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USGS Staff -- Published Research US Geological Survey

2002

Wyoming Toad (Bufo baxteri)

R. Andrew Odum

Paul Stephen Corn

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Odum, R. Andrew and Corn, Paul Stephen, "Wyoming Toad (Bufo baxteri)" (2002). USGS Staff -- Published Research. 91. https://digitalcommons.unl.edu/usgsstaffpub/91

This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Published in Amphibian Declines: The Conservation Status of United States Species, Edited by Michael Lannoo. University of California Press, 2005.

Park Zoo, New York; Cheyenne Mountain Lewis et al. (1985),Parker (1998),and oth- Zoo, Colorado Springs, Colorado; Detroit ers (see "Historical versus Current Distrib- Zoo, Detroit, Michigan; Houston Zoo, ution" above) report that Wyoming toads Houston, Texas; Sedgwick County ZOO,Wi- were abundant in the 1950s and 1960s, chita, Kansas; St. Louis Zoo, St. Louis, Mis- but by the mid-1970s experienced a dras- souri; and Toledo Zoo, Toledo, Ohio); at the tic decline (Baxter and Stone, 1985). A Bufo baxteri Porter, 1968 Saratoga National Fish Hatchery, Saratoga, 1980 survey revealed Wyoming toads in a WYOMING TOAD Wyoming, and the Wyoming Game and 5 km2 area (Van Kirk, 1980). In 1981, only R. Andrew Odum, Paul Stephen Corn Fish Sybille Wildlife Research Unit, Sybille, one male and one female were observed 1. Historical versus Current Distribution. Wyoming. These animals are managed co- during the breeding season (Baxter and Historically, Wyoming toads (Bit@ bnxreri) operatively in an AZA Species Survival Plan Meyer, 1982). No toads were observed in were abundant in the vicinity of Laramie, (SSP") to maximize genetic diversity in 1982; the discovery of two juveniles in Albany County, Wyoming (Baxter, 1952; captivity and produce offspring for reintro- 1983 represented the first evidence that Corn, 1991),where they were found in the duction (Spencer, 1999). reproduction had occurred since 1975 flood plains of the Big and Little Laramie Wyoming toads reflect relict popula- (Lewis et al., 1985). An intensive survey in rivers (Stebbins, 1985), an area of only tions of Canadian toads (B. hetniophrys) iso- 1984 located -30 males, but similar sur- 2,330 km2(Lewis et al., 1985). A rapid de- lated at higher altitudes (2,164 in) of the veys in 1985 and 1986 found no animals cline was observed in the mid-1970s Lramie Basin at the end of the Pleistocene (Stone, 1991). In 1987, a single popula- (Lewis et al., 1985) when they disappeared when glaciers receded and climatic condi- tion of Wyoming toads was located at from most of their range. Surveys in the tions wanned (Blair, 1972a). Currently, the Mortenson Lake (Anonymous, 1987). This early 1980s yielded few animals, and closest population of Canadian toads is is the last natural population of toads that Lewis et al. f 1985) reported their possible -750 km from the Lararnie Basin. Canadian has been located. extinction in 1983. Wyoming toads were toads are more aquatic than most toads and There is no evidence for what the size listed as Endangered under the Endan- are frequently found on the shores of small was of a "typical" population before the gered Species Act in February 1984. lakes (Conant and Colllns, 1991; see the decline of this species, but the size of the Mortenson Lake population has been monitored with varying degrees of rigor since it was discovered. Based on single Wyoming Toad (Bufo baxteri) surveys conducted each year about 1 Sep- tember, this population was thought to in- clude about 100-150 adults in 1987-89 (Withers, 1992; Corn, 1993a; Parker, 2000). Beginning in 1990, a capture-recapture study was conducted using photography to identify individual toads. In 1990, there were 120 (s.e. = 22) adult-sized toads (Corn, 1991). The population estimate in 1991 was 415 (s.e. = 99), but the majority of these were subadult toads that had metamorphosed the previous summer (Corn, 1992). The population estimate in 1992 was 155 (s.e. = 21), and a size- frequency analysis estimated that 36% of the population was composed of toads that were 2 yr old (Corn, 1993a). Few toads emerged in 1993, and only two adults were located during the annual September sur- vey; these were taken into captivity in an effort to salvage