COSEWIC Assessment and Status Report on the Great Plains Toad Anaxyrus cognatus in Canada – 2010

Table of Contents

Document Information

List of Figures

List of Tables


Document Information

Great Plains Toad

Illustration of the Great Plains Toad Bufo cognatus.

Special Concern – 2010

COSEWIC - Committee on the Status of Endangered Wildlife in Canada

COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows:

COSEWIC. 2010. COSEWIC assessment and status report on the Great Plains Toad Anaxyrus cognatus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 54 pp.

Previous report(s):

COSEWIC. 2002. COSEWIC assessment and status report on the Great Plains Toad Bufo cognatus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. v + 46 pp.

Didiuk, A.B. 1999. COSEWIC status report on the Great Plains Toad Bufo cognatus in Canada in COSEWIC assessment and status report on the Great Plains Toad Bufo cognatus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 1–46 pp.

Production note:
COSEWIC would like to acknowledge Janice James for writing the status report on the Great Plains Toad Anaxyrus cognatus in Canada, prepared under contract with Environment Canada. This report was overseen and edited by Ronald J. Brooks, Chair of the COSEWIC Amphibians and Reptiles Species Specialist Subcommittee.

For additional copies contact:

COSEWIC Secretariat
c/o Canadian Wildlife Service
Environment Canada
Ottawa, ON
K1A 0H3

Tel.: 819–953–3215
Fax: 819–994–3684
E–mail
Website

Également disponible en français sous le titre Évaluation et Rapport de situation du COSEPAC sur le crapaud des steppes (Anaxyrus cognatus) au Canada.

Cover illustration/photo:
Great Plains Toad -- Andrée Jenks, Guelph, Ontario.

© Her Majesty the Queen in Right of Canada, 2010.
Catalogue CW69–14/339–2010E–PDF
ISBN 978–1–100–15928–7

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COSEWIC Assessment Summary

Assessment Summary – April 2010

Common name
Great Plains Toad

Scientific name
Bufo cognatus

Status
Special Concern

Reason for designation
This species is widespread but has a scattered distribution of mostly small populations that fluctuate in numbers. It almost meets criteria for Threatened and could become Threatened because of ongoing loss and degradation of habitat, particularly loss of intermittent wetlands from cultivation, oil and gas development and increase in droughts. These threats increase fragmentation of populations and jeopardize their persistence.

Occurrence
Alberta, Saskatchewan, Manitoba

Status history
Designated Special Concern in April 1999. Status re–examined and confirmed in May 2002 and April 2010.

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COSEWIC Executive Summary

Great Plains Toad Anaxyrus cognatus

Species information

The Great Plains Toad is endemic to the North American prairies. Recently, the Great Plains Toad has been reassigned from the genus Bufo to a new genus, Anaxyrus. These toads are distinguishable from other species of toads in Canada by their relatively large size (47–115 mmsnout–vent length (SVL) adults), ‘L’ –shaped cranial ridges behind the eyes, and dark paired blotches with light borders on a grey, light–brown or olive–coloured back. Nocturnal and fossorial tendencies make the species difficult to document and monitor. The extremely loud call described as a ‘harsh explosive clatter resembling a jackhammer’ is easily discernible from those of all other amphibians that co–occur in Canada.

Distribution

The species reaches the northern periphery of its global range in the southern parts of the Prairie Provinces in Canada. Within Canada, records are most numerous in Alberta, with scattered records also existing across southern Saskatchewan and in the extreme southwestern corner of Manitoba. The distribution of the Great Plains Toad extends southwards from these areas, through the North American grasslands, to south–central Mexico.

Habitat

Great Plains Toads are associated with the grassland biome of North America. Adults are predominantly terrestrial and fossorial. Ephemeral pools are used as breeding habitat. Although they occur in some cultivated areas, most records for the species in Canada are associated with native grassland.

Biology

Great Plains Toads spend the majority of their lives underground. They emerge to breed and feed at night during the active season (April–September), especially in warm, humid conditions. The toads overwinter by burrowing below the frost line. Breeding occurs in spring in shallow temporary pools and is often stimulated by heavy rainfall. Individual toads may move more than 1 km during the active season.

Population sizes and trends

Population sizes and trends are unknown. Great Plains Toads are distributed in clusters at somewhat low densities. Reproductive success is highly variable from year to year and region to region. Under wet conditions, the toads may reproduce in enormous numbers, although they may not breed at all during drought years. Numbers of adults fluctuate widely from year to year reflecting this variation in precipitation during previous breeding seasons. In some parts of the species’ Canadian range, populations have become scattered and isolated. Populations are thought to be stable or declining across the entire range.

Limiting factors and threats

Cultivation, road mortality, herbicide and pesticide use, and oil and gas exploration pose threats to Great Plains Toad populations in Canada. Widespread use of herbicides and pesticides is thought to contribute to mortality by direct poisoning, through ingesting contaminated prey, and by reduced fitness. Intensive use of temporary wetlands by livestock may also reduce embryonic and tadpole survival due to contamination of the water with feces and trampling and the resultant suspension of sediment. Overwinter mortality from freezing is likely a limiting factor.

Special significance of the species

Populations of the Great Plains Toad in Canada represent relicts of the original fauna of the prairies and are important contributors to biodiversity of the grasslands biome. The unique physiological and behavioural adaptations of Canadian populations to the northern climate possibly distinguish them from members of the same species farther south. Their loud rattling breeding calls are distinctive.

Existing protection

COSEWIC and SARA designated Great Plains Toads as Special Concern in 2002. The General Status of Species in Canada (2005) lists the Great Plains Toad as “Sensitive” in Canada, as “May be at Risk” in Alberta and “At Risk” in Manitoba and Saskatchewan. The species is ranked as S2 in Alberta, S3 in Saskatchewan, and S2S3 in Manitoba by the respective provincial conservation data centres. No special protection is afforded to the species in Alberta and Saskatchewan, although it is considered protected under the Endangered Species Act in Manitoba.

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal–Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non–government science members and the co–chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2010)

Wildlife Species
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)
A wildlife species that no longer exists.

Extirpated (XT)
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)
A wildlife species facing imminent extirpation or extinction.

Threatened (T)
A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)*
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)**
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)***
A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
** Formerly described as “Not In Any Category”, or “No Designation Required.”
*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

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COSEWIC Status Report on the Great Plains Toad Anaxyrus cognatus in Canada – 2010

Species Information

Name and classification

The Great Plains Toad Anaxyrus cognatus Say belongs to the family Bufonidae of ‘true toads’ in Class Amphibia. All North and South American toads have long been classified together within the genus Bufo. Pauly et al.  (2004), in a recent examination of the global biogeography of Bufo suggested that all Nearctic (Greenland, Canada, United States to central highlands of Mexico) toads originated from an early invasion by toads from what is now South America. Following this publication, a separate investigation into the phylogeny of amphibians has recommended the division of the genus Bufo into a number of taxa (Frost et al. 2006). As a result, Bufo cognatus became a member of the new genus Anaxyrus (Frost et al. 2006). The latest version of the Standard Names List (Crother et al.  2008) has incorporated the change from Bufo to Anaxyrus.

The Great Plains Toad was first described by James (1823:190). The type locality is in “the alluvial fans of the [Arkansas] River” in Prower County, Colorado. The epithet ‘cognatus’, according to Krupa (1990), means ‘related or kindred’, which reflects its similarity in both appearance and habitat to B. musicus [= woodhousii and B. fuscus] (from James 1823). No subspecies are recognized (Krupa 1990). Anaxyrus cognatusis considered a distinct and readily identifiable species (Minton 2005).

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Morphological description

Great Plains Toads are relatively large, ranging from 47–103 mm in snout–vent length (SVL) for adult males and 49–115 mm SVL for adult females (Krupa 1990). Breeding females are generally larger than males (Bragg and Weese 1950). Dorsal colouration varies from a background of grey, light brown, or brownish–yellow, to olive (Krupa 1990; Russell and Bauer 1993; Stebbins 2003; Fig. 1). The toads are covered with irregular, dark paired blotches, which are outlined by lighter borders. A light stripe may be evident along mid–back (Krupa 1990; Russell and Bauer 1993; Stebbins 2003). Ventral colouration is light or pure white (Krupa 1990; Russell and Bauer 1993). The well–defined cranial crests are ‘L’ shaped and form a ridge behind each eye, coming together between the eyes to form a boss (Stebbins 2003). The parotoid glands, directly posterior to the cranial crests, are oval–shaped and large (Krupa 1990; Russell and Bauer 1993). In males, a pale–coloured flap of skin covers the dark, sausage–shaped vocal sac in the throat area (Krupa 1990; Stebbins 2003). When inflated, the vocal sac is approximately one third of the body length and extends in a curl to the anterior of the face (Stebbins 2003). There are two dark tubercles on the hind feet, the innermost of which is sharp–edged (Krupa 1990; Russell and Bauer 1993; Stebbins 2003).

Tadpoles have an arched dorsal fin (Russell and Bauer 1993). They may reach 28 mm total length (Bragg 1937b). The distinctive ‘V’–shaped cranial crests are present even in young toads and numerous small brick–red tubercles may also be evident (Stebbins 2003).

Figure 1. A Great Plains Toad in Saskatchewan. Photo by Candace Neufeld, courtesy of Andrew Didiuk, CWS Saskatoon.

Photo of a Great Plains Toad in Saskatchewan partly concealed by grasses.

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Genetic description/spatial population structure

Although the Great Plains Toad has been extensively studied in the United States, where it is more common and widespread, no basic research has been conducted on the species in Canada. Even in the USA, most genetic information available was derived from ‘pet–trade’ individuals (Frost et al. 2006) or animals from the ‘…southwestern USA’ (Chan 2006; Rogers 1973; Zorisadday et al.  2003) and did not produce useful information on population structure/isolation.

Krupa (1994) noted that breeding adults in Oklahoma would return annually to the same breeding sites with remarkable consistency. A high degree of site fidelity could significantly affect the genetic makeup of populations and play an important role in shaping the smaller, more isolated populations found at the northern periphery of the species’ range. Genetic structure and degree of isolation among Canadian populations are unknown.

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Designatable Units

There is no genetic or other evidence of differences that could justify subdivision of the species into separate Designatable Units (DU). Therefore, the species is treated here as a single DU.

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Distribution

Global range

The Great Plains Toad is aptly named. From the prairies of southern Alberta, Saskatchewan and Manitoba, it occurs throughout the central Great Plains’ states of the USA, and well into Mexico, south as far as Aguascalientes (Krupa 1990) and San Luis Potosì (Krupa 1990; Russell and Bauer 1990; Stebbins 2003; Fig. 2). The species’ distribution extends as far west as southeastern California across to western Minnesota, central Missouri and Iowa (Krupa 1990) in the United States. Krupa (1990) published a detailed map of the species’ entire range; Graves and Krupa (2005) made available the most recent range map for the species in the United States; NatureServe (2009) provides a global range map by county.

Figure 2. Global range of the Great Plains Toad (Anaxyrus cognatus). From NatureServe (2009).

Map of the global range of the Great Plains Toad.

