Species have always moved across the globe, but their introduction to a new environment does not always have the same outcome. While many species integrate without issue, some create profound disturbances. Understanding the criteria that define a species as “invasive” is important for recognizing its potential consequences for an ecosystem.
Defining Characteristics of an Invasive Species
An invasive species is described by two concurrent conditions: it is not native to the ecosystem, and its introduction causes or is likely to cause harm. This harm can be economic, environmental, or a threat to human health. Both elements must be present, as a non-native species causing no discernible negative impact does not meet the definition of invasive.
For instance, kudzu, a vine native to Asia, was introduced to the southeastern United States. It causes extensive environmental and economic damage by growing over and killing native plants, smothering forests, and damaging infrastructure. This combination of being non-native and causing harm makes it a clear example of an invasive species.
Several biological traits contribute to a species’ ability to become invasive. They often reproduce quickly and in large numbers, allowing their populations to expand rapidly; a single zebra mussel, for example, can produce up to one million eggs per year. Invasive species also frequently lack the natural predators and diseases that kept their numbers in check in their native lands, giving them a competitive advantage.
Invasive vs. Non-Native Species
The terms “invasive” and “non-native” are often confused but are not interchangeable. All invasive species are non-native, but most non-native species are not invasive. A non-native species is any organism living outside its natural geographic range, often introduced by human activity. The defining factor that separates them is the presence or absence of harm.
Many non-native species are harmless or even beneficial in their new environments. For example, common agricultural plants like tomatoes and peppers originated in South America but are not considered invasive in North America. Similarly, honeybees, brought from Europe, are essential for pollinating numerous crops and are not considered invasive. These species exist in new ecosystems without causing the widespread damage characteristic of an invasion.
While a non-native species might simply occupy a niche, an invasive one actively disrupts the ecosystem. This disruption is the core of the “invasive” label and the reason for concern. Without evidence of harm, a species is simply considered non-native or introduced.
The Impact of Invasion
The harm caused by invasive species manifests in several ways. Ecologically, they can alter food webs and habitats. Invasive species often outcompete native organisms for resources such as food, water, and space, which can lead to a decline in native populations and, in some cases, extinction. This competition is a leading threat to biodiversity.
Economically, the costs associated with invasive species are substantial. These costs include direct damage to industries like agriculture and forestry, where invasive insects or plants can destroy crops and timber. They also include the expenses of control, eradication, and restoration programs. Infrastructure can also be affected, such as when zebra mussels clog water intake pipes, disrupting municipal water supplies.