Bufo boreas Baird and Girard, 1852(b)
1. Historical versus Current Distribution. Western toads (Bufo boreas) occur throughout much of western North America. They are found throughout the Pacific Northwest, north through western Canada and into the southeastern portion of Alaska (Wright and Wright, 1949; Herreid, 1963; Stebbins, 1985; Norman, 1988a).
In the United States, western toads occur from north-central New Mexico (Campbell and Degenhardt, 1971; Stuart and Painter, 1994; Degenhardt et al., 1996) into western Colorado, where they have been found in most of the mountain ranges at elevations between 2,615 and 3,557 m (except in the Sangre de Cristo Range, Wet Mountains, and Pikes Peak; Hammerson, 1986, 1999; Livo et al., 1999). Their distribution extends north into the higher elevations (between 2,461 and 3,385 m) of western Wyoming (Luce et al., 1997) and into Montana (Maxell, 1999); northwest into Oregon and Washington, where they are found nearly statewide (except for the low-lying areas of the Columbia Basin [Washington], the Willamette Valley and the northern Coast Range [Oregon]; Leonard et al., 1993); west into the High Plateaus and the Wasatch Mountains of Utah (Ross et al., 1995; Oliver, 1997); across areas of high elevation on the Utah-Nevada border and throughout Nevada (Nevada Natural Heritage Program, 1999); and into northern California (Stebbins, 1985). Western toads are also found in the coastal forests of southeastern and south-central Alaska from the British Columbia border north to Prince William Sound (Hodge, 1976).
According to Crother et al. (2000), three subspecies of B. boreas are recognized: boreal toads (B. b. boreas), California toads (B. b. halophilus), and Amargosa toads (B. b. nelsoni). The systematics of this group is provided in Goebel (2003). However, this view is controversial (see Crother et al., 2003). In particular, most, perhaps all, workers familiar with Amargosa toads agree that they deserve species status. The arguments for this position and what is known about the conservation status of Amargosa toads are presented in this volume by Anna Goebel in the separate Bufo nelsoni account.
Severe declines and extirpations of many populations have occurred in areas where western toads were once abundant (Leonard et al., 1993; Carey et al., 2003; this volume). Between 1986 and 1988, Corn et al. (1989) searched 59 known western toad localities in Colorado and Wyoming and found them at only 17% of the localities. According to Loeffler (1999), western toads are documented to occur in only two counties in Wyoming. Western toads were found in 6 of 13 historical localities and in eight sites without previous records during surveys between 1987 and 1994 in Rocky Mountain National Park (RMNP; Corn et al., 1997). Surveys in 2000 and 2002 found toads in only 3 of 22 historical localities in RMNP and in one new locality (non-breeding; E.M., unpublished data). In New Mexico, a small number of western toad localities were documented in Rio Arriba County between the elevations of 2,775 and 3,200 m, however these populations have declined rapidly and are now thought to be extirpated (Stuart and Painter, 1994; Degenhardt et al., 1996; Loeffler, 1999).
There is no published evidence that western toads have declined in the mountains of the Pacific Northwest to the same extent as populations in the Rocky Mountains (Corn, 2000). According to Crisafulli and Hawkins (1998), toads currently are abundant within the blast zone of the Mount St. Helens 1980 eruption. However, Blaustein et al. (1994b) described mortality in large numbers of eggs, and Olson (1992) described mortality in adults in Oregon. In the Puget Sound lowlands of Washington and other lower elevations in the Pacific Northwest, toads are rare. Recent surveys have found western toads present at only 0–22% of the sites surveyed (Richter and Azous, 1995; Adams et al., 1998, 1999).
Factors suspected to be contributing to the decline of western toads include habitat degradation and destruction (Hammerson, 1999), fungal infections and other pathogens (Carey, 1993; Hammerson, 1999; Muths et al., in press), acid and mineral pollution from mine water drainage (Porter and Hakanson, 1976), and increased ultraviolet radiation (Kiesecker and Blaustein, 1995; Blaustein et al., 1998; but see Corn, 1998).