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Canadian range

In Canada, the Great Plains Toad is at the northern periphery of its global range (Fig. 3; Krupa 1990; Stebbins 2003). The species is primarily distributed east of Highway 36 and south of the Red Deer River in Alberta, across into southwestern Saskatchewan west of Regina, with scattered records across the southernmost remainder of that province. It is recorded only in the extreme southwest of Manitoba.

The earliest records in Alberta are from 1931 (Logier 1931; Logier and Toner 1961) and attributed to the Canadian Museum of Nature (Cook 1960; Alberta FWMIS2009). In 1960, Cook first documented Great Plains Toads in Saskatchewan. The species was first reported from Manitoba in 1986, when it was documented near Lyleton in the extreme southwestern corner of the province (Preston 1986). Approximately 5% of the global range of this species occurs in Canada.

Figure 3. Distribution of Great Plains Toad (Anaxyrus cognatus) in Canada. Data from Alberta FWMIS, Alberta NHIC, Andrew Didiuk (CWS) and Manitoba CDC.

Map showing the distribution of the Great Plains Toad in Canada.

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The Canadian distribution of the species is aggregated and primarily associated with areas of native prairie habitat. The distribution is best documented in Alberta, with limited information available from Saskatchewan and Manitoba (James 1998; Didiuk 1999a). In Alberta, Great Plains Toads are recorded from the Dry Mixedgrass subregion of the southeastern part of the province, with the bulk of relatively recent observations coming from the Canadian Forces Base (CFB) Suffield National Wildlife (NWA) area (Didiuk 1999b). Since the 1990s, many records have been added, most notably of a previously undocumented population locality near the town of Skiff.

All records in Alberta are east of 112° 08’ W longitude and south of 50° 58’ N latitude (Alberta NHIC database 2008; Alberta FWMISdatabase 2009). Currently there are 855 records of Great Plains Toads in the Alberta government database (Alberta FWMIS2009). Records in Alberta fall into four major regions (Fig. 3). The largest grouping of records covers the entire Suffield area, with the highest numbers of sites corresponding roughly to the Middle Sand Hills area in a broad southwest to northeast arc west of the South Saskatchewan River. It is bounded to the west by Highway 884 between the town of Suffield and Jenner, and stretches east to the Saskatchewan border. A notable population of Great Plains Toads was discovered at Many Islands Lake near Walsh in 2002, with additional sites between there and Highway 41 (Wershler pers. comm. 2009). Records in this region are all south of Highway 555 and the Red Deer River. The records in this grouping extend, in scattered clumps, south to the Highway 3 corridor.

The second largest group of records falls within the Tilley–Lake Newell–Vauxhall–Taber area. These records are bounded by Highway 36 to the west (with one historic exception) and Highway 1 to the north, and extend in a broad band to Highway 3 in the south. The records in this grouping are most numerous north of Highway 3 between Taber and Burdett.

The third group of records occurs near Onefour in the extreme southeastern corner of Alberta, south to the Montana border from the Milk River area to near the Wild Horse border crossing. The fourth and last major grouping of records is entirely recent and was discovered just west of the town of Skiff, within a highly cultivated area. This locality is smallest of all recorded Alberta groups. There is fossil evidence of the species having been distributed considerably farther to the north (Killam) in post–glacial times (Bayrock 1964; Krupa 1990).

In Saskatchewan, records are considerably fewer, and the distribution is more fragmented. There are 19 locality records in total (Didiuk pers. comm. 2008) with nine being historical (1959–1964) and 10 records more recent (1994–2008). The area along the Alberta border, west and south of the Great Sand Hills, has a number of records. The records near Elbow, to the northwest of Regina, are the most northerly known for this species (~510 04’ N). Lastly, there are two records along the North Dakota border, one central and one in the southeastern corner of the province. This latter record is perhaps indicative of some relationship with the Manitoba populations. The general dearth of records may reflect a lack of appropriately timed search effort for most areas.

In Manitoba, the area around the towns of Lyleton, Melita and Coulter, in the extreme southwestern corner of the province, constitute the entire known range. The 11 sites date from 1983 (initial record) to 1999 (MCDC 2008). The Manitoba/eastern Saskatchewan range may be contiguous in North Dakota; although no toads have been recorded in the U.S. counties along the border, almost all of the next set of counties to the south have records (Graves and Krupa 2005; NatureServe 2009).

Over most of the range of Anaxyrus cognatus in Canada, the species may be present, but due to limited investigation during appropriate conditions, the full extent of its occupancy may be under–documented (Didiuk 1999a). The extent of occurrence (EO) for this species in Canada has been calculated as 134,200 km² from the available records. The area of occupancy (AO) was calculated by placing each location in a 2x2 kmgrid. This gave an AO of 1276 km². However, it is likely this value will increase in the future as more locations are discovered.

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Habitat

Habitat requirements

Great Plains Toads are found in all types of grasslands throughout North America from short–grass to tall–grass prairie (Krupa 1990) and into desert areas (Stebbins 2003). They may also be found in creosote bush desert, mesquite grasslands and the desert riparian areas and desert ‘scrub’ habitats, such as sagebrush plains within the southern and western regions of the species’ range (Krupa 1990; Stebbins 2003). These toads have been reported at elevations up to 2400 m in Colorado (Hahn 1968). Following breeding in ephemeral wetlands, they move off to forage in surrounding upland sites (Ewert 1969).

In Canada, Great Plains Toads are exclusively associated with the grasslands of the southern Prairie Provinces. These xeric grasslands constitute the warmest and driest regions of the Prairie Provinces. The climate of the North American Great Plains is characterized by low rainfall, a high rate of evaporation, brief, hot summers, and long, cold winters. Mean summer temperature in the Mixedgrass ecoregion of the Prairie Provinces is 16°C with mean winter temperature being −10°C (ESWG 1996). Mean annual precipitation ranges from 250 to 350 mm (Environment Canada 1999). Wind is a dominant feature of the climate. Across the provinces, separate categorization systems have evolved to describe the different types of grasslands within the overall biome. The vast majority of Great Plains Toad observations in Canada fall within what is generally termed the Mixedgrass ecoregion (Environment Canada 1999). In southern Alberta, Great Plains Toads are found within the Dry Mixedgrass subregion (Natural Regions Committee 2006) and in southern Saskatchewan, within the Mixed Grassland ecoregion (SCDC 2007). In southern Manitoba, the species is found in the Aspen Parkland ecoregion (MCDC 2001).

The preference for clean, clear, shallow ephemeral pools and ditches of spring melt– and rainwater as breeding habitat is well documented for this species (Bragg 1937a, 1940, 1950; Bragg and Smith 1942; 1943). The importance of water cleanliness was emphasized by Bragg (1937a; 1940) who noted that fecal contamination of breeding pools by cattle in overgrazed pastures could result in mortality of egg masses. In the absence of rainfall, Great Plains Toads have been reported to breed in semi–permanent and permanent bodies of water, such as irrigation ditches in Arizona (Brown and Pierce 1967) and seepage pools from irrigation canals in Alberta (Wershler and Smith 1992). In Alberta, Wershler and Smith (1992) noted that these semi–permanent water bodies were especially significant during prolonged periods of drought, such as were documented in 1987 (Wallis and Wershler 1988) and 1990 (Wershler and Smith 1992). Wershler and Smith (1992) listed springs, irrigation projects, waterfowl management projects (i.e. Ducks Unlimited), and dikes in shallow drainages as more reliable breeding sites for this species than the shallow, temporary pools resulting from spring runoff and rains. Naugle et al. (2005a) recorded Great Plains Toads in all classes of wetlands surveyed in South Dakota: temporary, seasonal, semi–permanent, permanent, tillage, manmade (e.g. stock dams), and riverine wetlands. Brown and Pierce (1967) suggested that Great Plains Toads have benefited from the more reliable water sources provided by irrigation ditches, artificially flooded sites in fields, and cattle tanks. Historically, bison wallows may have been significant sites for temporary pool breeding amphibians such as Great Plains Toads (Gerlanc and Kaufman 2003). These toads avoid wetlands in wooded areas and those with cattails (Ewert 1969). Naugle et al. (2005b) emphasized the importance of ephemeral wetlands at the landscape level for facilitating periods of explosive growth in amphibian populations and renewing population levels, which facilitates survival through the next drought period. Reductions in the number of wetlands increase the distance between wetlands, isolating populations and increasing the probability of local extinctions (Semlitsch and Bodie 1998).

The quality of available breeding sites is important. Bragg (1940; 1950) noted that female Great Plains Toads will seek larger sites and will ignore males calling from unsuitable wetland sites. The suitability of breeding sites in Oklahoma was related to water depth, clarity, and length of persistence (Bragg 1950). Shallow (0.5–1.5 m depth), completely clear to semi–clear (visibility to 0.3 m) water, at temperatures between 11–20°C, in pools that dry up at least annually, were reported as preferred (Bragg 1950).

In surveys completed at the Suffield NWA (1994–1996) north of Medicine Hat, Alberta, breeding choruses of Great Plains Toads were found primarily affiliated with large, shallow, ephemeral pools, with a few calling males noted at the margins of dugouts -- and presumed to be making their way to the larger choruses nearby (Didiuk 1999a). Didiuk (1999a) also noted that steep–sided dugouts were entirely devoid of calling males unless connected to adjacent shallow, flooded depressions. Stock ponds, which are effectively small dams along drainage channels, were also noted to attract large numbers of calling males in the Suffield surveys, especially where extensive shallow water was available (Didiuk 1999a). Taylor and Downey (2002) found Great Plains Toad choruses near Onefour to be in “ephemeral ponds with clear water, approximately 1 m deep, with little or no aquatic vegetation”. Where emergent vegetation was present, toads were observed calling from “clumps of grasses and sedges” (Taylor and Downey 2002).

Wershler and Smith (1992) did not find any aggregations of Great Plains Toad in cultivated fields within irrigation areas. However, Didiuk (1999a) noted that successful metamorphosis of Great Plains Toads had been observed in wetlands within seeded pastures near the Suffield NWA in Alberta. Wershler (pers. comm. 2009.) found ponds in the Purple Springs/Bow Island area in 2002 that appeared to be within irrigated cultivation; however, upon closer inspection, at least some of these sites were within small remnants of native grasslands. Downey (2006) reported a small number of Great Plains Toads calling in the run–off pools and ditches in the “heavily cultivated” area between Wrentham and Skiff in 2005. In this area, the habitat used was within about 3 km of native coulee land (Downey pers. comm.  2009). A comparison of the distribution of records in Alberta (Fig. 3) with a map of the remaining intact native prairie (Fig. 4) shows that there is a distinct correspondence between the records and areas with a significant proportion of uncultivated, native lands. Ken DeSmet (pers. comm.  2008) noted that the records of Great Plains Toads calling in mid–May near Melita, Manitoba “…were primarily in low areas within cultivated fields, and …most or all of the water in the waterlogged fields was from spring meltwater.” The Manitoba area of the range is described as being a “mosaic of habitats, with native grassland, haylands and cropland” all interspersed (Downey pers. comm.  2009). Satellite images depict the range near Melita and Coulter in Manitoba as regions with considerably higher density of wetlands than surrounding areas. The area west of Lyleton appears in satellite images to be a region of largely uncultivated grasslands.

