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3 Invasive Animals That Have Permanently Changed the Delta

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Red-eared slider | Photo: Greg Hume/Wikimedia Commons/Creative Commons License

An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with for all the project's stories.

When American settlers started visiting the Delta, new animals came along with them. They reshaped the Delta. Over the course of the 19th and 20th centuries, the pace of introductions of new animals species increased.

There were our livestock, cattle and horses and sheep, our dogs and cats. Others we brought to release into the wild deliberately, hoping that they would be fruitful and multiply so that we could eat their descendants. And still more species stowed away in our ships, either hiding in the cargo or scooped up against their will in the ships' ballast water, to find new homes in the confluence of California's great rivers.

They made themselves right at home. It's hard to imagine the Delta without some of them now. Our attitudes toward them range from appreciation to resignation to attempted eradication. And all of them are likely to be part of the Delta for a very long time to come. Here are three of the most destructive. There are many more.

Red-eared slider

Red eared slider turtles are undeniably attractive and quite popular. Known to science as Trachemys scripta elegans, red-eared sliders are the most commonly kept pet turtle in the world.

In their native lands, the southeastern United States and northern Mexico, red-eared sliders are part of an ecosystem that has adjusted to their presence over millennia. Reaching an average maximum size of 12 inches long (measured along the shell, or "plastron," the turtles spend most of their time in fresh water, basking in the sun to warm themselves, and eating. Their preferred diet consists mainly of aquatic plants, though they're omnivorous and will readily eat small fish, frogs, and other animals.

Or, for that matter, canned dog food: mature red-eared sliders are relatively easily kept as pets. That is, if you know what you're doing, and have room in your life for an animal that can reach the size of a soccer ball and live for thirty years. And there's the issue: though many Californians over the last century have gladly taken home a slider or two when they're a couple inches across in the plastron, many of those youthful adoptees have been set "free" when they get to be too big, too old, or too much bother.

In some cities, adherents to certain religious schools of thought will purchase red-eared sliders and set them free as an admirable if ecologically destructive act of charity.

And once set loose in the wild, red-eared sliders can easily dominate whatever body of water they move into. They're larger than the native western pond turtle, and require essentially the same habitat. And that means the native turtles get shoved out of what had been their habitat.

There's also some conjecture that red-eared sliders spread disease to the native western pond turtles though the jury is still out on that.

But the larger invasive turtles definitely outcompete the smaller natives for available food, which has contributed to the western pond turtle's decline, along with our wholesale destruction of its habitat.

All of which means: if you love a red-eared slider, don't set it free.

American bullfrog

American bullfrog | Photo: Cornelier/Wikimedia Commons/Creative Commons License


This is another import that was originally native to the eastern parts of North America. Unlike the red eared slider, bullfrogs mostly weren't brought here as pets. Instead, gastronomes began to import the frogs as a food item once the native red-legged frog started getting scarce, partly because of the red-legged's popularity on 19th century menus and partly because we'd altered its habitat.

That habitat alteration doesn't bother bullfrogs nearly as much: the gigantic frogs, with the scientific name Lithobates catesbeianus, do just fine in reservoirs, polluted stockponds, and other such artificial or semiartificial habitats.

And that's a problem, because the kinds of native wildlife eaten by American bullfrogs is so extensive it's almost easier to list the wildlife the frogs don't eat. Adult American bullfrogs are what is occasionally referred to by biologists as a "gape-limited predator." That charming phrase essentially means the frog eats whatever it can wedge into its mouth. Given that bullfrogs, North America's largest frog, can reach six inches in length not including their hind legs, those gapes can be pretty wide. American bullfrogs are known to chow down on red-legged and yellow-legged frogs, native salamanders, Delta smelts, salmon and steelhead fry and eggs, mice, baby birds, moles, voles, lizards, snakes, butterflies, dragonflies... if it fits, write the obit.

That's a big enough problem right there: bullfrog mouths are huge black holes for native wildlife. The tadpoles are a problem, too: though they generally eat algae and similar vegetarian foodstuffs, American bullfrog tadpoles will occasionally turn away from the salad bar to eat the tadpoles of other amphibians such as red-legged frogs.

Bullfrogs pose a disease threat to native amphibians as well: they're carriers of, and resistant to, the devastating disease chytridiomycosis, which is considered a big reason for the global amphibian extinction crisis first noticed in the tropics in the 1980s, and is becoming an increasing problem in North America.

And to add insult to injury, bullfrogs offer one more apparent threat to the state's suffering red-legged frogs. Male red-leggeds are apparently prone to finding the much larger bullfrog females irresistably attractive, which means those males spend time courting bullfrog females instead of girls of their own species. That can lead to lower reproductive success among red-legged populations, not to mention the possibility that the male red-leggeds will get eaten by their ill-advised dates.

American bullfrogs are found almost anywhere there's fresh water throughout the state, aside from in lakes in the High Sierra, which is apparently too cold for them. In the Delta, that means freshwater sloughs and ponds, tule swamps and irrigation ditches, as well as in quieter water along the main stem of rivers such as the Sacramento and American.

Given all the above damage, you might be surprised to learn that it's still legal to import bullfrogs into the state of California: last year,. the California Department of Fish and Wildlife (CDFW) estimated that two million of the giant frogs are brought into the state annually, usually for sale in live food markets. Many of those animals escape, or are released into the wild by well-intentioned animal lovers.

That's prompted some official reactions as attempts to stem the influx of new live bullfrogs. CDFW moved in 2010 to require that live animal shops kill bullfrogs on site before customers left the premises. Many live animal markets failed to comply. In 2012, the city of Santa Cruz banned the sale of American bullfrogs within city limits, citing the danger to local populations of red-legged frogs and tiger salamanders. In 2014, CDFW stopped issuing long-term permits to import bullfrogs, and the California Fish and Game Commission has asked CDFW to propose longer-term measures to restrict import of the frogs, possibly including a total ban on new imports.

Mississippi silverside

Mississippi silverside | Photo: Dan Worth, California Department of Fish and Wildlife


Also called the inland silverside, Menidia beryllina is an attractive-looking six-inch native of the eastern United States, where it lives in lakes and streams accessible from the Atlantic Coast. In California, it was deliberately introduced in the 1960s both as a forage fish for larger, desirable game fish, or -- as in the case of the silverside's introduction to Clear Lake in 1967 -- to eat insects considered pests.

From those introductions, which also took place in the Bay Delta watershed, the silverside spread to the Delta, where in some places it's the most common fish.

Silversides eat zooplankton, the tiny floating aquatic animal fauna that includes not only animals that stay small their whole lives, like copepods, but the larval stages of larger fishes such as the Delta's Chinook salmon and Delta smelts. And silversides can tolerate water almost as salty as sea water, which means the brackish mixing zones where larval Delta smelts hang out are perfectly comfortable for adult silversides.

That pretty much means what it sounds like: Mississippi silversides are a major predator on Delta smelt. A 2012 study of silversides collected in the northern Delta found that almost half of the silversides the researchers collected in deeper water had eaten Delta smelt larvae within the last 36 hours.

Silversides aren't alone in that dietary choice: lots of other fish in the Delta, native or introduced, eat delta smelt. But the silverside's abundance makes this species one of the major predation threats to the Threatened smelt. And it looks as though the silverside, along with the American bullfrog and red-eared slider, is in the Delta to stay.

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