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Invasive Species Week: The Wild Pig

Wild pig in the hills near San Jose | Photo: Don McCullough/Flickr/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. So far this week we've covered the blue gum eucalyptus tree the Argentine ant, and the bullfrog.

Quite a few mammals have gone invasive after being introduced to California, some of them destructively so. Introduced red foxes live throughout the western half of California, eating everything they can catch -- including their Endangered cousins the San Joaquin kit fox on occasion. In our cities house mice, Norway and ship rats, and the ever-controversial house cats dominate the ecological landscape, with gray and fox squirrels not far behind.

But when it comes to just plain old damage to the landscape, it's hard to top the destructive power of California's population of introduced wild pigs.

The wild pig, Sus scrofa, is the same species of animal as the one revered by hipsters as the source of the world's bacon. Wild pigs in California are often called "feral pigs." Technically speaking, that's a bit of a misnomer. "Feral" connotes an animal that's escaped domestication, and pigs have been escaping domestication in California since the Mission days. But the animals now free-ranging the Californian countryside descend in part from populations of actual European wild hogs, the animals from which pigs were domesticated. Released into the wild by hunters in the 1920s and 1950s, those wild hogs made a distinct genetic contribution to the state's Sus scrofa population.

That contributes to the wildly variable appearances of individual wild pigs in California. Some of them look not all that different from domestic pigs. Others can appear fairly monstrous.

With three centuries of habitation behind them, it's not surprising that wild pigs have been reported from 56 of California's 58 counties. They're most abundant in places with surface water, which means the state's population mainly lives west of the mountains that separate coast from desert. Even with 300 years of tenure wild pig numbers have climbed dramatically in recent decades, especially in Southern California, which for decades had less than one percent of the state's wild pigs. Wildlife managers suspect part of the reason for the increase may be deliberate releases by would-be hunters, but facts are hard to come by.

Some invasive species do their damage in relatively subtle ways: displacing and inconveniencing native species little by little. But there's nothing subtle about the damage wild pigs do to the Californian environment. They regularly feed by rooting around in the soil for roots, bulbs, and insects; when a group of them -- a "sounder" -- descends on a valley grassland, what they leave behind them can look as though it's been thoroughly rototilled. That is, if a rototiller also ate all the snakes, lizards, baby birds, and other small animals as it churned the soil. And if a rototiller gave birth to half a dozen baby rototillers every year.

This feeding behavior isn't all that different from that of an extinct native, the California grizzly, and a few have suggested that pigs may actually be filling an ecological niche that's been vacant since the last griz died in California in the 1920s. But given that California grizzlies usually foraged alone rather in packs of more than a hundred individuals, the suggestion that pigs might be fulfilling the bears' old function seems unpersuasive.

In the state's remaining oak woodlands, wild pigs will vacuum up any acorn they encounter, often rooting up seedlings in the process. Oak woodlands are already under threat from fires, disease, and bulldozers, the pigs eating one next generation of trees after another doesn't help. Making matters worse for other local animals, pigs have apparently learned to raid others' caches of acorns, possibly consigning their neighbors to go hungry.

On hillsides, their rooting can cause serious soil erosion, silting up any streams down the hill. The pigs' habit of wallowing in open water or mud is fairly effective at clogging streams with silt as well. As they wallow, they defecate; wild pigs are frequently infected with Giardia and Cryptosporidum, which ends up in the water. After a 2006 outbreak of E. coli in people who'd eaten spinach from one Salinas Valley farm, the Food and Drug Administration reported that the spinach may have been contaminated by feces from wild pigs.

Wild pigs also pose problems for farms. They'll eat almost anything we do, which means that a herd of pigs happening on an unprotected field of lettuce or carrots or tomatoes can do hundreds of dollars' worth of damage per minute. Since many adult wild pigs in California weigh 200 pounds or more and are excellent diggers, a fence has to be pretty formidable to keep them out of places they really want into. And according to recent reports from San Jose, as the deepening drought hammers the backcountry, pigs are apparently wanting into well-watered suburban yards.

The animals have had other, less direct impacts on California's environment. Populations of pigs introduced to Santa Cruz and Santa Catalina islands turned out to attract golden eagles from the mainland. The eagles, which hadn't previously spent much time on the islands, were attracted by a steady supply of pig carcasses, and then turned their sights to the islands' Endangered foxes when the pigs didn't die fast enough.

After highly controversial eradication programs that attracted lawsuits from animal rights groups, the pigs were declared eradicated from Catalina and Santa Cruz islands within the last decade. Even on relatively small islands, getting every last pig was extremely difficult. Eradication from the California mainland is likely impossible. And that would be the case even if mysterious new introductions of the animals weren't happening in places where they hadn't been before, as happened somewhere around 2005 near Alpine in San Diego County.

Face it: they're here for the duration. It would just be too hard to get rid of them now. They're omnivorous, they're too clever for their own good, they're highly social, and when they throw their weight around it adds up to around 200 pounds, more or less.

Hmmm. Remind you of anyone?

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