California's Animal Invaders

Native species struggle for access to now-scarce resources

California is home to many invasive species that cover the entire plant and animal kingdom, from exotic fruit flies, bullfrogs, and feral mammals to escaped exotic plants. Invasive species threaten native species by dominating a habitat and competing for limited resources. Of all the states, California is hardest hit with over 670 invasives that cost state taxpayers millions in threats to the environment in general, and more specifically to agriculture, biodiversity, recreation, and fisheries. We'll look at a few of the pesky critters that cause environmental and economic damage.

European green crabs

In many cases, invasives can facilitate the habitat expansion of another invasive by eradicating a native species. Conversely, one invasive can lead another invasive to its demise. Hundreds of nonnative species inhabit California's coastal waters, with many more arriving each day in the ballast water of cargo ships. A 2001 state law requires ships to exchange ballast water at sea before entering ports to minimize the introduction of foreign sea life.

Particularly damaging to a native coastal clam population is the interaction of two nonnative species: the gem clam and the European green crab, with the native Nutricola clam. The Nutricola clam lives in the muddy bottom of Northern California's Bodega Bay and is a staple food of shorebirds. The gem clam, also a food source for shorebirds, arrived at the bay over 50 years ago, but its population was small and inconsequential. The European green crab was imported to the Bodega Bay area inadvertently around 1993 in boxes of fishing bait from New England. The 3-inch green crab invaded the bay waters, devouring the native Nutricola clams by the dozens per day. The gem clams quickly took over, displacing the decimated Nutricola clam's historic range and eventually may permanently displace it.

Bullfrogs

North American bullfrogs are expanding their range in the western U.S. while chomping away at literally any animal that fits in their mouths, according to wildlife refuge managers, the U.S. Geological Survey, the U.S. Fish and Wildlife Service, the National Wildlife Federation, and others.

The North American bullfrog was introduced to California around 1896 as a food source and game animal. Its historic range is in much of the eastern U.S., where predators such as alligators, snapping turtles, and large water snakes kept their numbers in check. Their high reproductive capacity (20,000 eggs at a time), increasing range, lack of predators west of the Rockies and voracious appetites make them a formidable wildlife management problem. Although native to the continent, the bullfrog's impacts in the western U.S. rival that of a foreign invasive species.

Two cases in point on the bullfrog's impact:

The California red-legged frog-featured in Mark Twain's 1865 short story 'The Celebrated Jumping Frog of Calaveras County'-is a dwindling amphibian species in California, and has been on the federally threatened list since 1996. Their drop in numbers came as a result of harvesting for human appetites during the late 1800s and early 1900s and widespread bullfrog predation that continues today.

Bullfrogs wiped out the threatened Chiricahua leopard frog, and nearly wiped out the Mexican garter snake on wildlife refuges in southern Arizona. Both of these native species were common refuge residents decades ago. Herpetologists are in an optimistic, but uphill struggle to control refuge bullfrog populations. A bullfrog will eat literally any living animal that fits in its mouth-from other frogs to turtles, snakes, birds, and bats.

Researchers are investigating unique, long-term control methods, as no single method used thus far has proven to be effective in bullfrog control. Toxic chemicals like rotenone kill all pond life indiscriminately. Researchers have also tried draining ponds during the dry season, killing the tadpoles and escaping adult frogs, but adults from distant intact ponds will take up residence as soon as the dry pond fills with summer monsoon rain. Predatory animals, like dragonfly nymphs, predaceous aquatic beetles, and bass, are being investigated to control the tadpoles.

Feral pigs

Introduced to the Channel Islands in the 1850s, feral pigs have uprooted vegetation and caused erosion. photo by Terry M. Smith

Domestic pigs were introduced in California in 1769 as a food source. Their range and numbers increased with the human population through the years, with small numbers escaping their domestic confines and reverting to a feral status. The feral pig population reached alarming numbers in the 1900s, resulting in their designation as a game animal by the California Dept. of Fish and Game in 1956. Their numbers were estimated in the 70,000-80,000 range statewide in the late 1980s.

Feral pig impacts on native vegetation are well-documented. They uproot vegetation, spread invasive weeds, and cause significant erosion. The increased damage over recent decades was publicized by the California Native Plant Society, leading to state legislation in 1991 to increase feral pig management efforts through hunting and depredation permits.

Santa Cruz Island, one of the Channel Islands off the Southern California coast, was subjected to some of the most dramatic feral pig damage since their introduction to the island as domestic farm animals in the 1850s. Native vegetation was stripped away, and the pigs' offspring were providing a new food source to golden eagles that had colonized the island in the 1990s. The eagles did not limit their mealtimes to piglets, but also began predating the native, cat-sized island foxes. The fox population declined not only on Santa Cruz Island, but also on nearby Santa Rosa and San Miguel Islands. The fox, once the dominant predator of the Channel Islands, is in such a severe decline that it's a candidate for the endangered species list, and has been displaced in numbers by another island mammal-a native spotted skunk.

The National Park Service began a major effort to save the island fox in 1999. A collaborative effort between the Nature Conservancy and the National Park Service aims to eradicate feral pigs from Santa Cruz Island. Captive fox breeding programs on San Miguel and Santa Rosa Islands will hopefully increase the numbers and genetic variability for re-introduction in the future. In the meantime, less than 100 foxes remain on Santa Cruz.