Europe’s towns and cities are particularly vulnerable to the threats posed by invasive alien species, and experts say that action needs to be taken to control them. Invasive alien species are plants or animals that are not native to an area and which therefore lack natural predators, meaning they are able to spread rapidly. Urban areas are at high risk from invasive species because of their large number of transport links, with many non-native animals and plants arriving accidentally at ports and airports. Some species also arrive through the plant and pet trades.