Process of Biological Invasion

An alien species is a species moved by human activity, directly or indirectly, beyond its natural past or present distribution. Biological invasion is a sequence in which the organism is transported and introduced, survives and reproduces to become established, and then disperses beyond the point of introduction. An alien species is regarded as invasive when its introduction or spread threatens biodiversity; therefore, not every introduced or established alien species becomes invasive.

Pathways of introduction

Introductions may be intentional, such as release for a specific purpose, or unintentional, as a consequence of trade, transport and travel. Standard international classification groups pathways into six broad categories.

• Release into nature includes deliberate introductions for purposes such as biological control, fisheries, hunting or habitat improvement.
• Escape from confinement occurs when species kept in agriculture, horticulture, forestry, aquaculture, gardens, zoos, farms or the pet trade enter the wild.
• Transport as a contaminant occurs when organisms accompany commodities such as seeds, nursery material, food, timber or other traded biological material.
• Transport as a stowaway occurs through ballast water, hull fouling, cargo, packaging, containers, vehicles and aircraft.
• Corridor pathways arise when human-made connections, such as canals or linked waterways, allow organisms to cross former geographical barriers.
• Unaided dispersal occurs when an alien species spreads naturally into a new area from a neighbouring region where it had previously been introduced.

Stages from introduction to spread

Progression is sequential, but many species fail at one or more stages and only a fraction complete the invasion process.

• Transport moves the organism beyond the geographical barriers that previously confined its natural range.
• Introduction occurs when the organism enters the wild outside its natural range through direct or indirect human agency.
• Survival requires the introduced individuals to tolerate local climatic, habitat and biotic conditions.
• Establishment occurs when the species reproduces successfully and maintains a self-sustaining population without continued human reintroduction.
• Spread occurs when the established population disperses and forms populations beyond the original site of introduction.
• Ecological impact follows when the alien species alters native species, communities, ecosystem processes or the benefits people obtain from nature.
• A lag period may separate establishment from rapid spread, so a currently restricted alien population may still expand later.

What determines progression

Each transition depends on the interaction among introduction effort, species characteristics, environmental suitability and chance events.

• High propagule pressure, large numbers of individuals or repeated introductions, generally raises the probability of survival and establishment.
• Reproductive capacity, dispersal ability and tolerance of varied environmental conditions can facilitate establishment and spread.
• Suitable climate, habitat and resource availability are necessary for long-term persistence.
• Disturbance can create vacant space or resources that facilitate some alien species.
• Competitors, predators, parasites, diseases and the availability of mates or pollinators can restrict establishment.
• Connected transport networks and repeated human-mediated movement can accelerate secondary spread.
• Spread alone does not establish the magnitude of harm; ecological and socio-economic impacts require separate assessment.

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