the species (Parker, 2000). Currently, Wyoming toads have been Canadian toad account). This aquatic Reproduction was recorded at Morten- reintroduced under a U.S. Fish and Wildlife tendency is shared by Wyoming toads. son Lake beginning in 1988 when at least Service recovery plan (Stone, 1991) to The presence of Canadian toads in the nine egg masses were found (Freda et al., Mortenson Lake, -23 km southwest of Laramie Basin was first recorded by Baxter 1988). Numbers of egg masses declined Laramie and the site of the last known pop- (1947, 1952) and included on the map in subsequent years, with two found in ulation of toads. This population has yet to accompanying the species account in 1989 (McLeary, 1989), three in 1990 return to its 1988 levels and may not be sus- Stebbins (1954a, p. 145). The Laramie (Chamberlin, 1990), and three in 1991 tainable (see "Historical versus Current population, with its long-term isolation (Withers, 1992). No reproduction was ob- Abundance" below). Attempts were also and distinctive characteristics, was later served in 1992 or 1993 (Corn, 1993a; made to reintroduce Wyoming toads at two elevated to subspecies status (8, hnnio- Parker, 2000). locations on the Hutton Lake National phw bmterk Porter, 1968) and then to full Corn (1993a) observed that with a Wildlife Refuge, 10 krn southeast of Morten- species status (B. baxteri; Smith, 1998).De- mortality rate of about 80'l-h for adults, the son Lake. These animals did not establish bate continues about whether Wyoming population was unsustainable. This was breeding populations and all released toads toads deserve separate species status supported by population modeling, which disappeared. The Mortenson Lake popula- (Packard, 1971; Smith et al., 1998). We fol- resulted in a negative population growth tion is considered the only known current low Crother et d. (2000) in designating rate and certain extinction within 10 yr distribution In nature for Wyoming toads Wyoming toads as a valid species. (Jennings et al., 2001). When the last nat- (Jennings eta]., 2001). urally bred wild toad was collected in Captive populations are being main- 2 Historical versus Current Abundance. 1994 at Mortenson Lake, Wyoming toads tained at seven American Zoo and Aquar- The occurrence of natural populations has probably became extinct in the wild (M. ium Association (AZA)institutions (Central dropped precipitously in the past 30 yr. Jennings, unpublished data). Efforts to establlsh a second popula- and small lakes (Baxter, 1952). Withers 10 m of water (Withers, 1992; Parker, tion were initiated in 1992 at Lake George, (1992)repoaed egg masses in water 3.56.3 2000). This may be due to the low humid- Albany County, Wyoming. Four "man- cm deep where mean daytime water tem- ity associated with the Laramie Basin up- aged breedings" in protective enclosures peratures ranged from 20.6-21.8 "C. lands and the need for toads to have were successful. The breeding stock came ii. Clutch size. Egg masses may contain access to moisture for rehydration. Histor- from animals collected at Mortenson Lake 1,000-6,000 individual 2-3 mm ova in ically, Wyoming toads frequented a vari- in 1990 and 1991 and held in captivity strings (Freda et al., 1988, Withers, 1992). ety of habitats including lakes, ponds, until the spring of 1992. These breedings One captive produced egg mass was esti- streams, marshes, roadside ditches, and resulted in large numbers of newly meta- mated at 8,000 eggs (M. Bock, personal floodplains (Stebbins, 1985; Luce et al., morphosed animals in July (Withers, communication). The female involved in 1997). At Mortenson Lake, toads use areas 1992), but a survey performed the follow- this captive breeding was especially robust along the shoreline as well as adjacent ing spring found only 22 individuals. This and this number may exceed the largest marshes and ditches (Corn, 1991; With- number decreased to two in the fall of clutch of eggs produced in the wild. Eggs ers, 1992). When disturbed, Wyoming 1993. Subsequent efforts to release animals may hatch in 46 d (Freda et a]., 1988; toads frequently seek escape by diving at Lake George have not been successful Withers, 1992) in the wild (2 d in a con- into open water and submerging (Parker, (total releases = 3,963 larvae or metamor- trolled