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Invasive Species Week: The Bullfrog

American bullfrog, Rana catesbiana. | Photo: The AlphaWolf/Wikimedia Commons/Creative Commons License

Next week might be Shark Week, but the California Department of Fish and Wildllife has declared August 2-10 California Invasive Species Action Week, an event intended to boost public awareness of the problems some non-native species of plants, animals, and even microorganisms can cause when they're imported into the state. We're celebrating, if that's the right word, by featuring five of California's most prevalent invasive species every day this week. The blue gum eucalyptus tree was featured Monday, and the Argentine ant on Tuesday.

Quite a few non-native species have been introduced to California because they added to our dinner tables. There are (usually non-invasive) cultivated crop plants, of course, almost none of which grew in the state prior to the 1700s. Aside from corn, beans, and squash, which were grown by some California Native people, most of the state's agricultural production encompasses introduced plants. Honeybees were imported in the 1850s for their honey, and opossums 40 years later by southeasterners nostalgic for the wild food of home.

And one of the most damaging invasive exotic species in California was first imported to the state to replace a native animal settlers had nearly eaten out of existence. And despite obvious evidence of the damage it causes to threatened native wildlife, we still import millions of them into the state each year.

The American bullfrog, Rana catesbiana, is the largest frog native to North America, with females reaching as much as eight inches in length, not counting the hind legs. Originally from the wetter regions east of the Rocky Mountains, the frogs were first imported to California after gold miners had eaten most of the state's native red-legged frogs.

As it turned out, the bullfrogs were even more adept at eating red-legged frogs than were the Argonauts. Bullfrogs are what biologists sometimes refer to as "gape-limited predators": if it fits in their mouth, they'll eat it. A lot of things can fit into a bullfrog's mouth. One in California was famously discovered with a 33-inch garter snake in its gullet. The list of what bullfrogs will eat is impressive: mammals, small birds, native frogs and salamanders, reptiles, fish, other bullfrogs, and pretty much anything else at hand of the appropriate size.

Now, the animals have spread throughout almost all of California west of the deserts, and they even inhabit a few places in the deserts where local water supplies are available: the Owens and Mojave rivers, the Imperial Valley, and a couple basins in the state's northeastern corner.

Bullfrogs affect California native animals in more ways than just eating them -- though they do that regularly, including legally protected species like the red-legged frog, California tiger salamanders, and coho and chinook salmon. They also outcompete with those species for food, an easy task given their overwhelming size and their fecundity. (A female bullfrog can lay 40,000 eggs.)

And bullfrogs are notorious vectors for a disease-causing pathogen that's done incredible damage to amphibian populations worldwide: the chyrtid fungus Batrachochytrium dendrobatidis, which causes chytridiomycosis. The disease has been linked to a global decline in amphibian populations over the last few decades, including in California. The two species of yellow-legged frogs in the Sierra Nevada are thought by some to have been driven to near-extinction by the disease.

And of millions of bullfrogs imported into the state each year, an estimated 62 percent are infected with chyrtid fungus. If they're released into local streams, that fungus can infect native amphibians.

Why do we import so many bullfrogs? They're still popular as food, for one thing. Though frog legs' popularity declined as a menu item among Californians of European descent over much of the 20th Century, that popularity is gaining again. The U.S. now accounts for 20 percent of total global consumption of frog legs. And the frogs never fell entirely out of gastronomic favor in the Asian-Californian community. It's no accident that most of the millions of bullfrogs imported into California come into San Francisco and Los Angeles.

That cultural phenomenon has posed a sticky problem for activists seeking to ban importation of bullfrogs. The now-suspended State Senator Leland Yee campaigned to keep the California Department of Fish and Wildlife from enforcing a 2010 ban on further importation of bullfrogs. According to Louis Sahagun in the Los Angeles Times, Yee claimed that such a ban was an affront to Chinese culture, adding;

For over 5,000 years, it has been the practice of both the Chinese community and the Asian American community to consume these particular animals. They are part of our staple. They are part of our culture. They are part of our heritage.

Which may be true of frogs in general, though the American bullfrog is a relatively recent addition to Chinese-Americans' diets likely dating no farther back than the Gold Rush in California.

That sentiment is by no means unanimous among California's Chinese-American communities. Yee similarly opposed a 2010 bill that would enact a statewide ban on shark fins, calling it an attack on Chinese culture as well. The bill was introduced by Assembly member Paul Fong of Cupertino, who said of the practice of shark finning "anything that is unhealthy, that the culture is practicing, we should stop doing it."

But due to such pressure, the Fish and Game Commission held "reconsideration hearings" on its bullfrog ban in 2010. Aside from a local ban on bullfrogs adopted by the city of Santa Cruz in January 2012, which Santa Cruz County adopted county-wide a month afterward, that's where things stand in 2014.

Environmental groups such as the Berkeley-based Save The Frogs have been pushing for reinstatement of the ban. Until they succeed, millions of bullfrogs will continue to be imported into California each year, with an unknown but probably high number released to the wild either by importers or retailers seeking to dump an unprofitable shipment, or by well-meaning freelance animal activists who take it upon themselves to act out an amphibian version of Born Free.

And the state's red-legged frogs, coho salmon, western pond turtles, and other beleaguered wildlife will pay the price.

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