The numbers of most frog species -- and most amphibians, for that matter -- are dwindling as habitats are diminished and waterways become increasingly polluted, but not the American bullfrog.
Native to the eastern and southern regions of North America, the American bullfrog has been introduced all across the United States and Canada. Most recently, it's been invading Montana -- showing up in increasing numbers along the Yellowstone River.
Because bullfrogs are large and can eat both vegetation and small animals, including each other, they make excellent invaders. As they use those invasion skills near Billings, Mont., wildlife officials and environment advocates are becoming worried the invading frogs could threaten more vulnerable native aquatic species.
Between 2010 and 2013, the number of bullfrog breeding sites quadrupled, and their territory more than doubled. At last count, wildlife officials found 45 breeding communities scattered along 66 miles of Yellowstone River.
"Bullfrogs were likely introduced to the Yellowstone River region for food, recreational hunting, bait and pest control, and as released pets," Adam Sepulveda recently told the Bozeman Daily Chronicle. Sepulveda is a scientist with the U.S. Geological Survey, currently conducting research at the Northern Rocky Mountain Science Center in Bozeman, Mont.
"The impacts of bullfrogs on native amphibians in the Yellowstone River are not yet known, but native Northern leopard frogs are likely to be most vulnerable to bullfrog invasion and spread because their habitats overlap," Sepulveda said.
Montana and federal conservation officials first tried to kill off the invading species, but the frogs proved too resilient. As a recent USGS report noted, "bullfrogs are extremely difficult to eradicate because they are highly fecund (females can produce up to 40,000 eggs per clutch) and are extremely mobile."
Now, scientists and wildlife officials are just trying to figure out a way to contain the frogs.