18 Plant, Animal, and Microbe Invasive Species in the United States and World

Approximately 50,000 plant, animal, and microbe invasive species are present in the United States, and an estimated 500,000 plant, animal, and microbe invasive species have invaded other nations of the world. Immediately, it should be pointed out that the US and world agriculture depend on introduced food crops and livestock. Approximately 99 % of all crops and livestock in all nations are intentionally introduced plants, animals, and microbes (Pimentel 2002). Worldwide, the value of agriculture (including beneficial non-indigenous species) is estimated to total $ 30 trillion per year. Other exotic species have been introduced for landscape restoration, biological pest control, sport, and food processing, also contributing significant benefits. Unfortunately, some invasive species are causing major economic losses in the United States and worldwide in agriculture, forestry, fisheries, public health, and natural ecosystems. Documenting the full extent of the environmental and economic damages caused by exotic species, and the number of species extinctions is difficult because little is known about the estimated 750,000 species that exist in the US and the estimated 15 million that exist worldwide (McNeeley 1999). Only an estimated 2 million species have been described worldwide. In the US, an estimated 40 % of those species forced to extinction can be accounted for by the impacts of invasive species (Pimentel et al. 2005). In some regions of the world, as many as 80 % of endangered species have been threatened and forced to extinction due to the pressures of nonnative species (Armstrong 1995). In addition, many other species worldwide, even if they have not been forced to extinction or endangered status, are negatively affected by various alien species or ecosystem changes caused by alien species. Calculating the negative economic impacts associated with the invasion of exotic species is difficult. For a few species, there are sufficient data to estimate