.5 million acres in Montana alone, costing ranchers about $40 million dollars annually (Alper).
The ballasts of ships used for overseas journeys may contain adult animals and any number of planktonic organisms, some of which are juvenile forms of mollusks, crabs, fish, or other sea animals. Mnllusks have been particularly detrimental in the United States. The zebra mussel, a ballast-transported organism, is estimated to cost the United States over $3 billion annually (Householder). The green mussel, a more recent migrant, is fast becoming another costly invasive species. Like the zebra mussel, it can attach to any hard surface, and was found in 2000 in the condenser tunnel of an electric cooling plant in Tampa, Florida (Benson). It has since spread and along with the zebra mussel now occupies large areas of dock pilings and waterway shores, displacing native animals that would normally occupy these areas. In addition, the zebra mussel has now been implicated in causing botulism poisoning in Lake Erie's native loon population, which injects the toxin through a contamination along the food chain (ôLakeàö).
Insects come over as well, lodging in a variety of places and spreading easily into a new landscape, unnoticed by humans until they begin to cause damage. The longhorn beetle is one such example, and is a severe pest insect of trees (Reed). Rats are brought within the holds of ships and spread out to human communities as well as natural communities, where they interact with other species (Schweiger). Thus, even when humans do not mean to transport other species, this inevitably results whenever humans travel. One only has to examine one's socks and shoes after a hike in the mountains and notice the dozens of seeds lodged there to realize that even small-scale human movements transport species.
Humans also consciously transport species. Colonists of new lands (modern as well as past) bring seeds and cuttings of their favorite...