Dogs, cats, horses, burros, cattle, sheep, pigs, goats, and deer--sounds like a pretty benign list of creatures. But when they take to the wild they become pests that prey on native animals, graze on vegetation and intensify soil erosion. Add in a few more malevolent immigrants--rats, for example--and the annual economic losses top $25 billion. Here are some examples:

Image: 4-H

GOATS (Capra hirus) introduced on San Clemente Island, California, have caused the extinction of eight plant species and endanger an additional eight.

Wild Horses
Image: Bureau of Land Management

WILD HORSES(Equus caballus) and burros (Equus asinus) number about 50,000 animals in the West. They graze heavily on native vegetation and diminish the food sources of native grazing animals and seed-eating birds. Pimentel estimates that they cost the nation $5 million a year.

Wild Hog
Image: Texas A&M

PIGS (Sus scrofa) from Eurasia and North Africa were introduced into some U.S. parks for hunting. In Hawaii, rooting pigs have destroyed 80 percent of the plant cover in areas where they live. In Florida, the wild pig population exceeds 500,000; the feral pig population for the entire U.S. is estimated to be 2 million. These pigs damage grain, peanut, soybean, cotton, hay, and various vegetable crops. They also transmit and are reservoirs for serious diseases of humans and livestock, like brucellosis, pseudobrucellosis, and trichinosis. Environmental damage aside, control efforts alone cost $200 million a year.

Image: JB's Website

RODENTS, including the European (black or tree) rat (Rattus rattus), Asiatic (Norway or brown) rat (Rattus norvegicus), house mouse (Mus musculus), are our constant companions. They are particularly pesky on farms, home to about a billion rats. In cities there is an estimated one rat for each person. Researchers estimate that each adult rat consumes about $15 worth of grain and other materials each year. At 1.25 billion rats, the total is $19 billion.

Ice crystal
Image: CHARLES DOUGLAS, Natural History Museum of Canada

INDIAN MONGOOSE (Herpestes auropunctatus) once seemed like a great way to rid sugarcane fields of rats. It was introduced into Jamaica in 1872 and soon after to Puerto Rico, other West Indian Islands, and Hawaii. Unfortunately, the mongoose also gobbled up ground nesting birds and amphibians and reptiles that were beneficial in controlling other pests. Seven to twelve species were driven to extinction and the mongoose emerged as the major vector and reservoir of rabies and leptospirosis. The Cornell researchers estimate that the mongoose still inflicts $50 million in damage each year.

Feral cat

DOMESTIC CATS number 63 million; another estimated 30 million run wild. Well fed or not, cats are lethal predators, destroying small mammals such as birds, amphibians, and reptiles. Cats are estimated to kill about 200 million birds each year. Assigning a value of $30 per bird produces a result of $6 billion per year.

Hunting dog
Image: Texas A&M

DOGS, like cats, share our homes, but have also escaped into the wild where man's best friend forms packs that kill deer, rabbits, domestic cattle, sheep, and goats. One investigator calculated that wild dogs cause $5 million in livestock losses in Texas each year. The Cornell group's estimate of $10 million for all the states is probably conservative. In addition, an estimated 4.7 million people are bitten by dogs annually. Medical costs associated with these attacks are estimated to total $56 million annually. Between 11 and 14 dog attacks result in death each year. According to the report, "if each of these deaths has a conservative value of $2.2 million the cost would be approximately $30 million per year."
Data: Excerpted from Environmental and Economic Costs Associated with Non-indigenous Species in the United States

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