Hungry American bullfrogs that have been known to eat other frog species and each other are invading Yellowstone River in Montana, the Associated Press reported.

The invasive frogs can be as long as 12 inches when stretched out and have spread to every state except North Dakota after being brought by humans from their native habitat in the eastern U.S.

The bullfrogs have been found in a 66-mile portion of Yellowstone River, where they pose a threat to native frog species as well as other wildlife. They now have at least 45 breeding sites, a number that almost quadrupled between 2010 and 2013.

"They are going to eat anything they can fit into their mounts. It doesn't matter if it's another frog or a bird or a mosquito," U.S. Geological Survey biologist Adam Sepulveda told the AP.

State and federal officials have been fighting to keep the invasive species at bay, but the frogs have been spreading quickly in spite of their efforts. It is believed that bullfrogs first came to western Montana in the 1920s when their legs were sold as food.

As an invasive species, American bullfrogs devour native frogs and infringe on resources such as food. They also spread a fungus that could be causing amphibians to decline.

The bullfrog was accidentally introduced during trout stocking and also spread through the aquarium trade and pest control, according to the U.S. Department of Agriculture.

The largest frog species in North America, bullfrogs are predators that can be damaging for native ecosystems including wetlands, according to the Washington Invasive Species Council. Their prey ranges from tree frogs to the western pond turtle to minnows to young snakes.  

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