Great Basin spadefoot (Spea intermontana) COSEWIC assessment and status report: chapter 6
Biology
Little specific information exists on the biology of S. intermontana in British Columbia, and the account below is supplemented with anecdotal observations and by studies in the United States on this and related species of spadefoots. Extrapolations to S. intermontana in British Columbia, however, must be carried out with caution, as habitats and conditions at the northern limits of their range are different from those farther south. In British Columbia, Leupin et al. (1994) reported on the distribution and life history of the species in the Thompson-Nicola regions and St. John (1993) in the South Okanagan Valley. Since 1998, work on the species has focused mainly on inventories within various parts of the species’ range (Sarell et al. 1998, Sarell and Alcock 2004, Rebellato 2005, Tarangle and Yelland 2005, Ashpole et al. 2006a, R. Weir, pers. comm.). Oaten (2003) conducted a laboratory study on burrowing behaviour.Ashpole et al. (2006a,b) studied species occurrence, early embryonic development, and pesticide exposure in the South Okanagan; this study is ongoing.
Life cycle and reproduction
Like other northern anurans, spadefoots have a biphasic lifecycle, consisting of aquatic eggs and tadpoles and terrestrial adults and juveniles. In British Columbia, adults begin to emerge from hibernation in early to mid-April and move quickly to breeding ponds where males begin to call (St. John 1993, Leupin et al. 1994). Females soon follow or may move to ponds at the same time as males (M. Sarell, pers comm.). Brown (1989) states that the breeding season in the northern parts of the species’ range lasts from April to June. At any given site, the length of the breeding season, as measured by the presence of calling males, can vary from one month to less than a week (St. John 1993). Sites occupied earlier in the season had calling spadefoots longer than those occupied later in the summer (St. John 1993). Females lay 300 – 800 black eggs (Stebbins 1951, Leonard et al. 1993) in clusters of 20 – 40 (Nussbaum et al. 1983), attached to sticks, pebbles, or aquatic vegetation in shallow water. Leupin et al.(1994) reported clusters of 4 – 5 eggs attached to submerged stems of foxtail barley (Hordeum jubatum) and alkali grass (Puccinellia nuttalliana) in the Thompson-Nicola regions, British Columbia.Surveys by Ashpole and Canadian Wildlife Service in 2003-2006 found clusters of 10 – 110 eggs, usually attached to sticks (C. Bishop, pers. comm.).
Spadefoot tadpoles have the shortest developmental time of all anurans (Buchholtz and Hayes 2002), an adaptation that allows them to effectively exploit ephemeral pools of water. Eggs of S. intermontana hatch in 2 – 3 days during warm conditions but can take 7 days or more in cool water (Nussbaum et al. 1983). Eggs in Nevada successfully hatched despite overnight low air temperatures of 0 – 7°C (Nussbaum et al. 1983). Under laboratory conditions, development of S. intermontana from egg to complete metamorphosis took 36 days at 23°C, but toadlets could leave the water at just over 28 days of age (Brown 1989). Other sources, probably referring to field data, give the larval development time as 6 to 8 weeks (Nussbaum et al. 1983, Green and Campbell 1984). Bragg (1961) observed eggs of Scaphiopus holbrookii, Bufo terrestris, and Rana pipiens laid simultaneously in a pond. The Scaphiopus eggs hatched in three days, the Bufo eggs in five days and the Rana eggs in seven, while the entire aquatic phase took 35 days, 55 days, and about 60 to 70 days, respectively. Environmental factors shown to accelerate development in at least some species of spadefoots include a reduction in water volume or food availability (Boorse and Denver 2003).
In British Columbia, most tadpoles are seen in May and most metamorphosed toadlets appear in July. If a pond begins to dry up, older tadpoles can accelerate metamorphosis to some extent but at the expense of attaining larger body size; small spadefoots (Scaphiopus couchii) lose water more quickly than larger ones (Newman and Dunham 1994). Young S. intermontana averaged 20.5 mm in snout-vent length at metamorphosis (Brown 1989) and often still have a substantial tail (18 mm) when they leave the water (Nussbaum et al. 1983). Bragg (1964) stated that Scaphiopus larvae “always emerge in nature before the tail has been visibly changed at all.” There is much variability in age and body size of S. intermontana at metamorphosis, and body size increases with increased larval food levels (Morey and Reznick 2004). Body size at metamorphosis affects survival of S. hammondii over the first year of life; in enclosures, larger metamorphs were more likely to survive to 1 yr of age than smaller juveniles (Morey and Reznick 2001). However, the size difference of surviving juveniles from different tadpole densities was negligible after one year and did not affect growth rates of older juveniles. There are no data on survivorship of S. intermontana.
Males become sexually mature at about 40 mm and females at about 45 mm SVL; they can reach this size in their second or third year (Nussbaum et al. 1983, Green and Campbell 1984). Spea intermontana may not breed each year if conditions are unsuitable (Leupin et al. 1993), but no information on frequency of breeding is available for British Columbia populations. The maximum longevity of S. intermontana is unknown, but for Scaphiopus couchii it is about 13 years for females and 11 years for males (Tinsley and Tocque 1995).
