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Western Toad - Anaxyrus boreas
Anaxyrus boreas boreas
State Rank Reason (see State Rank above)
Over the last few decades this species has undergone serious declines in abundance due primarily to infection with Chytrid fungus. While declines in breeding site occupancy appear to have stabilized in the last decade, changes to abundance across the species range within Montana remain unknown. Significant threats to the persistence of this species remain from continued impacts of disease and mortality of adults and young during breeding and local migration.
- Details on Status Ranking and Review
Western Toad (Anaxyrus boreas) Conservation Status Review
Review Date = 09/25/2018
ScoreF - 20,000-200,000 km squared (about 8,000-80,000 square miles)
Comment142,183 square Kilometers from Natural Heritage Program range maps
ScoreB - Large Decline (decline of 75-90%)
CommentVery few sites in drainages with suitable habitat are occupied. State populations are thought to have declined precipitously in the 1980s, possibly due to Chytrid fungus. Robust baseline data are not available to estimate the decline in abundance, but given that < 5% of seemingly suitable breeding sites are used within range and often by relatively few individuals, declines of up to 90% seem reasonable.
ScoreU/E - Unknown, but believed to be stable with population, range, area occupied, and/or number or condition of occurrences unchanged or remaining within ±10% fluctuation
CommentSurveys of known breeding locations across the Montana range did not find a difference from 2003-2005 surveys in the proportion of sites with evidence of breeding. Declines may have stabilized. However, this metric does not assess abundance, and changes in the number of animals using a site or area is possible.
ScoreA - Substantial, imminent threat. Threat is moderate to severe and imminent for most (>60%) of the population or area.
CommentFurther declines in abundance due to disease, vehicle caused mortalities on roads in proximity to migratory corridors and/or breeding sites
SeverityHigh - Loss of species population (all individuals) or destruction of species habitat in area affected, with effects essentially irreversible or requiring long-term recovery (>100 years).
CommentIf disease continues to impact populations, long term recovery could exceed 50 years if it is reversible with the continued presence of the chytrid pathogen. Declines may have isolated extant populations, requiring significant recovery to reconnect these
ScopeHigh - > 60% of total population or area affected
CommentAll of the species range is under threat
ImmediacyHigh - Threat is operational (happening now) or imminent (within a year).
CommentThreats are ongoing
ScoreB - Moderately Vulnerable. Species exhibits moderate age of maturity, frequency of reproduction, and/or fecundity such that populations generally tend to recover from decreases in abundance over a period of several years (on the order of 5-20 years or 2-5 generations); or species has moderate dispersal capability such that extirpated populations generally become reestablished through natural recolonization (unaided by humans).
CommentSlow maturation but can have high fecundity but low juvenile survival
ScoreB - Narrow. Specialist. Specific habitat(s) or other abiotic and/or biotic factors (see above) are used or required by the Element, but these key requirements are common and within the generalized range of the species within the area of interest.
CommentSpecies needs lentic waterbodies to breed
Raw Conservation Status Score
3.5 + 0 (geographic distribution) + 0 (environmental specificity) + -0.25 (long-term trend) + -1 (threats) = 2.25
Laid in long strings that are one to three (usually two) eggs wide in a zigzag pattern and contain 1,000 to more than 18,000 eggs (usually 6,000 to 12,000) (Livezey and Wright 1947, Samallow 1980, Olson 1988, Carey et al. 2000). Each ovum is black above, white below, and is surrounded by two jelly layers. Ovum diameters are 1.5 to 1.8 mm, but total egg diameters, including both jelly layers, are approximately 5 to 6.8 mm (Livezey and Wright 1947, Karlstrom 1962a, Maxell et al. 2002). In Montana, clutch size has been documented at 20,469 eggs in a 30 cm strand of two jelly layers. Egg were reported hatching in less than 7 days (Maxell et al. 2002).
Body and tail musculature are black or dark brown with either a black or gray belly (Maxell et al. 2009). The tail fins are both clear with dendritic pigmentation, with the dorsal tail fin having more pigmentation (Maxell et al. 2009). The anus is on the midline at the front end of the ventral tail fin. The eyes about midway between the dorsal midline and edge of the head. Labial tooth rows are 2/3, oral papillae are restricted to the sides of the mouth. Total length (TL) of 10-38 mm (Carpenter 1953, Corkran and Thoms 2006).
