Species are generally confined to their native geographical range. When an organism is found outside this natural, historical distribution, it signifies a change in biological geography, almost always mediated by human activity. Understanding this movement requires clear terminology to categorize organisms that have crossed significant ecological barriers. This article clarifies the vocabulary used to describe species living outside their natural home.
Defining Non-Native, Introduced, and Exotic Species
A non-native species, also referred to as an introduced, alien, or exotic species, lives in an area outside its historical or natural range. Its presence in the new location is a direct or indirect result of human action, whether deliberate or accidental. Organisms that arrive through natural means, such as being carried by a storm or migrating, are not considered non-native. The introduction must be a human-caused event that allows the species to overcome a natural barrier to dispersal.
These terms are often used interchangeably and serve as a neutral description of a species’ origin relative to a specific location. Many non-native species are considered benign, integrating into the local environment without causing measurable harm. For example, the common peony is non-native to North America but is seen as a harmless addition to the landscape. Similarly, many domesticated livestock and agricultural crops, like cattle, were intentionally introduced but do not disrupt the ecosystem.
The Critical Distinction: When Does Non-Native Become Invasive?
An invasive species is a non-native organism that meets two specific criteria: it is established outside its natural range, and its introduction causes or is likely to cause harm. This harm can be to the economy, the environment, or human health. This classification is applied only after the species has successfully established a self-sustaining population and begun to spread rapidly.
Non-native organisms often succeed because they lack the natural predators, parasites, or diseases that regulated their population in their native range. The Burmese Python in the Florida Everglades, for instance, became highly invasive by rapidly reproducing and preying on native mammals and birds. Unlike a benign non-native species, an invasive species like kudzu actively outcompetes and degrades the native ecosystem. The ability to cause harm is the factor that moves a species from merely being non-native to being classified as ecologically destructive.
Pathways of Introduction
The movement of non-native species across the globe is primarily facilitated by pathways linked to human activity and globalization. These pathways are generally categorized as either intentional or unintentional introductions. Intentional introductions occur when humans deliberately move a species for a perceived benefit, such as importing certain plants for ornamental landscaping or erosion control. The pet trade, the stocking of fish for recreational fishing, and the introduction of biological control agents also fall under this category.
Unintentional introductions occur as a byproduct of human travel and commerce. A major vector is the release of ship ballast water, which can carry countless tiny aquatic organisms, including larvae and microorganisms, from one port to another. Other accidental pathways include species “hitchhiking” on shipping containers, vehicles, or contaminated packaging materials. For instance, three species of rats have spread across the world by riding on ships, and pests can be inadvertently transported within imported agricultural products.
Ecological and Economic Impacts
When a non-native species becomes invasive, the consequences cause significant ecological and economic damage. Ecologically, invasive species are a leading driver of biodiversity loss worldwide, often by outcompeting native flora and fauna for finite resources. They can disrupt delicate food webs, alter the physical structure of habitats, and change biogeochemical cycles like soil composition and water quality. In some island ecosystems, invasive species are considered the main cause of extinctions.
The economic costs associated with invasive species are substantial, totaling at least $423 billion globally each year. This financial burden stems from damage to agriculture, such as crop loss and increased pest management expenses. Invasive animals can also interfere with human infrastructure, such as the zebra mussel, which clogs intake pipes for power plants and water systems. Governments and industries must spend billions annually on eradication programs, restoration efforts, and the long-term management of established populations.