Invasive Species

Non-native species that establish, spread, and cause ecological or economic harm — a leading driver of extinction, especially on islands.

Definition

An invasive species is a non-native organism whose introduction and spread cause, or are likely to cause, harm to native ecosystems, the economy, or human health. Not all introduced species become invasive; invasiveness requires successful establishment and damaging spread.

Scope

Covers the introduction, establishment, and spread of non-native species and the harm they cause to native biodiversity through predation, competition, hybridization, disease, and ecosystem alteration. Includes the invasion process, traits of successful invaders and vulnerable recipient communities, and approaches to prevention and control. Excludes broader habitat and exploitation threats (sibling topics) and the genetics of small populations (treated under conservation genetics).

Core questions

• Why are some introduced species harmless while others become invasive?
• Why are islands especially vulnerable to invasions?
• Through what mechanisms do invaders harm native species?
• How can invasions be prevented or controlled?

Key concepts

• Introduction pathways and vectors
• Establishment and spread
• Enemy release hypothesis
• Island vulnerability
• Competition, predation, and hybridization
• Biosecurity and control

Key theories

Stages of invasion

Invasion proceeds through transport, introduction, establishment, and spread; each stage acts as a filter, so most introduced species fail and only a subset become invasive, which focuses prevention on early stages.

Enemy release and naive prey

Invaders may thrive because they escape their native predators and pathogens, while native species lack co-evolved defences against novel enemies — a dynamic that helps explain the severe impacts of invasions on isolated biotas such as islands.

Clinical relevance

Invasive species are among the top causes of recorded extinctions, particularly of island birds, mammals, and freshwater species. Because eradication is far costlier than prevention, biosecurity at borders and early detection are central management strategies, and invasion ecology directly informs trade regulation and quarantine policy.

History

Charles Elton's 1958 book founded invasion ecology as a discipline, linking biological invasions to community stability and biogeography. The field expanded rapidly from the 1980s with international programmes documenting invasion impacts, and invasive species were formally recognized as a leading extinction driver in global biodiversity assessments.

Debates

How predictable is invasiveness?

The 'tens rule' and trait-based screening attempt to forecast which species will become invasive, but propagule pressure, recipient-community context, and time lags make prediction unreliable, fuelling debate over how to prioritize prevention.

Key figures

• Charles Elton
• Daniel Simberloff
• Mark Williamson

Seminal works

• elton1958
• primack2014
• groom2006

Frequently asked questions

Is every introduced species invasive?

No. Most species moved to new regions fail to establish, and many that establish remain harmless. 'Invasive' is reserved for non-native species that spread and cause ecological or economic harm.

Why are islands so vulnerable to invasive species?

Island species often evolved without certain predators or competitors and lack defences against them. Introduced rats, cats, goats, and snakes have therefore caused many island extinctions, especially among flightless birds and other endemics.