Basin Spadefoot the Common Named

¶ … Basin Spadefoot

The common named Great Basin Spadefoot is a ranked species in the animalia kingdom, and is known as Scaphiopus hammondi intermontanus and Scaphiopus intermontanus, Cope 1883 (Spea pp). The Taxonomic Hierarchy is as follows: Kingdom Animalia, Phylum Chordata, Subphylum Vertebrata, Class Amphibia Linnaeus, Subclass Lissamphibia, Superorder Salientia, Order Anura Merrem, Family Scaphiopodidae Cope (Spea pp).

The Great Basin Spadefoot is primarily a species of the Columbia Plateau Ecoregion, however the range extends into the Okanogan Ecoregion, and there has been a single report of a tadpole found in the Canadian Rockies Ecoregion from Stevens County across the Columbia River from Hudson (Great pp). They can be found from south British Columbia to California, and east to Colorado and northwest New Mexico (Great1 pp). Most observations in the Okanogan Ecoregion have been from the Columbia, Methow, and Okanogan river valleys (Great pp).

Great Basin Spadefoots live in forested areas and sagebrush flats, by digging burrows in loose soil or using the burrow of other animals (Great1 pp).

Although they occur mainly in shrub-steppe, a variety of aquatic habitats are used for breeding such as slow flowing springs, seasonal pools, irrigation ditches and ponds (Great pp). Primarily nocturnal, they can be occasionally found abroad in daylight foraging for insects and can sometimes be brought to the surface (Great1 pp). The adults forage at night for earthworms and insects, particularly ants, beetles, and grasshoppers and are especially active on rainy or damp nights (Great2 pp). Burrowing Owls, herons, crows, snakes and coyotes feed on the spadefoots (Great2 pp). Spadefoot tadpoles munch on algae and aquatic plants, and the occasional dead fish (Great2 pp). Although some spadefoot species have carnivorous larval morphs, a genetic variant, that eat brine shrimp and sometimes even their own kind, this behavior has not been detected in Great Basin Spadefoots (Great2 pp).

Spadefoots hibernate from October to early April, remaining dormant until warm weather and rain return, however during extremely hot and dry weather they retreat again to wait for more comfortable conditions (Great2 pp). They may travel long distances between foraging, breeding, and hibernation sites, yet little is known about their movement patterns (Great2 pp). Spadefoots emerge from hibernation in early April to breed, with the males gathering and calling at small ponds, and females joining them to mate (Great2 pp). The females lay hundreds of eggs, which attach to sticks and pebbles underwater and hatch within a week in cool weather, or as quickly as two days if it is warm (Great2 pp). The eggs are laid in small loose packets of ten to forty, approximately 15-20 mm long axis length (Great pp). Egg packets are irregular in shape with each egg distinguishable from the others, somewhat like a cluster of grapes (Great pp). Individual eggs can be easily separated from the mass and are small with the ovum and gel together measuring less than 5 mm in diameter (Great pp). The tadpoles transform into toadlets six to eight weeks after hatching and become mature in their second or third year, and may live up to ten years (Great2 pp).

Transformed spadefoots are nocturnal and completely terrestrial, and only return to water for breeding (Great pp). They can survive in arid climates by spending long periods of time buried under ground and are able to quickly bury themselves in loose soils by using their hind legs in a circular motion to back into the soil (Great pp). They are able to remain buried for a period of month and are able to tolerate high levels of water loss (Great pp). Activity is reported to be primarily associated with rains and periods of high humidity, although in many areas of the Columbia Basin, it is common to spot them on roads at night when precipitation is low (Great pp).

Great Basin Spadefoots are not dependent upon vegetation for cover, and although fire would alter species composition of their primarily arthropod prey base, overall numbers of arthropod prey would most likely not change (Biota pp). Because they are not dependent on any particular anthropod species as prey, they would be able to find food in a post-fire environment (Biota pp). Moreover, due to runoff, nutrient levels of breeding pools may actually increase after fire, thus benefiting tadpoles by encouraging growth of bacteria, algae, and other tadpole foods, yet, high levels of sediment may adversely impact tadpoles by reducing oxygen levels (Biota pp). Therefore, even if fire does render breeding pools in a given basin inhospitable to tadpoles, fire is not likely to have a serious impact on the Great Basin Spadefoot population of the basin (Biota pp).

