Species have always spread into new areas via natural processes. But in the Anthropocene, humans are moving them faster, further and in greater numbers than ever before. Invasive species affect every region of the world - even Antarctica has been invaded by around 200 species.
Most don't become establish, but many that do spread and increase rapidly, often at the expense of native species. They may compete with them for space and food, they may prey upon them or alter habitats so much that natives cannot persist. Many are undeniably bad, such as cane toads in Australia, the brown tree snake on Guam and grey squirrels in the UK. That invasives are bad news was drummed into me at university. However, the debate is hotting up in the conservation world, as more and more evidence emerges that not all invasives are as bad as they are made out to be, and may even be beneficial, by providing new habitats or sources of food. Conservationists are having to rethink their approaches, and its a fascinating subject to follow.