Invasive Species

Invasive species are living organisms—such as plants, animals, fungi, or even microorganisms—that are introduced, either intentionally or accidentally, to regions outside their native range. When they arrive in a new area, some of these species can thrive, reproduce rapidly, and begin to spread in a way that causes harm to the environment, the economy, or even human health.

The process starts with introduction. This can happen through many pathways. People move species around the world deliberately, with examples including bringing ornamental plants to gardens, transporting agricultural crops, or moving animals for livestock, pets, or hunting. Accidental introductions are also common, such as when seeds, insects, or other organisms hitch a ride on international cargo, in ballast water from ships, or even on travelers’ clothing and gear.

Not all introduced species become invasive. For a species to cause environmental problems, it generally must first survive the new conditions, establish a breeding population, and then spread widely. If it overcomes these hurdles, it may become “invasive.” One reason a species becomes invasive is because its natural predators, competitors, or diseases are left behind in its native habitat, which gives it a tremendous advantage in the new ecosystem. Free from these biological checks and balances, some species reproduce quickly and outcompete local species for resources such as food, sunlight, water, or nesting space.

Environmental problems caused by invasive species are numerous and can be severe. One major issue is the reduction of biodiversity. Invasive species can crowd out native species, leading to the decline or even extinction of local plants and animals. This happens in many ways. Some invasive plants grow so thickly they completely block sunlight from reaching native plants. Certain invasive animals may prey on native species that have not evolved defenses against them—like when rats or snakes are introduced to islands with ground-nesting birds that have no experience evading those kinds of predators. Other invaders may compete for the same food or breeding grounds, gradually pushing out native species that can’t compete as effectively.

These disruptions can ripple through entire ecosystems. When a key species is eliminated or greatly reduced—especially so-called “keystone” species that play a central role in their environment—it can affect many other organisms that depend on them, leading to what scientists call a “trophic cascade.” For example, when invasive zebra mussels arrived in the Great Lakes in North America, they filtered plankton from the water at such a high rate that it threw the entire aquatic food web out of balance, affecting fish and native mussel populations, and even changing water quality.

Invasive species can also alter the structure of an ecosystem. Some invasive plants, like kudzu in the southeastern United States, can grow over and smother entire forests, while others—like cheatgrass in the American West—dry out quickly and fuel wildfires, making fires burn larger and more often than they did historically. This in turn changes the landscape, making it even harder for native species to recover and opening the door for further invasions.

Another problem is the spread of disease. Sometimes invasive species bring with them pathogens that can infect local species, domestic animals, or even people. For instance, imported mosquitoes have spread diseases like West Nile virus and Zika virus to new regions. Novel pathogens carried by invasive species can be devastating because native species often have no immunity to them.

The arrival of invasive species can also have substantial economic costs. They may damage crops, forests, or fisheries, leading to reduced yields and higher management expenses. For example, the emerald ash borer, an invasive beetle in North America, has killed millions of ash trees, resulting in significant costs to cities and homeowners for removal and replacement. In agriculture, invasive weeds and pests can lower productivity by competing with crops or killing livestock, leading to increased use of herbicides and pesticides, which may themselves cause further environmental harm.

In response to these challenges, efforts have grown worldwide to detect and prevent the introduction of invasive species, to control and remove those already present whenever possible, and to restore damaged habitats. This often involves a combination of public education, tighter controls on trade and travel, the development of early warning systems, and the use of biological control agents—other species introduced to keep invasives in check, though this must be done very carefully to avoid new problems.

Ultimately, the problem of invasive species highlights how interconnected our world is, and how our actions—whether intentional or accidental—can have far-reaching impacts on the environment. Invasive species are a leading cause of extinctions, biodiversity loss, and ecosystem degradation worldwide. Addressing this issue requires ongoing cooperation between scientists, policymakers, industries, communities, and individuals to both prevent new invasions and limit the damage from those already under way. Awareness, thoughtful management, and prompt action are essential to protecting the world’s diverse environments from the many threats posed by invasive species.