What are introduced species? How are they a threat to biodiversity?
Introduced species (also called invasive species) are species that did not previously occur naturally in that environment and can predate or outcompete native species.
Introduced species may have found their way to a new habitat naturally or been introduced by humans. Introduced species can devastate populations by disrupting the food chain, predator-prey dynamics and by outcompeting native species. They are particularly devastating on islands as islands are often home to ground dwelling birds due to the lack of mammalian predators. For example, when humans have moved to islands they often bring cats for companionship, however the ground dwelling birds will not have evolved to cope with mammalian predators and so may go extinct.
Grey squirrels in the UK are a well known example, they were introduced and are more resilient than the native red squirrels. Red Squirrel populations have declined rapidly as grey squirrels have outcompeted them for resources.
Introduced species are not always animals, they can be plants too. In the 19th Century prickly pears (a species of cactus) were introduced to Tenerife for agriculture and they are very resilient plants that can grow in all manner of conditions. The plants grew abundantly and use up the natural resources needed by the native plants.