Physiology
Spadefoots have a variety of physiological adaptations for living in a dry environment, including the ability to survive the loss of up to 48% of their body mass in water, compared to a tolerance of only 31% in the frog genus Rana (Nussbaum et al. 1983). While the permeability of the amphibian integument is often thought of as a detriment to life in an arid environment, it does allow spadefoots to absorb water directly from the soil while burrowed (Ruibal et al. 1969). This is accomplished by accumulating urea in the blood plasma, thus reducing the water potential of body fluids to allow absorption of water through the skin.
Water quality is often highly variable in the small ponds used by spadefoots. While temperature tolerances have not been studied in Spea intermontana, Brown (1967) found that eggs of Spea hammondii near hatching could withstand temperatures of up to 39 or 40º C, but freshly laid eggs died at 37º C. Leupin et al. (1993) reported that while water temperature in small breeding ponds fluctuated greatly within a 24-hour period (from 33º C to 12º C on some occasions), there were no apparent effects on tadpole survival of S. intermontana. The species has been recorded at breeding sites with pH of 7.2 – 10.4 (Hovingh et al. 1985, Leupin et al. 1993). Anecdotal evidence suggests that Spea intermontana may be intolerant of the very high pH levels common to many ponds within the Canadian range of the species. In the Thompson-Nicola area, Dave Low (pers. comm.) has found spadefoots clustered around seepage areas where fresh water was entering small ponds. The pH was around 8.5 at the seepage sites and about 10 elsewhere in the pond where the species was absent. The plasma osmolarity of Spea hammondii tadpoles was consistently higher than that of bullfrog (Rana catesbeiana) tadpoles, an apparent adaptation to osmotic concentrations found in drying pools (Funkhouser 1977).
Terrestrial ecology and hibernation
Spadefoots occupy open, semi-arid to arid habitats not normally associated with aquatic anurans. They cope with this lack of water by burrowing underground and remaining dormant through dry and/or cold periods. They emerge when a combination of warm weather and wet soil (either the result of rainfall or snowmelt) provides suitable conditions for survival above ground, although Wright and Wright (1949) noted that they will emerge in the absence of rain if conditions are otherwise suitable.
Foraging occurs at night, especially during rain or when humidity is high (Hallock 2005, Matsuda et al. 2006). Spadefoots shelter underground during the day, thereby minimizing water loss. Svihla (1953) described diurnal retreats of Spea intermontana around breeding ponds in central Washington. Adults dug backward away from the pond in sand, leaving tracks and “pretzel-shaped ridges” marking their temporary burrows. Svihla also found many Spea intermontana that had burrowed under flat rocks, about 30 cm², and within 0.6 to 6 m from the breeding pond. Wright and Wright (1949) stated that adults could be surveyed by stamping on the ground near their breeding grounds, causing the animals to emerge from their shallow burrows. Homing to specific burrows has been reported for the Eastern Spadefoot, Scaphiopus holbrookii (Pearson 1955).
In laboratory experiments, Scaphiopus holbrookii burrowed most easily in sand and were unable to burrow in sod; recently metamorphosed juveniles were unable to burrow in gravel or sod (Jansen et al. 2001). Experiments with a small number of juvenile Spea intermontana from British Columbia, showed they could burrow into sandy clay loam, fine gravel, sand, and Brown Chernozemic soils prevalent in the grassland habitats, but tended to prefer sandy clay loam and gravel over clay and sand (Oaten 2003). The toadlets showed no preferences for substrates with pre-existing holes (Oaten 2003), but rodent burrows and crevices may be important in areas with coarse or compact substrates (Sarell 2004).
Not surprisingly, hibernating individuals probably burrow to relatively great depths. In other species of spadefoots, Ruibal et al. (1969) found that hibernating Spea hammondii burrowed to an average depth of 54 cm (n=6) in February in southeastern Arizona; the maximum depth observed was 91 cm. Some spadefoot species can obtain enough energy in as few as seven feedings (Spea multiplicata) or even just one feeding (Scaphiopus couchii) to survive a year of dormancy (Dimmit and Ruibal 1980), but similar data are not available for Spea intermontana. Scaphiopus couchii and Spea hammondii can remain dormant for two or more years waiting for suitable foraging and breeding conditions (Seymour 1973), but it is unknown whether Spea intermontana has this ability.
Dispersal and migrationmovements
Spadefoots undertake seasonal migrations between terrestrial foraging and hibernation habitat and aquatic breeding habitat. There is very little information on movement distances of S. intermontana in British Columbia or other areas. Based on anecdotal information on spadefoots in general, Hammerson (2005) noted that spadefoots will move several hundred m or more from breeding sites and thus, in the absence of more specific information, it can be assumed that spadefoots use terrestrial habitat up to a minimum of 500 m around breeding sites and probably up to 1 km, depending upon the terrain.