2. Historical versus Current Abundance. Western toads are considered Endangered in several states, and their status is under review in other states. The U.S. Fish and Wildlife Service lists Rocky Mountain populations of western toads, specifically in Colorado, New Mexico, and Wyoming as a candidate species (U.S. Department of the Interior, 1991). The Colorado Division of Wildlife and the New Mexico Department of Game and Fish lists western toads as Endangered, and western toads are a Protected Species in Wyoming. According to Loeffler (1999), they are believed to occur in 15 counties in Colorado and possibly one county in New Mexico. The Utah Division of Wildlife Resources as well as the U.S. Bureau of Land Management has listed this species as Sensitive in Utah, which is defined as "declining population, distribution and/or habitat" (Oliver, 1997).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Adults emerge from hibernation sites and migrate to breeding wetlands. In Washington and Oregon, breeding occurs as early as February–April in the lower elevations west of the Cascade Mountains, and from May–July in the Cascades (Leonard et al., 1993). Males can sometimes exceed females by 20:1 at breeding sites and actively search for gravid females during the night.
Male western toads lack vocal sacs (Hammerson, 1999), although they will emit a chirp when grasped either by other toads or by researchers (Hammerson, 1999; E.M. and P.S. Corn, personal observations). According to Blair (1972a), western toads have lost their mating call. Western toad vocalizations are not typical anuran advertisement calls. There is some controversy about calling in western toads; Awbrey (1972) recorded a putative advertisement call produced by an individual of the western toad subspecies in California (California toad; Bufo boreas halophilus). The calling male was atypical of western toads because it had a vocal pouch, and the call produced was audible at greater distances than the release call (Awbrey, 1972). Whether the call of western toads is simply a release call or functions in the formation of breeding aggregations of males and the attraction of females is unresolved.
In Colorado, breeding depends on the timing of snowmelt, occurring at higher elevations from mid May to mid July (Hammerson, 1999; E.M., personal observations). In the vicinity of Juneau, Alaska, breeding occurs between May and July (J. Van Denburgh, in Wright and Wright, 1949; Hodge, 1976). Although female western toads have been found in Colorado at elevations from 3,462–3,557 m, there is little evidence of successful breeding at these elevations (Campbell, 1970d). In Rocky Mountain National Park, however, western toad eggs and tadpoles have been found in a pond at an elevation of 3,380 m (Corn et al., 1997).
ii. Breeding habitat. Breeding sites are in still or barely moving water, typically ponds and small lakes, streams, rain pools, and ditches (Hodge, 1976; Luce et al., 1997; Hammerson, 1999).
i. Egg deposition sites. In many western toad populations, egg deposition occurs after snowmelt when breeding ponds are refilled (Leonard et al., 1993; Degenhardt et al., 1996). In the Blue and Wallowa Mountains of northeastern Oregon and southeastern Washington, egg deposition can sometimes occur as late as June–July (Leonard et al., 1993). In New Mexican montane populations of western toads, eggs are laid in mid to late July (Degenhardt et al., 1996). Clutches have been laid as late as August in Colorado (Fetkavich and Livo, 1998). Eggs are deposited in shallow water, generally not > 15 cm deep (Hammerson, 1999).
ii. Clutch size. Eggs are laid in strings of double rows of up to 12,000 eggs/clutch (Wright and Wright, 1949; Samollow, 1980; Leonard et al., 1993) with an average of 5,200 eggs/clutch (Carey, 1976). Eggs are large; the vitellus has a diameter of 1.5–1.75 mm (Wright and Wright, 1949).
At an elevation of about 2,440 m (8,000 ft), Ferguson (1954a) observed an instance of western toad egg masses being washed ashore due to large chunks of snow-pack and ice falling into the breeding lake. He suspected that this phenomenon may be common at these and higher elevations and may affect reproductive success.
i. Length of larval stage. Egg and larval development is dependent on the temperature of the water (Smith-Gill and Berven, 1979; Ultsch et al., 1999). Western toad eggs hatch in 3–10 d (Leonard et al., 1993) but often take longer at higher elevations. The duration of the larval stage is 30–45 d, with metamorphosis occurring between May and September (Wright and Wright, 1949; G. Fellers, personal communication). Forelimbs appear at Gosner (1960) stage 42, which generally indicates the onset of metamorphosis. In California, metamorphosis occurs from mid May to late September (G. Fellers, personal communication). In Alaska, metamorphosis occurs between July and August (Hodge, 1976) and has been observed to occur as late as October in Colorado (E.M., personal observations). In Wyoming, tadpoles will metamorphose before the end of summer at elevations below 3,050 m (10,000 ft; Luce et al., 1997). Juvenile toads emerge at 1.0–1.6 cm in length (Leonard et al., 1993; Hammerson, 1999). At higher elevations, tadpoles may not metamorphose before early autumn freezes occur, and overwinter survival of larvae has not been observed during extensive monitoring in Colorado (Fetkavich and Livo, 1998). In Washington, metamorphosis at high elevations may occur 1–2 wk prior to snow accumulation covering the breeding pools (Leonard et al., 1993).