JUVENILES AND ADULTS
The skin is dry with a dorsal base color of olive green or light or dark brown with reddish or light brown color on the warts and small black spots over everything (Maxell et al. 2009). The warts may be reddish-brown and encircled by dark pigment. Ventral color is cream to tan mottled with numerous dark blotches. A white stripe extends down the center of the back in older individuals but may be absent or inconspicuous in juveniles (Maxell et al. 2009). Parotid glands are oval and larger than the eyes, located behind the eye and tympanum. Cranial crests are absent or indistinct. The eyes have horizontal pupils. The hind feet each have two light brown digging “spades” on their soles, but the spades lack a sharp cutting edge (Black 1970b, Maxell et al. 2009). Mature males have a dark patch on the inner surface of the innermost digit ("thumb") during breeding. Males lack a vocal sac; however, they may produce a repeated chirping sound. Snout-vent length (SVL) of 11-118 mm; with males rarely exceeding 95 mm SVL and females rarely 110 mm (Black 1970b, Maxell et al. 2009). Recently metamorphosed toadlets measure about 10 to 16 mm SVL but can be 16 to 20 mm (Maxell et al. 2002). Juveniles 20 to 35 mm SVL often are present in wetlands adjacent to breeding sites (Nussbaum et al. 1983, Hammerson 1999).
Calls of this species are soft, high-pitched chirps likened to young geese. It is unsure if these calls are for attracting females or territorial communication between males (Werner et al. 2004). Calls typically do not carry far and can be heard a distance of 20 meters (approximately 66 feet) (Bryce Maxell, personal communication).
Adult Western Toad lack the prominent cranial crests found on the other species of Montana toads. The Woodhouse’s Toad (Anaxyrus woodhousii
) have parallel cranial crests on the snout and behind the eyes in the shape of an “L”. Western Toad have horizontal rather than vertical pupils. Eggs and larvae of Western Toad tadpoles lack visible white or gold flecks on the back that are present in Woodhouse's and Great Plains Toad (Anaxyrus cognatus
) tadpoles (Werner et al. 2004). Woodhouse's Toad ova are enclosed in a single jelly layer, not two, and Great Plains Toad eggs are in strings that are noticeably pinched between each egg (Bragg 1937a). However, eggs and tadpoles of Western and Woodhouse's Toads are very similar and may be indistinguishable in some cases. Distribution is a useful character for all life stages. The geographic range of Great Plains Toad does not overlap with the geographic range of Western Toad. See the geographic range of Woodhouse’s Toad for limited areas of possible overlap in a narrow area north of the Beartooth and Absaroka mountains (Maxell et al. 2003).
Western Hemisphere Range
The Western Toad is currently recognized as two subspecies that range from the Rocky Mountains to the Pacific Coast and from Baja Mexico to southeast Alaska and the Yukon Territory at elevations up to 3,640 m (11,940 ft) (Stebbins 2003, Hammerson 1999). One subspecies, the Boreal Toad (Anaxyrus boreas boreas), is currently recognized as occurring in Montana. However, mitochondrial DNA analysis indicates that four main phylogenetic groups exist, and each may warrant recognition as separate species: (1) a southern Rocky Mountain group in Colorado and southern Wyoming; (2) a southern Utah group; (3) a northwest group including all specimens in Montana, northern Idaho, and northern Wyoming; and (4) a southwest group composed of individuals currently recognized as the California Toad (Anaxyrus boreas halophilus), the Black Toad (Anaxyrus exsul), and the Amargosa Toad, (Anaxyrus nelson) (Goebel 1996). If these phylogenetic groups are eventually recognized as full species it is likely that populations across Montana and the Pacific Northwest will be recognized as the Western Toad, (Anaxyrus boreas). In Montana the species has been documented across the mountainous portion of the state west of the Beartooth Plateau, and the eastern edge of the Castle, Little Belt, and Highwood Mountains (Black 1970b, Black 1971).
Maximum elevation: 2,810 m (9,220 ft) in Gallatin County (Maxell et al. 2003).
