Invasive alien species reduce local biodiversity by outcompeting native species, preying on them, or introducing diseases and parasites. These pressures reduce native population sizes and may cause local extinction, simplifying communities and weakening ecosystem resilience.
Level: SL and HL Subtopic 3.2 Human impact on biodiversity.
An alien species occurs outside its natural geographical range because of human activity, whether deliberate or accidental. It becomes invasive when it spreads and causes ecological, social or economic harm. A common misconception is that every alien species is invasive; many introduced species survive without spreading significantly or damaging ecosystems.
Invasive species may initially lack their normal predators, parasites and diseases. This allows rapid population growth, increasing their impacts on native populations.
| Mechanism | Effect on local biodiversity | Named example |
|---|---|---|
| Competition | The invader uses limited food, space, light or nesting sites, reducing the survival and reproduction of native species. | Grey squirrels in the United Kingdom compete with native red squirrels for food and habitat. |
| Predation | Native prey may lack effective defences against an unfamiliar predator, causing rapid population decline or local extinction. | The introduced brown tree snake on Guam caused severe declines and local extinctions among native forest birds. |
| Disease or parasites | The invader carries pathogens to which native species have little resistance. | Grey squirrels transmit squirrelpox virus, which is often fatal to red squirrels but usually does not seriously affect grey squirrels. |
The loss of native species reduces species richness and may also reduce evenness. If a keystone species is affected, food-web disruption can produce indirect effects across the ecosystem and reduce its resilience.
Management can create conflicts between conservation agencies, landowners and communities, especially when removal is costly or affects livelihoods. This connects biodiversity conservation to perspectives and environmental justice.
Exam technique: For an “explain” question, identify a mechanism, show the causal chain, and add a named example. Do not merely state that invasive species are “harmful”; explain precisely how they reduce native survival, reproduction or population size.