ii. Larval requirements.
a. Food. Larvae will feed upon filamentous algae, detritus, and may even scavenge carrion (Leonard et al., 1993). Western toad tadpoles have been observed to feed on a fish carcasses and on the bodies of conspecifics (E.M., personal observations).
b. Cover. Larvae generally seek out the warmer, shallower portions of their habitat during the day and retreat to deeper waters as temperatures drop. They will swim to cover, rocks, vegetation, or shadows when disturbed.
iii. Larval polymorphisms. Unknown and unlikely.
iv. Features of metamorphosis. Large metamorphic aggregations will form at the edge of the pond, sometimes two or more individuals deep (Lillywhite and Wassersug, 1974; L.J. Livo, personal communication) probably to conserve moisture (Livo, 1998).
v. Post-metamorphic migrations. Upon completion of metamorphosis, juveniles move from natal wetlands to nearby terrestrial sites or to other nearby wetlands (Hammerson, 1999; E.M., personal observations). Hammerson (1999) notes that juveniles may stay to overwinter along the border of their natal wetland.
D. Juvenile Habitat. The habitat characteristics of juvenile western toads are unknown. Presumably, they use the same habitats as adults, but probably use wetland habitat more than terrestrial adults because they are more susceptible to desiccation (Livo, 1998). Because of their small size, newly metamorphosed individuals can use small cracks in the substrate and other very tiny refugia to overwinter, in contrast to adults that typically use larger ground squirrel burrows or other underground hibernacula (see Mullally, 1952).
E. Adult Habitat. Western toads are often found at the water's edge or basking on partially submerged logs in the spring and early summer (Muths and Corn, 1997; Hammerson, 1999). Later in the year, they are often found in more terrestrial habitats, although often in damp areas or near water (Hammerson, 1999). Historically, western toads were thought to be more terrestrial except when breeding, and tolerant of dry habitats (Ruthven and Gaige, 1915; Wright and Wright, 1949).
Bartelt (2000) documented extensive (> 75% of the time) use of terrestrial habitats by western toads in Idaho. He found that conditions at microsites with toads were more humid but approximately the same temperature as randomly chosen microsites without toads (Bartelt, 2000).
In California, western toads are found from valley floors (sea level) to high mountains (> 2,740 m [9,000 ft]) in grassy tussocks near lakeshores, streams, and in mountain meadows (Grinnell and Camp, in Wright and Wright, 1949). In Wyoming, western toads are found in the wet areas of higher elevations between 2,461 and 3,385 m in the foothills, subalpine zones, and mountainous regions (Luce et al., 1997).
During cold weather (temperatures below 3 ˚C), western toads (Bufo boreas halophilus) will use gopher and ground squirrel holes as retreats, where temperatures remain between 4.8 ˚C and 7 ˚C at varying depths even when freezing temperatures occurred above ground (Mullally, 1952). At higher elevations, western toads hibernate in rock-lined chambers near creeks (Campbell, 1970b), in ground squirrel (Spermophilus lateralis) burrows (Jones and Goettl, 1998), in and under root systems of evergreen trees (E.M. and P.S. Corn, personal observations), and possibly in beaver dams (Goettl et al., 1997). Smits (1984) reported that western toads were diurnal following emergence and just prior to hibernation, with an abrupt switch to nocturnality in June. Smits (1984) also observed that western toads avoided extreme temperatures (hot or cold) by retreating to burrows. Experimental results from Smits and Crawford (1984) suggest that thermal cues in western toads direct their daily emergence from subterranean refugia. Lillywhite et al. (1973) found that the preferred body temperature among western toads in the laboratory is approximately 26–27 ˚C. Their results also demonstrated that basking versus seeking cooler temperatures is dependent on both food and moisture availability (Lillywhite et al., 1973).
