For more than a quarter century, biologists in Arizona have battled a voracious invasive species: the American bullfrog.
Humans transported the bullfrogs – prized as a delicacy — to the western United States in the 19th and early 20th century. They stocked ponds with the frogs, which can grow to seven inches in length and weigh up to one-and-a-half pounds.
“Bullfrogs are amazing animals where they belong – east of the Continental Divide,” said David Hall, a senior wildlife biologist at the University of Arizona. “There they have predators that have evolved with them, and they keep everything in balance. Here, the predation pressure is just totally absent and they just eat everything that they see and that they can cram into their mouth – bats, songbirds, rodents. It’s amazing what they can eat, what we’ve found in their stomachs.”
In the high desert of northern Arizona, the bullfrogs invaded the habitat of native leopard frogs, including the threatened Chiricahua leopard frog. Since the early 1990s, biologists traveled from pond to wetland, attempting to eradicate the bullfrogs with gigs and rifles.
“Nobody had really tried to do that before, and it’s difficult,” Hall said. “In those early days, we spent a lot of resources, and a lot of labor and effort, and we failed miserably. But we learned from those failures to the point (that) now in certain areas we can be pretty effective at removing bullfrogs.”
Today, as waterways are cleared of the invaders, researchers and field managers are tracking the return of native species using eDNA technology.
Rather than relying on visual or audible cues, researchers can take water samples to determine which species have returned and in what numbers. That’s particularly important for species that can be difficult to locate.
“There might be species in here that are highly cryptic and we don’t see them,” Hall said. “eDNA sampling is really beneficial because otherwise you have to do repeated surveys and even then, detection probabilities are low for a lot of these species.”
The technology can help track the American bullfrog as well. The frogs can produce clutches containing up to 40,000 eggs, and ensuring that a waterway has been cleared can require multiple visits over the course of several years. As eradication efforts continue, Hall and other biologists are watching to see which species return and thrive.
“As more markers are developed for eDNA, I think that’s a new frontier,” Hall said.