Soil type, especially friability, is also presumably of great significance to Great Plains Toads, as they dig shallow burrows for daily shelter and more deeply for overwintering (Ewert 1969). The relationship between given soil types and the distribution of Great Plains Toads remains unclear. Wershler (pers. comm.  2009) believes that “sandhill areas and sand plains are examples of surficial deposits that are important [for the species].” Ewert (1969) found Great Plains Toads sought out elevated areas such as roadsides within their individual home ranges for overwintering.

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Habitat trends

A thorough consideration of the status of available habitat is still premature. A general overview is provided here.

Cultivation has been, and continues to be, the most evident and significant of human impacts on all grassland habitats. Widespread cultivation has resulted in the prairies becoming the region of Canada most extensively altered by humans (Environment Canada 1999). Cultivation affects wetlands in either permanent or transitory ways. Wetlands may be permanently lost by being drained, filled in, or cultivated. Additionally, they may be affected by the reduction or elimination of grassland margins surrounding them. More transitory effects include burning, haying, or grazing. Some of the effects of livestock include hummocking of wetland margins, muddying of water by wading, and contamination of water with feces. Turner et al. (1987) reported that evidence of agricultural impacts was apparent in 66% of wetland basins and 93% of wetland margins in Alberta. A reduction in wetland margins and the grazing of wetland margins were the most common of impacts for the remaining wetlands in southern Alberta. In Saskatchewan, Turner et al. (1987) reported that 59% of wetland basins and 78% of wetland margins had been impacted. Manitoba was found to have 48% of basins and 64% of wetland margins impacted (Turner et al. 1987). Less than 19% of mixed grass prairie remains in its native state in Canada (McLaughlin and Mineau 1995). Grasslands in the Prairie Provinces have declined from 29,200,000 ha to 6,740,600 ha (76.9%) according to Samson and Knopf (1994; as cited in Alberta EP1997). Alberta has lost 61%, but Saskatchewan 81.3% and Manitoba almost all (99.9%) of its native prairie (Samson and Knopf 1994).

Native grasslands continue to be lost to cultivation across the Prairie Provinces, although at a reduced rate. There are a number of possible reasons that could lead to the further expansion of cultivation in southern Alberta. Across the Prairie Provinces, there is the potential for increased crop production for bio–fuels, which may contribute to expansion of irrigated croplands. The weakening of the cattle industry following the ‘mad–cow’ incidents earlier this decade may have resulted in formerly uncultivated, or seldom–cultivated, pastures becoming cultivated, as cattle populations have declined across Alberta. The potato industry has expanded, due to suitable (sandy) soils, available irrigation, and an increase in the number of processing facilities. Wershler (pers. comm. 2009) perceives the major threat to Great Plains Toads as cultivation of native grasslands on sandy soils for crops such as potatoes. Grasslands in Saskatchewan and Manitoba have been more widely cultivated than those in Alberta.

Oil and gas exploration and development are also considered a threat to habitat quality and cause localized habitat alteration and destruction, but the potential impacts have not been studied (Fig. 5). Both ephemeral wetland habitats as well as upland foraging areas may be compromised by such disturbances.

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Figure 4. Map of native prairie remaining in Alberta. Notice that the Alberta range of Great Plains Toads (from Fig. 3; approximated by black outlines) generally corresponds with larger areas of native prairie. The large, relatively intact block north of Medicine Hat is CFB Suffield. Map courtesy of Alberta Prairie Conservation Forum (PCF) 2008.

Map showing the distribution of native prairie in Alberta. The map also indicates the Alberta range of the Great Plains Toad.

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Figure 5. Map of transportation network and oil and natural gas well–sites in grasslands of Alberta illustrates the level of habitat fragmentation. From Alberta EP (1997).

Map showing how the transportation network and oil and natural gas wells have fragmented habitat in Alberta’s grasslands.

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Habitat protection/ownership

The Alberta Prairie Conservation Forum (2008) lists the grassland natural region in Alberta as covering 9,694,650 ha. Of this, 4,143,960 ha, or nearly 43%, remains in an essentially native state (Alberta PCF 2008). Of the native prairie remaining, 2,328,630 ha are under Crown ownership, while 1,815,060 ha are privately held (Alberta PCF 2008). The following table documents native lands remaining in Alberta in the Mixedgrass and Dry Mixedgrass subregions and the proportion that is held by the Crown and private landholders (Alberta PCF 2008).

Table 1. Amount of Alberta’s native Dry Mixedgrass and mixedgrass subregions held privately and publicly. Data from Prairie Conservation Forum (2008).
Alberta native grasslandsDry MixedgrassMixedgrassTotal ha
Total Crown Lands
1,983,373 ha
411,924 ha
2,395,297 ha
Total Native Prairie
2,576,667 ha
573,855 ha
3,150,522 ha
Native Prairie Crown
1,694,456 ha
326,270 ha
2,020,726 ha
Private Range
882,211 ha
247,585 ha
1,129,796 ha

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Major activities in the Dry Mixedgrass subregion area are farming, ranching and energy procurement. Approximately one half (51%) of the land in the Dry Mixedgrass subregion of Alberta is privately owned (Alberta EP 1997). The Dry Mixedgrass subregion is the largest of the grassland subregions in Alberta (47,000 km²). About 42% of the subregion is Crown land, 36% is provincially owned, and 5.7% is federally owned (Alberta EP 1997). Of the grassland subregions in Alberta, this subregion has the lowest road density (0.41km/km²) but the highest well–site density (1.11 well–sites / km²) (Alberta EP 1997). About one third (33%) of the subregion has more than 76% native prairie remaining but nearly half (47%) of the area has less than half of its native prairie intact; 25% has no native grasslands left at all (Alberta EP 1997).

The majority of Great Plains Toad habitat in Alberta is on Crown land. One of the areas known to have significant populations of Great Plains Toads in Canada is the CFB Suffield NWA in southeastern Alberta (Didiuk 1999a). The NWA is a large tract (62,762 ha; Government of Canada 2003) of uncultivated prairie, the largest remaining in western Canada. Other grassland areas that remain under public jurisdiction across the southern parts of the Prairie Provinces are provincial community and cooperative pastures, Prairie Farm Rehabilitation Act pastures, provincial and federal parks, and leased provincial Crown lands. On the remaining areas that are privately owned, land use practices are generally dictated by water availability with cultivation being most extensive in areas with access to irrigation.

Scattered remnants of intact native prairie persist in Saskatchewan. Grasslands National Park, the Cypress Hills Interprovincial Park area, and a number of small sites remain uncultivated. The area where Great Plains Toads are found in Manitoba is primarily cultivated, privately held land, with uncultivated grasslands evident to the west of Lyleton only.

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Biology

Great Plains Toads are well adapted to the xeric environment of the grasslands. They avoid desiccation and predators by seeking refuge underground during the day and are active mainly at night. Feeding and breeding are primarily nocturnal activities. Burrowing underground during extended dry periods helps the toads avoid dehydration. They are most active in periods of warm, wet and humid weather, often following rainfall. Occasionally, they may be active diurnally, but generally only on overcast, humid days. Breeding activity has been described as ‘explosive’ when appropriate conditions occur.

Krupa (1990) published a comprehensive literature review of Great Plains Toads. The previous COSEWIC report on this species (Didiuk 1999a) provided an excellent overview of biological details about this species in Canada up to 1998. Graves and Krupa (2005) compiled a more recent overview of the species’ status in the United States. The following is a summarization of relevant information with inclusion of Canadian information where available.

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Life cycle and reproduction

Emergence and initiation of breeding activity

Ewert (1969) observed emergence from overwintering sites in Minnesota beginning April 22 and continuing over a 5–wk period, with some individuals commencing breeding activity before others had emerged. No other recorded dates for emergence were found.

Reproductive activity in the Great Plains Toad has been well studied in Oklahoma. Early season breeding activity may begin in March and may continue until June (Krupa 1994). There, breeding activity is initiated almost exclusively by spring rainfall events (Krupa 1994). A great deal of variation has been observed in the amount of precipitation required to stimulate breeding activity (Bragg 1946; Krupa 1994), such that breeding activity may occur at any point during the active season (Bragg 1942). For example, in years when early season rainfall was insufficient, breeding activity did not take place until September in Oklahoma (Bragg and Smith 1943). Bragg (1940) noted that breeding occurred once air temperatures had reached 12°C. In Oklahoma, the period of calling is widely variable. Krupa (1994) reported that most males had arrived at breeding pools the first night of calling, and that most had departed after the third night of calling. In contrast, Bragg (1950) recorded a prolonged period of breeding during a wet and rainy May, with calling almost every night for the entire month.

Rainfall may also stimulate breeding activity in Alberta populations. Wershler and Smith (1992, p. 7) noted that breeding activity began in the ‘irrigated portion of the range’ in Alberta after two days of ‘showers and light rain’ in 1987. However, Didiuk (1999a) noted that the high water levels associated with spring runoff in 1994 and 1996 seemed sufficient to stimulate breeding activity in Suffield NWA after the ‘first warm day in May’ even without rainfall. Taylor and Downey (2002) reported that Great Plains Toads in southern Alberta, near Onefour, began calling in mid–June (14th) following 3 d of >40 mm rainfall in 2002. Manitoba Great Plains Toads were observed to be calling in mid–May from pools of spring meltwater (K. DeSmet, pers. comm.). The earliest record for calling in the Alberta provincial database is May 1 (Alberta FWMIS2009). Wershler and Smith (1992) documented that male Great Plains Toads have been reported calling from May 2 to June 17 in Alberta. One record from the Manitoba Conservation database indicates that calling was heard on May 25 (1999). Didiuk (1999a) also noted calling occurred over prolonged periods in 1994 and 1996, for approximately 1 mo in both instances.

Site fidelity

Some breeding adults in Oklahoma were documented to return annually to the same breeding sites, and with apparent independence from site quality because they would return annually to poor sites even when potentially better sites were available (Krupa 1994). In contrast, some males moved from pool to pool between nights if the pools were in ‘close proximity’ (Krupa 1994). Ewert (1969) reported that some individual toads would return to specific breeding sites annually, but that other individuals were more opportunistic in choice of calling sites. The degree to which site fidelity plays a role in Canadian populations is unknown. However, the potential for males to move between breeding ponds from night to night must be taken into account when designing and conducting surveys for this species.

Calling

The call of Great Plains Toads is extremely loud and has been described as a “metallic–sounding trill” (Krupa 1990), a “repeated harsh clatter” (Russell and Bauer 1993) and as a “harsh explosive clatter resembling a jackhammer” and “almost deafening when large numbers are heard” (Stebbins 2003). The sound carries great distances over the flat topography of the prairies: Bennett (2003) commented that Great Plains Toads could be heard at least 2 km away on still nights. Call duration varies from 5–50+ sec (Stebbins 2003), with average duration of each trill about 25 sec (Krupa 1990). Body size influences the sound frequency of calls, which range from 1900–2600 Hz (Krupa 1990). Temperature affects pulse rates, which range from 10–19 pulses per sec (Krupa 1990).