Observations in Montana Natural Heritage Program Database
Number of Observations:
(Click on the following maps and charts to see full sized version)
Map Help and Descriptions
(Observations spanning multiple months or years are excluded from time charts)
No information is available specific to Montana. Elsewhere it is known that the Western Toad migrates between aquatic breeding and terrestrial nonbreeding habitats. In Colorado, movements of 900 meters (with 95 meters change in elevation) to 4 kilometers have been reported (Hammerson 1999), and radio-tracked females in Idaho have been observed to move up to 2.4 kilometers from breeding ponds (Koch and Peterson 1995). Movement patterns are highly variable, with some individuals remaining in the same location for several days, then moving 50 meters or more on several consecutive nights.
The Western Toad is known to utilize a wide variety of habitats, including desert springs and streams, meadows and woodlands, mountain wetlands, beaver ponds, marshes, ditches, and backwater channels of rivers where they prefer shallow areas with mud bottoms (Brunson 1952, Carpenter 1953, Black 1970b, Campbell 1970c, Nussbaum et al. 1983, Baxter and Stone 1985, Russell and Bauer 1993, Koch and Peterson 1995, Cavallo 1997, Hart et al. 1998, Hammerson 1999). Forest cover around occupied montane wetlands may include Aspen (Populus tremuloides
), Douglas-fir (Pseudotsuga menziesii
), Lodgepole Pine (Pinus contorta
), Engelmann Spruce (Picea engelmannii
), and Subalpine Fir (Abies lasiocarpa
); in local situations it may also be found in Ponderosa Pine (Pinus ponderosa
) forest. They also occur in urban settings, sometimes congregating under streetlights at night to feed on insects (Hammerson 1999). Normally they remain close to ponds, lakes, reservoirs, and slow-moving rivers and streams during the day, but may range widely at night.
Habitats used by Western Toads in Montana are similar to those reported for other regions, and include low elevation beaver ponds, reservoirs, streams, marshes, lake shores, potholes, wet meadows, and marshes, to high elevation ponds, fens, and tarns at or near the treeline (Rodgers and Jellison 1942, Brunson and Demaree 1951, Miller 1978, Marnell 1997, Werner et al. 1998a, Boundy 2001). Forest cover in or near encounter sites is often unreported, but Western Toads have been noted in open-canopy Ponderosa Pine woodlands and closed-canopy dry conifer forest in Sanders County (Boundy 2001), Willow (Salix
spp.) wetland thickets and Aspen stands bordering Engelmann Spruce stands in Beaverhead County (Jean et al. 2002), and mixed Ponderosa Pine/Cottonwood/Willow sites or Douglas-fir/Ponderosa Pine forest in Ravalli and Missoula counties (Paul Hendricks, personal observation).
Ecological Systems Associated with this Species
- Details on Creation and Suggested Uses and Limitations
How Associations Were Made
We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for
vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
- Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
- Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
- Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
- Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use.
In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system.
However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system, even if
point observations were associated with that system.
Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature.
The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association.
If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.
Suggested Uses and Limitations
Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning.
These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at: mtnhp.org/requests
) or systematic surveys for species and evaluations of habitat at a local site level by trained biologists.
Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales.
Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade.
Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections).
Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.
- Adams, R.A. 2003. Bats of the Rocky Mountain West; natural history, ecology, and conservation. Boulder, CO: University Press of Colorado. 289 p.
- Dobkin, D. S. 1992. Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34. Missoula, MT.
- Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
- Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998. Montana atlas of terrestrial vertebrates. Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT. 1302 p.
- Hutto, R.L. and J.S. Young. 1999. Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32. 72 p.
- Maxell, B.A. 2000. Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species. Report to U.S. Forest Service Region 1. Missoula, MT: Wildlife Biology Program, University of Montana. 161 p.
- Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath. 2004. Amphibians and reptiles of Montana. Missoula, MT: Mountain Press Publishing Company. 262 p.
- Commonly Associated with these Ecological Systems
Forest and Woodland Systems
Human Land Use
Recently Disturbed or Modified
Shrubland, Steppe and Savanna Systems
Wetland and Riparian Systems
- Occasionally Associated with these Ecological Systems
Forest and Woodland Systems
Human Land Use
Recently Disturbed or Modified
Shrubland, Steppe and Savanna Systems
Adults feed on a variety of invertebrates including spiders, daddy longlegs, and millipeds, but rely most heavily on ground dwelling coleopterans and hymenopterans. They are also known to eat smaller vertebrates including smaller individuals of their own species (Cunningham 1954, Moore and Strickland 1955, Mullally 1958, Livezey 1961, Campbell 1970a, Miller 1975, 1978, Hansen and Thomason 1991).
