Great Basin spadefoot (Spea intermontana) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Persistence of spadefoots in particular areas depends on the availability of both suitable breeding sites and surrounding upland habitat, coupled with suitability of weather conditions and chance events (Greenberg and Tanner 2005). The main threat to S. intermontana within its Canadian range is from loss and degradation of habitat due to human activities. Dry grasslands of the Okanagan, especially those in the southern part of the valley, are under tremendous development pressures, both from intensive agriculture such as vineyards and urbanization, and habitat continues to be lost (Hlady 1990; see Habitat Trends). In California, causes of population declines of S. hammondii were consistent with local habitat destruction, rather than with other hypotheses tested (pesticide drift, UVB-radiation, climate change), and the number of occupied breeding sites was reduced in areas surrounded by urban or agricultural developments within a 5-km radius (Davidson et al. 2002). Similarly, in the eastern Mojave Desert, the lowland amphibian fauna has changed drastically over the past century in conjunction with severe habitat loss and modification; Spea intermontana is known only from historical records and appears to have disappeared from this area (Bradford et al. 2005).

Loss and degradation of breeding ponds

Perhaps the most critical environmental variable to an amphibian living in an arid environment is the availability of a water source for breeding. Loss of water bodies occurs from infilling and drainage associated with urban developments or conversion of land into agricultural uses. Overdrawing of water for irrigation or other human uses can result in complete drying of small ponds used by spadefoots or reduce the amount of time that the ponds contain water. Reports exist of mass mortality of S. intermontana tadpoles in drying ponds (Leupin et al. 1994).

Livestock use, pesticides, herbicides, and fertilizers can all degrade water quality in ponds where spadefoots breed. In dry grasslands, cattle almost always congregate around any water source, including the small ponds used by spadefoots for breeding. Cattle could have a negative effect on breeding pools for several reasons. Feces from cattle congregating in small pools could seriously affect water quality (Orchard 1985). Also, deep cattle hoof-prints left in the bottom of muddy temporary pools create a multitude of tiny pools as the pond dries up instead of a single, larger pool. Very small pools could seriously restrict the foraging ability of larvae and result in entrapment.Leupin et al. (1994) found that nearly all breeding ponds examined in the Thomson-Nicola regions showed signs of livestock use. Additionally, Leupin et al. (1994) speculated that trampling of newly metamorphosed toadlets by cattle might be an important potential source of mortality. 

Preliminary results of ecotoxicological studies on pesticide exposure and spadefoot egg development in the Okanagan Valley during 2003 – 2006 indicate that where pesticide concentrations are highest in pond water, hatching success is lowest (Ashpole et al. 2006b.). In the South Okanagan, a large population of S. intermontana has been reported from a waste-water effluent pond (St. John 1993), but reproductive success and survivorship patterns at this site are unknown.

Nussbaum et al. (1983) suggested that irrigation projects may have benefited the species over much of its range by providing dependable breeding ponds in otherwise dry areas. However, in the South Okanagan,water is often removed from the ponds before spadefoot tadpoles have completed their development, and these ponds act as ecological sinks (C. Bishop, pers. comm.). Agricultural projects also have serious effects on the amount and quality of foraging habitat available to spadefoots by converting shrub steppe habitat to monocultures of agricultural crops.

Introduced species

Where S. intermontana breeds in permanent water bodies, it is vulnerable to predation by sport fish, which are a serious threat to many amphibians throughout British Columbia (reviewed in Wind 2005). Non-predatory, introduced fish can also pose a threat to amphibians through habitat modification, competition for food, and as vectors for disease. The introduced Bullfrog (Rana catesbeiana) has been reported from a few localities in the South Okanagan (Ashpole et al. 2005b, 2006c) and could be a threat to native amphibians if allowed to spread. Although these ponds are not known breeding sites of spadefoots, the species has been caught in fence-traps surrounding ponds occupied by bullfrogs (C. Bishop, pers. comm.). In 2006, bullfrogs were found in the Okanagan Lake for the first time, raising concerns that the species might be more widely spread than previously thought and that eradication might be very difficult (Ashpole et al. 2006c).

Foraging habitat and hibernation sites

The quality of habitat in remaining grasslands within the species’ range in British Columbia is difficult to address. Little is known about the foraging needs of Spea intermontana, and the effects of grazing or other disturbance on terrestrial habitat are unstudied. Almost nothing is known about the hibernating and short-term retreat sites used by Spea intermontana. Concern has been raised over the impact of soil compaction by cattle trampling, which would reduce the water content of the soil as well as make it difficult for burrowing. Effects of ground compaction associated with orchard and vineyard development are also unknown.

Spadefoots are able to burrow into a variety of substrates (Oaten 2003), but coarse gravel and sod substrates hinder burrowing at least in Scaphiopus holbrookii (Jansen et al. 2001); burrowing is essential for survival of spadefoots in the terrestrial habitat.

Fragmentation of habitat and road mortality

Fragmentation of foraging habitat has almost certainly disrupted traditional dispersal routes used by spadefoots between breeding, foraging, and hibernating sites. Increased road density and the amount of traffic on these roads could significantly increase spadefoot mortality in British Columbia (M. Sarell, pers. comm.). The spadefoots are particularly vulnerable to road mortality during mass migrations to and from breeding sites, and many reports of road mortality exist from several sites (Leupin et al. 1994; Sarell 2004; records compiled for this report). During surveys by Ashpole and the Canadian Wildlife Service in the South Okanagan, spadefoots were regularly observed crossing or attempting to cross HWY 97, while moving from higher elevations to breeding ponds in the valley in the spring (C. Bishop, pers. comm.). They were frequently observed feeding and thermoregulating on roads. Both habitat fragmentation and road mortality are difficult to quantify, and their significance to local populations remains unstudied.

Disease and parasites

Disease outbreaks have not been documented for S. intermontana, but epidemic disease must now be regarded as a serious potential threat to all amphibian populations. Chytridiomycosis is an emerging infectious disease of amphibians that has been linked to precipitous declines and extirpations of many species worldwide (Daszak et al. 1999, Speare 2005), Lethal iridoviruses have also been associated with amphibian epizootics (Daszak et al. 1999). In the South Okanagan, the Tiger Salamander (Ambystoma tigrinum) has suffered mass mortality at least at one site within the past decade, possibly as a result of disease (Sarell 2004). S. intermontana and A. tigrinum sometimes use common breeding sites. Fungal infections may have a significant effect on the survival of eggs (Nussbaum et al. 1983). Bragg and Bragg (1957) reported a die-off of Spea bombifrons tadpoles in a temporary pond due to the water mould Saprolegnia but they felt it was a rare event. Saprolegnia can be carried from hatchery fish to amphibians (Kiesecker et al. 2001), a potential concern where spadefoots breed in permanent water bodies or their immediate vicinity.

Climatic factors

As a dry-land amphibian at the northern edge of its range, S. intermontana requires a delicate balance of climatic variables. A series of hot, dry summers could cause a population decline through the reduction of low elevation breeding sites but conversely could open up new habitats at higher elevations. A series of cool, wet summers might have a negative effect through the reforestation of grassland habitat, reducing the extent and quality of foraging habitat and likely increasing competition with other anurans. Compounding effects of human water use, water tables in the Okanagan basin are predicted to drop as a result of climate change, and small ponds used by amphibians may experience premature or complete drying (Cohen et al. 2004, Graham 2004).

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