table dropped 4 m between 1978 and 1988 (Northcote 1992), and it dropped a similar amount at Mahoney Lake, an alkaline lake in the South Okanagan Valley, between 1982 and 1994 (Lowe et al. 1997). Although these drops are at least partly due to reduced precipitation, the accelerating human development in these areas and associated increase in water consumption are probably lowering the water table as well. Predicted increases in droughts associated with climate change will further diminish and alter wetland habitats in the area (Cohen et al. 2004).

Land alienation of grasslands in British Columbia has accelerated since the 1970s, particularly in the Okanagan Valley (Hlady 1990). Redpath (1990) calculated that less than 9% of grassland habitat remained undisturbed by 1990 in the South Okanagan and Similkameen valleys. The human population grew exceedingly rapidly throughout the Canadian range of S. intermontana from 1986 to 1996; the growth slowed down somewhat in 1996 – 2005 but is predicted to continue to increase steadily over the next decades, putting pressure on already scarce land and water resources (Government of British Columbia 2006; Table 1). Within the past two decades, population growth has been greatest in the south, particularly in the Okanagan-Similkameen region, and lowest in the Cariboo region in the north. Over the next two decades, the greatest growth rates are predicted for the Central Okanagan. Concomitant with increasing human population, terrestrial and wetland habitats will continue to be lost as more of the land base is converted into housing developments and other human uses and as demand for water increases.

Table 1.  Human population growth in regions of British Columbia inhabited by Spea intermontana (rates calculated from Government of Canada 2006). Change (%); calculated from census estimates
Region 1986-1995 1996-2005 2006-2016 Projected change (%)
2016-202
Okanagan-Similkameen
28.8
5.6
10.2
8.6
Central Okanagan
52.7
18.6
18.4
15.3
North Okanagan
30.4
8.5
12.3
9.9
Thompson-Nicola
21.4
4.3
8.3
5.5
Cariboo
10.5
0.7
10.4
4.7

Extensive habitat loss has also occurred within the range of S. intermontana in the United States. For example, more than 90% loss of native shrub-steppe grassland in Oregon and in southwestern Washington has been lost (The Nature Conservancy 1992); more than 99% of basin big sagebrush (Artemisia tridentata tridentata) habitat in the Snake River plain of Idaho has been converted to agriculture (Hironaka et al. 1983); 99.9% of Palouse prairie throughout the species’ range in Idaho, Oregon, and Washington have been lost to agriculture (Tisdale 1961); 52% loss of wetlands in Nevada and 30% loss of wetlands in Utah occurred from 1780s to 1980s (Dahl 1990).

Habitat protection/ownership

Most of the habitat suitable for S. intermontana is unprotected. About 68% (40,000 ha) is within Indian Reserves or is privately owned (Sarell 2004). Two of the largest known populations are partially protected. The Haynes’ Lease Ecological Reserve (ER#100) provides 100 ha of secure habitat for part of the large population of S. intermontana breeding and foraging in the marsh and shrub steppe habitats at the north end of Osoyoos Lake. The adjacent South Okanagan Wildlife Management Area provides additional habitat for that population as well, but at a lower level of protection. Lac du Bois Grasslands Protected Area (15,000 ha) near Kamloops in the Thompson area provides protection for another population. The Nature Trust of British Columbia has acquired a number of properties with suitable habitat for the species, including the White Lake Ranch west of Okanagan Falls, providing protection for several smaller populations. Two provincial parks with suitable habitat for the species were established in 2001: White Lake Grasslands Protected Area (3,741 ha), which is contiguous with other protected areas around Vaseux Lake,and South Okanagan Grasslands Protected Area (9,364 ha). These areas are protected from development, but livestock grazing occurs at least at the White Lake and South Okanagan grasslands protected areas (C. Bishop, pers. comm.). In the 100 Mile House District, South Cariboo, all recent records and majority of ponds that provide potential habitat are from provincial Crown lands that are under grazing licences (R. Packham, pers. comm.).

Habitat for a large population of S. intermontana at the Osoyoos sewage lagoon (about 1,000 breeding males in early the 1990s; St. John 1993) remains unprotected, and much of the terrestrial habitat surrounding the lagoon has been converted to housing and golf course expansion within the past 10 years (Sarell 2004).

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