Invasive Alien Species

An introduced or alien species is a species moved by human activity outside its natural geographical range; introduction alone does not make it invasive. An invasive alien species establishes and spreads in the new environment, threatening native biodiversity or ecosystem functioning.

Introduction, establishment and spread

The distinction concerns what happens after arrival, not simply the species’ foreign origin.

• An introduced species may fail to survive or remain dependent on repeated human introductions.
• Naturalisation occurs when an alien species maintains a self-sustaining population; this alone does not demonstrate harmful invasion.
• An invasive alien species combines establishment and spread with harmful ecological effects, extending beyond its initial introduction site.

Ecological effects that make invasion harmful

The decisive feature is a threat to native biodiversity, rather than abundance or rapid growth alone.

• Invaders may compete with native species for resources, prey on them, or introduce pathogens, reducing native populations.
• Some invaders alter ecosystem processes, such as nutrient cycling, fire regimes or habitat structure, thereby changing conditions for other species.
• NCERT identifies Lantana, carrot grass and water hyacinth as alien plant invasions associated with threats to native species.

Traits and conditions favouring invasiveness

Successful invaders often have high reproductive capacity, effective dispersal and broad environmental tolerance. These are facilitating traits, not a universal checklist or sufficient proof of invasiveness.

• Ecological flexibility can help an alien species exploit resources and survive under varied conditions.
• Escape from predators, parasites or pathogens that regulate populations in its native range can favour expansion.
• Disturbance and available resources can increase an ecosystem’s susceptibility, so invasiveness depends on the interaction between species traits and the receiving environment.