Invasive Species

Introduced, established, and causing harm.

An introduced species is one moved outside its native range. An invasive species is an introduced one that establishes, spreads and causes ecological or economic harm. Most introductions fail; a small fraction become invasive, and those few account for enormous damage.

Why they succeed

• Enemy release — the predators, parasites and diseases that limited them at home were left behind.
• Rapid reproduction and broad tolerance of conditions.
• Naive prey — native species have no evolved defences against an unfamiliar predator.
• Disturbed habitat — disruption opens ground that established natives would otherwise hold.

Islands are hit hardest

Island species evolved in isolation, often without mammalian predators, and lost costly defences such as flight. Introduced rats, cats and goats have therefore driven a disproportionate share of recorded extinctions. The brown tree snake on Guam eliminated most of the island's native forest birds within a few decades.

Control

Prevention is far cheaper than removal, which is why biosecurity inspection at borders is the main line of defence. Once established, options are mechanical removal, chemical control, and biological control — introducing a natural enemy. Biological control carries obvious risk: the cane toad was introduced to Australia to control beetles and became a far worse problem than the beetles.