Alien Species : Alien species are often referred to as nonnative species. In plants, those that are nonnative are known as weeds. Alien species are those that migrate into an ecosystem, or are deliberately or accidentally introduced to an ecosystem.
1) Accidental Introduction:
b) Bark Beetle - Accidentally introduced via contaminate lumber from overseas.
2) Deliberate Introduction:
a) African Bees - Introduced from Africa to Brazil to increase brazilian honey production. Some escaped, and have spread northward to the US and are now known as "Killer Bees".
b) Rabbits were brought from England to Australia as food and for hunting. Some escaped, and having no native predators there, spread rapidly, soon overgrazed much of southern Australia's plantlife.
b) Scot's Broom - Introduced to the USA from Scotland as a decorative plant, scot's broom has now spread across much of the western US as a weed.
c) Crops - Many of our crops are nonnative. Grains like wheat and oats were introduced from eastern Europe.
3) Biological Control: A biological control is typically an alien species that is introduced to control another alien or weedy species.
a) The mongoose was introduced to Hawaii to control rats, which had previously been accidentally introduced. Unfortunately, the mongoose also eats many native Hawaiian animals.