Plant, animal, and disease transmission
Species are limited in their ability to move to new continents by geographical isolation, and this limits the majority of the world's species from becoming invasive species on another continent. Dispersal overcomes the isolating mechanisms that prevent the movement of species from one continent to another, and species have varying levels of abilities to disperse across the barrier and successfully open invasion windows. Species can move by natural dispersal vectors including wind, animal, and water transport.
After a plant species successfully disperses and establishes in a new continent, many species can vegetatively reproduce. One successful propagule can start a new population, and many plant species spread solely or mostly via asexual means (e.g., bulbils of Dioscorea opposi-tifolia, plant fragments of Eichhornia crassipes (Figure 4), Salvinia molesta, Elodea canadensis, and H. verticillata). Wind-dispersed invasive species with prolific seed production can also spread rapidly after their initial introduction (e.g., Phragmites spp. and Typha spp.)
Historically, invasive species have been very successful in migrating between continents after human travel and migration increased. Hitchhiking species cling to travellers' luggage, mud on shoes, and probably even the tires of airplanes. Recirculating air in airplanes transfers germs from one passenger to another on transcontinental flights, after which the travelers themselves deliver the novel diseases to new continents. Invasive species passively cling to transport vessels. Cargo and other ships carry
organisms in ship ballast and transport exotic pests; Zebra mussels (Dreissena polymorpha) cling to the outsides of ships, and have been transported since 1988 throughout the Great Lakes region of the United States and Canada. Antarctic now has at least 207 alien species, and these invasive mostly originate from ballast discharge.
With the current level of international travel, new introductions of invasive species are inevitable. However, international agreements attempt to address the problems associated with the international transport of organisms. The Global Ballast Water Convention is an international agreement passed in 2004, which will eventually require ships to comply with discharge limits, and establishes inspection and enforcement procedures.
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