Stanford team leaps at chance to study frogs invading Golden Gate pond

Researchers will suggest methods of controlling aggressive species

Photo: Ruthann Richter

Veterinarian Sherril Green wades through the tepid waters of the Lily Pond in San FranciscoÂ’s Golden Gate Park in hopes of snagging some of the African clawed frogs that have invaded the pond. Green and her frog-catching team plan to return later this month to capture more of the fast-breeding, carnivorous amphibians.

Veterinarian Sherril Green, DVM, PhD, wearing chest-high waders, is trolling the Lily Pond at Golden Gate Park looking for the infamous African clawed frogs that have invaded the pond;’s once-peaceful waters. The frogs are in evidence everywhere, but the adults are slippery creatures, hard to catch.

Suddenly one jumps into Green’s net -– specimen No. 1 for a Stanford project to study the habits of these fast-breeding, carnivorous amphibians that have taken over this popular spot in the park.

For about a year, state and local officials have been plotting a war against the foreign frogs, which pose a threat to native species. They gobble up any fish and other amphibians that cross their paths and may be carriers of disease that could spread to native species, said Green, an associate professor of comparative medicine and a frog expert.

She and her team are working with the state Department of Fish and Game to help contain the spread of the frogs, which are virtually impossible to eliminate. Over the next year, the Stanford researchers will determine how many of the aggressive species are living in the pond, observe their breeding and eating habits and examine whether they carry bacteria, parasites or other diseases that could harm local species.

“This species has never been eradicated from anywhere it has invaded before, but there hasn’t been a place where studies have been conducted over the long term,” Green said. “By counting the number of frogs and seeing how many survive from one season to the next and looking at the predators, we can help provide some ideas about controlling the spread to other ponds and lakes.”

The frogs, an African species known as xenopus laevis, are black- and brown-spotted creatures that can grow up to 5 inches in length. They have webbed hind feet and clawed front legs. They are commonly found in medical research laboratories because they serve as good subjects for studies in developmental biology, medicine and physiology, Green said. They were probably released into the pond by well-meaning scientists who wanted to save them from destruction, she said. But whoever left them there had little notion of the havoc the frogs could cause.

A year ago, the state Department of Fish and Game hatched a plan to eliminate the interlopers by draining the pond, which covers about a third of an acre, but the cost was too high. Moreover, it wouldn’t have done the job, Green said. The tadpoles would just burrow in the mud and wait out the winter until they could spawn again.

Green, who has studied infectious diseases in the frogs for eight years, received permission from the state to remove a few hundred of the species for study. And so she and four other researchers from the Department of Comparative Medicine set out on the morning of June 29 to net some good specimens. The group included veterinary pathologist and associate professor Donna Bouley, DVM, research associate Sushama Varma and visiting veterinary students Sally Davis and Brielle Hecht.

The researchers donned waders and ventured into the stagnant, tepid waters of the 3-foot-deep pond, located near the California Academy of Sciences. Armed with a grid of the pond, they had expected to mark the hangouts for the tadpoles, but found the fast-moving, grey specimens slithering everywhere by the thousands. The adult frogs proved harder to find at first, as they had found cover under the lily pads.

Then Green spotted five adults jumping above the surface and netted one, the first catch of the day. But the slimy frogs proved elusive until representatives from the San Francisco Recreation and Parks Department volunteered use of their makeshift traps -– black metal cages with chicken livers as lure. Sally Davis laid the first trap and immediately saw it shudder with activity -– seven or eight large frogs aggressively angling for a good meal.

Green said the African frogs will eat virtually anything, including their own kind. The researchers found little other wildlife in the pond -– a single large carp, three mallards, some mosquito fish and a few red-eared slider turtles. But the stickleback fish that once populated the pond were gone. “It seems like they’ve eliminated everything from the pond,” she said.

The team members packed their captures, 17 adults and eight large tadpoles, in buckets for transport back to the Animal Research Facility. Initial visual studies suggest that the frogs carry no infectious disease and few parasites, though final results are not yet in, Green said. She said the stomach contents of the frogs prove they’ve been feeding on each other and on local fish.

Green’s team will recommend measures for controlling the creatures, even something as simple as a fence around the pond to prevent people from ferrying the frogs to other locations. It may also be possible to introduce fish species that are natural predators to help keep them in check, she said.

The researchers plan to return later this month with their own minnow traps in hopes of capturing more frogs for study.