In the 1920s, Arizona’s State Game Department introduced bullfrogs for food and sport; the predators now eat young northern Mexican gartersnakes and move up to 7 miles

Roughly a century ago, Arizona wildlife officials set out with a simple goal: give hunters and anglers something new to catch and eat. Beginning in the 1920s and continuing at least until 1982, the state’s game department stocked lakes and rivers with American bullfrogs in hopes that frog legs would become a popular local dish, according to the Tucson Herpetological Society. That decision has become one of the biggest threats to the northern Mexican gartersnake, a shy, water-loving reptile that once thrived across the American Southwest, according to an assessment by the U.S. Fish and Wildlife Service. Almost a century later, the frogs are still spreading, and the snake is still losing ground.

A food craze that got out of hand

The American bullfrog isn't native to Arizona. It's a frog of the east, living naturally east of the Rocky Mountains, in wetter, greener states. Wildlife agencies wanted a game species, something people could hunt and cook, so they took bullfrogs west and put them out. No one thought to ask what a big, fast-breeding frog might do to the native wildlife already there. Most native amphibians are tiny compared to bullfrogs. Once a pond is colonized, recovery is difficult without major human intervention.


A predator that eats almost anything that moves

Northern Mexican gartersnakes rely on frogs and smaller fish for their diet, hunting at the margins of creeks and cienegas, which are wetlands found throughout the Southwest region. The bullfrogs disrupt the food chain in two ways. As tadpoles, they are not eaten by native fish species, which struggle to keep pace with a female bullfrog capable of laying about 20,000 eggs at once, National Geographic reports, helping bullfrog populations grow unchecked. Grown into adults, they become predators themselves capable of eating snakes, birds, small mammals, and any other frogs.

This places pressure on the population of gartersnakes. Juvenile gartersnakes fall prey to adult bullfrogs before reaching breeding age, while bullfrogs and their tadpoles compete with gartersnakes for resources. Officials note that there are times when gartersnakes feed on some larvae and juveniles of bullfrogs, although this is not significant enough to balance out the number of gartersnakes eaten by the bullfrogs.

Hopping for miles to find the next pond

Bullfrogs travel far for an amphibian. Young bullfrogs tagged near Arizona's Buenos Aires National Wildlife Refuge have ranged more than 6 miles in a matter of weeks, National Geographic reports. A separate U.S. Fish and Wildlife Service–backed study tracked bullfrog dispersal across grassland in southeastern Arizona and recorded a maximum straight-line distance of approximately 6.3 miles within a single year, with some unmarked frogs appearing nearly 8 miles from likely source ponds. In wetter years, biologists have seen the frogs travel even farther, hopping between small ponds until they reach a larger body of water.

Bullfrogs are notoriously hard to control because of their mobility. Wildlife managers can drain one pond of every adult frog, and within a season new bullfrogs may arrive from other ponds miles away. That kind of reinvasion leaves little room for a species like the northern Mexican gartersnake, which relies on a handful of remaining wetlands.

A snake running out of places to hide

The effects have been severe. Federal assessments conclude that the gartersnake has been extirpated from, or reduced to very low densities across, about 90 percent of its historical range in the United States. Populations that used to occur throughout Arizona and into New Mexico are now limited to a few isolated pockets, and those pockets are in decline, with fewer young snakes being recorded over time. It's not just bullfrogs. Water diversions, urban development and loss of habitat are also to blame. But, according to wildlife biologists, nonnative predators, especially bullfrogs, are one of the most immediate and direct threats to the species.

Can the gartersnake still bounce back?

There is a ray of hope. Since the 1990s, teams from the University of Arizona, state wildlife agencies and federal partners have worked to physically remove bullfrogs from key wetlands, sometimes pond by pond, using nets, traps and even by draining stock tanks entirely, according to the U.S. Fish and Wildlife Service. In areas like the Cienega Creek complex, where bullfrogs were successfully eradicated in 2013, native leopard frog populations have grown in the years since, the Fish and Wildlife Service reports. It's a slow, resource-heavy process, and the bullfrogs keep finding their way back in, but it shows that a mistake made a century ago, when officials merely wanted better fishing and hunting, does not have to be permanent.