The prevalence of wild-growing water hyacinth （Eichhornia sp.）can block the passage of boats and disrupt shipping. Photo：Li Qingsong

The globalisation of flora and fauna has changed biodiversity more significantly than any other phenomenon apart from species extinction. Human activity has introduced animals and plants to regions previously inaccessible to them. In some cases this has been an intentional process，while in others species have been introduced accidentally and frequently unnoticed at first.

Alien species can be found amongst animals，plants and micro-organisms. Alien or exotic animal species are known as “neozoa”，and plants as “neophytes”. Not all succeed in becoming established or spreading to any great extent. Only a few give cause for concern in terms of the environment，the economy or human health，or have the potential to harm biological diversity. Such species are termed “invasive”. The distinction between “alien” and “invasive” species is not always clear-cut and ultimately depends on the damage they cause.

The “tens rule” suggests that of all the introduced species，about 10 percent escape to the wild. Of these，about 10 percent become established in their new environment. And a tenth of these in turn become invasive and pose an ecological，economic or health hazard. Although this rule has been challenged and modified several times，it nonetheless shows that compara-tively few species become invasive. However，it is extremely difficult to predict whether or not a new species will become a pest due to the complex interactions of ecological systems.