Invasive species aggressively invade new continents so that these species become dominant in their new geographical areas. Benign components of their original habitats, invasive species include plants, mammals, birds, fish, amphibians, reptiles, arthropods, mollusks, and plant and animal diseases. Some invaders familiar in North America include the black rat, house sparrow, and kudzu (Rattus rattus, Passer domesticus, Pueraria montana var. lobata). Once established, invasive spe cies cause many problems for humans in that they degrade natural communities, and damage agricultural species with pests and diseases. Invasive aquatic species can cut off local commerce by cutting off boating along rivers, and cause local electricity emergencies by clogging the operation of hydro electric dams, particularly in countries such as New Zealand that rely on hydroelectric power.
Invasive species are those species that arrived on con tinents after the sixteenth century after human global travel, commerce, and migration increased. While species have moved between continents for millennia, global travel by humans has greatly accelerated the rate of intercontinental movement of species. New invasions of species have paralleled the movements of humans world wide, and the associated explosion of invasive species has caused the decline of native species on their continents of origin. This relatively free movement of biota between continents has created a 'New Pangea'. Continents will have more species as new invasive species arrive, but the displacement and extinction of native species caused by invasive species ultimately will cause the overall world wide number of species to decrease. Exotic species introductions are generally unintentional, although there are many documented cases of species being transported to other continents for horticultural or agricultural purposes.
From a philosophical perspective, invasive species are not just another species for the species richness list because invasive species cause environmental degrada tion. While some authors criticize the furor over invasive species as being akin to xenophobia, the perspec tive that invasive species are problematic in natural areas is fundamentally dissimilar to the idea that foreigners can cause harm to a society. The concern of ecologists over invasive species is due to the damage invasive species cause to natural plant communities.
In North America, there are 50 000 nonindigenous species, 3000 of which are invasive. Hawaii has more than its share of invasives with 860 invasive species. Each year, US$137 billion per year are spent to eradicate invasive species in the United States (Figures 1 and 2), and US$26 billion are spent to combat crop weeds.
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