“Smiley said all a frog wanted was education,” the long-winded raconteur says in Mark Twain’s “The Celebrated Jumping Frog of Calaveras County,” and sure enough, Dan’l Webster, the athletic amphibian of the title, just needs a little coaching to bring out his greatness. But biologists studying the threatened red-legged frog—long assumed to be Daniel Webster’s breed—would add one or two things to the list of the frogs’ needs. Like habitat, for starters, preferably without bullfrogs.
Even among amphibians—the world’s most threatened class of vertebrates—red-legged frogs have it rough. Eaten with gusto by Californians in the 19th century, they’ve since lost 70 percent of their historical habitat, largely because of land conversion. They’ve all but vanished outside of the Central Coast; the wetlands of Elkhorn Slough are one of the remaining strongholds. And even here many of the seasonal ponds they call home have been invaded by bullfrogs, which eat red-legged frogs.
Nina D’Amore, a researcher with the Elkhorn Slough National Estuarine Research Reserve, laid it out for a small crowd of nature nerds last Thursday, Aug. 18, at the Santa Cruz Museum of Natural History’s last companion lecture to the art exhibit “Endangered Neighbors” (the show runs through Sept. 10). Her years of research in the Elkhorn Slough area suggests that red-legged frogs are much pickier than bullfrogs (or the much smaller, ubiquitous Pacific chorus frogs) about where they’ll lay their eggs. They don’t seem to like water tainted by agricultural runoff, as D’Amore and other researchers learned when the Estuarine Reserve bought some land, took out the strawberries being grown there and saw red-legged frogs return to the ponds on the land the next year. And they don’t like bullfrogs.
“We started doing bullfrog removal,” she said, “and suddenly there were red-legged frogs in these ponds.” D’Amore cautions, though, that it’s not quite as simple as it may seem; in fact, the presence of bullfrogs seems to change the way the red-legged frogs use their habitat, not necessarily drive them from it.
Fewer bullfrogs also means fewer wasted mating efforts by confused red-legged males, which, in some ponds, spend more time trying to mate with juvenile bullfrogs than with red-legged females.
“I’ve seen them hanging on to limp, dead juvenile bullfrogs,” D’Amore said. “I’ve seen them hang on for 10 days, and they’re wasting their time.”
Just to make sure it didn’t devolve into a bullfrog-bashing affair, UC-Davis herpetologist Sean Barry suggested that while the perfect management plan would get rid of the invasive bullfrogs—imported in the 19th century, ironically, to give the red-legged frogs a break—that they in fact serve a purpose: as food for the beautifully colored turquoise-red-and-black San Francisco Garter Snake.
“San Francisco garter snakes eat bullfrogs like a smorgasbord,” Barry said, adding that they have “been the salvation of San Francisco garter snakes on the watershed” of the San Francisco Bay.
The San Francisco garter snake was the first snake listed as threatened under the Endangered Species Act. (Barry insists they’re not actually rare, just very good at hiding.)
Still, their habitat is small and under pressure from development and water management issues.
Ninety-five percent of their diet is frogs (mostly Pacific chorus frogs, but also juvenile bullfrogs and red-legged frogs).
“Bullfrogs colonize new aquatic habitat very early and make it possible for reproductive populations of San Francisco garter snakes to become established early,” he said.
