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Frogs on the Move
California red-legged frogs were still so common at the end of the 19th century that tens of thousands ended up on dinner plates each year. But after urbanization, and the introduction of predators like the bullfrog and various exotic fish, the frog disappeared from approximately 70% of its historical range. Although it hangs on in small numbers where its habitat hasn’t been destroyed, little is known about its behavior and needs.
At the East Bay Regional Park District, biologists Steve Bobzien, Joe DiDonato and Pete Alexander have been trying to learn more about the frog on the district’s lands—where it breeds and what types of habitat it uses. One area they’ve focused on is upper Alameda Creek, where they fitted 27 frogs with radio transmitters and pit tags (a scannable tag) and tracked them for several seasons. "People used to think these frogs acted like a bunch of bricks," says Bobzien. "But we’re finding that their behavior is more variable and more puzzling than we thought."
Last summer, when Alameda Creek dwindled to a trickle with a few remnant pools, the frogs were on the move. After disappearing for three weeks, one male frog turned up 132 meters from where he had been tagged, high up a dry, rocky tributary, under a log amid some rattlesnake skins. Several of the tagged females traveled great distances too—one moved 640 meters downstream—think 10 city blocks—to lay her eggs. Another male was discovered 60 meters away from the creek, beneath some leaf litter. No one is sure quite why the frogs are traveling so far or why they are moving away from the creek, although Bobzien thinks they may be dispersing in order to find food. "People used to think the frogs stayed close to the streams, near water," says Bobzien. "But these frogs have evolved in our Mediterranean climate. As long as their skin can stay cool and moist, they seem to be able to hide out in unusual places—ground squirrel burrows or cracks in the mud in drying ponds."
Red-legged frogs are also often thought of as preferring pond or marsh-type habitat, says Bobzien. So he was excited to document for the first time frogs breeding in the creek. "The early 1990s were drought years, and later we had El Niño flood flows, which red-leggeds don’t do well in. So last winter may have been the first ideal breeding year for them in the creek," he explains.
The scientists were also surprised to learn that in certain areas of the park district, red-legged frogs over-wintered as tadpoles (typically, a tadpole will develop into a small frog within its first growing season). The biologists aren’t quite sure why this is happening, but speculate that cool water (and lack of food associated with the cooler temperatures) may temporarily inhibit metamorphosis. The district is collecting water temperature data to further study this unusual phenomenon.
Bobzien is encouraged by what they have learned so far. "Before we did the studies, we thought maybe we had a few remaining adults trying to breed occasionally. But instead it appears that certain years are really nice for breeding—especially years in which we’ve had relatively mild winter stream flows, which creates ideal late winter-early spring breeding conditions." One challenge for the district is educating the public about the need to protect the frogs. "As soon as a golf course or urban development comes in nearby," says Bobzien, "someone throws a bunch of bullfrogs or exotic fish into the ponds. We’re trying to encourage people not to put non-natives in the ponds."
Bobzien hopes that predators can be controlled to the point where the frogs, already present in the park in greater densities than in many areas of the state, will make a comeback over time. He predicts that with this year’s early heavy rains, they will breed early in the ponds (which are already full to the brim) but delay breeding, until flows have subsided, in the creeks. But that’s just a guess. If he’s learned anything about red-legged frogs, he says, it is "not to be surprised by the surprises."
Contact: Steve Bobzien (510) 544-2347
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