Removing Invasive Bullfrogs to Restore Wetlands in Eight Southwestern Parks

By Benjamin Randall, National Park Service Natural Resources Office of Communications, May 2024.

The National Park Service is restoring native amphibians in more than 20 sensitive wetlands across eight national parks in the Southwest. The main obstacle is a single animal: the American bullfrog.

Bullfrogs can grow much larger than most native frogs and toads. They eat insects, worms, fish, other frogs, toads, snakes, small mammals and birds, and where they are not native they do great damage to aquatic ecosystems. NPS / Tani Hubbard.

An eastern frog out of place

Despite its name, the American bullfrog (Lithobates catesbeianus) is not native to the West. It comes from the eastern United States and is common in the states east of the Mississippi River. In the West it upsets fragile wetland ecosystems and puts native amphibians in real danger, for three reasons:

• Appetite. Bullfrog tadpoles and adults eat the eggs of native fish, reptiles, amphibians and water birds, and adults eat animals as large as small mammals.
• Dominance. Once a population is established, bullfrogs compete directly with native species for scarce food.
• Disease. Park managers and recent research have found that the bullfrogs carry chytrid fungus (Batrachochytrium dendrobatidis), which can kill some amphibians.

The male's call sounds like a bull's roar, which gives the frog its name. The species is easy to recognise by the large eardrums (tympana) just behind its eyes. NPS.

Dr. Erin Zylstra, a quantitative ecologist with the Tucson Audubon Society and a close partner in the project, explains that bullfrogs are especially hard to control because they breed prolifically, are voracious predators, and can spread over long distances. The team hopes to build on what other groups fighting bullfrogs in Arizona — among them the University of Arizona and the Arizona Game and Fish Department — have learned, to design control methods suited to each site and set up long-term monitoring of aquatic species.

Eight parks, one project

Funded by the Inflation Reduction Act and carried out with many partners, the project aims to eliminate bullfrog populations and bring native species back into balance at:

• Carlsbad Caverns National Park
• Chiricahua National Monument
• Coronado National Memorial
• Fort Bowie National Historic Site
• Gila Cliff Dwellings National Monument
• Montezuma Castle National Monument
• Saguaro National Park
• Tuzigoot National Monument

The status of bullfrogs in the eight parks where the aquatic recovery work will take place. NPS / Elizabeth Schnaubelt.

Once the bullfrogs are gone, the pathogens they carry find it harder to survive and stop wreaking havoc on these ecosystems. Removing them takes pressure off native amphibians and reptiles and lets the wetlands recover naturally.

A neighbourhood problem

"Invasive species are a neighborhood issue," says Andy Hubbard, program manager of the Park Service's Sonoran Desert Network and the project's lead. "In this case, it's a really big neighborhood when we talk about the whole project area – and that 'neighborhood' includes parks, local and area residents, and a variety of non-profit conservation groups."

"It doesn't make sense for us to go to a small park like Gila Cliff Dwellings and deal with 'our' local problem when there might be a large population of bullfrogs right next door," he explains. "Those nearby bullfrogs would simply move into now-vacant habitat and we've solved nothing. The only way to do this is in a holistic, collaborative fashion with our partners."

Elizabeth Schnaubelt, a Sonoran Desert Network Scientist in the Park, has led much of the public outreach. The partners include the Arizona Frog Team at the University of Arizona, other federal agencies and state departments, park neighbours and private landowners. "It's important to us that we have a relationship with all parties and work together to develop action plans that benefit everyone," she says. Park neighbours are the newest partners, because the project involves them so directly.

Finding frogs with eDNA

The first stage gathered baseline data to guide later control and restoration. Experienced herpetologists ran traditional surveys to find bullfrog populations and check on native species in each park. The project added environmental DNA (eDNA) sampling, which Hubbard calls "a groundbreaking tool in the fight against invasive species."

eDNA sampling matches traces of DNA in water to known target species — here, bullfrogs, chytrid fungi and ranaviruses. It reveals not only bullfrogs but also the native species present, including very rare or hard-to-find ones. Combined with acoustic recordings of the animals' distinctive mating calls, it gave the team several ways to monitor and assess species.

Backpack eDNA samplers collect water continuously as the tube is moved through the water. The samples are processed right on the stream bank, ready to ship to the lab. NPS / Elizabeth Schnaubelt.

"We have several sites where we did not detect some of our target species using traditional visual encounter surveys, but eDNA samples collected at the same time provided reliable detections of their current use of that wetland," Hubbard says. The results show which sites need treatment, which still have native species, and which are still being invaded.

Spreading the word

The team has used many ways to alert the public: news coverage, social media, stickers and colouring books, public events and education programmes for visitors and supporters. One was the Natural History of the Gila Symposium, held from February 28 to March 1, 2024, which opened the Gila Centennial, marking 100 years of the wilderness idea in the United States.

A winter solstice morning at Gila Cliff Dwellings National Monument. NPS.

Zylstra calls the project a rare funded chance to think big about conservation. The main threats to native amphibians and reptiles in the Southwest — shrinking surface water and spreading invasive species — work across whole landscapes, she says, and the more varied the partners, the better the chance of strategies that work locally and regionally.

With the funding, the project will recruit advanced graduate researchers to lead assessments of native species recovery and to help the Arizona Frog Team write control and recovery plans for each site, while the team keeps reaching out to park neighbours.

Sources

• Benjamin Randall, "Collaboration & Innovation For Ecosystem Restoration in the Southwest", National Park Service, May 2024. https://www.nps.gov/articles/ira-imr-restoreamphibians.htm
• Corrections: the source lists "Tuzigoot National Historical Park"; the park is Tuzigoot National Monument, as the source's own map shows. It says bullfrogs eat the eggs of "even small mammals"; mammals here do not lay eggs, and the source's own caption lists small mammals among what adult bullfrogs eat. "Tempani" is corrected to tympana, and "wrecking havoc" to wreaking havoc.
• The source page's portrait of the author is not reproduced.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.