captive situation; R. A. O., personal 2000). Adults tend to be nocturnal (Luce phosed animals, 56 juveniles [I yr olds]; observations). et al., 1997) or diurnal (Parker, 2000), with M. Jennings, unpublished data). C. Lawae/Metarnorphosis. activity levels largely dependent on tem- In 1994, 77 Wyoming toads were pro- i. Length of larval stage. Wyoming toad perahw (Parker, 2000). duced in the first captive breeding of this tadpoles are darkly pigmented and 5-7 mm F. Home Range Size. Has not been species at the Sybille Wildlife Research in length at hatching. Length of larval clearly delineated. At Mortenson Lake, Unit, Sybille, Wyoming. These animals stage is approximately 1 mo, with meta- Withers (1992) suggested a post-breeding were held in captivity for further breeding morphosis in early July (Withers, 1992). migration from the north shore to the stock. In 1995. a concentrated effort to Larval periods are reduced under more south shore. This migration would require reintroduce toads back to Mortenson Lake thermally homogeneous conditions in the toads to move several hundred meters. commenced from captive produced off- captivity (R. A. O., personal observations). Parker (2000) observed a maximum 1 d spring. In 1995, 1,300 Wyoming toad ii. Larval requirements. movement of 151.8 m and a mean daily young (< 4 mo) were released. This was a. Food. Although not well studied, movement of 5.05 m. followed by 2,638 young in 1996 and 213 Wyoming toad tadpoles are generalized G. Territories Unknown, but unlikely. in 1997 (M. Jennings, unpublished data). suspension feeders, grazing on the organic H. Aestivation/Avoiding Desiccation. In 1998, free-ranging toads from the 1995 and inorganic matter associated with Parker (2000)observed that adult toads ac- releases at Mortenson Lake produced four rock, plant, and log surfaces. tive during the day often sought "night egg masses, the first known wild breedings b. Cover. Larvae select areas of higher refuge" under dense grass on moist sub- since 1991 (Parker, 2000). The same year, temperature as high as 31 "C (Withers, strate. While in these nocturnal refugia, these breedings were augmented with the 1992). Beiswenger (1978) collected tad- toads were observed in a state of torpor. release of 3,428 captive-born young. poles from shallow water at 30.5 "C and Wyoming toads also use rodent burrows Three wlld breedings were observed at noted that Wyoming toad tadpoles seek for refuge (Withers, 1992; Parker, 2000). Mortenson Lake in 1999 and the popula- out warm regions in thermally stratified I. Seasonal Migrations. Seasonal migra- tion was estimated to be 159 adults. 'ltYo environments. In one case, shallow de- tions involve breeding (see "Breeding mi- thousand captive bred toads were also re- pressions described as "water cow paths" grations" above), post-metamorphic (see leased in 1999. It appeared that the popu- were areas of congregation for tadpoles UPost-metamorphicmigrations" above), lation was on its way to recovery; from three separate clutches (Withers, and overwintering (see "Torpor [Hibema- however, the following year reproduction 1992). In the laboratory, Beiswenger tion]" below) movements. was limited to two egg masses (Parker, (1978)observed that Wyoming toad tad- J. Torpor (Hibernation). Only anecdotal 2000). In 2000, no reproduction was ob- poles aggregate in temperatures between data exist on the hibernation sites of served, census numbers were very low, 28-34 "C (preferred temperature was Wyoming toads. Withers (1992)observed and sick and dead toads were observed. -31 T)when placed in a stratified thermal toads near pocket gopher or ground squir- The outcome for this reintroduct~oneffort environment. rel burrows in the spring and fall and as- is still guarded. iii. Larval polymorphisms. Do not occur. sumed these structures were used for iv. Features of metamorphosis Metamor- hibernation. One captive bom and re- 3. Life History Features. phosis usually takes place in early July, leased toad with an implanted radio trans- A. Breeding Reproduction is aquatic. which corresponds to the annual bloom mitter was found in the fall burrowed in a i. Breeding migrations. Adult toads first of small black flies (Diptera), possibly a badger excavation. The toad was at a emerge from hibernation when air