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A Berkeley resident of my acquaintance has a bullfrog in her garden pond. She’s not sure how it got there, but it’s been in residence for a couple of years. Usually she just sees its periscope eyes. Sometimes, though, it ventures out of the water, leaving wet frogprints on the pondside tiles. 

When she first told me about her frog, she seemed a bit apologetic, having presumably heard how environmentalists feel about the critters. Bullfrogs, like possums and cowbirds, are a native North American species, but never occurred naturally in California. I have nothing against bullfrogs in the right place, and that place is east of the Rockies. Out here, they’ve been a disaster. 

Bringing the bullfrog to California must have seemed like a good idea at the time. It was a commercial proposition: frog legs for the tables of San Francisco and other growing cities. 

Our state used to have native frogs in abundance. The most popular for culinary and recreational purposes (Twain’s “Celebrated Jumping Frog of Calaveras County”) was the California red-legged frog, which once ranged from the North Coast all the way to Baja, west of the Cascade-Sierra crest. It was by all accounts a tasty amphibian. Mary Dickerson, writing just before the San Francisco quake of 1906, said westerners esteemed it “the best edible frog in North America,” finding the flesh of the bullfrog “tough and coarse” in comparison. Dickerson also described the red-leg as “very alert and intelligent-looking,” for whatever that’s worth. 

Intelligent or not, red-legged frogs were harvested in appalling numbers from the Gold Rush to about the turn of the twentieth century. French cuisine was in vogue—one San Franciscan recruited 40 Parisian chefs at one go—and frog legs were on all the best menus. The annual catch ran as high as 118,000 in the peak year of 1895. This, along with habitat loss as wetlands were drained for farming, took its inevitable toll, and the red-leg is now an endangered species. But Californians hadn’t lost their taste for frog legs. Enter the bullfrog. 

The state’s first froggery was founded in 1896 in El Cerrito, with 4 artificial ponds and 36 Florida and Maryland bullfrogs. Some of their descendants may have escaped, and other bullfrogs may have just been released in lakes and streams to fend for themselves and to be collected when needed. They did spectacularly well: they’re now found all over the state, even on Santa Catalina Island. 

The bullfrog, Rana catesbiana (named for 18th century naturalist-artist Mark Catesby, who discovered it in the Southeast), is a survivor. Bullfrogs are fecund in the extreme, their tadpoles seem to be unpalatable to fish, and they can tolerate a broader range of water conditions and temperatures than native frogs. With feral populations in Europe, Asia, South America, the West Indies and Hawaii, the bullfrog is a successful agent of ecological 

globalization. 

The trouble with bullfrogs is their appetite. They’ll consume pretty much anything they can get into their capacious mouths. You wouldn’t think of turtles as frog food, but hatchling western pond turtles are vulnerable to bullfrog predation. This once common species, the West Coast’s only aquatic turtle, is now in trouble, despite bullfrog-bashing efforts in some of the areas where it still persists. 

Snakes, too. Arkansas bluesman Sonny Boy Williamson’s song about fattening frogs for snakes represents the normal situation, but it’s sometimes reversed: big bullfrogs will eat small snakes. I once watched a bullfrog in Tilden Park swallow a garter snake of impressive size. Bullfrogs have been implicated in the decline of the gorgeously marked San Francisco garter snake and the Central Valley’s giant garter snake, which, when young, is not too gigantic to be a meal for a bullfrog. 

But perhaps the most drastic bullfrog impact has been on the native western frogs. It is a frog-eat-frog world. Bullfrogs will devour red-legged frogs, mountain yellow-legged frogs (despite their garlic odor, or maybe that just whets the appetite), foothill yellow-legged frogs, Cascades frogs, spotted frogs. Not that the frogs don’t have other problems: diseases, pesticides, holes in the ozone layer. But the bullfrogs have certainly not helped. 

There’s another aspect to the relationship between the bullfrog and the red-legged frog, and it’s an ironic one. Frogs have evolved species-specific mating calls so the appropriate partners can get together. However, male frogs appear to be less discriminating than females. (This tendency may reach its ultimate in the notorious cane toad, which will attempt to copulate with road kill). Male red-legged frogs have been found to prefer bullfrogs to females of their own kind. The larger bullfrogs appear to provide a kind of superstimulus. These misalliances have not produced fertile hybrid offspring, so there’s no risk of genetic swamping here. But it’s a distraction that reduces the threatened red-leg’s reproductive success. 

The bullfrog is one more instance of the ramifying consequences of introducing an exotic organism—even one from the same continent—into a new ecosystem. Bullfrogs, starlings, star thistles, and hundreds of other alien animals and plants have transformed California, mostly for the worse. My friend’s bullfrog, though, has so far led a celibate existence and seems unlikely to contribute to the problem.