The Great Basin Spadefoot reaches the northern limit of its distribution in the dry valleys of southern Interior British Columbia, and althought he total numbers in this area are probably higher than 10,000, the population trends are unknown (Great3 pp). They are on the provincial Blue List, which means it is a species that is considered vulnerable to human actions, and has been designated Special Concern by the Committee on the Status of Endangered Wildlife in Canada (Great2 pp). Because human beings also enjoy living in warm dry climates, the spadefoot's habitat is under great pressure, since the dry grassland habitat is one of the rarest types in British Columbia, accounting for only six percent of the province's land area (Great2 pp). Spadefoots are restricted to those areas that have access to breeding pond, and only three such ponds were found to contain over half the total population of calling males, however two of these ponds are protected (Great2 pp). Their number is believed to be declining due to the loss of breeding and foraging habitats in the Okanagan Valley (Great2 pp). Since breeding and foraging sites must be connected by movement corridors to be of use to spadefoots, fragmentation of the habitat is a serious concern (Great2 pp). Furthermore, grazing cattle may compact the soils, thus making it difficult for the spadefoots to burrow, as well as having a detrimental effect on water quality in breeding ponds (Great2 pp). Most disturbing however, is the intensive human demands on water resources in the Canadian range of the Great Basin Spadefoot which have lowered the water table significantly at numerous sites and reduced the number of breeding ponds (Great2 pp).

Many areas of the Columbia Basin exist where no Great Basin Spadefoots have been recorded in the Washington Department of Fish and Wildlife Herp database (Great pp). The last records from Spokane, Garfield and Asotin counties were in 1937, 1958 and 1947 respectively (Great pp). Although no obvious threats exist at this time, lack of systematic documentation at sites where they were historical present makes interpretation difficult (Great pp). Conversion of shrub-steppe, that contained seasonal aquatic habitats historically, provides some justification for refining the basis of existing information (Great pp).

Within the United States, the Great Basin Spadefoot lacks special state or federal status (Great pp). They occur throughout the Columbia Basin and are locally common in many areas, and within Washington state, there have been no declines documented (Great pp). Spadefoots apparently can tolerate some habitat alteration, which often persists in irrigated agricultural lands (Great pp). Moreover, it is believed that they may have actually increased in abundance due to the prevalence of breeding sites provided in some areas by irrigation water, however no systematic surveys have been conducted to document such patterns (Great pp).

This species is a small, rotund amphibian, that is grey or olive green in color, with very large, golden yellow eyes that are set on the sides of the head with vertical pupils (Great2 pp). The tympana, ears, are small and inconspicuous and they have a bump between the eyes which give the head a distinctive shape (Great2 pp). Adults are 4 to 6.5 centimeters long, and the females are generally larger than the males (Great2 pp). Limbs are short and stubby and the body is plump, thus when a spadefoot is sitting still on the ground it gives the impression of being a large pebble (Great2 pp). They have bumps, or tubercles, on the skin that are small and dark brown or reddish in color, and the skin also has other spots and patches of color that are not raised (Great2 pp). The spadefoot has light-colored stripes down the sides of its back, and the skin on the stomach is pale (Great2 pp). On the first toe of each hind foot is a small, black spade, which is its most distinctive feature, and hence is the source of its name (Great2 pp). This hardened tissue gives them the ability to dig into loose soil for shelter (Great2 pp). The spade and the cat-like vertical pupils are what set the Great Basin Spadefoot apart from the Western Toad, which has horizontal pupils and distinct paratoid glands that appear as large swellings at the back of the jaw (Great2 pp). To attract females during the breeding season, males use a call that sounds like "gwaaa, gwaa" and will also use this call in response to other males, who join in forming a chorus that can be heard several hundred meters away (Great2 pp). These calls are done in a rapid series of low-pitched throaty notes (Great1 pp).

A study titled, "A Comparative Analysis of Plasticity in Larval Development in Three Species of Spadefoot Toads," reported by David Reznick in the June 01, 2000 issue of Ecology, evaluated four salient features of the Wilbur and Collins (1973) model for amphibian metamorphosis (Reznick pp) H.M. Wilbur and J.P. Collins offered an evolutionary explanation for the labile nature of amphibian metamorphosis (Reznick pp). Their model has provided the most important framework for interpreting phenotypic plasticity in age and size at metamorphosis (Reznick pp). This model is attractive due to its simplicity, and the fact that it focuses on selection at the larval life stage, is time invariant, and ignores complex relationships between larvae and their predators (Reznick pp).

Reznick study performed an experiment on three species of spadefoot toads derived from environments that differ in their degree of ephemerality, in order to evaluate the existence of a threshold, or minimum size, for attaining successful metamorphosis and to evaluate the influence of growth rate, mass, and stage of development on the definition of this threshold (Reznick pp). They further characterized the rate of development after the threshold and the nature of differences in the threshold and post-threshold development among species (Reznick pp).