Research outside the state shows that metamorphosed individuals feed on various small invertebrates, including seven orders of flying insects, spiders, mites, daddy longlegs, snails, crayfish, sowbugs (terrestrial amphipods), and earthworms (Nussbaum et al. 1983, Hammerson 1999). Larvae filter suspended plant material or feed on bottom detritusand other dead tadpoles or adults (Black 1970e; Franz 1971; Loeffler 1998).
Eggs and larvae develop in still, shallow areas of ponds, lakes, or reservoirs or in pools of slow-moving streams, often where there is sparse emergent vegetation. Basking groups of up to 1000 young-of-year have been observed in Waterton National Park; basking sites may be important at high elevations (Black and Black 1969). They are active day/evening in early summer and late evening/night when it's hot (e.g. August) (Miller 1975).
Adults and juveniles apparently use olfactory and celestial cues, respectively, to orient (Tracy and Dole 1969a; Tracy 1971). The active period typically begins in April or May and extends to September or October, depending on elevation and latitude (Russell and Bauer 1993, Koch and Peterson 1995, Hammerson 1999), although adults may be active in January and February in coastal populations (Nussbaum et al. 1983). In Montana, records extend from late April to early October (Rodgers and Jellison 1942, Brunson and Demaree 1951, Black and Brunson 1971, Hendricks and Reichel 1996a, Boundy 2001). Smaller juveniles are active almost exclusively diurnally and adults are usually active at night except during the spring and at high elevation (Mullally 1958, Lillywhite et al. 1973, Sullivan 1996, Sullivan et al. 1996, Maxell 2000). Adults may move more than 800 m in a night, may move more than 4 km away from water after breeding, and can remain away from surface water for relatively long periods of time (Pimentel 1955, Tracy and Dole 1969, Campbell 1970b, Loeffler 1998). Juveniles may disperse up to or more than 4 km from their natal site (Sornborger 1979). Adult and juvenile toads are freeze intolerant and overwinter and shelter in underground caverns, or more commonly in rodent burrows (Mullally 1952, Black 1970e, Nussbaum et al. 1983, Smits 1984, Koch and Peterson 1995, Hammerson 1999, Jones 1999).
Predators of adult Western Toads include Raccoon (Procyon lotor
), domestic dog, Coyote (Canis latrans
), Red Fox (Vulpes vulpes
), Short-tailed Weasel (Mustela erminea
), American Mink (Mustela vison
), Marten (Martes americana
), Badger (Taxidea taxus
), American Black Bear (Ursus americanus
), Northern Pygmy-Owl (Glaucidium gnoma
), Black-billed Magpie (Pica hudsonia
), Common Raven (Corvus corax
), American Crow (Corvus brachyrhynchos
), Steller's Jay (Cyancitta stelleri
), Canada Jay (Perisoreus canadensis
), American Robin (Turdus migratorius
), Loggerhead Shrike (Lanius ludovicianus
), and Northern Shrike (Lanius borealis
) (Salt 1979, Corn 1993, Olson 1989, Brothers 1994, Koch and Peterson 1995, Hammerson 1999, Jones et al. 1999). Predators of Western Toad tadpoles include Mallard (Anas platyrhynchos
), Spotted Sandpiper (Actitis macularius
), Terrestrial Gartersnake (Thamnophis elegans
), Tiger Salamander (Ambystoma mavortium
), Wood Frog (Lithobates sylvaticus
) tadpoles and diving beetle larvae. Predators of Western Toad in Montana are not documented.
The reproductive biology of Western Toads in Montana is poorly described. The known breeding period extends from April to mid-July. Breeding aggregations of more than 40 adult males have been reported in mid-May (Black and Brunson 1971). Timing of breeding is dependent on temperature, snowmelt, and/or the presence of surface water from flooding and takes place from May to July in shallow areas of large and small lakes, beaver ponds, temporary ponds, slow-moving streams, and backwater channels of rivers (Metter 1961, Black 1970b). Water chemistry at most breeding sites generally has a high pH (>8.0), high conductivity, and high acid-neutralizing capacity (Koch and Peterson 1995).
