An alien species is one that was introduced into a new environment, but does not have harmful effects. Such alien species are considered invasive species as they begin to have harmful effects on the new habitat. Invasive species can cause damage to the ecosystem, economic harm or human health effects.
Three invasive species are as following:
Rats that came on board ships to the Virgin Islands infested the islands' sugar cane fields and caused significant crop damage.
The woolly adelgid hemlock is an invasive Asian insect which rapidly kills infested hemlock trees.
Zebra moulds in the Great Lakes can easily cover submerged surfaces, obstructing water intakes at water treatment plants and power plants