According to the Reznick study, the threshold in larval development exists in all three species that separates an early larval period during which the larvae are not able to metamorphose due to a decline in food availability and a later period in development when they can metamorphose (Reznick pp). After attaining the threshold, larvae respond to a cessation of feeding by speeding up the rate of development and metamorphosing at an earlier age, and relative to fed controls, are smaller in size (Reznick pp). This response is consistent with the idea that such plasticity is adaptive since it will result in an earlier escape from a deteriorating environment (Reznick pp). After the threshold is exceeded, the degree of delay is relative to the increases with food availability and growth rate, thus rapidly growing individuals remain in the favorable environment longer and grow to a larger size (Reznick pp). Larvae that were exposed to a cessation in food availability after exceeding the threshold continued to respond by accelerating the rate of development throughout the remainder of the larval period (Reznick pp).

In all areas, the responses of Scaphiopus larvae to a cessation of feeding are consistent with the predictions of the Wilbur-Collins model for adaptive plasticity in amphibian metamorphosis (Reznick pp). The net result being that the larvae will metamorphose at an earlier age if they encounter a decline in growth opportunity, if they have exceeded a critical threshold (Reznick pp). One consequence is that they metamorphose at a smaller body size, and if the growth environment is favorable, then they will extend their period of development for a longer period, take advantage of the greater growth opportunities, and metamorphose at a larger body size (Reznick pp).

Reznick's results further demonstrated that the properties of the threshold differed among species in a way that is consistent with its being an adaptation to ephemeral environments (Reznick pp).

Reznick assumed that the growth rate was a good index of environmental quality and that a reduction in growth rate should serve as a good general cue of the degradation of the larval environment (Reznick pp).

The issue was whether simply removing all food was a reasonable surrogate for the signal that a tadpole would receive in nature (Reznick pp). A recent study on Scaphiopus hammondi suggests that this is true (Reznick pp). Researchers simulated a drying environment by either reducing water depth or by inserting netting and thus restricting the larvae to being close to the surface (Reznick pp). All of the experiments were conducted in a constant temperature room, so that water depth was not confounded with temperature (Reznick pp). Either treatment resulted in a rapid cessation of feeding, caused by a hormonal stress response (Reznick pp). Researchers further observed a threshold with identical properties to Reznick's findings, yet with a slight shift in the Gosner stage associated with the threshold (Reznick pp).

Six earlier studies investigated the existence of thresholds for metamorphosis with a species, and in all six studies, growth rate was manipulated by switching larvae from a high food supply to a low food supply and vice versa at different ages, and found that there is a threshold in the ability of larvae to respond with changes in age and size at metamorphosis (Reznick pp). However, if larvae were switched to low food after the threshold, there was no change in the age at metamorphosis, yet there was a decline in the size (Reznick pp). But if larvae were switched to low food before the threshold, then they metamorphosed at a later age and smaller size (Reznick pp). Moreover, a switch from low to high food caused earlier metamorphosis at a larger size if the switch was made before the threshold, yet only larger size if it was made after the threshold (Reznick pp).

Reznick found that the majority of the information for predicting whether or not an individual larva will respond to a loss of growth opportunity by successfully completing larval development is contained in the two correlated variables, body mass and developmental stage (Reznick pp). The relative contributions of these two variables varies across a range of growth rates, with body mass contributing more at high growth rates and developmental stage contributing more at low growth rates (Reznick pp). Although the two variables are often highly correlated, and it would seem that either one should be equally suitable as a measure of the threshold, this is, however not the case for two reasons (Reznick pp). First, the threshold stage of development is progressively earlier in faster growing tadpoles, indicating that size and/or condition play an important role, and second, body mass becomes a poorer predictor of the threshold in extremely low growth environments, therefore making developmental stage more important in those conditions (Reznick pp). Thus, the threshold is defined by both the body mass and the developmental stage (Reznick pp).

Analyses of patterns of age and size at metamorphosis would be greatly facilitated if an external morphological marker for the threshold condition could be found (Reznick pp). Some researchers have suggested that the threshold is marked by the asymptote of the growth curve, and this is probably a fairly accurate marker when growth rate is very low, but is clearly a very poor marker of the threshold when growth is high (Reznick pp).

Within the mountains that divide Saugus from Canyon Country are the Cruzan Mesa vernal pools where the endangered Riverside fairy shrimp flourish (Aidem pp). According to a study by the Los Angeles County Department of Regional Planning, the surface water in these pools is home to two plants on the federal endangered species list, as well as the spadefoot toad (Aidem pp). And in the Great Basin-Mojave Desert region, amphibians are one of the rarest animal groups because of their high water requirements, however four native species of frogs and toads are widely distributed throughout, including the Great Basin spadefoot, western toad, Pacific chorus frog, and northern leopard frog (Great4 pp). Of the twenty-two amphibian species, ten, or forty-five percent, are species of concern or candidates for federal listing, and several others seem to be declining (Great4 pp). According to recent surveys in northern Nevada, leopard frogs, spotted frogs, western toads, chorus frogs, and spadefoots are now difficult or impossible to find, yet they had been abundant earlier in the century (Great4 pp).