An invasive species is a species of animal, plant, bacteria, etc. that was brought to a place from somewhere else. For example, the snakehead, a fish that can live on land for up to four days at a time, was brought to North America from a different place. It is believed that someone had it as a pet, then it was dumped in a local pond. It produced offspring, and now snakeheads thrive all throughout North America, especially the United States and Canada. Invasive species spread through mating. They produce offspring with other fish. Also, they can spread through a certain place if they are brought to other parts of that place. For example, if snakeheads, let's say, were native to Maine, and some were caught, the people that caught them could return to New Jersey with some snakeheads. Those snakeheads have babies, then the owners dump the babies in a local lake, then those babies produce offspring, and are caught and moved to a different part of the country, etc. Through transportation this way, a species located in one part of a country could populate the whole country in a matter of twenty or so years. Hope this helped. EDIT: Yes, Peter is correct. I forgot to include the fact that since invasive species have no natural predators, spreading of the species is made easier, since nothing's going after them. In fact, the invasive species can bully the inhabitants of their new habitat, and maybe even control that whole area, therefore affecting their habitat. That is why invasive species are particularly bad. If you bring a great white shark to the Atlantic Ocean, it can destroy the whole fish economy. The sharks would most likely breed and breed and breed until the population of great white sharks in the Atlantic Ocean was massive. They might endanger a certain type of fish, therefore destroying those companies that depend on that certain type of fish. Not to mention the attacks on humans...

Invasive species arrive through a variety of methods. Most of the examples that follow are for North America, as I am a resident and familiar with the impact of several introduced species. Accidentals: - A non-native species is introduced into a new habitat by accident, usually as a side-effect of human activities, e.g., Zebra Mussels through ballast water in ships. - A non-native species establish themselves in a new habitat on their own, e.g., off-course migratory birds. Introductions: - A non-native species is introduced into a controlled habitat by humans, but escapes into the wild, e.g., Purple Loosestrife that was sold as a garden perennial, but escaped into the wild. - A non-native species introduced into a new habitat by humans on purpose, to control other species, as a source of food, for sport hunting, or for other reasons, e.g., House and Eurasian Tree Sparrows (Weaver Finches), Rock Dove (common pigeon). Introduced species may or may not have any natural enemies in their new habitat. The spread of the introduced species may be quite rapid, even if it has predators in the new habitat, as there may be too few to control the alien species effectively. More rarely, an introduced species has no natural predators in their new range. However, this often has devastating consequences, e.g., the introduction of rabbits in Australia. For example, prior to the arrival of humans in New Zealand some 1500 years ago, many ecological niches on the islands were occupied by birds that had evolved in isolation on the islands over an extended period of time. The arrival of mammals, both human and those that accompanied the humans, has caused several unique species to become extinct. In some cases, non-native species are introduced in an attempt to control a native species or, more frequently, another introduced species. This practice has been successful in some cases and spectacularly unsuccessful in others. On a more positive note, a species that has no natural predators in a new environment may eventually become a source of food for native species. However, this process can take many generations of the species involved and may never provide an effective control. Introduced species often displace local species that occupy the same habitat to such an extent that local species become endangered or extinct. Introduced species may also prey on local species to such an extent that they upset the local predator / prey relationship, potentially causing both the local predator and its prey to become endangered or extinct. Introduced species may also breed with native species, a process known as hybridization. Hybrids may then drive out native species. Non-native species may alter their new habitat to such an extent that native species become endangered or extinct. This process goes on all around us in the world naturally, as species move on their own to new ranges. Humans, however, have caused some serious problems through either ignorance or carelessness. The introduction of alien species into new habitats because of human activities or carelessness has become a serious environmental problem around the entire globe. Additional information on invasive species in Canada can be found at: http://www.hww.ca/hww2.asp?id=220 (No comments, please, from greying Second City viewers.)