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Economic and Ecological Impacts of Invasive Species

In the United States, the annual cost of invasive species (including plants and aquatic organisms) is estimated at more than $138 billion (Pimental et al. 2000). Horses and burros introduced into western states number over 50,000 animals, which overgraze vegetation and decrease food for native animals. The Bureau of Land Management spends about $22 million annually to manage these animals. Feral pigs cause damage nationwide of about $800 million/year (Pimental et al. 2000). Feral dogs cause about $9-10 million in losses to cattle and sheep each year (Bergman et al. 2002, Pimental et al. 2000) and feral cats kill about 465 million birds per year at an estimated cost of $14 billion (Pimental et al. 2000). Invasive mammals cause large agricultural losses. Nutria are pests in 15 states, causing over $6 million per year (Bergman et al. 2002) in damage to sugarcane. Rat destruction of stored grains in the U.S. averages more than $19 billion per year (Pimental et al. 2000).

The pigeon is the most serious pest bird in the United States, with yearly damages estimated (Pimental et al. 2000) at $1.1 billion to property and agricultural crops; pigeons can also spread over 50 human and livestock diseases (Weber 1979). House Sparrows, introduced into the country to control canker worms, are now pests because they consume agricultural crops and ornamentals, displace native birds from nesting sites, and can spread 29 human and livestock diseases (Weber 1979). European starlings are agricultural pests on grain and fruit crops, consume or contaminate livestock feed at feedlots, and are implicated in the spread of 25 diseases (Weber 1979).

The brown treesnake was accidentally introduced on the U.S. territory of Guam after World War II with military cargo. Snake populations 30 years later reached densities of 100/ha and caused the extinction of 10 of 13 native forest birds, 2 of 3 native mammals, and 9 of 12 native lizards. Snakes also cause frequent power outages by shorting out utility lines, resulting in $1 million damage yearly (Pimental et al. 2000). The cost to control snakes on Guam and limit their dispersion to other parts of the Pacific is estimated at $6 million per year.

Invasive species can change ecosystems through their effects on vegetation. Feral pigs introduced into U.S. states for hunting now number about 4 million and damage both crops and the environment.

Invasive species have caused the extinction or endangerment of numerous native species throughout the world. Introduced rats and other mammalian predators are the major cause (42%) of bird extinctions on islands, with 54% attributed to rats, 26% to cats and the remainder to mongooses, weasels, stoats, and other species like goats and pigs. About 42% of the almost 1000 species listed under the U.S. Endangered Species Act are at risk because of invasive species (Nature Conservancy 1996). The mongoose is a classic case of biological control run amok. Beginning in 1872, it was introduced into Jamaica, Puerto Rico, other West Indian Islands, and Hawaii for control of rats in sugarcane; it preyed heavily on native reptiles, amphibians, and ground nesting birds, causing extinction and endangerment of many species.

Mating or competition between introduced and native species can lead to extinctions (Simberloff 1999). Mallards, introduced to Hawaii and Florida for hunting hybridized extensively with the endangered Hawaiian duck, and the Florida mottled duck, threatening their existence. In the United States, the introduced starling and house sparrow outcompete native songbirds, leading to a long-term decline in songbird species.

Introduced species also propagate diseases. In Hawaii, introduced Asian songbirds are host to avian pox and avian malaria (Simberloff 1999), which have contributed to the elimination of many native birds. Small rodents introduced worldwide act as vectors of salmonellosis, leptospirosis, plague and murine typhus. Feral pigs spread brucellosis, pseudorabies, and trichinosis and the mongoose is a vector for rabies and leptospirosis in Puerto Rico and other islands (Pimental et al. 2000).