What Is Biological Invasiveness and Why Does It Matter?

Biological invasiveness describes a subset of species introduced to a new environment. A non-native species is an organism living outside its natural geographic range due to human activities. An invasive species is a non-native that not only survives but also spreads and causes ecological, economic, or human health harm. While many introduced species fail to establish or remain in small, harmless populations, invasive species proliferate to the detriment of the native ecosystem.

Traits of Invasive Organisms

A defining characteristic of many invasive species is their capacity for rapid growth and reproduction. For instance, rabbits introduced to Australia in the 19th century multiplied at a rapid rate due to their short gestation period and large litter sizes. This reproductive efficiency allows a small founding population to quickly grow, overwhelming native species and resources.

Another trait is high dispersal ability, which allows a species to spread far from its point of introduction. Seeds of invasive plants may be carried long distances by wind or animals, while aquatic organisms can travel in the ballast water of ships. This mobility enables the species to colonize new areas and expand its range rapidly, making containment difficult.

Ecological competence is the ability to thrive in a variety of conditions. Invasive species are often generalists that consume a wide range of food sources and tolerate diverse environmental conditions. The wild boar, for example, is highly adaptable, capable of living in forests, grasslands, and semi-arid regions, and eating everything from roots to small animals. This flexibility gives them a competitive advantage over native species that may have more specialized needs.

The Invasion Process

The invasion process unfolds in three stages: introduction, establishment, and spread. Introduction is the initial arrival of a species in a new area, often facilitated by human activities. Pathways include the transport of goods, where insects might hide in shipping crates, or the intentional release of pets into the wild.

Following introduction, the species must establish a self-sustaining population. This requires the organism to survive the new conditions, find resources, and reproduce. Many non-native species fail at this stage, unable to adapt to the local climate, find food, or avoid predators. Successful establishment means the population is viable without further introductions.

Once established, the species enters the spread phase, where its population grows and expands. The rate of spread depends on the organism’s reproductive rate, dispersal mechanisms, and the new environment’s characteristics. During this phase, the population expands from a localized group to a widespread presence, and its invasive characteristics become apparent.

Consequences of Biological Invasions

Biological invasions have extensive ecological and socioeconomic impacts. Ecologically, invasive species are a driver of biodiversity loss by outcompeting native organisms for resources like food and space. For example, the zebra mussel in the Great Lakes depletes the plankton native fish rely on. Invasives can also be predators, like the brown tree snake in Guam, which has driven several native bird species to extinction.

Invasive species can also alter entire habitats. The kudzu vine in the southeastern United States grows over forests, blocking sunlight and killing native trees, which transforms the ecosystem’s physical structure. These disruptions ripple through the food web, affecting many species and diminishing the ecosystem’s overall health.

Socioeconomically, the costs are substantial. Invasive insects and weeds can decimate crops, causing financial losses for farmers. In aquatic environments, species like the zebra mussel clog water intake pipes for power plants and water supplies, requiring costly maintenance. The global cost of managing invasive species and their damage is estimated to be in the trillions of dollars.

Ecosystem Vulnerability and Resistance

The success of an invasion depends on both the organism’s traits and the ecosystem’s characteristics. Ecosystems with human-caused disturbances like logging or urban development are more vulnerable. These disturbances create openings and reduce competition from native species, providing opportunities for invaders to establish themselves. Similarly, ecosystems with naturally low biodiversity, such as islands, are highly susceptible as native species may lack defenses against new competitors or predators.

Conversely, healthy and biodiverse ecosystems are more resistant. A rich community of native species creates a complex web of competition, leaving few available resources or niches for a new organism to exploit. Intact, undisturbed habitats lack the opportunities invaders need to become established. Conserving biodiversity and maintaining healthy ecosystems is a primary defense against biological invasions.