An introduced species is an organism that is not native to a given place. It has brought there by human activities. This can cause problems because introduced species can damage the ecosystem they are introduced into. This damage can be to other animals, plants and also human activity.
An example of an introduced species is a fish, the tilapia. It comes from warm freshwater areas of Africa, North and South America, India and Sri Lanka. It has been taken to other areas because it is good for fish farming, and it also is a good pet for an aquarium. When the fish is released into other parts of the world, it quickly takes over from the native fish. It can do this because it breeds quickly, eat all the available food, and eat the eggs and babies of native fish. This can cause the water to become muddy, and only the tilpia can keep living in these conditions. It has been listed as one of the 100 worst introduced species. A person owning or selling a tilapia in Australia could be fined up to AU$11,000.