An introduced species is an organism living outside its native habitat, having been transported there through deliberate or accidental human activity. This human-mediated transport separates introduced species from those that spread naturally. These non-native species can have a wide range of effects on their new environment, from benign to significantly harmful.
Pathways of Introduction
The routes species use to arrive in new environments are categorized as either intentional or unintentional. Increased global travel and commerce have accelerated the rate at which organisms are moved around the world, creating more opportunities for introductions.
Intentional introductions occur with an expected benefit to humans. Agriculture is a primary example, involving the transport of crops and livestock far beyond their native ranges. The horticultural and pet trades also move many plants and animals globally, some of which are released or escape. Other examples include species introduced for sport fishing or misguided attempts at biological control.
Unintentional introductions occur when species are moved accidentally. The ballast water of large ships is a major vector, transporting microscopic organisms, larvae, and small fish across oceans. Species can also hitchhike in shipping containers, on nursery plants, or within agricultural shipments. Vehicles like airplanes and boats can inadvertently carry seeds or insects to new locations.
Distinguishing Between Introduced, Naturalized, and Invasive Species
The terms used for non-native species reflect their environmental impact. “Introduced” is the initial classification for any species transported by humans outside its natural range. This term carries no implication about the species’ ability to survive or its ecological effect.
A subset of introduced species becomes “naturalized” by establishing a self-sustaining population in the wild without further human intervention. These species reproduce and maintain their presence but do not cause significant ecological or economic disruption. For example, the European honeybee in North America was introduced for honey production and now thrives in the wild, benefiting agriculture.
A small fraction of introduced species become “invasive.” An invasive species establishes a naturalized population, spreads aggressively, and causes harm to the environment, economy, or human health. They often outcompete native organisms, disrupt food webs, and alter the physical structure of habitats. This distinction helps direct management efforts toward the species that pose the greatest threat.
Ecological and Economic Consequences
Invasive species cause significant ecological disruptions and economic losses. These impacts stem from their ability to proliferate in a new environment, often free from the natural predators and diseases of their native range. The resulting effects can permanently alter ecosystems and burden human industries.
Ecologically, invasive species are a leading driver of biodiversity loss. They can prey on native wildlife, like the Burmese python in the Florida Everglades, which has decimated mammal populations. Invasive plants like kudzu can smother native vegetation, blocking sunlight and outcompeting local flora. Some invaders also change ecosystem processes, such as zebra mussels that filter vast amounts of water, altering water clarity and nutrient cycles.
The economic toll of invasions is immense. In agriculture, invasive insects and weeds destroy crops, reduce yields, and require costly pest control. Infrastructure can also be damaged; zebra mussels clog water intake pipes for cities and industrial facilities, leading to millions in removal costs. The damage extends to industries like forestry, fisheries, and tourism, with the combined costs of management and damages estimated at billions of dollars annually in many countries.
Methods of Management and Control
Managing invasive species requires strategies that depend on the invasion’s scale and stage. Prevention is the most cost-effective method, aiming to stop species from arriving. This includes regulations requiring ships to exchange ballast water at sea and rigorous inspections of international cargo. Public awareness campaigns about the risks of releasing pets or moving firewood also contribute to prevention.
When prevention fails, early detection and rapid response (EDRR) is the next step. If a new invasive population is found while small and localized, eradication may be possible. Eradication involves the complete removal of every individual from a defined area, a goal that is most feasible for new incursions in a limited space.
For widespread invasive species, the goal shifts from eradication to long-term control. Common management approaches include:
- Physical or mechanical control, which involves methods like hand-pulling weeds, trapping animals, or using barriers to limit spread.
- Chemical control, which uses pesticides and herbicides carefully to avoid harming non-target species and the broader ecosystem.
- Biological control, which introduces a natural enemy from the invasive species’ native range after extensive research to ensure the agent is safe.