Females wrap egg strings around emergent vegetation or clumped loosely in shallow (usually less than 15 cm) waters (Black 1970b; Hammerson 1999). Eggs are laid in two jelly strings from early May to late June and hatch in approximately 5 days. Tadpoles commonly form dense aggregations in shallow warmer waters from late May to early September. Tadpoles metamorphose en masse in 40 (Maxell et al. 2009) to 75 (Loeffler 1998) days and metamorphs can be found in dense aggregations adjacent to breeding grounds (Turner 1952a, Black and Black 1969, Lillywhite and Wassersug 1974, Devito et al 1998). Recently metamorphosed toadlets have been reported from early June to late August (Brunson and Demaree 1951, Werner and Reichel 1994, Reichel 1995a, Hendricks and Reichel 1996a, Marnell 1997, Boundy 2001, Burton et al. 2002). Size of one clutch in the Bitterroot Valley of Ravalli County was 20,000 eggs. These eggs were laid in late May and produced metamorphosed toadlets by July 11, about 40 to 49 days after oviposition (Maxell et al. 2002).
In other areas of the species' range, the breeding period is known to be variable depending on location. In the mountains, breeding follows the melt of winter snowpack, and in some cases, eggs may be laid when ponds are still rimmed with ice. Water temperature may be as low as 7.5 °C but usually more than 9.0 °C (Salt 1979). At a given site, breeding may extend over several weeks, although a peak usually occurs.
Breeding aggregations may be quite large (more than 2,000 adults), but most often are much smaller, with males outnumbering females. Typical clutches in Colorado contain 3,000 to 9,000 eggs (mean of 5,200 eggs), although in the Pacific Northwest clutches of 12,000 eggs are considered normal, and may reach 16,000 eggs (Nussbaum et al. 1983, Koch and Peterson 1995, Hammerson 1999). Eggs hatch in 3 to 12 days. Tadpoles are about 1.0 cm total length at hatching and grow to about 2.5 to 3.0 cm. They sometimes are found in huge aggregations; one in Yellowstone National Park, Wyoming measured 0.5 m wide and more than 36 m long, and another in the Oregon Cascades was 1.0 m wide by 300 m long (Nussbaum et al. 1983, Koch and Peterson 1995).
Tadpoles metamorphose in the first summer. They can take two months or more to reach metamorphosis, depending on the water temperature. At high elevation sites near the treeline in Wyoming and Colorado, tadpoles may fail to metamorphose (Baxter and Stone 1985, Hammerson 1999). Persistence of these populations may be dependent on immigration of juveniles and adults from lower elevations as there are no observations of tadpoles overwintering. Metamorphosis usually occurs during August in Colorado and Oregon but may occur in late July to mid-September. Toadlets may overwinter along the borders of the pools where they developed or move to other nearby wetlands. The minimum age of breeding males is four years, and six years for breeding females; captive animals have lived up to 35 years (Russell and Bauer 1993, Hammerson 1999).
The following was taken from the Status and Conservation section for the Western Toad account in Maxell et al. 2009
Within the last twenty-five years populations of Western Toad have undergone population crashes
in Colorado, Utah, southeast Wyoming, and New Mexico (Stuart and Painter 1994, Ross et al. 1995, Corn et al. 1997, Loeffler 1998). (Anaxyrus boreas
) is now listed as endangered by the State of Colorado and considered a candidate species which is warranted, but precluded, for federal listing by the USFWS in the southern Rocky Mountains (Colorado, southeast Wyoming and northern New Mexico) (Ross et al. 1995, Loeffler 1998). The estimated cost of implementing the first four years of the recovery plan for the Southern Rocky Mountain population is one million twenty-five thousand dollars (Loeffler 1998). Reports of declines in Western Toad populations have also been reported in Oregon and California (Blaustein et al. 1994a, Stebbins and Cohen 1995, Drost and Fellers 1996, Fisher and Shaffer 1996).
