The following is shared from the frequently asked questions page on the Center for Invasive Species Research website…
How do invasive species move from place to place?
Invasive species reach new areas outside of their home range in one of two ways: (1) self introduction on their own, or (2) with human assistance that may be deliberate or accidental.
Self-introduction of species into new areas is not a new phenomenon. This process has been happening for millions of years at a very slow rate, and often introductions occur between close neighbors. For example, New Zealand has acquired bird, plant, and insect species that are carried by winds across the Tasman Sea from Australia.
Humans have greatly altered the speed at which species are moved around the world and species introductions are occurring between areas that are separated by vast distances and across natural barriers (e.g., oceans and mountains) that had previously prevented the long-distance movement of species. The speed at which long-distance spread can happen has greatly accelerated due to air travel which allows people to reach most places on earth within 72 hours or less. Short travel (hours on a plane as opposed to weeks or months on a boat at sea) times have greatly increased the survival chances of invasive species traveling with humans.
Humans have deliberately moved an incredible number of plant and animal species around the globe either for food, as part of international commerce (e.g., the pet and nursery trade), or for sport (e.g., hunting and fishing). It is estimated that there are 50,000 non-native species of plants and animals living in the USA.
Occasionally, some of these species that were once beneficial while under human control (e.g., weeds that were originally garden plants) become problematic when they escape and start to colonize and breed in areas where they are not wanted. For example, plants like salt cedar from Eurasia have invaded the desert southwest of the USA because humans deliberately moved them there for the control of constantly eroding desert sands.
Often, invasive species are moved accidentally by humans. This can occur through hitch-hiking unnoticed on plants that are being moved (e.g., tiny insects or diseases on leaves or in potting), in ballast water that is used to stabilize large transport ships, or inside other animals (e.g., diseases that kill birds have been spread by the commercial trade in exotic pet birds.)
Air Traffic: The above video shows world air flight traffic over a 24 hour period. Notice how the number of planes flying changes between daytime and nighttime. Even at night there are still lots of planes flying around the world. Every flight can potentially move an invasive species into a new area.
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