What Is an Invasive Species?

Old World Climbing Fern smothering a cypress stand in Florida. This vine from Asia and Australia shades out native vegetation. USDA

One of our top environmental issues, invasive species get relatively little attention. First, we need to differentiate a few terms. A species referred to as alien or non-native is found outside of its natural geographic range. Exotic means practically the same thing. The alien designation generally implies that humans were instrumental in moving it to its new location. Some species naturally expand into new areas, and those are not considered alien.

Another term frequently employed is feral. Feral animals are wild individuals belonging to a species that is domesticated. There exists colonies of feral cats, packs of feral dogs, and many regions have problems with feral pigs, and even with feral goats and cattle.

An invasive species is an alien species that heavily colonizes an area, causing harm to the environment, to human health, or to the economy. Not every organism has the potential to become invasive if transplanted in a new area. Some characteristics facilitate that kind of behavior. For example, invasive plants tend to grow rapidly, produce seeds quickly and abundantly, and have the ability to disperse far and wide (think of dandelion seeds).   

Just as organisms vary in their capacity to become invasive, ecosystems vary in their vulnerability to invasive species. Most likely to harbor invasive species are islands, areas that have been disturbed (for example, road sides), and places that are highly diverse.

How Do Invasions Happen?

One or more factors may be at play, allowing an alien species to become invasive. Sometimes a species makes it to new shores without the predator or competitor that holds them in check in their native range. For example, a marine alga, , is invasive in the Mediterranean, but is controlled by a snail and by other grazers in its native Caribbean Sea. Other species exploit resources that are unavailable to local species. Tamarix, or saltcedar, is an invasive tree in the desert Southwest US, and it uses its long tap roots to reach zones saturated with groundwater but too deep for other plants.

Invasions rarely take off after just a handful of plants or animals of one species are introduced in a new area. The species is often present in really small numbers for many years before it suddenly expands its range. Scientists are not certain why, but it could be that this lag time may allow the species to adapt to the new environment, perhaps hybridizing with a native species. Over that period of lag time, new individuals continue arriving, providing more genetic material and thus better equipping the invasive species for conditions in the new environment.

What Drives Invasions?

We use the term vector to describe the method by which invasive species make it to new areas. Many plants arrive through agricultural or horticultural activities. Sometimes called escapees, ornamental outdoor plants can start growing outside of the landscaped front yard they were planted in. Boxes and containers holding cargo can hold stowaways, as we are periodically reminded when we hear news stories of shaken customers finding tropical spiders in their grapes or bananas. The emerald ash borer, an insect decimating ash trees in North America, probably arrived from Asia in wooden pallets and boxes used as cargo crating. In the marine world, ships’ ballast tanks are often blamed for holding water containing alien species that can become invasive. This is probably how zebra mussels made it to North America.

Ultimately, the main driver of invasions is trade. Increased purchasing power, reduced trade barriers, and delocalized manufacturing centers have all led to an increasingly global economy. Net US imports have grown by over ten times since the 1970s, facilitating the movement of cargo and people around the world, along with many plants and animals eager to get a fresh start somewhere new.