Males aggregate and start calling from near the periphery of temporary, rain–filled pools about 30 min prior to sunset in Oklahoma (Bragg 1937a, Krupa 1994). Krupa (1994) reported that choruses seemed most intense during the first 3 hr of calling, and then gradually tapered off, with sporadic calling between 01:00 h and sunrise. At low densities, calling males are spaced at intervals of at least 1 m apart along the shoreline (Sullivan 1982). Sullivan (1983) compared this to a lek–like breeding system, with females choosing males as mates by the attractiveness of their call. In high–density aggregations, some non–calling males, or ‘satellites’ will hover near a calling male to intercept any females moving towards that calling male (Sullivan 1982). Silent, satellite males are generally 0.5 m (Sullivan 1982) and never more than 1 m from a calling male (Krupa 1989). Up to 95% of calling males are parasitized by one to five non–calling males (Sullivan 1982). Males may undertake satellite behaviour if their call is less intense than that of other calling males in a large chorus, where the din renders individual calls less distinguishable (Krupa 1989). As well, it may also serve a role in energy conservation, as indicated by a large proportion of breeding males using such behaviour (Krupa 1989).

Didiuk (1999a) found that Great Plains Toads at Suffield NWA usually did not begin calling until approximately 45 min after sunset, although Wershler (pers. comm. 2009) reported calling well before dusk following intensive rains on several occasions. Didiuk (1999a) noted that in Suffield NWA calling continued all night on warm nights, only dwindling as dawn approached. Males call from perches in shallow water, such as algal clumps (Wershler and Smith 1992) or emergent vegetation. Didiuk (1999a) noted the occurrence of satellite behaviour at intermediate densities in Suffield. Satellite behaviour could result in the underestimation of breeding male numbers during call surveys.

Breeding activity

There is some minor disagreement as to which night of a chorus the most eggs are laid. Bragg (1937a, 1950) reported that females came to breeding ponds on the first night, but waited until the second night to mate, whereas Krupa (1994) believed most mating was accomplished on the first night of calling. Krupa (1994) reported that over 90% of females had arrived at breeding sites within the first 3 h that males called, whereas Sullivan (1983) reported females were observed only on the same nights that they laid eggs. Sullivan (1983) reported that a maximum of 21% of males mated per night, with 90% of males remaining in a given chorus for only one or two nights at locations in New Mexico and Arizona. Following the first night of breeding activity, toads remained in the water, burrowed into the damp soil, or made shallow burrows near the pool during the day, facilitating a rapid start to the chorus on the second night (Bragg 1937a, 1950).

In Oklahoma, the majority of coupling took place within the first 3 h of mating, with eggs being laid following sunrise the next morning (Krupa 1994). Males clasp any toad they happen to touch, but will ‘chirp’ in protest if clasped themselves, whereas females do not (Bragg 1937a). Amplexus observed in six pairs lasted 7.5–17.2 h Krupa 1994). In laboratory studies, fertilization efficiency was 96% (Krupa 1988). Hybridization with Texas Toad (Bufo speciosus; Rogers 1973) Woodhouse’s Toad, (B. woodhousii; Bragg 1937c; Gergus et al.  1999) and with the Red–spotted Toad (B. punctatus; Sullivan 1990) has also been documented, though rarely. None of these species co–occur with Great Plains Toads in Canada.

Eggs are typically laid in two strings simultaneously over a period of several hours (Bragg 1937a). Strings of eggs are wound around vegetation and over the bottom of shallow pools, covering an area of approximately 1–4 per clutch (Bragg 1937a; Krupa 1994). Krupa (1994) reported a conspicuous tendency for communal egg–laying within specific areas of each breeding pool. Toads showed high fidelity to these sites both annually and between rainfall events (Krupa 1994). The advantages of the particular sites chosen for communal egg laying were not evident (Krupa 1994). Despite of large numbers of toads calling from larger pools, the actual number of clutches laid, per pool, may be density–limited (Bragg 1937a). Krupa (1994) found an average–sized breeding female would produce over 9300 eggs per clutch, with large females (>80 mm SVL) capable of producing more than 45000 eggs per clutch. Such large clutches may represent as much as half of a female’s body mass (Krupa 1991). Larger females may lay multiple smaller clutches over the entire breeding period or fewer, larger clutches (Krupa 1986, 1994). Such reproductive bet–hedging, by way of multiple clutch production, likely increases survival in the unpredictable environment that Great Plains Toads inhabit (Krupa 1986).

Tadpole development

Bragg (1936; 1940) stated that hatching occurred after 50–53 h at ‘ordinary temperatures’. Newly hatched tadpoles are 3.0 mm in length, reaching a head–body length of 11.2 mm just prior to metamorphosis (Bragg 1940) or a total length of 28 mm (Bragg 1937b). Tadpoles develop rapidly, and once they begin metamorphosis, pass through the phase quickly (Bragg 1937b). Tadpole survival was found to be highest in low–density pools. Metamorphosis and successful emergence from natal pools was achieved in only a few cases (e.g. nine of 26 pools; Krupa 1994).

In Oklahoma, the larval period from egg to metamorphosis varied with the season. Early season (March–April) clutches took longer (up to 49 d) to develop than late season (June) clutches (as few as 18 d) due to higher temperatures (Krupa 1994). A high rate of egg and tadpole loss is due to drying–up of breeding ponds prior to the completion of metamorphosis (Bragg 1937a; Krupa 1994). Throughout the breeding season, tadpole survival is very low (Krupa 1994). High tadpole densities and low temperatures prolong development and increase the chances of desiccation and mortality (Krupa 1994). Bragg (1940) suggested that drying of pools is by far the most significant factor contributing to egg and larval mortality. He noted that eggs and tadpoles can survive water temperatures from the freezing point up to and exceeding 37°C. Zweifel (1977) reported that tadpoles could survive up to 6 h at 40.5°C. Ballinger and McKinney (1966) reported the lethal thermal maximum for eggs of B. cognatus to be over 34.5°C but below 39°C, at constant temperatures. Normal egg development was documented to occur in 6 to 13 d at temperatures from 15.9 to 20.8°C. The developmental period declined to 4 to 5 d at temperatures from 21.6 to 25.7°C and to 3 days at 29.4–34.5°C. The range of tolerance for embryonic development was reported as 16°C to 33.5°C (Zweifel 1968). Bragg (1940) mentioned freezing weather and snow falling on breeding ponds, with embryos surviving. Adaptations to such a range of environmental temperatures is to be expected for a species with a wide distribution in a variable environment.

Metamorphosis and dispersal

Metamorphosis of Great Plains tadpoles in Oklahoma is reported to occur synchronously (Krupa 1994). Bragg (1940) describes metamorphosis in some detail for this species in Oklahoma. He reported an average SVL of 12 mm and average weight of 0.15 g upon emergence. Growth following emergence can be rapid, with some individuals reported to increase their weight by 200% after 1 wk (Bragg 1940). Newly emerged juveniles are diurnal and remain near natal ponds unless they dry up (Bragg 1940).

Didiuk (1999a) reported synchronous metamorphosis of Great Plains Toads at two sites in the Suffield NWA in 1996. Wershler and Smith (1992) reported finding small young–of–the–year on June 28 and July 18 in Alberta. Didiuk (1999a) noted finding newly metamorphosed toadlets on July 15. Taylor and Downey (2002) reported a mean SVL of 26.2 mm and a mean mass of 2.5 g from 25 young–of–year individuals captured in southern Alberta in mid–August, 2002.

Growth and longevity

In the southern USA, male toads may reach sexual maturity at 2 yr; females take 2 (Sullivan and Fernandez 1999) or 3 yr(Bragg and Weese 1950). In the cooler climate of the Canadian prairies, age at maturity is likely delayed by 1–2 years, so that maturity occurs at 3–5 years. Growth rates have been reported as highly variable among cohorts and individuals, depending on available food resources and environmental conditions (Bragg and Weese 1950). Males begin to issue protest calls when handled at about 15 wk and have developed a gular (vocal) pouch by the end of their second summer (Bragg and Weese 1950). Females are larger at maturity (SVL: 60–115 mm) than males (SVL: 56–98 mm) (Bragg and Weese 1950), eat more (Smith and Bragg 1949), and may grow more rapidly in their first year (Krupa 1994). Life span is thought to be up to 10 y or more (Krupa pers. obs. as cited in Graves and Krupa 2005). A captive survived for more than 10.5 yr (Snider and Bowler 1992). Sullivan and Fernandez (1999) reported that the oldest toads in a Sonoran desert population were 6 yrold. However, survival rates may be low; only 6.7% of males and 7.5% of females were recaptured in a 3–yr study in Oklahoma (Krupa 1994). Alternatively, reproduction may not occur annually, or some individuals may move to new breeding habitats frequently (Graves and Krupa 2005).

Burrowing for daily shelter

Great Plains Toads are seldom active during daylight unless it is during times of high humidity, or rainfall, and heavily overcast conditions (Bragg 1937a). These toads have been documented to burrow into the soil to avoid desiccation and to hide during the day. Burrows are very difficult to locate and may range from complete submersion beneath the soil surface, to mere depressions in moist conditions (Bragg 1940; Ewert 1969). Ewert (1969) reported that Great Plains Toads remain in these shallow burrows (< 5 cm) from 1 to 6 d. In New Mexico, Creusere and Whitford (1976) observed juvenile toads seeking daytime refuge under surface objects, such as clumps of dead vegetation, cattle dung, or pieces of wood, or in cracks in the drying soil. The most successful retreats were those beneath the soil surface, as high surface temperatures during the day proved fatal to more than half of the young toads that remained above ground. Once the upper 20 cm of soil had dried up, Great Plains Toads ceased all activity and remained inactive until a rainfall occurred (Creusere and Whitford 1976). Tihen (1937) reported a Great Plains Toad from 213–244 m (700–800 ft) into a cave.

Burrowing for overwintering

Great Plains Toads burrow deeply to overwinter. Ewert (1969) found that individual toads in Minnesota spent from 63–77% of the year in dormancy. Tihen (1937) reported a description of the burrows of Great Plains Toads as recounted by C.E. Burt. He stated that the burrows are “…dug from holes in bare spots in sandy areas; each dwelling of a toad was in the form of a question mark with the entry from the long end and the resting place was formed as an expansion of the short end, which was large enough to easily accommodate the toad; the sand surrounding this resting place was packed, but moist”. Burrowing activity of Great Plains Toads was described by Ewert (1969) as rapid, in comparison with Bufo americanus, and was accomplished using the ‘spades’ on the hind feet to dig the toad downwards, posterior–first. Ewert (1969) also commented that toads maintained a “grossly inflated condition” as they burrowed.

In Minnesota toads selected roadside berms and other sites with a good drainage to overwinter (Ewert 1969). It has been suggested that for successful hibernation juveniles may be reliant on the softer soils of ant nests (Whitford and Meltzer 1976) or the burrows of fossorial mammals (Bragg 1940; Lomolino and Smith 2004). Ewert (1969) noted that some adults used badger excavations along the roadside to facilitate burrowing into a roadbed. Ewert (1969) also found that Great Plains Toads adjusted the depth of their burrow over the course of the winter, burrowing more and more deeply (up to 104 cm) until winter (November to March) and gradually ascending during the early months of spring (late March to mid–April). Great Plains Toads initially burrowed for overwintering between August 10 and September 13 in Minnesota (Ewert 1969). Overwintering survival was low in the Minnesota study (six of 18 survived); only individuals that burrowed to a maximum depth of 74–104 cm survived. Individual toads burrowed vertically and horizontally during the overwintering period as they sought to remain below the frost line, yet above groundwater levels (Ewert 1969). Creuse and Whitford (1976) found survival rates of young pelobatids and bufonids in New Mexico to be about 10% over each of two winters.