F. Home Range Size. Western toads with embedded radioactive tags moved 900 m from their summer habitat to their hibernacula, then emerged in the spring to return to their summer habitats (Campbell, 1970b). Western toads were studied at ponds and wetlands at a heavily disturbed site (due to Molybdenum mining) in Colorado. Data from this location suggest that toads use a variety of habitat types from upland aspen/conifer stands to rocky areas (Jones, 2000). Bartelt (2000) reported daily movements up to 439 m and found males to travel shorter distances from breeding ponds than did females. Home range estimates using a fixed kernel method (Worton, 1989) for males (n = 11) and females (n = 12) are 7.09 ha and 16.96 ha respectively (Jones, 2000). In Rocky Mountain National Park, toads have moved over 4 km between breeding locations in 1 yr (E.M. and P.S. Corn, unpublished data). Female toads move farther from breeding sites and use larger home ranges than males (Muths, 2003) and tend to be found more often at drier sites (E.M., unpublished data).
G. Territories. Although there is no evidence of territoriality in western toads, amplexed males will defend females from approaching males (Black and Brunson, 1971). Black and Brunson (1971) observed a male of an amplexing pair kick away six males that approached. During the breeding season, Black and Brunson (1971) also observed males to actively approach and briefly wrestle with newly arriving males.
Male western toads displaced from their breeding sites returned rapidly and directly to the site of capture, under both clear and overcast skies (Tracy and Dole, 1969). Tracy and Dole (1969) found under test conditions that olfactory cues appear to play a large role in orienting displaced animals towards their breeding sites (Tracy and Dole, 1969) and immature toads are reported to use celestial cues in dispersal (Tracy, 1971).
H. Aestivation/Avoiding Dessication. Not documented.
I. Seasonal Migrations. Western toads emerge from hibernation and travel to breeding pools in the spring, move to terrestrial sites during the summer, and return to hibernacula in the autumn (Hammerson, 1999). In Colorado, western toads generally emerge from hibernacula in May (Hammerson, 1999) and begin to move toward hibernacula in late August to early September (Campbell, 1970b; Hammerson, 1999). Activity varies among locations and seasons. Western toads are active in California from February (in some years) to late October (Mullally, 1952; G. Fellers, personal communication). Toads in a controlled environment were observed to stay close to their burrows during seasonal transitions (immediately pre-hibernation/post-emergence or between diurnal and nocturnal activity; Smits, 1984).
J. Torpor (Hibernation). According to Pinder et al. (1992), in amphibians that are not freeze tolerant, the metabolism of hibernating animals does not appear to decrease more than is expected from temperature effects alone. Campbell (1970b) found western toads hibernating beneath or near large boulders along a spring-fed brook supplemented by extensive snow accumulation to maintain the flow of groundwater. The toads were found with feet and venters in contact with moist substrate, not in physical contact with other toads.
K. Interspecific Associations/Exclusions. Western toads will breed in association with spadefoot toads (Spea sp.; Cope, in Wright and Wright, 1949). They will also breed in association with Pacific treefrogs (Pseudacris regilla), Yosemite toads (B. canorus), and several species of Rana (G. Fellers, personal communication). Western toads will naturally hybridize with red-spotted toads (B. punctatus) in the southwestern United States and in Baja, Mexico (Feder, 1979).
L. Age/Size at Reproductive Maturity. Sexually mature adult males range from 56–108 mm SVL; females from 60–125 mm (Wright and Wright, 1949). Western toads become sexually mature at 4–6 yr old (Carey, 1976; Carey et al., 2001).
M. Longevity. Western toads that survive to adulthood have a long life expectancy (Campbell, 1970c, in Samollow, 1980; Hammerson, 1999). Snider and Bowler (1992) report a wild-caught western toad lived almost 6 yr, 3 mo. Western toads in Colorado live at least 9 yr (Campbell, 1976) and probably much longer (Muths and Corn, 2000).
N. Feeding Behavior. Prey items are ingested by a quick extension of the tongue, termed a "zot," to snap up a prey item (Tracy, 1973). Primary food sources include spiders, worms, ants, moths, beetles and other arthropods (Campbell, 1970a,c; Barrentine, 1991b; Leonard et al., 1993; Luce et al., 1997). Billbug weevils (Sphenophorus sp.) are also ingested frequently. However, Barrentine (1991a) found that approximately 68% of ingested billbug weevils survived through the digestive tract and emerged from fecal pellets; thus, little nutrition is derived from ingested billbug weevils. Cunningham (1954) documented an instance of adult cannibalism in western toads.