temper- food resource for young toads (Withers, depth of 35 cm and a temperature of 13 "C. atures reach 21 'C (usually in early to mid- 1992; A. Anderson, personal communica- The badger had broken through the hard May; Stone, 1991; Withers, 1992; Parker, tion). soil crust, allowing the toad to dig into the 2000). Males usually start calling a week v. Post-metamorphic migrations. Follow- softer substrata (Parker, 2000). late^ The call of Wyoming toads is a harsh ing metamorphosis, young toads disperse K. InterspedflcAssocMons/Exduslons. trill. Calling peaks when air and water along or near the shoreline. By late sum- Wyoming toads may hybridize with temperatures are between 21.1-26.7 "C, mer, many migrated from the north side American toads (B. americanus; Henrich, decreases as temperatures fall, and ceases of Mortenson fake to the east and south- 1968). at 10 OC (Withers, 1992). east sides (Withers, 1992). L. Age/Size at Reproductive Maturity. ii. Breeding habitat. Withers (1992) re- D. Juvenile Habitat. Similar to those of Wyoming toads are smaller than Cana- ported that vegetation at sites selected by adults. dian toads. Smith et al. (1998) recorded a calling males was a sedgefrush (Carex E Adult Habitat. Wyoming toads are maximum length of only 59.5 mm, with sp./Scirpus amricnnzis) mix. These plants found dose to water in-short &ass prairie ordy 3 of 23 preserved specimens measur- were also associated with egg masses. (Stebbins, 1985; Luce et al., 1997; see ing over 56 mm. However, Corn (1993a) B. Eggs. Withers, 1992 for a more detailed analysis measured live toads and recorded a maxi- i. Egg deposition sites. Wyoming toads of habitat). Adults almost always restrict mum length of 68 mm. Sexually mature deposit their eggs in shallow areas of ponds their habitat use to the shoreline within males and females are dimorphic for size.

BUFONIDAE 391 In 1992 at Mortenson Lake, the average 4. Conservation. known pedigrees that can be traced back size of 1-yr-old males was estimated to be Wyoming toads were estirpated in the wild to the original collection of animals in the 48 mm and the average size of 2-yr-old in 1994 and have been reintroduced. early 1990s. All other animals are derived males was 52 mm. One-yr-old females Chytrid fungus appears to have been the from these earlier collections, and Group were the same size (47 mm)as males of the proximate cause of the decline of these A represents all of the known diversity for same age, but the average size of 2-yr-old toads (Taylor et al., 1999; A. Pessier, per- the species. The second group (Group B) is females was estimated to be 61 mm. sonal communication). Disease still re- composed of animals that were collected Growth after metamorphosis is rapid, and mains important and impacts Wyoming as wild-born offspring of reintroduced males reach adult size by August of their toad recovery efforts (Jenningset al., 2001). Wyoming toads from Group .4. Although second summer and breed the following Other factors have been hypothesized Group B represents less gene diversity spring (2 yr old, similar to Canadian as responsible for the precipitous Wyoming than Group A, they are the offspring of toads; Breckenridge and Tester, 1961). Fe- toad decline, including the aerial applica- successful survivors in nature, which may males most likely require an additional tion of baytex Ifenthion), an organophos- indicate increased fitlws. year to reach maturity (Baxter, 1952; phate pesticide used for mosquito control; Wyoming toad recovery is still largely Corn, 1991; Withers, 1992). In the artifi- predation by California gulls iLtlrrrs cnlitbr- dependent upon an influx of individuals cial environment of captivity, 1-yr-old ni~-11s).American white pelicans fPrI~'ctm~u.from the captive populations. Large num- toads have produced offspring (D. Roberts, rrytlzrorhyrdzos), and raccoons (Pruqon bers of animals have been released at personal communication). lotor), which have been increasing in num- Mortenson and two other sites (Lake M. Longevity. No long-term studies bers in this region; and habitat modifica- George and Rush Lake at Hutton Lake have been performed to determine the tion due to the irrigation regions of the NWR). Only the Mortenson Lake site has maximum longevity of Wyoming toads in native hay meadows (see