Are all non-native species bad?

No. Many species we rely on, including most crops and farm animals, are non-native and cause no harm. A species is only called 'invasive' when it spreads and damages the new ecosystem, economy or human health. The harm, not the origin, is what defines an invasive species.

What was the cane toad disaster in Australia?

Cane toads were brought to Australia in the 1930s to eat beetles damaging sugar cane. The toads barely touched the beetles but bred rapidly and spread across the continent. They are poisonous, so native predators that eat them die. It is a classic example of a well-meaning introduction going badly wrong.

Invasive Species Explained | Nature

What is an invasive species?

An invasive species is a living thing — plant, animal, fungus or microbe — that has been moved to a new region where it does not naturally belong, and which then spreads and causes harm. The key words are spreads and causes harm. Plenty of species live outside their original range without any problem; we call those simply non-native or introduced. An organism only earns the label "invasive" when it damages its new environment, economy or human health.

It is also worth remembering that the species itself is not "evil." It is simply doing what living things do — eating, growing and reproducing. The problem is that it is doing so in a place that has no defences against it.

How invasive species spread

Almost all invasions are caused by humans, on purpose or by accident. As global trade and travel have grown, so has the movement of species:

Hitchhikers in cargo and ships. Insects travel in wooden packing crates; sea creatures ride in the ballast water that ships pump in and out, or cling to ships' hulls.
Escaped pets and garden plants. Released pet fish, snakes and plants dumped from gardens can take hold in the wild.
Deliberate introductions gone wrong. Sometimes a species is introduced on purpose to solve a problem, only to become a worse one — as you will see below.

A species that took millions of years to be confined to one continent can now cross the planet in a matter of days.

Why invaders thrive

Back in its native home, every species is kept in check by predators, diseases and competitors that evolved alongside it. When it arrives somewhere new, it often leaves all of those enemies behind. With nothing to control it and plenty of food, its population can explode.

At the same time, native species have no defences against the newcomer. They did not evolve with it, so they may not recognise it as a threat, cannot escape it, or are simply outcompeted for food, light and space. This is the same reason invasive species appear in the "HIPPO" list of threats to biodiversity and conservation.

The damage invaders do

Invasive species are one of the leading causes of extinction worldwide. The harm comes in several forms:

Predation. Rats, cats and snakes carried to islands have wiped out countless native birds and reptiles that evolved with no such predators. The brown tree snake, accidentally introduced to Guam, eliminated most of the island's native birds.
Competition. Aggressive plants like Japanese knotweed or kudzu smother native vegetation and starve it of light.
Disease. Newcomers can carry illnesses that native species have never faced, such as the fungus devastating amphibians around the world.
Disrupted food webs. Removing or replacing one species can ripple through entire food chains and ecosystems.
Economic cost. Invasive pests damage crops, block waterways and clog pipes, costing economies enormous sums every year.

Islands are especially vulnerable. Their native animals often evolved in isolation, without large predators, so they are easy prey when rats or cats arrive.

Famous examples

Cane toads in Australia. Introduced in the 1930s to control beetles, they failed at the job, bred explosively, and now poison the native predators that try to eat them.
Zebra mussels in North America. Arriving in ships' ballast water, these tiny mussels coat surfaces, clog water pipes and filter out food other species need.
Lionfish in the Atlantic and Caribbean. Beautiful but voracious, with venomous spines and no local predators, they devour reef fish and breed rapidly.

Fighting back

Dealing with invasive species is difficult, but several approaches help:

Prevention is by far the most effective and cheapest. Biosecurity measures — inspecting cargo, cleaning boats, banning risky imports — stop invaders before they arrive.
Early detection and rapid response. Catching and removing a new invader while its population is small can prevent a full-blown invasion.
Control and eradication. Established invaders may be reduced by trapping, removal, or carefully tested biological control — introducing a natural enemy. This is risky, though, which is exactly how some invasions began.

Try it yourself: model an invasion

You can model why invaders spread so fast with a simple game.

Use a board game or a grid drawn on paper, plus two colours of counters (or two kinds of dried bean) to represent a native species and an invasive one.
Start with the board mostly filled with native counters and just two invasive ones.
Each "round," every invasive counter places two new invasive counters in neighbouring squares, replacing any native counter there. Native counters add just one new counter each round.
Play several rounds and watch the board.

The faster-reproducing invader quickly takes over, just as a real invasive species can dominate an ecosystem once it escapes the predators that once held it back. This shows why stopping invaders early matters so much.

Invasive Species Explained

Key takeaways