The second arrival: when a transported species rewrites an ecosystem

Movement is not the same as invasion

People move species deliberately for farming, gardens, pets and biological control, and accidentally in cargo, ballast water and attached to vehicles. Most introduced organisms do not become invasive. The term applies when an alien species establishes, spreads and causes harm to nature and often to people.

The 2023 IPBES assessment identifies invasive alien species as one of the five major direct drivers of biodiversity loss. Their effects include predation, competition, disease transmission, hybridization and changes to fire or nutrient cycles. Islands and freshwater systems can be especially vulnerable because native species evolved with strong geographic barriers.

The harm is a relationship

A species is not morally “bad”. The problem comes from ecological context. A predator harmless within its native food web may encounter prey with no effective defence elsewhere. A plant valued in gardens may escape into disturbed land. A pathogen may travel with a host that shows few symptoms.

This distinction matters for public communication. Removing an invasive population can protect native wildlife, but management should be evidence-led, humane where possible and designed to avoid damaging non-target species.

Prevention is conservation before crisis

Biosecurity, inspection, clean equipment, responsible pet ownership and early reporting can stop an introduction before it becomes a landscape-scale problem. Once a species is widespread, eradication may be impossible and long-term control expensive. Early detection changes that equation.

Use the Species Explorer to compare threat factors. “Invasive species” on a profile is a summary; the next question is which organism, which pathway, which life stage and what evidence links it to decline.

Good policy addresses pathways rather than relying only on lists. It also monitors results: native recovery, reinvasion risk and unintended effects. The goal is not to freeze ecosystems in time, but to reduce preventable harm caused by human-assisted movement.

Sources and further reading

• IPBES invasive alien species assessment
• IPBES assessment primer
• SpeciesWatch Sources