Even worse, they're kind of cute — and thus more likely to be whisked away by children and dumped into other ponds, where they spread even more.
"They are a threat," said Dr. David Wake, an emeritus professor of integrative biology at the University of California-Berkeley. "They change the environment quite profoundly."
Native to Kenya, the frogs are able to live under ice, in the ground and in salty water. They alter ecosystems by gobbling up insects, fish, lizards and even birds that fit into their large, tongueless mouths. They also burrow into the ground to survive dry conditions and prey on the state's endangered red-legged frog.
The African frogs, outlawed as pets in California several years ago, are used in medical and biological research. Some theorize that researchers might have released the animals into Golden Gate Park's Lily Pond and parts of Southern California to save the frogs from destruction.
Pet stores and collectors wary of being slapped with fines of up to $1,000 also might have released them into local creeks and ponds.
Eric Mills of Oakland-based Action for Animals, which has lobbied the state to fight the spread of invasive species, said the only way to prevent the frogs from spreading is to kill the population in Lily Pond.
"They spent millions of dollars a few years ago in San Diego trying to get rid of these frogs," Mills said. "If they get loose in the San Francisco delta, it will be devastating to get them out."