What Are Invasive Alien Species and Why Are They a Threat?

Invasive alien species are a significant global issue with far-reaching consequences for both ecosystems and economies. When non-native organisms are introduced to new environments where they lack natural predators, they can spread aggressively. This expansion can cause widespread environmental harm and result in billions of dollars in economic damage, representing a persistent and costly challenge worldwide.

Defining an Invasive Alien Species

An “alien species” refers to any plant, animal, fungus, or microorganism living outside its natural geographic distribution. Humans are the primary vector for these introductions, moving species across oceans and continents to areas they could not reach on their own. Many of these non-native species are harmless or even beneficial, such as agricultural crops like potatoes and corn, which were introduced to new regions without causing damage.

The term “invasive” is applied only when an alien species begins to spread and cause harm. An invasive alien species is one whose introduction threatens biological diversity, ecosystem stability, or results in economic or environmental damage. The vast majority of alien species do not become invasive, and the transition occurs when a species successfully out-competes native organisms.

Certain biological traits increase the likelihood of a species becoming invasive. These characteristics include rapid growth and reproduction, a high capacity for dispersal, and the ability to tolerate a wide variety of environmental conditions. An additional factor is the absence of natural predators, competitors, and diseases that would normally keep its population in check within its native habitat.

Pathways of Introduction

Species are introduced to new territories through either unintentional or intentional pathways. Unintentional introductions happen when species are transported accidentally. For example, microscopic larvae of aquatic organisms like the zebra mussel can be carried in the ballast water that ships use for stabilization. Insects such as the emerald ash borer, which has devastated ash tree populations, can hide in wooden packing materials used for international shipping.

Intentional introductions occur when humans purposefully bring a non-native species into a new area. The horticulture industry has introduced many plants for ornamental purposes, such as Japanese barberry, which subsequently escaped from gardens to invade natural forests. The pet trade is another significant pathway, with examples like Burmese pythons becoming established in the Florida Everglades after being released by owners. Some species were introduced for agriculture or sport, like wild boars, which now cause widespread damage to crops and natural landscapes.

Ecological and Economic Consequences

The arrival of an invasive species can trigger negative effects within an ecosystem. Invasive plants can outcompete native vegetation for sunlight, water, and nutrients, sometimes altering the physical structure of a habitat, as seen with kudzu infestations that blanket entire forests. Invasive animals may prey on native species that have no natural defenses against them, leading to severe population declines. The entire food web can be disrupted, affecting the availability of food for other animals and leading to broader ecosystem instability.

These ecological disruptions translate into substantial economic costs. In the European Union, the estimated annual economic damage from invasive species is around €12 billion. The agricultural sector suffers from crop destruction by invasive insects and competition from invasive weeds. Forestry is impacted by tree mortality caused by non-native pests, and fisheries can collapse due to invasive aquatic species. Beyond these direct damages, governments and private entities spend billions of dollars annually on control and management measures.

Management and Prevention Strategies

The most effective and least costly approach to dealing with invasive alien species is to prevent their introduction in the first place. This involves implementing strict policies and procedures at borders and ports of entry. Preventative measures include regulations for the treatment of ship ballast water and rigorous inspection of international cargo and wood packing materials.

When prevention fails, the next line of defense is early detection and rapid response. The goal is to identify and eradicate a newly introduced invasive population before it has a chance to become firmly established. This requires monitoring programs to survey high-risk areas and a clear action plan to quickly remove any detected invaders. Eradication is far more likely to succeed when the population is small and localized.

Once an invasive species is widely established, eradication is often no longer feasible, and efforts shift to long-term control and management. Control methods include physical removal, such as trapping animals or pulling weeds. Chemical controls, like the targeted application of herbicides or pesticides, are also used. Another approach is biological control, which involves introducing a natural enemy of the invasive species from its native range to help suppress its population.

Individuals have a role to play in preventing the spread of invasive species.

• Boaters and anglers can follow the “Clean, Drain, Dry” protocol for their equipment to avoid transporting aquatic invaders.
• Gardeners can choose to plant native species instead of potentially invasive exotic plants.
• Pet owners should never release unwanted animals into the wild.
• Anyone who spots a suspected invasive species can report it to local environmental authorities.