Bullfrogs are invading Sheridan, threatening native species

Bullfrogs are invading Sheridan, threatening native species

Biologists once found a crayfish with an entire garter snake and several fish inside a female bullfrog near Sheridan. That may sound like an impressive amount of food for a two-pound frog, but it’s actually pretty standard for the voracious amphibians. Wendy Estes-Zumpf, the state’s herpetological coordinator, once saw a picture of a bullfrog that choked to death trying to eat a duck. 

Bullfrogs consume nearly every living thing they find.  Their menu is limited only by the size of their mouths — as the aforementioned duck would suggest, sometimes not even that —  and they are wreaking havoc on the West’s native species. 

To make matters worse, once introduced to an area, bullfrogs can be nearly impossible to eradicate. Their potential to damage ecosystems is so great, the Wyoming Game and Fish Department passed a new regulation that took effect Jan. 1 prohibiting anyone from importing or possessing live bullfrogs. 

“A lot of the landowners in the Sheridan area said they had lots and lots of leopard frogs and now they have only bullfrogs,” said Estes-Zumpf. “There are frog species that are now endangered in other states and the primary cause are bullfrogs because the natives just cannot compete.”

A bullfrog caught near Sheridan in 2019 had two native species — a wandering garter snake and a calico crayfish — in its stomach, among other things. (Wyoming Game and Fish Department)

Eaters and carriers

Like carp, feral pigs and crayfish, bullfrogs aren’t all bad. They play important roles in the areas where they’re from. In the eastern part of the U.S., bullfrogs are critical as predators and prey and distribute nutrients from wetlands into more terrestrial areas. 

“They are just an amazing species,” said JJ Apodaca, executive director of the Amphibian and Reptile Conservancy. Also often called pond frogs, bullfrogs are the largest true frog species in the U.S.

They have abundant predators like smallmouth bass and snapping turtles in their native habitats, and the waterways are much more complex. Ponds back East tend to vary in depth and vegetation and streams snake between other ponds offering other frog species, tadpoles and fish plenty of places to hide. 

Bullfrogs are also hardy, big and incredibly efficient travelers, able to cover tens or even up to 100 miles over dry land to reach new water. 

“They get big and have chunky chicken thigh legs,” Estes-Zumpf said. “They are probably the number one frog-leg frog,” and they taste like chicken.

As a result, they were introduced across the world, sometimes as a food source, and other times as stowaways in fish stocking efforts. More recently, well-meaning classroom teachers have dumped easy-to-raise bullfrogs into ponds and creeks after they’ve run their course as classroom pets. 

How, exactly, they entered the Sheridan region, no one knows for sure. 

Game and Fish first received reports of bullfrog sightings in 2018. The next year, biologists surveyed the area and found bullfrogs everywhere. 

Estes-Zumpf knows a landowner outside Sheridan introduced bullfrogs and fireflies to his property to remind him of home back East, but she’s guessing as widespread as they’ve become in the region, they were likely introduced elsewhere as well. 

A comparison of a newly metamorphosed American bullfrog, on the left, and a young adult northern leopard frog, on the right. (Parker Loew/Wyoming Game and Fish Department)

Bullfrogs have occupied a few streams in far eastern Wyoming for decades. They’ve been there so long, in fact, biologists don’t know the impact they’ve had because no records exist of native species prior to their arrival. 

The problem in the Sheridan area is the bullfrog’s crusade to consume nearly every northern leopard frog and tadpole in sight. Northern leopard frogs are stable in portions of their range. They exist everywhere from Maine to Washington down to eastern Arizona, but were petitioned for Endangered Species Act listing in some portions of the West where their populations are struggling

Leopard frogs can coexist with bullfrogs in eastern water systems because they’ve found plenty of ways to hide and survive, but they’re not so well adapted out here, said Apodaca. 

Potentially more concerning, bullfrogs carry, but are immune to, the deadly chytrid fungus that is suffocating and killing native frogs and salamanders across North America.

“When it becomes a real concern for us in the conservation community is when [bullfrogs] throw ecosystems out of alignment and actively harm and damage native species,” Apodaca said.

Keeping them out

Once established, it’s exceptionally hard to eradicate bullfrogs, Estes-Zumpf said, though Wyoming’s harsh winters could help. 

One approach includes draining ponds, trapping adults and killing any tadpoles and eggs in the process. But the collateral damage of annihilating everything in the water limits its use and is not practical once bullfrogs spread to complex water systems with streams and connecting waterways. 

A bullfrog tadpole found in a pond west of Sheridan. (Andy Gygli)

Biologists with more invasive-bullfrog-management experience elsewhere have found ways to minimize their damage. 

Officials in Boulder County, Colorado, for example, have been able to control bullfrogs in priority wetlands where northern leopard frogs breed. 

Efforts include targeting adult bullfrogs, Apocada said, as well as strategically removing eggs since bullfrogs reproduce later in the summer than northern leopard frogs. 

“To get rid of bullfrogs is a continuous battle because they’re on the landscape and will come back,” he added. 

That’s why Game and Fish decided to ban them from the state. The fewer bullfrogs here, the lower potential for release. Estes-Zumpf encourages anyone with a hankering for bullfrog legs to feel free to hunt them (though not at night with lights, because that’s illegal), remembering that they can’t be kept alive as pets or released back into the water again. 

With any luck, Estes-Zumpf said, the bullfrog invasion can be contained to the Sheridan region. 

Anyone who hears the telltale croak — it may sound similar to a cow mooing — or sees or captures a bullfrog should call their local Game and Fish office. The earlier bullfrogs can be caught and contained the better for the rest of Wyoming’s species, fish, frogs and ducks included, Estes-Zumpf said.

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