The American bullfrog (Rana catesbeiana) is an effective and ravenous predator, now thriving outside its natural range and posing a threat to other native frog and toad species. In Minnesota, bullfrogs are native only to the backwaters of the Mississippi River in Houston and Winona counties. Unfortunately, people are buying bullfrogs for their backyard ponds and water features, and there is nothing to keep these animals from hopping into nearby ponds, lakes, and wetlands.
Nonnative bullfrogs often harm native frog and toad populations, as the bullfrogs prey on these smaller animals, compete with them for food and habitat, and introduce diseases that are deadly to native species. During the past few years, volunteers for the DNR's annual Minnesota Frog and Toad Calling Survey have identified bullfrog colonies in 18 counties where bullfrogs are considered nonnative. In Colorado and California, introduced bullfrogs are believed to be partly responsible for population declines of some native frog species.
"Recent research has also found that introduced bullfrogs frequently carry a fungus that is deadly to other frog species," says Rich Baker, acting supervisor of the DNR Natural Heritage and Nongame Wildlife Research Program. "The presence of bullfrogs in a water body will often result in the death of all other frog species in that water body."
In Minnesota, it is illegal to import frogs without a permit or to take and sell frogs from the wild without a license for any reason other than to use them as bait. "Some pet shops, aquariums, and garden centers have been obtaining bullfrogs and bullfrog tadpoles illegally," says Baker. The DNR believes that people are buying them mainly for their backyard water features and that the bullfrogs are subsequently either escaping or being released into the wild. Yvette Monstad, coordinator of the Minnesota Frog and Toad Calling Survey, says she thinks milder winters in the state may be a critical factor in the increased ability of these bullfrogs to survive in northern counties beyond their native range.
Voracious eaters, bullfrogs can grow up to 8 inches long and weigh up to 1.5 pounds. Minnesota's largest frog species eats mainly insects and small invertebrates, but it will also eat frogs, toads, snakes, small mammals, and birds, including ducklings and migratory songbirds. "The adult bullfrog will eat anything that it can fit into its mouth," says Baker.
Baker says that while bullfrogs or their tadpoles are not generally used for bait in Minnesota, anglers who use them should take extra care to avoid releasing them alive, whether in their native range or elsewhere in the state. A DNR license is required to take bullfrogs and tadpoles from the wild and sell them for any purpose other than for use as bait, including the eating of their legs, and harvested bullfrogs must be more than 6 inches long.
If you see or hear bullfrogs outside of their native range, please call the DNR at 651-259-5100.
Fran Howard, volunteer
Minnesota Frog and Toad Calling Survey