There are some species that are introduced by human activities to new areas where they do not naturally occur. Such species are also known as exotic, non native or even alien. Non native species are not necessarily harmful. The majority of them actually prove to be beneficial to their new environment. But when they do cause ecological or even economic harm, they are termed as invasive species. They can broadly be divided into four groups; invasive aquatic animals, invasive terrestrial animals, invasive aquatic plants and invasive terrestrial plants.