Site selection and other aspects of overwintering for this species in Canada remain unclear. The latest observation of an active Great Plains Toad in Alberta in the provincial database is from September 20 (Alberta FWMIS2009). Overwintering sites must be below the frost line as Great Plains Toads are considered to be freeze–intolerant (Swanson et al. 1996), a factor that probably limits its distribution in Canada to southern parts of the Prairie Provinces.

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Diet

No information on the diet of Canadian populations of Great Plains Toads is available; however, the topic has been well studied in the United States. Tadpoles feed mainly on algae, although they will also scavenge, consuming insect or animal remains and cannibalizing other dead tadpoles (Bragg 1937a, 1940). Described as “almost social” in feeding habits, adults feed mostly during wet weather and then aestivate in hot, dry weather (Bragg and Smith 1943). Adults feed at night and primarily on beetles and ants, although they are opportunistic and take a variety of prey depending on availability (Smith and Bragg 1949) and prey size (Bragg (1950). Larger female toads consume many more prey items than males (Smith and Bragg 1949). Earthworms are not preferred prey items (Smith and Bragg 1949). Bragg (1950) surmised that all species of Bufo eat more in the earlier part of the active season, spring and summer, than later on, nearer to hibernation.

Young Great Plains Toads will eat almost any small arthropod they find. Juveniles feed mainly on mites, in addition to ants and small beetles, until they reached the larger size class of 21–40 mm SVL, at which point mites were no longer consumed, and they switched to primarily ants and beetles (Bragg and Smith 1950). Smith et al. (2004) reported that newly metamorphosed Great Plains Toads fed on scarab beetles and formicid ants. Flowers and Graves (1995) investigated diet changes during postmetamorphic ontogeny and found that juveniles fed on mites, springtails and beetles. Newly emerged Great Plains Toads inhabiting grassland watersheds had a much more diverse diet than those using playa with cultivated watersheds (Smith et al. 2004). Flowers and Graves (1995) noted there were distinct differences in diet between study years, possibly related to differences in prey availability due to weather conditions and competition. Foraging for insects by Great Plains Toads is of considerable benefit as insect control (Bragg 1943; Smith and Bragg 1949).

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Predation

As tadpoles, Great Plains Toads are preyed on by larval Spadefoot Toads (Spea bombifrons), which may contribute to the eradication of the Great Plains Toad tadpoles from some breeding pools (Bragg 1940). Bragg (1940) also noted predation by diving beetles (Hydrophilus triangularus). Stuart (1995) reported that Bullfrogs (Rana catesbeiana) will consume Great Plain Toads, but these are not found within the species’ range in Canada. Competition with other species of anurans may be most crucial during the larval stage. Predation and competition may play significant roles in preventing temporary–pond anurans from breeding in permanent ponds (Woodward 1983).

Plains Spadefoots are known to share habitat with Great Plains Toads in Alberta, although the former have a comparatively larger distribution and broader habitat preference (Stebbins 2003) in Alberta. Wershler (pers. comm. 2009) observed that Plains Spadefoots and Great Plains Toads often use different parts of a wetland where they co–occur in Alberta. Water beetle (dytisicid) larvae are known to eat larval Great Plains Toads (Bragg 1940) and may be significant predators of anuran larvae in Canada. These larvae were abundant in ponds in Suffield NWA(Didiuk 1999a). Tiger Salamander (Ambystoma tigrinum) larvae co–existed with Great Plains Toad larvae at all breeding sites located in Suffield NWA, but it was not determined if they preyed on Great Plains Toad tadpoles (Didiuk 1999a). Juvenile and adult Great Plains Toads serve as prey for a number of terrestrial predators. As a defence, they are nocturnal and cryptically coloured, and like all true toads, make use of noxious skin secretions as protection from predators. Toads are probably most vulnerable as newly metamorphosed juveniles and as adults during breeding activity. For example, American Crows (Corvus brachyrhynchos) have been observed picking up young toads at the edges of buffalo wallows, and Swainson’s Hawks (Buteo swansonii) may also prey on toads (Bragg 1940). Snakes, such as hog–nosed snakes (Heterodon spp.), or gartersnakes (Thamnophis elegans vagrans; Didiuk 1999a; T. radix ; Flowers and Graves 1997) are also possible predators. Plains Hognosed Snakes (Heterodon nasicus) make use of similar habitats and have been considered as frequent predators of adult Great Plains Toads (Platt 1969). These toads will inflate when attacked by hog–nosed snakes (Krupa pers. obs. cited in Graves and Krupa 2005).

In Canadian populations, Black–billed Magpies (Pica pica), Great Blue Herons (Ardea heterodias), and Black–crowned Night Herons (Nycticorax nycticorax) may also feed on Great Plains Toads during diurnal activity or during the breeding season (Didiuk 1999a). Plains Hog–nosed Snakes have been reported to eat Great Plains Toads in Alberta (Wershler and Smith 1992). Wershler (pers. comm. 2009) reported five Plains Hog–nosed Snakes in the vicinity of a Great Plains Toad breeding site near Hatton, Saskatchewan, following a rain episode. It is unlikely that any of these predators rely exclusively upon Great Plains Toads but feed on them opportunistically.

Some evidence of predator–avoidance has been documented in this species. For example, juvenile Great Plains Toadswere found to detect and avoid chemical cues from gartersnakes (Flowers and Graves 1997). Graves et al. (1993) found that the aggregation–forming tendencies of young Great Plains Toads was an anti–predator behavioural response with juveniles being attracted by sight and by chemical cues to other members of their cohort.

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Physiology

The ability of Great Plains Toads to survive in the extremely hot, dry environments of North American deserts has resulted in a number of investigations into their considerable capacity to avoid desiccation (Walker and Whitford 1970; Hillyard 1976; Yokota and Hillman 1984). A number of authors have addressed the significance of the urinary bladder as a water reservoir for this species (Ruibal 1962; Schmid 1962, 1969; Shoemaker 1964). Burrowing and aestivation has been reported to result in reduced metabolism and heart rate (Whitford 1969) or, alternatively, oxygen stress, associated with a shift to anaerobic metabolism (Armentrout and Rose 1971). Toad muscle tissue exhibits tolerance for high concentrations of urea, likely an adaptive trait associated with aestivation (McClanahan 1964). Aspects of respiration have also been addressed in a number of studies (Hutchinson et al. 1968; Hillman and Withers 1979; and Withers and Hillman 1983). Boehm and Secor (2003) investigated energy budget models for Great Plains Toads.

Great Plains Toads are tolerant of a relatively wide range of temperatures. Tester et al. (1965) concluded they were more tolerant of high temperatures than Bufo hemiphrys and B. americanus. Schmid (1965) reported that Great Plains Toads had a higher upper thermal maximum than B. hemiphrys and found that Great Plains Toads would usually burrow at air temperatures above 38°C with ambient temperatures over 40°C being lethal in under 1 h. Sievert (1991) reported that Great Plains Toads were capable of elevating their body temperature, through thermoregulation, to over 30°C but that their mean preferred body temperature was 27.7°C. Great Plains Toads are dehydration–tolerant, but not freeze–tolerant (Edwards et al. 2004). Brattstrom (1968) reported a critical thermal minimum of −4°C for this species.

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Dispersal/migration

Following metamorphosis and emergence, young Great Plains Toads have been reported to stay in the vicinity of the natal pool, seeking soft soil, sometimes in agricultural fields, to burrow into during the day (Smith and Bragg 1949; Bragg 1950; Bragg and Weese 1950). Originally, Bragg (1940) described a step–wise dispersal from the natal pool, with newly metamorphosed juveniles remaining near the water for the first 2 d, then up to 20 m from the water over the next 3 d, and up to 40 m from the pool in the following week. Later descriptions by Bragg (1950) suggested that dispersal occurred if the pool evaporated or as the young toads reached greater maturity at 2 to 3 months. Young of the year have been recorded to move as far as 914 m (3000 ft) within their first year (Ewert 1969).

Bragg and Brooks (1958) reported mass migration and the formation of aggregations of young Great Plains Toads in Oklahoma. No discernable environmental stimulus was evident. Mass migrations, described as “eruptive”, were primarily comprised of juveniles, which are the largest proportion of the population following metamorphosis. Smith and Bragg (1949) reported that such migrations tended to be along roads, and that all the toads moved in the direction of the road in one of either possible directions. These observations were made in years of heavy rainfall. Such aggregations of juvenile Great Plains Toads have since been interpreted as an anti–predator behaviour (Graves et al. 1993). Great Plains Toads have been observed on unpaved vehicle tracks in Suffield NWA and on the roads in Walsh, Alberta during a major rain event (Wershler pers. comm. 2009). However, there have been no reports of mass migrations in Canada.

Daily movement and home range

In Minnesota, activity patterns following breeding consisted of a series of short–distance movements from breeding areas directly to overwintering sites (Ewert 1969). Movements were punctuated by stops for foraging and with retreats into shallow burrows for several days at a time (Ewert 1969). Adult toads were estimated to have elongate home ranges of at least 610 m (2000 ft) in length (Ewert 1969). Most movements by individuals during the early summer were short (43%; 0–3m), with an even higher proportion (69%) of movements being short distances through late summer and into fall (Ewert 1969). Total distances that individuals moved during the active season averaged 615 m (Ewert 1969). Creuse and Whitford (1976) reported adult Great Plains Toads moved up to 1600 m from the nearest breeding site. Some authors have suggested that they may be able to move much farther (Maxell 2000). The movement of an individual toad a distance of 808 m (2650 ft) in 1 d, reported by Ewert (1969), lends support to that hypothesis. Dormancy is an important facet of Great Plains Toad life history; Ewert (1969) reported that 81% of the year passed with at least one of the study toads dormant.

Diurnal activity

Ewert (1969) reported that diurnal activity was frequent for the Great Plains Toad but was associated with movements around overwintering sites. Adult Great Plains Toads have been observed to be active diurnally in some Alberta populations following breeding activity (Wershler pers. comm. 2009). Both diurnal and nocturnal activity was observed in the Hatton area of Saskatchewan in 2008, with as many as 20 adult toads in the vegetation surrounding a small breeding pool for up to 3 consecutive days, even in clear sunny weather (Wershler pers. comm. 2009). Adults gradually dispersed from the site over a period of several days.

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Interspecific interactions

Great Plains Toads are thought to make use of the burrows of prairie dogs in the USA. Prairie dog towns have been implicated as important for providing shelter for Great Plains Toads within old burrows (Sharps and Uresk 1990; Kotliar et al. 1999). Lomolino and Smith (2004) found Great Plains Toad numbers to be positively associated with Black–tailed Prairie Dog (Cynomys ludovicianus) towns. However, as prairie dogs are entirely absent from Alberta and Manitoba, and found only in one location in south–central Saskatchewan, it is more likely that Richardson’s Ground Squirrels (Spermophilus richardsonii) or Northern Pocket Gophers (Thomomys talpoides) would provide burrows available to Great Plains Toads in Canada.

