According to the U.S. Fish and Wildlife Service, the northern Mexican gartersnake has vanished from roughly 90 percent of its historical range in the United States, driven in part by American bullfrogs introduced by wildlife officials in the 1920s to provide a food source for hunters and anglers. These large, fast-breeding amphibians outcompete native species and prey directly on juvenile snakes, creating an ongoing ecological imbalance across Arizona and New Mexico wetlands.
How a 1920s Wildlife Stocking Program Created an Arizona Crisis
Beginning in the 1920s and continuing at least until 1982, Arizona wildlife officials stocked lakes and rivers with American bullfrogs, hoping to establish a local supply of frog legs for hunters and anglers, according to the Tucson Herpetological Society. At the time, the wider ecological consequences of moving the large amphibians into unfamiliar western ecosystems were not fully understood.
While commercial frog farming ultimately proved financially risky and challenging due to disease outbreaks and high food demands, the escaped and stocked bullfrogs successfully established themselves in the wild, proliferating across Arizona ponds, streams, and cienegas.
The Dual Threat Facing the Northern Mexican Gartersnake
The northern Mexican gartersnake relies heavily on wetland habitats like streams, ponds, and springs, where it hunts small fish and frogs along the water’s edges. The arrival of American bullfrogs disrupted this habitat by introducing direct competition and heavy predation, according to assessments from the U.S.
Female bullfrogs can produce roughly 20,000 eggs in a single breeding event, according to National Geographic, overwhelming native predators that cannot consume enough tadpoles to check population growth. While young gartersnakes occasionally eat small bullfrog larvae, adult bullfrogs easily consume juvenile snakes. This dynamic prevents young gartersnakes from reaching reproductive age, leading to steep population declines even in wetlands where adult snakes are still observed.
Did you know?
American bullfrogs are exceptionally mobile. Fish and Wildlife Service recorded tagged frogs traveling up to 6.3 miles in a single year, with some unmarked individuals appearing nearly 8 miles from their starting ponds during wetter weather.
Why Mobile Bullfrogs Hamper Wetland Conservation Efforts
Conservation teams, state wildlife agencies, and federal partners have worked since the 1990s to reduce bullfrog numbers at critical sites using nets, traps, and strategic pond-draining. However, local eradication is frequently undermined by the species’ high mobility. Young bullfrogs tagged near Arizona’s Buenos Aires National Wildlife Refuge traveled more than six miles within weeks, according to National Geographic.
This rapid dispersal allows bullfrogs to quickly reinvade wetlands cleared by wildlife managers. A striking contrast exists between the success at specific localized sites—such as the Cienega Creek complex, where bullfrogs were eradicated in 2013 and native leopard frog populations subsequently rebounded—and the broader regional trend of decline documented across Arizona and New Mexico by federal regulators.
Frequently Asked Questions
Are American bullfrogs native to Arizona?
No. American bullfrogs are native to the eastern United States, but state wildlife officials introduced them to Arizona lakes and rivers starting in the 1920s as a potential food source for hunters and anglers.
Why are bullfrogs dangerous to the northern Mexican gartersnake?
According to the U.S. Fish and Wildlife Service, adult bullfrogs prey directly on juvenile northern Mexican gartersnakes, while bullfrog tadpoles compete with native species for limited food and resources in wetland habitats.
How far can American bullfrogs travel between wetlands?
Fish and Wildlife Service have tracked bullfrogs moving up to 6.3 miles in a year, with some individuals traveling nearly 8 miles, particularly during wet weather when temporary pools serve as stepping stones.
What conservation methods are used to control bullfrogs?
Wildlife managers use nets, traps, and the periodic draining of artificial stock tanks or wetlands to remove bullfrogs and prevent them from reproducing, a strategy that successfully eradicated bullfrogs at the Cienega Creek complex in 2013.
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