Bullfrogs are bullying their way up the Yellow Stone River, invading the floodplains of Montana, according to a new study. And experts are saying that this may be one of the hardest invasions to fight.

In most of the country, and even the world, amphibians are on the decline. Favoring wet marshlands, elevating temperatures and intense drought conditions are eliminating areas where these delicate creatures can mate and raise their offspring.

However, the American bullfrog is one amphibian who didn't get the memo. Instead of being on the decline like its many cousins, this bullfrog has been thriving in recent years, rapidly spreading up and down the Yellowstone River with little competition.

The US Geological Survey (USGS) calls the American bullfrog an "extremely successful invader and threat to biodiversity" as they are exceptionally adaptable, reproduce rapidly, and eat practically anything.

"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," USGS scientist Adam Sepulveda said in a statement.

Sepulveda is the lead author of a new study that was recently published in the journal Aquatic Invasions.

The study details the results of field surveys conducted between 2010 and 2013 of American bullfrog prevalence amid large bodies of water in the Yellowstone River. According to Sepulveda, while the frog was only first documented in the floodplain in 1999, it has seen what looks to be an aggressive population explosion.

Bullfrogs expanded from roughly 37 miles in 2010 to about 66 miles in 2013. The dispersal occurred both up and downstream, with the number of breeding sites having increased from 12 sites in 2010 to 45 sites in 2013.

In a previous report, the USGS said that "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."

However, Sepulveda said all they can do to maintain biodiversity now is to try to contain the large amphibians.