Until the late 1990’s many biologists believed that populations in the northern Rocky Mountains had not undergone similar declines. However, surveys in the late 1990’s revealed that populations of the Western Toad were absent from a large number of their historic localities and that although they were still widespread across the landscape they occupied an extremely small proportion of suitable habitat (less than 10% in most cases, but usually less than 5%) (Werner and Reichel 1994, 1996a, Reichel 1995a, 1996, 1997b, Koch and Peterson 1995, Koch et al. 1996, Hendricks and Reichel 1996a, Werner et al. 1998a, reviewed by Maxell et al. 1998). As a result of these findings the USFS listed the Western Toad as a sensitive species in all Region 1 Forests (USDAFS 1999) and initiated a regional inventory program in Montana. The systematic inventory of standing water bodies in 40 randomly chosen 6th level Hydrologic Unit Code (HUC) watersheds across western Montana during the summer of 2000 also found Western Toad populations to be widespread, but extremely rare. Of the 40 watersheds that were surveyed Western Toads were found in 11 (27%), and of the 33 watersheds that contained suitable breeding habitat they were found breeding (eggs, larvae, or metamorphs observed) in 7 (21%). However, of the 347 standing water bodies that were surveyed within these watersheds, Western Toads were only found at 13 (3.7%) and were found breeding at only 9 (2.6%). Furthermore, at sites where Western Toads were observed, only small numbers of adults or relatively small numbers of eggs or larvae were observed. Similarly, in an inventory of approximately 400 standing water bodies in Glacier National Park during the summers of 1999 and 2000, Western Toads were found and bred at approximately 5% (Steve Corn, USGS BRD Aldo Leopold Institute, pers. comm.). Similar patterns have been observed on the Flathead Indian Reservation where years have been skipped between breeding events (Kirwin Werner, Salish Kootenai College, pers. comm.). Thus, the evidence to date suggests that Western Toads have either undergone a decline in the 1980s and are now in the process of recovering, or they have undergone a decline and are continuing to decline because populations are small, isolated, and/or subject to one or more factors that are impacting populations separately or synergistically.
Risk factors relevant to the viability of populations of this species are likely to include all the risk factors described above. As a supplement to this information managers may wish to refer to Loeffler (1998) who, for the recovery of Western Toad populations in the Southern Rocky Mountains, reviews these and other general risk factors and provides management guidelines to mitigate their impacts. Individual studies that specifically identify risk factors or other issues relevant to the conservation of Western Toads include the following. (1) Carey (1993) observed the disappearance of several populations of Western Toads in the West Elk Mountains of Colorado between 1974 and 1982 and during this period found many Western Toads with symptoms of red-leg disease, a common bacterial infection in amphibians and fish. She hypothesized that an unidentified environmental factor had caused sublethal stress of the Western Toads, which caused immune response to be suppressed leading to the systemic infection and death of Western Toads. More recently the chytrid fungus (Batrachochytrium dendrobatidis
), which is suspected to be responsible for declines of amphibians in Australia, Central America, and the western United States has been found to have caused mass mortalities in Western Toad populations in Colorado during the summer of 1999 (Berger et al. 1998, Daszak et al. 1999, 2000, Morell 1999, Milius 1999, 2000, Carey et al. 1999b). As was observed for declines in the late 1970’s and early 1980’s only metamorphosed individuals died (Carey et al. 2000). The fungus only seems to attack keratinized tissues, so metamorphosed individuals with lots of keratinized tissues die and tadpoles with keratinized tissues only around the mouthparts survive until metamorphosis (Berger et al. 1998, Morell 1999). Another line of evidence to suggest that the chytrid fungus was responsible for declines in the late 1970’s and early 1980’s is that Northern Leopard Frog (Lithobates pipiens
) populations in Colorado crashed at the same time that Western Toad populations did and museum specimens of Northern Leopard Frogs that were collected during these time periods have now been found to have the chytrid fungus (Daszak et al. 1999, Milius 2000). Thus, the chytrid fungus may be the most likely cause of declines of Western Toads and the near extirpation of Northern Leopard Frogs in western Montana in the late 1970’s and early 1980’s and clearly represents a threat to populations today. Another fungus, Saprolegnia ferax
