While species have naturally moved across geographical barriers throughout Earth’s history, human activities have significantly accelerated this process. This has led to organisms being transported far beyond their natural distribution, bringing about complex ecological changes in new environments.
Defining Introduced Species
An introduced species is any organism living outside its native range due to human activity, whether deliberate or accidental. These are also known as non-native, alien, or exotic species. They differ from native or indigenous species, which naturally occur in an area without human intervention. Introduced species can establish populations in new environments, becoming “naturalized” if they reproduce and maintain their numbers without human assistance.
How Species Are Introduced
Species are introduced into new environments through intentional or unintentional pathways. Intentional introductions involve the deliberate movement of species for purposes like agriculture, ornamental gardens, pets, or biological pest control. Unintentional introductions occur when species are inadvertently transported as a byproduct of human activities. Significant pathways include global shipping, where organisms travel in ballast water or by attaching to ship hulls (hull fouling). Other routes include contaminated goods, packing materials, or hitchhiking on travelers or their vehicles.
When Introduced Species Become Problematic
Not all introduced species cause harm; many integrate into new environments without significant negative effects. However, a subset becomes problematic, leading to ecological, economic, or human health issues. These are classified as invasive species, characterized by their ability to spread widely and cause damage.
Invasive species often arrive in new habitats without the natural predators, parasites, or diseases that regulated their populations in their native range. This gives them a competitive advantage over native species, allowing rapid growth and reproduction. High reproductive rates and adaptability also contribute to their success.
Invasive species cause ecological disruption by outcompeting native species for resources, preying on them, altering habitats, or introducing new diseases. This can lead to declines or extinctions of native populations. Economically, invasive species impose substantial costs, damaging agriculture, infrastructure, and tourism industries. Global costs are estimated in the hundreds of billions of dollars annually, with figures quadrupling each decade. Some invasive species also affect human health by acting as disease vectors, spreading pathogens, causing allergies, or inflicting physical injuries.
Examples of Introduced Species
The Burmese python, originally from Southeast Asia, is an invasive predator in the Florida Everglades. It has caused significant declines in native mammal and bird populations due to its voracious appetite and lack of natural predators. Similarly, zebra mussels, native to Eastern Europe and Western Asia, are highly invasive in North American freshwater systems like the Great Lakes. They filter plankton, disrupting the food web, and attach to infrastructure, causing billions in damage.
The Asian tiger mosquito, an invasive species, is a vector for diseases such as West Nile virus and dengue fever, impacting human health. However, many introduced species, like common agricultural crops or ornamental garden plants, do not become invasive and contribute positively to human economies and landscapes. For example, kudzu was introduced to the southeastern United States for erosion control but subsequently became invasive, covering native vegetation.