Creusere and Whitford (1976) investigated the ecological relationships in a desert anuran community and found that juvenile toads remained active for longer than adults. Juvenile toads that inhabited the drying bottom of the natal playa (ephemeral lakebed in arid region) were able to remain active for longer periods of time (up to 55 d) and therefore grew more rapidly than those juveniles inhabiting the drier fringe of the playa. The added benefits of the higher moisture levels available on the playa bottom enabled them to eat more, and therefore be better able to survive under the dry conditions. They described the activity of the adults of all toad species as “… in spurts of short duration following rains.” They also noted that the Great Plains Toad, as a ‘larger’ species, tended to prefer habitats that were more distant from the playa.

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Adaptability

Great Plains Toads may travel widely during their active season (Ewert 1969). Their mobility and larger body size may enable them to seek resources farther from breeding sites than smaller, less widely roaming anurans. Some metapopulation theories suggest that species that are more prone to movement are better able to contend with habitat fragmentation or loss (e.g. Hanski 1994). However, their vagility increases the likelihood of traffic mortality (Carr and Fahrig 2001). Ewert (1969) reported that Great Plains Toads were killed in three (14.2%) of 21 paved road crossings monitored in Minnesota.

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Population Sizes and Trends

Search effort

Most records of Great Plains Toads in Canada have been collected opportunistically. There have not been dedicated surveys for the species across its presumed range in Canada.

Earlier concerns about a perceived decline in Great Plains Toads in Alberta began during a period of drought in the late 1980s (Cottonwood Consultants 1986; Butler and Roberts 1987) and resulted in a recommendation that the species be considered for endangered status (Wallis and Wershler 1988). Field surveys were done in the spring of 1990, and a provincial status report for the species was completed (Wershler and Smith 1992) that recommended the species be listed as “Threatened” in Alberta.

Opportunity for more in–depth study arose in the mid–1990s when several multi–year wildlife surveys, associated with the designation of a portion of the Suffield Military Base as a National Wildlife Area (NWA), were commissioned, and, importantly, precipitation levels were higher. The Suffield NWA surveys of 1994–1996 found breeding populations of Great Plains Toads to be ‘widespread’ within many seasonal wetlands in both the eastern and western sections of the military base (Didiuk 1999a;b). Based upon this experience, Didiuk (1999a) further suggested that the extent of occurrence and the population level of Great Plains Toads had been significantly underestimated in Alberta in earlier studies. He suggested that the widespread breeding observed in the Suffield NWA, especially under the favourable conditions of 1996, illustrated the resiliency of this species, despite previous years of drought conditions.

Suffield NWA is a large tract of relatively undamaged native grassland with limited levels of human disturbance. Whether or not the information learned from this area can be extrapolated over the entirety of the range of Great Plains Toads in Alberta is debatable given the higher level of human disturbance in some parts of the range (i.e. cultivation, traffic on roads). There have been an additional 590 distribution records for this species documented in Alberta since 1997, more than doubling the number of records available up to 1998 (265) in Alberta (Alberta FWMIS2009). The occurrence of peak amounts of precipitation in some recent years, such as 1998 and 2002 (see Fig. 6) and, presumably, the eruption of breeding activity in this species as a result, have enabled observers to expand the number of records in Alberta.

Figure 6. Total annual precipitation (mm) at Medicine Hat, Alberta 1985–2006 from National Climate Data, Environment Canada (2008). Medicine Hat lies within the range of the Great Plains Toad in Alberta.

Chart tracking total annual precipitation (millimetres) at Medicine Hat, Alberta, from 1985 to 2006.

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Most recent location data for Great Plains Toads in Alberta have been collected during surveys for other amphibians (e.g. MULTISARAmphibian surveys of 2002, 2006; Taylor and Downey 2002; Downey et al. 2006) or other wildlife (e.g. Ord’s Kangaroo Rat (Dipodomys ordii) studies; Gummer and Robertson 2003; Gummer 1997; Gummer and Gummer 1997). The Alberta Amphibian Monitoring project provided a significant number of records (ref.). As general awareness of the species has improved, along with the ease of collecting GPS location data, the number of records has also increased. More researchers are working in remote parts of the prairies, reflecting the increases in oil and gas exploration and related environmental assessments. As a result, more reports for this species have been generated. If dedicated surveys were to be implemented, it is important to recognize the significance of conducting surveys for this species in wet years only, following significant rainfall events, and using suitable methods (e.g. accounting for satellite behaviour and inter–pool movements by males). Data collected from an assortment of sources may also harbour skewed data. For example, a large number of records for Great Plains Toads have been submitted from the Middle Sand Hills habitat area within the Suffield NWA, due to collections made by Ord’s Kangaroo Rat researchers (Gummer and Robertson 2003) in that area. More suitable habitat and higher abundance is found in more southern parts of the Suffield NWA and into the adjacent CFB Suffield (A. Didiuk, pers. comm.).

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Abundance

Wershler and Smith (1992) estimated that the Alberta population could be as high as 2000 adults, based upon the limited distribution records they had available at the time. However, Didiuk (1999a) later suggested that perhaps the total Alberta population could be in the “many tens of thousands” of toads based on extrapolation from the findings at Suffield in 1994–1996. Given the expansion of records for this species in the past decade, the population is undoubtedly higher than originally predicted. Given that individual large females could produce up to 45,000 eggs (Krupa 1994), there could easily be tens of thousands of young produced under excellent conditions.

Estimating the total Alberta population for this species would require more extensive surveys of potential breeding sites under appropriately wet conditions to first delineate its range more clearly within the province. As the government databases grow, they will contribute to the understanding of the true range of this species in Alberta. Estimation of population sizes in Saskatchewan at this time would be completely unrealistic given the paucity of records. Manitoba populations, given the more limited distribution area, might be more feasibly documented with appropriate surveys.

Great Plains Toads were once described as the most common toad in the southwest (King 1932) but more currently are described as less common and localized in distribution (Graves and Krupa 2005; NatureServe 2009). Black (1971) illustrated the range of this species in Montana as the entire eastern two thirds of the state east of the Rockies and contiguous with the Canadian border, east from about the Port of Del Bonita. Later, Reichel and Flath (1995) illustrated the range in Montana as more restricted and stated that there are large gaps in the known range. They illustrated an isolated population to the southeast of Shelby, with the remainder of the range delineated as a wide swath running north and south over the Fort Peck Lake area in the eastern half of the state. Again, it is not clear if this is indicative of more clustered distribution or incomplete occurrence data.

In all documented reports of this species in Alberta (855 records), the number of individuals reported per site is low, with the vast majority being single digit records (<10 individuals). Of the records available for Alberta, seven are of 50 or more individuals, with the single highest number being 124 individuals, from a 1957 record (Alberta FWMIS2009). This does not include most of the Suffield records, as the numbers of individuals from that survey effort are not available in this dataset. However, Didiuk (1999a) used an average of 20 adults per wetland in Suffield NWAin his generalized calculation to illustrate the potential for higher population levels than previously presumed. From the dataset, it seems that Great Plains Toads in Alberta are potentially widespread, but clustered and at somewhat low densities.

Of course, there may be considerable errors within the database as well. For example, it is difficult to discern the number of calling males without visual observation once a full chorus has been established, due to the overlapping, extremely loud calls. Some researchers saw little value in recording chorus numbers for this species, due to the high potential for error, and thus many records from Suffield have no numerical values associated with them. The potential for errors within the databases are made all the more probable by the assorted contributors and collection dates. This became evident from the number of outlying records (5) within the Alberta database that, upon investigation for the most recent provincial and federal status reports, were determined to be incorrectly plotted or misidentified (i.e. juvenile spadefoots, later determined from photographs). Finally, the occurrence of satellite behaviour by calling males could have confounded the calling records, without direct observations to verify numbers of breeding males.

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Fluctuations and trends

Population trends of the Great Plains Toad in Canada are unknown. Fossorial habits and intermittent, explosive, breeding episodes make population trends difficult to monitor (Ewert 1969; Graves and Krupa 2005). Population density fluctuates widely with precipitation levels (Graves and Krupa 2005). Bragg and Weese (1950) noted that Great Plains Toad populations in Oklahoma were subject to years of enormous breeding successes followed by years of complete reproductive failure, which were attributed to low levels of precipitation. Sullivan and Fernandez (1999) also found that Great Plains Toads did not reproduce in years without sufficient rainfall in the Sonoran desert. Such episodic population explosions have been likened to pulses of growth that sustain the overall populations during years of drought (Naugle et al. 2005b). Overall, reproductive success is highly variable both among years and locations (Graves and Krupa 2005).

It is reasonable to believe that populations in Alberta, Saskatchewan, and Manitoba are also subject to similar fluctuations in population levels, dependent upon precipitation levels and site quality. In addition, given the toads’ intolerance to freezing, the more northern climate would impose additional limitations due to higher levels of overwintering mortality. Finally, their populations may be more susceptible to human influences than are some other amphibian species with which they co–occur (i.e. Plains Spadefoot Toads; Bragg 1960), so widespread distribution should not necessarily be equated with commonality.

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Rescue effect

The range map published by Graves and Krupa (2005) portrays five counties in Montana along the Canadian border with records of the Great Plains Toad; Toole county (also in Black 1971), south of Coutts and Writing–on–Stone Park in Alberta, and Blaine, Philips, Valley and Sheridan counties, all south of southwestern Saskatchewan. None of the counties adjacent to Canada in North Dakota are known to have Great Plains Toads, and only one county in the north westernmost corner of Minnesota is documented to have them (NatureServe 2009; Graves and Krupa 2005). The range documented as adjacent to Manitoba in Minnesota does not correspond with the known Manitoba population, but rather abuts with the southeastern corner of Manitoba in an area where the species has not been recorded as present in Canada (See Fig. 2; Graves and Krupa 2005; NatureServe 2009). The considerable distances between documented populations across the border and the fragmentation and human use of habitat between them, would make natural recolonization of Canadian localities from USAsources difficult.

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Limiting Factors and Threats

Environmental conditions

Bragg (1940) originally considered desiccation of breeding pools to be the most limiting factor for this species. However, Great Plains Toads are well adapted to the drought–prone and generally xeric conditions of North America’s grassland biome as their widespread range attests. Natural climatic, and perhaps geological, factors have delineated the historic range of Great Plains Toads in Canada, as elsewhere. Bayrock (1964) reported fossil evidence of Anaxyrus cognatus near Killam, Alberta, less than 100 km southeast of Edmonton. Such evidence indicates the prairie biome ranged much farther north in the post–glacial climatic optimum that followed the last ice age.

Bragg (1940) pointed out that Great Plains Toads appear to be well adapted to over–winter survival in Oklahoma, with large numbers returning to breeding sites, even after 2 yr of unsuccessful breeding efforts. In Canada, at the northernmost range extent for the species, over–wintering mortality is much more likely to be a significant range–limiting factor especially in areas, or under moisture conditions, that make digging more difficult (i.e. clay soils, extreme drought). Overwintering resulted in significant mortality (67%) in a Minnesota study, the most northerly study available (Ewert 1969).