Dispersal from natal ponds occurs en masse (Hallock 2005). For example, in British Columbia hundreds of small, newly metamorphosed spadefoots were seen crossing Inkaneep Road near Oliver on a wet night in late July 1990 (pers. obs. by R. Cannings). In the Eastern Spadefoot (Scaphiopus holbrookii) movements of metamorphs from natal ponds was rapid and completed within a week from transformation (Greenberg and Tanner 2004). There are no data on dispersal distances at any life history stage.
Diet and predation
Adults of Spea intermontana feed on a variety of invertebrates, including earthworms, ants, beetles, crickets, grasshoppers, and flies (Nussbaum et al. 1983, M. Sarell, pers. comm.). Bragg (1956) found that adults of the similar Spea bombifrons ate a wide variety of insects, including flies, small wasps, moths and beetles.
Larvae are voracious feeders on algae and aquatic plants and scavenge dead fish and even their own feces (Green and Campbell 1984). Bragg (1960) successfully raised Spea bombifrons tadpoles on boiled lettuce. Martha Hett (pers. comm.) found that S. intermontana tadpoles living in a garden pool in Vernon readily ate boiled lettuce and frozen bloodworms (midges).
In the South Okanagan, cannibalism among tadpoles of S. intermontana was frequently observed in ponds where densities were high (C. Bishop, pers. comm.).
A specialized carnivorous morph has been described for tadpoles of other spadefoot species (Spea bombifrons: Orton 1954, Bragg 1956, 1964, 1965, Bragg and Bragg 1959; Spea multiplicata: Pfennig 1990) but has not been documented for S. intermontana. Carnivorous S. bombifrons tadpoles are characterized by larger heads and more serrated beaks, enlarged jaw muscles, and modifications of the labial denticles. This morph apparently eats other tadpoles, including conspecifics (Orton 1954), and fairy shrimp, Artemia (Farrar and Hey 1997). Carnivorous S. multiplicata tadpoles have larger heads and mouths and specialize in eating fairy shrimp. This morph has an extremely rapid development (12 days) and is favoured in ephemeral water bodies with high densities of fairy shrimp (Pfennig 1990).
Adults are eaten by snakes (such as the Western Garter Snake, Thamnophis elegans) and burrowing owls (Athene cunicularia; Leupin et al. 1994), and likely by larger predators such as great blue heron (Ardea herodias) and coyotes (Canis latrans; Leupin et al. 1994, Leonard et al. 1993). Mike Sarell (pers. comm.) observed an adult Tiger Salamander (Ambystoma mavortium) eating an adult S. intermontana in captivity.
Tadpoles are likely eaten by ducks, and killdeer and ravens have been seen feeding on dying larvae in a drying pond (Leupin et al. 1994). Carp have been observed consuming tadpoles in the South Okanagan (C. Bishop, pers. com.). Black (1970) suggested that aggregations of Spea bombifrons tadpoles were an adaptation to avoid predation by cannibalistic tadpoles and water beetles, which attacked tadpoles at the periphery of the aggregations but not those in the centre of the group. Painted turtles (Chrysemys picta) may be important predators of tadpoles (St. John 1993). Adults have noxious skin secretions that appear to deter some predators (Stebbins and Cohen 1995, Matsuda et al. 2006).
Interspecific interactions
Leupin et al. (1994) noted a complementary pattern of breeding site occupancy between the Pacific Treefrog, Pseudacris regilla, and S. intermontana in the Thompson-Nicola area: at several sites containing suitable habitat for both species, only one species was abundant, while the other was either absent or occurred in very low numbers. They suggested that this pattern might have resulted from interspecific competition and possibly from acoustic interference by the louder treefrogs. Surveys in the South Okanagan by Ashpole and the Canadian Wildlife Service found a similar negative association of the two species in breeding ponds (C. Bishop, pers. comm.). Leupin et al. (1994) found that the Western Toad (Bufo boreas) usually far outnumbered S. intermontana at sites where the two co-occurred and suspected that interspecific competition for food may occur among tadpoles. Spadefoots may be excluded from permanent ponds that have high painted turtle populations (M. Sarell, D. St. John, pers. comm.).
Adaptability
Spadefoots are able to use a variety of open and semi-open habitats and breed opportunistically in a wide range of aquatic habitats, including very small pools and artificial ponds (Leupin et al. 1994, Sarell 2004). Seasonal migrations to and from breeding sites and reliance on naturally scarce water bodies for breeding in arid habitats increase the vulnerability of S. intermontana in fragmented, human-modified landscapes.These characteristics affect their ability to persist in human modified landscapes provided that key features of both the aquatic and terrestrial habitat are retained. In the United States, the species persists in some areas within the Columbia Basin that have been converted into irrigated agricultural land (Hallock 2005). However, David Cunnington (pers. comm.) has observed that ponds created by irrigation run-off may serve as ecological traps for spadefoots. A large number of animals were observed breeding in a run-off pond in a hayfield but once the hay was harvested the pond rapidly dried, destroying the entire cohort of tadpoles.
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