It might seem odd to be killing animals on a nature reserve, but that’s exactly what a team of volunteers was doing at the Santa Rosa Plateau near Murrieta one Saturday last month.

The target: bullfrogs.

An invasive carnivorous species, bullfrogs gobble up anything they can fit in their mouths, including sensitive native species such as southwestern pond turtles and the California red-legged frog, which is listed as threatened by the U.S. Fish and Wildlife Service.

The threat is not just hypothetical. In a specimen jar in her office, Santa Rosa Plateau Ecological Reserve manager Carole Bell has a southwestern pond turtle hatchling — designated a California species of concern — that was discovered in the belly of a bullfrog captured on the reserve in 2008.

Stretched out, a full-grown bullfrog can easily measure 16 inches long.

Wearing high-water wading suits, two volunteers ventured into the muck of a reed-filled pond in the Sylvan Meadows area of the plateau one morning, guiding a large net through the water in an attempt to capture tadpoles and adult frogs. Bell waited with more volunteers on the opposite shoreline with buckets at the ready to scoop up bullfrogs.

“I hope there’s no crocodiles in here,” joked Steve Fuller, of Menifee, wading into chest-high water.

They didn’t manage to snag any full-grown frogs, but about 20 hapless 4-inch-long tadpoles with bulbous heads found their way into the net, along with an assortment of bugs and a garter snake, which they released.

By the end of the morning, the group was splattered head to toe with pond mud.

Bell said volunteers remove bullfrogs from ponds on the plateau at least once each year. She said they will take another crack at the bullfrogs in a few weeks when the water is lower and they can round up more volunteers.

In previous years, they have collected thousands of tadpoles across the plateau.

It has been about eight years since a red-legged frog was last seen on the plateau, but Bell said she hopes to reintroduce the frogs one day.

“They don’t have a chance of survival with bullfrogs in the watershed,” Bell said.

Widespread Invasion

A decade ago, when there was still hope to save the plateau’s red-legged frogs, reserve officials were more aggressive with their efforts to curb the bullfrog invasion. They even visited neighboring properties, helping residents to clear bullfrogs from their backyard ponds.

Despite the threat to native species, some people are reluctant to kill the frogs, Bell said.

“I don’t blame people. They just don’t feel comfortable killing anything. Who does?” she said.

But one neighbor changed her mind when she realized the ravenous amphibians were responsible for the mysterious disappearance of her ducklings.

Bullfrogs are native to the United States east of the Rockies. They were brought to California in the late 19th century as food for humans and are now a widespread problem in several western states. Bullfrogs have been introduced for commercial frog-leg farming in many parts of the world, including Europe, Asia and South America, where they often prey upon and out-compete native amphibians.

Richard Fisher, a research biologist with the U. S. Geological Survey in San Diego, said the mining boom marked the beginning of major troubles for the red-legged frog, which is the largest native frog species in the western United States. The mining process damaged frog habitat and the miners themselves had a hankering for frog legs.

As the numbers of red-legged frogs began to decline, bullfrogs were imported for farming. Since then, though, the robust and voracious amphibians have made themselves at home in waterways and ponds across the state.

Big Frog, Big Appetite

Unlike some native frog species that have suffered from human development, bullfrogs fit right in.

Bullfrogs also carry — but are apparently not affected by — the chytrid fungus, which has been driving many species of amphibians worldwide to extinction.

Among the other critters bullfrogs have been known to eat: coast range newt larvae, 3-foot garter snakes, tree frogs, birds, voles, snails and smaller bullfrogs. Bell once found a full-size pocket gopher in a bullfrog.

While eradicating bullfrogs is, at this point, hopeless, Fisher said thinning their numbers in more isolated habitats such as the plateau can help native species.

Fisher said he has studied bullfrog removal efforts in the Santa Margarita watershed on Camp Pendleton.

Several years ago, the military was attempting to eradicate bullfrogs as part of an environmental mitigation plan. When military officials ended the program, the numbers of bullfrogs shot up dramatically.

“There were just a ton of bullfrogs,” Fisher said.

The team he worked with would hit the streams at night grabbing full-grown bullfrogs — frozen in the beam of a flashlight — with their bare hands.

Fisher has a video in which an endangered arroyo toad that had been eaten by a bullfrog is rescued, still alive, in an emergency in-field dissection. The arroyo toad puffs up its cheeks in distress as the scientists cut the stunned but otherwise unharmed amphibian from the bullfrog’s stomach.

The team also discovered many arroyo toads that were not so lucky in the frogs’ stomachs.

Fisher said they estimated that bullfrogs were consuming 120 adult arroyo frogs per month per kilometer of river.

Thinning the numbers of bullfrogs “was having a huge positive effect in that system,” he said.

“They were eating everything. Just everything.”

To volunteer with Team Stream, which works on a variety of projects on the plateau, e-mail cbell@tnc.org.