An introduced species is any organism that is established in a location outside of its natural, historical geographic range due to direct or indirect human activity. These species, also known as non-native or alien species, have been transported across ecological barriers that they could not have crossed on their own. The unifying factor is that their presence in the new environment is a result of human movement and global trade. The impacts of these introductions are highly variable, with some non-native species integrating seamlessly into their new ecosystems while others cause profound disruption. Understanding the ways these species arrive and the distinctions in terminology are necessary to address the complex ecological and economic consequences that can follow.
Defining Introduced, Native, and Invasive Species
The terminology used to describe species based on their origin and impact is specific and often misunderstood by the public. A native species is one that has originated and evolved within a specific region or ecosystem without human intervention, adapting to the local environmental conditions over a long period. These indigenous organisms form the foundation of a region’s biodiversity, having co-evolved with other local species.
A non-native or introduced species is any species that is living outside of its native range because of human activity, which can be deliberate or accidental. Importantly, an introduced species is not inherently harmful; many commonly grown crops, domesticated animals, and garden plants fall into this category. Some may even become “naturalized” by establishing stable populations without causing significant problems.
The third category, the invasive species, is a specific subset of introduced species that causes or is likely to cause ecological, economic, or human health harm. Only a small fraction of introduced species actually become invasive, but those that do often spread aggressively, outcompeting native organisms for resources. The distinction is based entirely on the negative impact; a non-native species is only labeled invasive when its spread actively damages the new environment. This harmful characteristic is frequently linked to a lack of natural predators, parasites, or competitors in the new region, allowing for unchecked population growth.
Pathways of Arrival
Introduced species arrive in new locations through two broad types of human-mediated pathways: intentional and unintentional.
Intentional Introductions
Intentional introductions occur when humans deliberately move a species outside its native range with the express purpose of establishing it in a new environment. These actions are often motivated by the perceived benefits of the species for human use. Examples include introducing species for agriculture, such as new forage grasses or fruit trees, or for ornamental purposes in horticulture. Species have also been introduced for biological control, where a natural enemy is released to manage a pest population, or for recreational hunting and fishing.
Unintentional Introductions
Unintentional introductions involve the inadvertent movement of organisms as a byproduct of global human travel and trade activities. These species are often stowaways hitching a ride on transport vehicles or in commercial goods. A major pathway in marine environments is the discharge of ship ballast water, which can release aquatic organisms, including fish, invertebrates, and microorganisms, into new ports. Terrestrial accidental pathways include the movement of pests in wood packing materials or in shipments of plants and agricultural products. The movement of recreational watercraft between bodies of water also serves as an accidental pathway, transporting species like zebra mussels attached to propellers and hulls.
Ecological Shifts Caused by New Species
When an introduced species becomes invasive, it can trigger dramatic shifts in the structure and function of the local ecosystem. These ecological impacts are often categorized by the way the invasive species interacts with the native flora and fauna.
Competition
One significant mechanism is competition, where the introduced species rapidly outcompetes native organisms for limited resources like light, water, nutrients, and space. Invasive plants, for example, often grow faster and produce more seeds than native species, allowing them to dominate forest understories or grasslands quickly. This intense resource competition can push native species toward decline and local extinction. The invasion of certain plants has even been demonstrated to alter biogeochemical cycles, affecting how carbon and nitrogen move through the soil.
Predation
Another mechanism of impact is predation, where an introduced animal directly preys upon native species that have not evolved defenses against the new hunter. The brown tree snake (Boiga irregularis), for example, caused the extinction of numerous bird species on the island of Guam because the native birds had no defense mechanisms against the snake. This predatory pressure can have a more immediate and severe effect on native populations.
Habitat Alteration
A third form of ecological damage is habitat alteration, in which the physical or chemical environment is fundamentally changed by the invader. Some invasive plants, such as saltcedar (Tamarix spp.), can deposit large amounts of salt onto the surrounding soil, making the habitat inhospitable for native plant life. Other invaders can change a region’s fire regime; the dense growth of downy brome (Bromus tectorum) in Western U.S. grasslands has increased the frequency and intensity of wildfires. These alterations can dramatically reduce the resilience of the ecosystem.
Economic Consequences
Invasive species inflict substantial and measurable financial costs on human economies globally. These economic consequences stem from damage to resources, losses in productivity, and the significant expenses required for control and management efforts. Globally, the annual economic cost of biological invasions now totals at least \(423 billion.
Agriculture
The agricultural sector is consistently one of the most heavily affected by invasive species. Pests and pathogens can cause massive crop damage and livestock losses, reducing yields and requiring expensive chemical or biological control measures. In the United States, costs to the agricultural sector are a major component of the overall annual economic damage, estimated to be over \)21 billion.
Infrastructure
Invasive species also cause significant damage to human infrastructure and public services. Aquatic organisms like zebra mussels (Dreissena polymorpha) are notorious for clogging water intake pipes, filtration systems, and power plant cooling systems, generating hundreds of millions of dollars in annual damages in regions like the Great Lakes. Other species, such as the brown tree snake, have been known to cause costly power outages by climbing on electrical lines.
Public Health
Invasive species can pose direct threats to public health, leading to costly control measures and medical expenses. Certain invasive mosquito species can act as vectors for diseases that are not endemic to a region, spreading illnesses such as West Nile virus, dengue, and Zika. The costs associated with controlling these insect populations and treating the resulting diseases represent a tangible economic burden.