Jennings and An- had any recent breeding activity and con- the wild. Corn (1993a) observed that few derson, 1997). However, there is little evi- tinues to have animals present. The recov- adult toads lived >2 yr at Mortenson Lake, dence that any of these factors are the ery of Wyoming toads is stilt uncertain. but that was in a population afflicted with primary cause of toad declines. Parker Considering the limited numbers, low ge- chytrid fungus. In captivity, one female (2000) found radio-tagged toads preyed netic diversity, reliance on captive breed- toad with an estimated birth date in 1989 upon in his study and suggested additional ing, and continuing problems with disease, (studbook #5) lived in captivity from 1994 predators include skunks, weasels, badgers, Wyoming toads are the most endangered until its death in 1997 (Callaway, 1998). coyotes, mink, domestic cats, and herons. anuran in the United States, if not the This animal produced large numbers of Another factor impacting Wyoming world. healthy young from 199496. toad recovery is the small amount of ge- N. Feeding Behavior. Adults feed on netic diversity remaining in the species. ants, beetles, and other small insects and The entire captive, and thus the reintro- invertebrates (Luce et a]., 19971. Stomach duced wild, population contains the equiv- contents and fecal pellets of adult toads alent genetic diversity of -2.4 unrelated contained substantial numbers of ants, animals (Founder Genome Equivalents). which appear to be the main component of This is equal to 79.4% of the expected the adult diet (G.Lipps, personal commu- gene diversity that should have been con- nication; R. A. O., personal observations). tained in the original wild population. 0. Predators. Predation may be an im- The mean inbreeding coefficient (F) of portant cause of mortality in Wyoming the population is 0.155, which is greater toads (Parker, 2000; see "Conservation" than a first cousin cross. Inbreeding depres- below). sion has not been conclusively documented P. Anti-Predator Mechanisms. Noctur- in Wyoming toads, although there is some nal activity patterns. Wyoming toads will correlation between inbreeding and lower swim out into the water away from shore fecundity (Lipps and Odum, 2001). when frightened (Parker, 2000). Attempts have been made to manage Q. Diseases An overview of diseases of the captive and reintroduced populations Wyoming toads is presented in 'Ikylor et to maximize the retention of gene diver- a]., 1999. A substantial number of deaths sity. Using the studbook data, mean kin- (71% in cases where cause was known) re- ship is calculated to determine which pairs sulted from mycotic dermatitis with sec- will maximize the amount of gene diver- ondary bacterial infections. The authors sity retained in the next generation. Un- misidentified the primary pathogen as Bu- fortunately due to husbandry difficulties, sidiohhrs rtlnnnm, which was later deter- this method has not been entirely success- mined to be &rtraclzoc/z)~triutnrientlrobutiiii~ ful. Many scheduled priority breedings (chytrid fungus; A. Pessier, personal com- have not produced offspring and other less munication; E. Williams, personal commu- important pairings have. Analysis of the nication). The majority (90%) of these husbandry data (G.Lipps and R. A. O., un- deaths in free-ranging toads occur in Sep- published 2000 data) show that there is a tember-October when temperatures start slightly less than significant negative cor- to fall. Chytrid fungal infections are also relation between the inbreeding coeffi- the most frequent cause of mortality in cient of the offspring produced by a captive populations and again are most pairing and breeding success (as number of prevalent in autumn. A treatment for this offspring produced from the breeding). disease has been developed for captive ani- These data are still preliminary, and results mals (A P'essier, personal communication). from 2001 appear to be contradictory. R. Parasites. Unreported in wild popu- Captive Wyoming toads currently are lations. In captivity, a wide variety of pro- being managed as two separate popula- tozoan and helminth parasites have been tions. The first group (Group A toads) is isolated (R. A. O., personal observations). composed of animals that have fully