What Is an Invasive Species? Arriving Is Not the Problem

The vocabulary is used loosely in public and precisely in the literature, and the distinctions decide policy:

• •Native, meaning present in a region through natural processes rather than human introduction
• •Introduced or non-native, meaning moved by people deliberately or accidentally, which covers the great majority of crops, livestock and garden plants and is not in itself a problem
• •Naturalised, meaning established and reproducing without human help, which most introduced species never achieve
• •Invasive, meaning spreading rapidly and causing measurable harm to ecosystems, economies or health, which is a small fraction of the naturalised ones
• •The rough rule of thumb used in the literature is the tens rule: about ten percent of introduced species escape, ten percent of those establish, and ten percent of those become invasive, which gives roughly one in a thousand and is an approximation rather than a law
• •Native species can behave invasively when conditions change, which is why the harm rather than the origin is what the definition turns on

Several explanations are supported and none covers every case. The enemy release hypothesis holds that a species arriving without its predators, parasites and diseases devotes resources to growth and reproduction that it previously spent on defence, which is well supported and explains some spectacular cases. Empty niche arguments apply where an island or a lake lacks any species doing a particular job. Propagule pressure, meaning how many individuals arrive and how often, is one of the strongest predictors, which is why shipping routes and the pet trade matter more than the characteristics of any individual species. Disturbance helps, since a disrupted system with bare ground, altered nutrients or removed predators offers opportunities that a stable one does not. And a set of traits recurs among successful invaders: rapid growth, early reproduction, many offspring, broad tolerance of conditions and generalist diet, which is the profile of a weed in the broadest sense.

The costs fall into categories that are easier to list than to add up. Predation on species with no evolved defences is the most destructive on islands, and the brown tree snake, introduced accidentally to Guam after the Second World War, eliminated almost all the island's native forest birds within a few decades. Competition displaces natives, as grey squirrels have displaced red in Britain, helped by a virus they carry and survive. Disease introduction can be catastrophic, with chestnut blight removing an entire dominant tree species from eastern North American forests and a chytrid fungus implicated in amphibian declines worldwide. Habitat alteration occurs where an invader changes the physical system, as water hyacinth does by covering lakes and as invasive grasses do by introducing fire cycles to systems that never burned. Economic costs include agricultural losses, infrastructure damage and control spending, and a 2021 synthesis put the global total across the previous half-century in the hundreds of billions, with the caveat that such figures depend heavily on what is counted.

The pathways are well characterised and most are commercial. Ballast water taken on by ships in one port and discharged in another moves marine organisms in enormous numbers, which is how zebra mussels reached the Great Lakes and why an international convention now requires ballast treatment. Hull fouling moves species attached to the outside. Horticulture is the largest pathway for plants, since a species selected for vigour, tolerance and attractive growth is a species selected to be a good invader, and many of the worst plant invaders were deliberately planted. The pet and aquarium trade supplies releases, which is how Burmese pythons reached the Florida Everglades and how lionfish reached the Atlantic. Deliberate introductions for pest control have a particularly poor record, with the cane toad brought to Australia in 1935 to control a beetle it could not reach, and then spreading across the continent poisoning everything that tried to eat it. Timber, soil, packing material and food imports carry insects and pathogens.

The economics of control are stark and drive the strategy. Prevention is cheaper than everything else by an order of magnitude, which is why biosecurity inspection, treatment requirements and import restrictions are where the effort should sit, and why New Zealand and Australia, as isolated islands with much to lose, run the strictest regimes. Early detection and rapid response is the next best, since eradication is achievable while a population is small and localised and rapidly becomes impossible. Established invaders usually require permanent management rather than removal, which is a continuing cost with no end point. Eradication has succeeded most often on islands, where rats, goats, cats and rabbits have been removed from hundreds of islands with dramatic recovery of seabird populations, and it works there precisely because reinvasion can be prevented. Biological control, introducing a natural enemy, has produced both notable successes and notable disasters, and is now preceded by extensive host specificity testing. Genetic approaches including gene drives are proposed and unreleased.

Most species moved to a new place fail, and roughly one in a thousand becomes invasive, which is defined by causing measurable harm rather than by origin. Success is predicted by how many individuals arrive and how often, by escaping the predators and diseases of the home range, by disturbance in the receiving system and by fast-growing generalist traits. Damage includes predation on defenceless island species, competition, introduced disease and physical alteration of habitat. Prevention costs an order of magnitude less than control, and eradication succeeds mainly on islands.