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Threats

Habitat loss and fragmentation

Agricultural practices are the primary influence over most of the land area of the southern Prairie Provinces, followed by oil and gas development. Fragmentation, therefore, is widespread, with cultivation, irrigation, roads, wellsites and pipelines contributing to varying levels of habitat disturbance and degradation. Great Plains Toads are likely to suffer higher than normal levels of mortality in cultivated areas due to their fossorial tendencies (Bragg 1960). Ewert (1969) documented 20 encounters of 14 individual Great Plains Toads with agricultural activities (mowing, raking, baling, discing and ploughing). He found that toads responded to haying activities with a short movement, or no movement at all. Discing and ploughing (four observations), however, resulted in immediate dispersal from the area, or remaining underground for several hours. How individual toads could survive cultivation was not clear, but the author suggested that because Great Plains Toads tend to respond to being approached with “a sudden burst of hopping for a distance of 3–10 feet … often at right angles to the approaching person” (Ewert 1969, p. 73) they might be evading the machinery in the same manner, as toads can undoubtedly hear and feel the machinery approaching. If the disc or plough was not particularly wide, this evasive maneuver might be sufficient. It is important to note that most cultivation equipment currently used on the Canadian prairies is vastly wider than 3 m (10 ft). Cultivation also reduces the persistence of ephemeral wetlands (Didiuk 1999a). The impact of activities associated with oil and gas exploration and development remains unclear. The relatively large tracts of intact native grasslands in Alberta probably serve as reservoirs for populations in that province. There are far fewer, and smaller, tracts remaining over the same ecological region in Saskatchewan and Manitoba.

Great Plains Toads seem to be somewhat opportunistic with some human disturbances given that they have been documented to breed in flooded areas in cultivated fields and ditches and may opportunistically use irrigation floodwaters when available. The occurrence of this species in at least one widely cultivated area within Alberta, suggests that they have thus far endured the local cultivation practices, although without direct study it is impossible to say to what extent this population has been affected. However, the degree to which losses of natural breeding wetlands, altered prey bases, effects of pesticides and herbicides, and mortalities from cultivation and vehicles are offset by any possible increase in breeding opportunities will dictate the long–term viability of this species in such areas. The general co–occurrence of this species within areas of considerable tracts of intact native range, suggests that although it may persist in some cultivated areas, populations in Canada are more robust where habitat remains intact.

Anderson et al. (1999) found no effect of surrounding land use, either cultivated or grassland, on occurrence of anurans in playa wetlands in Texas and New Mexico. Gray et al. (2004) found that abundance of Spadefoots (Spea multiplicata and S. bombifrons) was actually higher for wetlands surrounded by cultivation in Texas, perhaps due to increased productivity from chemical fertilizers. However, the abundance of Great Plains Toads was not significantly different between wetlands in cultivated areas and those in uncultivated areas (Gray et al. 2004). Degenhardt et al. (1996) suggested that Great Plains Toads tolerate drier conditions and agriculture better than most bufonids. Lannoo et al. (1994) suggested that there has been an eastward expansion of Great Plains Toads into northwestern Iowa since Bailey and Bailey’s (1941) documentation, although they did not explain how or why this might have occurred.

Post–metamorphic body size for Anaxyrus cognatus was found to be notably higher at wetlands surrounded by intact grasslands, compared to wetlands embedded in cultivated areas (Gray and Smith 2005). As body size is positively correlated with survival, this relationship indicates a negative association between cultivation and survival of Great Plains Toads (Gray and Smith 2005). Where habitat remains relatively undisturbed, amphibian and reptile populations are likely to persist with little change in distribution of species (e.g. Hossack et al. 2005). The robustness of Great Plains Toad populations within the Suffield NWA likely reflects this, illustrating the significance of this area as a biological reservoir.

Some populations of Great Plains Toads in the Mojave Desert have been eliminated due to habitat losses stemming from dams on the Colorado and Virgin Rivers used to create Lakes Meade and Mojave (Bradford et al. 2005). The drastic drop in Great Plains Toad populations from “enormous numbers” to “only small congresses” of breeding adults in the area near Norman, Oklahoma, over 25 yrwas documented by Bragg (1960) and thought to be attributable to cultivation and traffic. Of the 15 species of amphibians he documented over this time period, Anaxyrus cognatuspopulations appeared most impacted by human activities. Eight species maintained their populations, and of the remaining six species (excluding B. cognatus), populations fluctuated, but persisted at far lower numbers (Bragg 1960). McLeod (2005) found Great Plains Toads absent from their Nebraska study area in the 1990s, whereas they had been documented as present in the 1970s. Busby and Parmelee (1996) also found no Great Plains Toads on a military reserve surveyed in Kansas, although they were present in 1927.

Habitat loss to urbanization is of relatively limited scope in the southern Prairie Provinces. In Alberta, Medicine Hat is the only major centre in the Dry Mixedgrass subregion; in Saskatchewan, Regina; and in the Manitoba portion of the range, the only urban centres are the small towns of Melita, Coulter, and Lyleton. On a relative scale, the proportion of available habitat affected by urbanization is low.

Herbicides and pesticides

Amphibian species, due to their permeable skin and reliance upon water for breeding, are often considered to be indicator species for ecosystems (Halliday 2000). The most likely source of environmental contamination on the Canadian prairies is the widespread use of herbicides and, perhaps less often, pesticides. Pesticides, such as organophosphate insecticides, travel widely within the environment, often accumulating in areas distant from their point of application (LeNoir et al. 1999). Exposure to atrazine, a common broad–leaf herbicide in the United States, has been implicated in the feminization and hermaphrodization of frogs (Hayes et al. 2002). Nitrites, from the nitrogen–based fertilizers widely applied across much agricultural land in North America, have been found to adversely affect larval amphibians (Marco and Blaustein 1999). Kiesecker (2002) found evidence that exposure of Wood Frogs (Lithobates sylvatica) to agricultural pesticides, even at low levels, led to reduced immune system capacity and resulted in a higher level of infection with trematodes, in turn causing increased rates of limb malformations.

Fertilizer and herbicide application is a routine annual event on almost all cultivated areas of the Canadian prairies. Glyphosate (e.g. Round–Up) is by far the most widely used pesticide in Alberta (Alberta Environment 1998). Glyphosate has been demonstrated to lead to extremely high rates of mortality in some amphibians (Relyea 2005). The herbicides most frequently detected in rainfall in Alberta are 2,4–D, MCPA, bromoxynil, dicamba and mecoprop with 2,4–D being most common in southern Alberta (Hill et al. 2002). There are no records of toxicology for the Great Plains Toad in the RATL database (Pauli et al. 2000).

Traffic mortality

For more than seven decades, the impacts of vehicles have been evident upon Great Plains Toad populations in the United States. Breckenridge (1938) noted that “…thousands of them were killed on the graveled and paved highways...”. As well, Bragg (1940 p. 348) stated “The automobile kills hundreds of toads each year. I have seen as many as fifty dead toads along a mile of paved road after a rain. Many are killed also in the unpaved country roads about Norman, especially during the breeding season.” Bragg (1960) also surmised that cultivation kills many of these toads while they are underground in shallow burrows. It is probable that these toads face similar threats from vehicles in Canada, especially where wetlands occur on both sides of the road (Langen et al. 2009). It is important to note that paving roads often increases levels of traffic volumes and could result in higher levels of mortality for this species in some areas (Wershler pers. comm. 2009).

Diseases and parasites

Like other amphibians, diseases and parasites also affect Great Plains Toads. Climate change, in conjunction with other human caused impacts, may be promoting the occurrence of the pathogenic chytrid fungus (Batrachochytrium dendrobabatidis) and hastening amphibian extinctions (Pounds et al. 2006). There is suggestion that the spread of this fungus has been human–facilitated. Livo (2004) reported Great Plains Toads as among the species within which Batrachochytrium dendrobabatidis had been positively detected in Colorado. Chytrid infection has not been reported in this species in Canada.

Redleg (Aeromonas spp.) was attributed as the primary cause of mortality for the majority of overwintering Great Plains Toads in a Minnesota study (Ewert 1969). Shively et al.  (1981) documented that infections of the skin, intestine and respiratory tract with Mycobacterium marinum could be fatal for Great Plains Toads. There are reports of infections with protozoa (Trowbridge and Hefley 1934; Bragg 1940), nematodes, cestodes (Goldberg and Bursey 1991; Goldberg et al. 1995) and trematodes (Miller et al. 2004). However, earlier studies had found no trematodes or cestodes (Ulmer 1970; Ulmer and James 1976; Brooks 1976). These toads, like other primarily terrestrial anurans, spend a small proportion of their lives in direct contact with aquatic habitats, a trait that may contribute to the limited reports of parasitic infection (Brandt 1936).

Entrapment

Other human impacts on the landscape may also impact amphibian populations. Bragg (1940) pointed out that toads become trapped in ‘pits’ created by human activities, such as post–holes (Bragg 1940). In Alberta, similar reports of toads, and other small animals, becoming trapped in sunken natural gas well caissons have been noted (Didiuk 1999a; b; G.L. Powell, cited in James 1998). Didiuk (1999a) pointed out that the trenches dug for pipelines likely entrap any toads that may try to move across them; this would be especially true if near breeding areas. The deep, steep–sided hoof prints created by cattle in the mud around wetland margins can also act as unmonitored pit–fall traps for amphibians (J. James. pers. obs.). Apparently the species is trafficked in the pet trade in the USA as well (Frost et al. 2006).

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Special Significance of the Species

Great Plains Toads are endemic to North America and a representative species of the prairie biome. Most members of the public, even within the recognized range, appear to be unaware of the species. That new populations of this species continue to be documented (Alberta FWMIS2009; Alberta NHIC 2008) is testament to their cryptic habits and restricted activity levels.

The populations of this species that survive in southern Canada, like other species occurring on the periphery of their ranges on the northern Great Plains, are probably most significant for their physiological, and perhaps behavioural, adaptations to the northern climate. These populations are likely remnants of a much more widespread range that previously extended much farther north, following the retreat of the last glaciation (Bayrock 1964).

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Existing Protection or Other Status Designations

The 2002 COSEWIC assessment and status report on the Great Plains Toad in Canada designated the species as “Special Concern” (A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats) (COSEWIC 2002). NatureServe (2009) lists the Global Rank of this species as “S5” (Secure) and the Rounded Global status, as “G5–Secure”, with the specified reasons as “common and widespread in western and central North America [with] no major threats”. These designations were last reviewed in February of 2003. The US National Status is N5 (Secure – common, widespread, and abundant in the jurisdiction) and the Canadian National Status is N3 (Vulnerable – vulnerable in the jurisdiction due to a restricted range, relatively few populations, recent and widespread declines, or other factors making it vulnerable to extirpation) as of 2000 (NatureServe 2009). The Global Short Term Trend is listed as “Declining to stable (± 10% fluctuation to 30% decline) by NatureServe (2009). NatureServe (2009) also notes that populations of Great Plains Toads have likely been reduced in some regions by intensive cultivation and herbicide/pesticide use. The IUCN Red List category for this species is “LC” or of Least Concern; species that are widespread and abundant fall into this category (IUCN2008). However, the population trend is listed as ‘decreasing’ (IUCN2008).

