Wild or feral pigs were implicated in at least one E. coli incident in a vegetable field in the Salinas Valley. Some have called wild pigs a minor contributor to the E. coli threat, pointing to domestic animals as larger sources for E. coli in California's food crop.

However, domestic animals are far easier to see and photograph by the media than wild hogs. A University of North Dakota wildlife ecologist working in California says the wild pig problem is far greater than what the eye and camera captured and it is growing, posing a threat not only to crops, but to California's natural habitat.

Using computer-aided mapping and records of hunting tags, Rick Sweitzer, a wildlife ecologist at the University of North Dakota supported by the University of California Exotic/Invasive Pests and Diseases Research Program has calculated how far wild pigs have expanded their range in California to encourage using alternative methods to control their spread.

Wild pigs have long been considered a threat to native species and especially native plants in California, Sweitzer says, “Unless we find better ways to manage wild pigs, California will risk losing many of its unique plants and animals. Equally important, agricultural losses might become enormously costly if wastes from wild pigs spread into croplands.”

Sweitzer and his research team compiled a database of more than 70,000 wild pig harvest locations, which they used to determine the pace of range expansion by the species in California over the last 13 years. When hunters in California apply for tags, the tags include a portion that requires hunters to provide the California Department of Fish and Game with a brief description of where the pigs are killed. Preliminary results indicate they expanded their range by more than 7,000 square miles between 1992 and 2004.

Overturn vegetation

Wild pigs travel in herds, and create wallows, overturning native vegetation as they dig for food. Their rooting also damages the habitat of animals that live on or under ground such as amphibians, reptiles, mammals, and ground nesting birds. Rooting loosens soil, which may then be washed into streams and creeks, compromising water quality.

Sweitzer's study has demonstrated that wild pigs are continuing to expand in California, even under very liberal hunting regulations of year-round hunting with no bag or possession limits. “Our results indicate that 97 types of vertebrates and plants identified as threatened, endangered, or rare are exposed to rooting and other activities of wild pigs.”

Since 1956, the California Department of Fish and Game, has classified wild pigs as big game mammals; the goal is to use sport hunting to reduce their spread. “The problem is that more than 85 percent of the current statewide population of wild pigs is in areas where hunting access is limited, such as state and federal parks, reserves and natural areas, and private lands,” says Sweitzer. “Many private landowners simply don't want people on their properties, which means that sport hunters can't harvest and reduce populations in the areas where most of the wild pigs live.“

There are other legal ways to manage and remove wild pigs that are causing damage to natural areas, private properties, and agriculture. The California Department of Fish and Game has made it relatively easy to obtain depredation permits that allow park managers and other people to use traps and trained hunters to remove wild pigs. This approach can be time consuming and costly, however, and is not always effective.

Smart animals

“One reason wild pigs are so popular among hunters in California is because they are smart,” says Sweitzer. “But the negative side of this is that they quickly learn to avoid traps when they sense danger, and they have even been known to play dead when being hunted from helicopters in areas like the Australian outback.”

Because it may be difficult to halt the spread of pigs by sport hunting, Sweitzer is taking a proactive approach and has used computer modeling to identify areas with suitable habitat conditions that are currently unoccupied by wild pigs. Results suggest that about 17,000 square miles of mixed hardwood and conifer woodlands in northern and eastern California are vulnerable, specifically areas in the western Sierras, Cascade Range, and along the margins of the Central Valley.

The third component of Sweitzer's research involves a major effort to estimate how much damage feral pigs cause to California agricultural and ecological areas.

The ecologist and his team developed two surveys. One survey addresses damage to natural areas and was sent to national, state, regional, and private parks, monuments, and preserves, USDA National Forest districts, Bureau of Land Management field offices, USDI Army Corps of Engineer recreation areas, and tribal officials for Native American Tribal Lands and Reservations in California.

Sweitzer sent the second survey to all California county agricultural commissioners, including questions related to the history and trends of wild pig-related agriculture and livestock damage. Sweitzer also designed and sent commissioners maps of their counties, and asked them to mark the specific areas where damage is occurring.

“Once the areas with agricultural damage are linked up with the mapped locations of hunter-killed wild pigs, we will be able to identify statistical linkages between regions of important agricultural damage and wild pig population abundance.”

Results from these two surveys will provide the first look ever at how much damage wild pigs are causing across the state. Sweitzer's hope is that once actual dollar numbers are assigned to the damage being caused by this non-native mammal, policy makers might think seriously about other approaches to managing them.