Invasive Vertebrate Species

Alien vertebrate species can be introduced in many ways. Some are introduced intentionally by humans, as in the case of feral pigs, European starlings, American bullfrogs, Virginia opossums, and wild turkeys. Others are native to this continent but have spread outside their normal range due to changes brought by civilization, as in the case of the brown-headed cowbird.

Feral pig (Sus scrofaIn California, feral pigs are hybrids of free-roaming domestic pigs and European wild boars. Free-ranging domestic pigs began with the Spanish missions and were perpetuated first by European settlers, then by houndsmen who dispersed the animals to start populations for hunting. European wild boars were brought to the United States from Poland and transplanted to the Smokey Mountains in the late nineteenth century. Animals were removed from that population in 1925 and introduced onto the Moore Ranch in Monterey County, and then to the Hearst Ranch in San Luis Obispo County soon after. By 1991 they occurred in 49 of the 58 counties in California, and they continue to spread. In 1956, they were adopted as a game animal by the California Fish and Game Commission.

Feral pigs are notoriously fecund, capable of producing multiple litters of up to a dozen piglets each per year. They are voracious consumers of oak mast, forbs, and seeds, and can leave a devastating wake in their path as large sounders of up to 60 pigs feed through an area. Feral pigs also root for bulbs, roots, insects, and earthworms, and disturb soils so heavily that they greatly increase erosion and runoff of topsoils. They have recently been discovered to be opportunistic predators of many other vertebrate species (Wilcox and Van Vuren, 2009; Jolley et al. 2010).

On the Mitsui property, pigs are deterred from entering in large sounders by a sturdy perimeter hogwire fence. A few places provide small opening for the occasional incurrence, but feral pigs betray their presence by rooting the earth—after which they are normally hunted heavily until they temporarily subside. Hunters kill approximately half a dozen pigs per year on the Mitsui property.

European starling (Sterna vulgareEuropean starlings were introduced, after several attempts, into New York City’s Central Park in the late nineteenth century. The starlings quickly spread throughout the continent and have become naturalized in California. European starlings out-compete native birds, especially cavity-nesting birds, by beginning nesting attempts early and aggressively driving away potential competitors. They compete directly with woodpeckers, western bluebirds, oak titmice, and several swallow species, particularly in oak woodlands. In winter they congregate in large flocks, which provides protection from predation and increases their chances of finding food.

American bullfrog (Lithobates catesbeianusAmerican bullfrogs are native to the southeastern United States. They were imported into California to replace the diminishing population of California red-legged frogs that were over-harvested by the restaurant industry for frog legs during and after the Gold Rush. American bullfrogs are voracious predators and will eat literally anything live they can fit in their mouths. They are prolific egg producers and their larvae out-compete native larvae in ponds, lakes, and streams by growing very large and persisting two years in the aquatic environment, where they out-compete native larvae for food (algae). On land, they attain a larger size than native frogs and consume them whenever the opportunity presents itself.

On the Mitsui property, American bullfrogs are largely confined to Leaky Lake but occasionally attempt to expand their range; in the fall of 2010 they were discovered at Dolcini and Turtle Ponds. They were culled in both places, during which time it was discovered that they were preying upon juvenile California red-legged frogs (Wilcox, in press).

Virginia opossum (Didelphis virginianusVirginia opossums are native to the southeastern United States but were brought to the West Coast as pets in the early twentieth century. They have a highly varied diet and raise large litters of young. They can tolerate many climate types, except for long periods of freezing weather. Virginia opossums eat plants and animals and are efficient predators. They are occasionally seen on the Mitsui property.

Wild turkey (Meleagris gallopavoThe wild turkey is native to North America east of the prairie midlands. In the 1960s and ‘70s, the California Department of Fish and Game sponsored widespread introduction of the species across the state as a game species, and today it flourishes here. Wild turkeys are common on the Mitsui property, both in open pasture and under oak woodland canopy. They feed on insects, acorns, and assorted other seeds, nuts, and berries.

Brown-headed cowbird (Molothrus aterBrown-headed cowbirds are native to the eastern United States and were initially inhibited from spreading west by the wide-open plains and deserts of the Heartland. Human habitation, and the subsequent planting of food crops and woodlots, has since allowed them to move west.

Female brown-headed cowbirds parasitize the nests of other birds by laying eggs in the native nests, especially in riparian areas. The native bird unwittingly incubates the cowbird egg, and when the chick hatches, it kills or pushes out the native chicks; eventually, the native mother is feeding only the interloper. The female cowbird may lay up to three dozen eggs in a season, and so successful is the species’ invasive strategy that native nests usually don’t fledge any native chicks; only those of the cowbird.