, has been found to cause 95% mortality of an estimated 2,496,000 Western Toad embryos at a site in Oregon (Blaustein et al. 1994c). Spread of the fungus between egg strings is enhanced by the behavior of Western Toads because females often deposit eggs communally (Kiesecker and Blaustein 1997a). (2) Hailman (1984) found that Western Toads tended to congregate around roads in the late evening and early morning. Cunningham (1954) found hundreds of Western Toads flattened on a highway the morning following a summer thunderstorm. (3) Olson (1989, 1992a) reports that Common Ravens killed large number of breeding Western Toads (20% of the entire breeding population at one site) at three sites in the Oregon Cascades. The author speculates that human activity near these sites may serve to concentrate Common Raven activity in the area and subsequently leads to Western Toad predation. Similarly, Brothers (1994) found Western Toads being preyed on by American Crows and Beiswenger (1981) found tadpoles being preyed upon by Canada Jays. Furthermore, Jones et al. (1999) report predation of Western Toad tadpoles, metamorphs, and adults by a number of avian and mammalian species that may be attracted to areas of human activity and/or subsidized by the presence of humans, subsequently leading to increased rates of predation. These animals included Mallards, Spotted Sandpipers, American Robins, Red Fox, Raccoon, and a domestic dog. Sherman and Morton (1993) also report high levels of predation on breeding aggregations of the closely related Yosemite Toad (Anaxyrus canorus
) by Clark’s Nutcrackers (Nucifraga columbiana
), California Gulls (Larus californicus
), and Common Ravens. Fisher and Shaffer (1996) implicate introduced American Bullfrog (Lithobates catesbeianus
) and fish predators in the decline of Western Toads in the Sacramento and San Joaquin Valleys in California. However, Jones et al. (1999) found that neither Cutthroat Trout (Oncorhynchus
sp.) or Brook Trout (Salvelinus fontinalis
) would prey on tadpoles and Licht (1968) found that Western Toad eggs were not palatable to fish. Furthermore, both Drost and Fellers (1996) and Corn et al. (1997) found Western Toads breeding at sites with and without fish. (4) After what may have been the first successful reproductive event at a site in southeastern Idaho in 10 years Bartelt (1998a) documented the deaths of thousands of Western Toad metamorphs when 500-1,000 sheep were herded through the drying pond the toadlets were concentrated. He found that hundreds of animals had been directly killed underfoot and hundreds more died soon afterward as a result of desiccation because the vegetation they had been hiding in had been trampled to the point that it no longer provided a moist microhabitat. (5) Antimycin and rotenone, two commonly used piscicides, are both toxic to Western Toad tadpoles (Loeffler 1998). (6) Johnson and Prine (1976) exposed juvenile Western Toads to the insecticides Abate, fenthion, chlorpyrifos-methyl, chlorpyrifos-ethyl, methylparathion, and the insect growth regulator Altosid for 24 hours at one half the concentrations usually applied in the field. They found that Western Toads exposed to the insecticides reduced their activity levels and had lower tolerance to high temperatures than Western Toads in the control group. (7) Porter and Hakanson (1976) found that a variety of heavy metals found in drainage water from mines in Colorado were highly lethal to Western Toad larvae. Furthermore, they found that lethal pH for tadpoles ranges from 3.1 to 4.0. Other studies report that no significant embryo mortality is observed for Western Toads until pH falls below 4.9, but embryos have an LC50 at pH less than or equal to 4.5 (Corn et al. 1989, Corn and Vertucci 1992, Vertucci and Corn 1996). (8) In Oregon, Blaustein et al. (1994d) found that survival rates for Western Toad embryos was lower when they were exposed to ambient UV-B radiation than when they were shielded from UV-B radiation and attributed this to the presence of low levels of photolyase, an enzyme that is known to repair UVB damage to DNA. However, Kiesecker and Blaustein (1995) found that UV-B may only be impacting embryo survival as a result of a synergistic interaction with the fungus Saprolegnia ferax
. They found that embryos had 95-100% survival rates when exposed to ambient UV-B radiation in the absence of Saprolegnia
. However, when embryos were infected with Saprolegnia
survival dropped to 50% at ambient UV-B levels. Similarly, Corn (1998) failed to find a relationship between exposure to UV-B and embryo survival to hatching in Colorado and noted that a number of other studies have also failed to find a convincing impact of ambient levels of UV-B radiation on amphibian embryos. At artificially high levels of UV-B exposure Worrest and Kimeldorf (1975) report a decline in larval survivorship of Western Toads from 94-100% in controls to 0%, 17%, and 41% for UV-B treatments exposed to 0, 2, and 4 hours of photoreactivating (>315 nm) light following UV-B exposure.
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