In Alberta, the general status of Great Plains Toads is ‘May be at Risk’ (Alberta SRD 2005). The last evaluation of the species rank was based on the report by James (1998) and was completed by the Endangered Species Conservation Committee at that time (Alberta FWD 2008). An update of status is currently underway in Alberta (Pearson 2009, in prep.). Alberta Natural History Information Centre (ANHIC 2008) lists Anaxyrus cognatus as S2;G5(Imperiled -- Imperiled in the jurisdiction because of rarity due to very restricted range, very few populations, steep declines, or other factors making it very vulnerable to extirpation from jurisdiction). The Great Plains Toad is currently listed as a ‘Non–game’ animal under Schedule 4, Part 5 of the Wildlife Act Regulation 143/97 in Alberta (Government of Alberta 2005). There are no special protection or conservation efforts underway for this species in Alberta other than to collect occurrence records.

In Saskatchewan, Great Plains Toads are currently not included as a species at risk under The Wildlife Act (Government of Saskatchewan 1998) and are therefore accorded no protection. The Saskatchewan Conservation Data Centre ranks Great Plains Toads as “S3”, (Vulnerable -- vulnerable in the jurisdiction due to a restricted range, relatively few populations, recent and widespread declines, or other factors making it vulnerable to extirpation) (SCDC 2007).

In Manitoba, Great Plains Toads were listed as ‘Threatened’ in 2001 and are therefore accounted for under Manitoba’s Endangered Species Act (Government of Manitoba 2008). In Manitoba, a species which has been listed as threatened or endangered is protected under the Endangered Species Actmaking it “unlawful to kill, injure, possess, disturb, or interfere with the species; destroy, disturb the habitat of the species; or damage or destroy, obstruct or remove a natural resource on which the species depends for its life and propagation” (Government of Manitoba 2008). The Manitoba Conservation Data Centre lists Great Plains Toads as S2S3 (between “Imperiled” and “Vulnerable”) (MCDC 2001).

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Technical Summary

Anaxyrus cognatus

Great Plains Toad – crapaud des steppes
Range of occurrence in Canada: AB, SK, MB

Demographic Information

Generation time May mature at 3–4 years and live up to
0 years (See Growth and Longevity)
~ 5 years
Is there an [observed, inferred, or projected] continuing
decline in number of mature individuals?
Unknown
Estimated percent of continuing decline in total number of
mature individuals within [5 years or 2 generations]
Unknown
[Observed, estimated, inferred, or suspected] percent
[reduction or increase] in total number of mature individuals over
the last [10 years, or 3 generations].
Unknown
[Projected or suspected] percent [reduction or increase]
in total number of mature individuals over the next
[10 years, or 3 generations].
Unknown
[Observed, estimated, inferred, or suspected] percent
[reduction or increase] in total number of mature individuals
over any [10 years, or 3 generations] period, over a time
period including both the past and the future.
Unknown
Are the causes of the decline clearly reversible and understood and ceased?N/A
Are there extreme fluctuations in number of mature individuals? There are extreme fluctuations in “local” populations dependent climate. Few breed during drought years and many breed and have enormous numbers of eggs during wet years. However, if these climate changes vary over the species’ range then conceivably dry years in some areas could be offset by moist years in others. This possibility has not been tested. Also, adults could withstand a few years of drought by remaining buried underground. (See sections under Life cycle and reproduction, and also Fluctuations and Trends) Yes, but size of fluctuations are unknown

Extent and Occupancy Information

Estimated extent of occurrence: Records have more than
doubled in AB in past decade (>850 total). 19 records
in SK and 12 locations in MB. Range in SK still thought
to be widely under–documented with large gaps. Range
in MB is in confined area of extreme southwest of province.
Although range is larger than previously estimated in Canada,
populations appear clustered and density may be low.
(calculated as a minimum convex polygon using records from 1998–2008)
134,200 km²
Index of area of occupancy (IAO):Data from Alberta Fish and Wildlife Information System (2009); Andy Didiuk (CWS, Saskatchewan) and Manitoba Conservation Data Centre (2008);(calculated as the area of grid cells of 2x2 km² containing sites 1998–2008) that is, it is an index of AO (IAO). 1,276 km² However, the IAO may be much larger than this because many more populations MAY be discovered.
Is the total population severely fragmented? Unknown
Number of “locations* Unknown but above thresholds
Is there an [observed, inferred, or projected] continuing
decline in extent of occurrence?
Unknown
Is there an [observed, inferred, or projected] continuing
decline in index of area of occupancy?
Unknown, but may be declining
Is there an [observed, inferred, or projected] continuing
decline in number of populations?
Unknown, likely declining
Is there an [observed, inferred, or projected] continuing
decline in number of locations?
Possible decline
Is there an [observed, inferred, or projected] continuing
decline in [area, extent and/or quality] of habitat?
Likely declining
Are there extreme fluctuations in number of populations? Probably not, but not certain
Are there extreme fluctuations in number of locations*? Probably not, but not certain
Are there extreme fluctuations in extent of occurrence? No
Are there extreme fluctuations in index of area of occupancy? No

*See definition of location.

Number of Mature Individuals (in each population)

Population N Mature Individuals
Alberta:Not known
  • Suffield–Medicine Hat area: largest ‘cluster’ of populations in Canada
  • Onefour area: more clustered population
  • Skiff area: small, clustered population in cultivated area
  • Tilley–Vauxhall–Taber–Grassy Lake area: scattered records, appear clustered in Taber–Grassy Lake – Purple Springs area; second largest population area

Saskatchewan: estimated range, from 19 records:

  • Western area: along AB border, from Kindersley area in north to Maple Creek in the south, and extending east to just west of Moose Jaw.
  • Extreme southeastern corner of the province – includes region west of Estevan to MB border in east; north to perhaps Hwy 361.

Manitoba:

  • Extreme southwestern corner near towns of Lyleton, Coulter and Melita.
Total Unknown

Quantitative Analysis

Probability of extinction in the wild is at least [20% within 20 years or 5 generations, or 10% within 100 years]. N/A

Threats (actual or imminent, to populations or habitats)

  • Widespread and expanding cultivation with resultant loss and fragmentation of native prairie habitat (main threat)
  • Direct mortality from farm machinery, oil and gas operations, and road traffic
  • Herbicides and pesticides: direct mortality and impaired reproduction
  • Extensive and increasing frequency of drought
  • Loss of habitat and other effects of oil and gas drilling

Rescue Effect (immigration from outside Canada)

Status of outside population(s)?
USA: N5 (National Rank: Secure -- Common, widespread, and abundant in the jurisdiction.)
Montana: S2 (Subnational Rank: Imperiled -- Imperiled in the jurisdiction because of rarity due to very restricted range, few populations, steep declines, or other factors making it vulnerable to extirpation from jurisdiction.)
North Dakota: SNR (Subnational Rank: Unranked -- National or subnational conservation status not yet assessed.)
Minnesota: SNR (Subnational Rank: Unranked -- National or subnational conservation status not yet assessed.)
Is immigration known or possible? Unknown but possible
Would immigrants be adapted to survive in Canada? Probably
Is there sufficient habitat for immigrants in Canada? Possibly
Is rescue from outside populations likely? Probably not without human intervention, given distance to nearest known USA populations, widespread cultivation throughout intervening areas and S2 rating in Montana the state closest to most Canadian populations

Current Status

COSEWIC:Special Concern (April 2010)

Status and Reasons for Designation

Status: Special Concern
Alpha–numeric code:N/A

Reasons for designation:
This species is widespread but has a scattered distribution of mostly small populations that fluctuate in numbers. It almost meets criteria for Threatened and could become Threatened because of ongoing loss and degradation of habitat, particularly loss of intermittent wetlands from cultivation, oil and gas development and increase in droughts. These threats increase fragmentation of populations and jeopardize their persistence.

Applicability of Criteria

Criterion A (Decline in Total Number of Mature Individuals): Does not meet criterion as there is no evidence of large range–wide decline.

Criterion B (Small Distribution Range and Decline or Fluctuation): Does not meet criterion. Although IAO is less than the 2,000 km² threshold for Threatened and there is ongoing habitat loss and degradation, there are more than 10 locations, there is no severe fragmentation, and no extreme fluctuations have been documented.

Criterion C (Small and Declining Number of Mature Individuals): Does not meet criterion as total number of mature individuals exceeds thresholds.

Criterion D (Very Small or Restricted Total Population): Does not meet criterion as population size much larger than thresholds.

Criterion E (Quantitative Analysis): Not conducted.

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Acknowledgements and Authorities Consulted

Many thanks to Ken DeSmet (Manitoba Conservation), Andy Didiuk (CWS – Saskatoon), Brad Downey (Alberta Conservation Association), and Cleve Wershler (Sweetgrass Consultants) for their help and information. Thank you also to Kim Pearson (Updated Alberta GPT report) who also provided much help. Drajs Vujnovic (Alberta Natural Heritage Information Centre) and Lonnie Bilyk (Alberta Sustainable Resource Development) provided me with updated provincial datasets. Jeff Keith (Saskatchewan Conservation Data Centre) sent what information he had. Nicole Firlotte (Manitoba Conservation) provided the Manitoba records for plotting. Wayne Roberts (University of Alberta) pointed me in the right direction to access the U of A Museum records. Many thanks are also due to Robin Gutsell (Alberta Sustainable Resource Development) and Sue Peters (Alberta Sustainable Resource Development) for answering questions. The indispensable Alain Filion (COSEWIC secretariat) composed the maps and calculated EOs and AOs.

Thanks to all the reviewers of the first draft of this report for providing valuable feedback. Your contributions were greatly appreciated and added a great deal to the final version.

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Information Sources

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Biographical Summary of Report Writer

Janice James completed her Master’s thesis on maternal thermoregulation in short–horned lizards in southern Alberta at the University of Calgary in 1997. In 1998 she wrote the provincial status report on Great Plains Toads in Alberta. A former resident of southern Alberta, she has had an affinity for Great Plains Toads since 1990, when she first encountered them south of Lake Newell.

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Collections Examined

Data used in this report came from multiple sources. Nicole Firlotte of Manitoba Conservation forwarded the table of element occurrences for that province. The sources were not listed.

All Saskatchewan locations are courtesy of Andrew Didiuk of the Canadian Wildlife Service, Saskatoon and Cleve Wershler. His records were listed as compiled from: National Museum of Canada specimens, Bill Houston (PRFA Regina), Candace Neufeld (CWS Saskatoon) and Jennifer Neudorf (CWS contractor, Saskatoon), Rick Lauzon (AXYS consultants), Secoy and Vincent, 1976, SAMP report.

The Alberta FWMISrecords are extensive: the Canadian Museum of Nature, Provincial Museum of Alberta, University of Alberta, Alberta Amphibian Monitoring project members, Lloyd Bennett, Larry Powell, Andrew Didiuk, Cleve Wershler, Cliff Wallis, Ord’s Kangaroo Rat Study, Jonathan Wright, Wayne Smith, Rick Lauzon, Shannon Lord, Brad Taylor, Brad Downey, and Darryl Jarina were major contributors. Many others also contributed a limited number of individual records to the database. Eastern Irrigation District information was available at the level of township only through the Alberta FWMIS and Rick Martin (EID). The Alberta Natural History Information Centre (ANHIC) also provided data courtesy of Draj Vujnovic in Edmonton. University of Alberta Natural Science collection records were also accessed, but had been previously included in the FWMISdataset.