The Wyoming toad (Anaxyrus baxteri), a glacial relic and one of the most endangered amphibians in North America, is endemic to the Laramie Basin in Albany County, Wyoming. Once abundant throughout their limited range, the species experienced a rapid decline in the 1970s. The exact cause for their decline remains unknown, but suspect causes include irrigation practices that changed the nature of the floodplains, aerial spraying of fenthion for mosquito control, and amphibian chytrid fungus (Batrachochytrium dendrobatidis).
The Wyoming toad was listed by the Endangered Species Act in 1984. Once presumed extinct, a single population was discovered at Mortenson Lake in 1987, and egg masses were also discovered at the lake the following year. The population continued to dwindle and the last known ten remaining toads were brought into human care in 1989; the Wyoming toad was declared extinct in the wild in 1991 (IUCN).
Conservation efforts began in 1993 with the Nature Conservancy’s purchase and founding of the Mortenson Lake National Wildlife Refuge. That same year, an ex situ breeding program began at the Sybille Wildlife Research and Conservation Center.
Successful reproduction occurred in 1994, allowing for the first reintroductions within the species’ native range in 1995. The Wyoming Toad Species Survival Plan® joined recovery efforts in 1996, and has since worked closely with the recovery team to promote long-term sustainability of the ex situ population to support future reintroduction efforts while maintaining a sustainable assurance colony.
Every year, new hatchlings in the ex situ population are assessed for their genetic representation; offspring from under-represented founder lineages are prioritized for breeding. With a small founder base and no opportunities to supplement the ex situ population with additional unrelated animals, it is critical that, through carefully managed breeding, extant gene diversity is preserved.
Husbandry is complex. A single egg mass can contain more than 2,000 eggs. The high clutch sizes require careful management that prevents sudden over-representation of some founder lineages and the possible loss of others. Transfers must be completed by October due to weather constraints and breeding pair selections are assessed in very early spring.
Successful breeding is dependent on a five-week hibernation phase prior to June to stimulate egg development in the females. Breeding in early June results in suitable conditions for tadpole release in the wild. Metamorphosing from July to August, animals are chosen for release or retention in the SSP. Interim recommendations facilitate the transfer of toadlets for the upcoming breeding season, and the cycle starts again. Any delays in transfers could result in a loss of breeding opportunities that must occur in a sensitive time window.
The Wyoming Toad SSP and its partners (including USFWS, Wyoming Game and Fish Department, University of Wyoming, Laramie Rivers Conservation District, Wyoming Natural Diversity Database, local ranchers) have had significant success with nearly 7,000 toadlets produced since 1996 and successful reproduction observed in offspring after ten generations (> F10).
Today, the ex situ population hovers around 600 animals, and to date, over 200,000 tadpoles and toadlets have been released to the wild. Without the support and expertise of zoo professionals working with federal facilities, a sustainable in situ population could not be maintained. Success is made possible by the strong collaboration among the recovery team, and the partnership with the Wyoming Toad SSP.
Photos credit: © Omaha's Henry Doorly Zoo and Aquarium
Sarah Armstrong is the Wyoming Toad studbook keeper and the elephant manager at Henry Doorly Zoo and Aquarium.
Asako Navarro is an adjunct population biologist at the San Diego Zoo Wildlife Alliance.