bullfrog invasion
October 5, 2014

Invasive Species Of Bullfrog Spreading Along Montana’s Yellowstone River

Chuck Bednar for redOrbit.com - Your Universe Online

Native species living along the Yellowstone River in Montana are being threatened by a growing invasion of voracious American bullfrogs known to eat just about anything (including other bullfrogs), a team of researchers led by US Geological Survey biologist Adam Sepulveda claim in a new study.

Writing in the journal Aquatic Invasions, Sepulveda and colleagues from the USGS’s Northern Rocky Mountain Science Center, the Montana Natural Heritage Program, the Bureau of Land Management and the Montana Fish Wildlife and Parks concluded that this invasive species is now thriving and rapidly spreading in the river’s floodplain.

According to BBC News reports, the study authors found that the number of American bullfrogs living in the region has nearly quadrupled over the past four years. The frogs, which can grow to be up to 12 inches long, are typically native to the eastern US but have spread to nearly every state – and now they have been found along a 66 mile stretch of the Yellowstone River.

The bullfrogs are believed to be one of the reasons that multiple amphibian and reptile species all over the world have been declining in numbers, the USGS explained in a statement Thursday. The creatures’ size, combined with their mobility, their appetite, their ability to reproduce at a rapid rate and the fact that they are carriers of amphibian diseases, makes them a threat to biodiversity.

“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, adding that the invasive species (which had not been observed in the region prior to 1999) were likely introduced to Montana “for food, recreational hunting, bait and pest control, and as released pets.”

In order to get a grasp on the degree to which the bullfrogs have spread, the USGS said that scientists conducted field surveys in 2010, 2012 and 2013. Those researchers used visual encounter surveys to search for adults, egg masses and larvae, and calling surveys to listen for calls of breeding males.

Their work was performed while walking or slowing driving down roads that were adjacent to wetlands, large bodies of water or while on the river itself. The investigation found that bullfrogs had expanded from about 37 miles in 2010 to about 66 miles in 2013, and that the number of breeding sites had increased from 12 to 45 sites in that time.

The results, the agency said, “indicate that bullfrogs are firmly established in the Yellowstone River floodplain and can rapidly spread to new habitats.” While the spreading was found upstream, most of it was downstream, which suggests that the spread could be accelerated by river flow. The bullfrogs were also found in publicly accessible areas with deeper waters and emergent vegetation, indicating that they were likely introduced by humans.

The bullfrogs have caused problems by “preying on native frogs, out-competing other animals for food, and spreading a fungus that's suspected as a cause of a widespread decline in amphibians,” according to Associated Press (AP) reporter Matthew Brown. While state and federal agencies initially tried to halt the species’ spread by killing them off, they were forced to give up after “the number of bullfrogs overwhelmed the effort,” he added.

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