O. Predators. Western toads are preyed upon by garter snakes (Thamnophis sp.), coyotes (Canis latrans), raccoons (Procyon lotor), and some birds (Leonard et al., 1993). Badgers will also eat boreal toads (Long, 1964). Ravens (Corvus corax) eviscerate toads and leave them partially eaten, presumably to avoid the toxins in the skin (Olson, 1989; Corn, 1993b). Beiswenger (1981) reported tadpole predation by gray jays (Perisoreus canadensis), with no indication that the birds found the tadpoles to be distasteful (see "Anti-Predator Mechanisms" below). Predaceous diving beetle larvae (Dytiscus sp.) and garter snakes were observed to prey upon western toad larvae (Livo, 1998), as were spotted sandpipers (Actitis macularia) and mallard ducks (Anas platyrynchos; Jones et al., 1999).
P. Anti-Predator Mechanisms. When threatened by predators, adult western toads secrete a mild toxin from the parotoid glands (Cei et al., 1968; Stebbins, 1985). In Bufo sp., this toxin is secondarily deposited in the ova and thought to afford some protection to newly laid, fertilized eggs, although the toxicity of the eggs is thought to decrease as the zygotes develop (Brodie et al., 1978). Generally, reports on bufonid larvae show that invertebrate predators find them palatable, while many vertebrate predators find them unpalatable (Kruse and Stone, 1984). Hews and Blaustein (1985) demonstrated that chemoreception can elicit an alarm response in western toad tadpoles and thus may contribute to predator avoidance.
Q. Diseases. Amphibian chytridiomycosis (chytrid fungus) is a recently described disease caused by infection by the fungus Batrachochytrium dendrobatidis. Chytrids, fungi in the phylum Chytridiomycota, live in water and soil and are considered to be ubiquitous. There are many genera and species of chytrid fungus, most of which are beneficial saprobes. Batrachochytrium dendrobatidis, cultured, identified, and named by Longcore et al. (1999), is the first member of this phylum to be described as a parasite or pathogen of a vertebrate species. Batrachochytrium dendrobatidis has been implicated as a contributor to amphibian declines in Australia, Central America (Berger et al., 1998), and the United States (Nichols et al., 1998; Daszak et al., 1999). This chytrid fungus has been identified in declining wild populations of western toads in Colorado and Wyoming (Muths et al., in press; D.E. Green, personal communication).
Upon infection, Batrachochytrium dendrobatidis lives inside cells in the superficial layer of amphibian skin (Longcore et al., 1999; Pessier et al., 1999). Amphibians rely on their permeable skin for a variety of uses. Severe infection by this chytrid fungus causes a thickening, or hyperkeratosis, of the skin. Because amphibians obtain moisture by absorbing it through their skin, infection may compromise water absorption and/or osmoregulation enough to cause death (Nichols et al., 1998). Alternately, B. dendrobatidis infection may affect immune systems and may damage the skin sufficiently to allow micro-organisms such as bacteria to cause lethal secondary infections (D.E. Green, personal communication). Information on sublethal infections of or spontaneous recovery from chytridiomycosis in western toads is lacking.
Olson (1989) found that disease caused by the bacteria Aeromonas hydrophila (often called "red-leg") was fairly common among western toads. Pathologists suspect that bacterial infections in the 1970s and 1980s may have been secondary to chytrid fungus infections (D.E. Green, personal communication). Carey (1993) proposed that suppression of the immune system leaves toads vulnerable to bacteria such as Aeromonas, but there has been no convincing evidence of what might be causing immunosuppression.
R. Parasites. James and Maslin (1947) report parasitization of western toads by fly larvae and Ingles (1936) discusses "worm parasites" of California amphibians including western toad species. More recently, Paperna and Lainson (1995) reported parasites in marine toads (Bufo marinus), but specifics about western toads are not known.
4. Conservation. Western toad populations were last documented in New Mexico in 1986. Causes of this extirpation may include increased competition due to the presence of introduced fathead minnows (Pimephales promelas; Carey, 1987), mortality due to natural predators (Corn, 1993b), mortality due to trampling from domestic livestock usage of wetlands (Bartelt, 1998), and decreased hatching success due to UV-B radiation exposure (Blaustein et al., 1994c, see also Degenhardt et al., 1996; Corn, 1998). High altitude populations in Colorado and elsewhere in the southern Rocky Mountains are in severe decline (for a discussion of reasons, see Carey et al., 2003; this volume).
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Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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