African clawed frogs, outlawed as pets in California several years ago, are proliferating in San Francisco-area ponds, alarming California biologists who already are fighting a host of other invasive species.

Native to Kenya, the frogs alter ecosystems by greedily eating insects, fish, lizards and even small birds. They also prey on the state's endangered red-legged frog. The hardy African frogs can live under ice, in salty water and in the ground, where they burrow to survive dry conditions.

The frogs are used in medical and biological research. Some people believe that researchers released the animals into Golden Gate Park's Lily Pond and other parts of Southern California to prevent their demise. Others believe that pet retailers and collectors released them into local creeks and ponds to avoid fines of up to $1,000.

Some groups are lobbying to destroy the frogs as the only way to prevent their spread.

The California Department of Fish and Game dropped plans to dry out Lily Pond last summer, however, to focus on the northern pike, a gluttonous fish that has taken over Lake Davis in the Sierra Nevada.