The movement of plant species, largely facilitated by human activity, has profoundly affected biodiversity globally. Plants are routinely transported across geographical boundaries, often establishing themselves in new ecosystems where they did not evolve. Understanding the definitions and classifications of these introduced organisms is necessary for effective ecological management. This article clarifies what non-native plants are, details their classifications, and explains how they are introduced to new environments.
Defining Non-Native Plants
A non-native plant, also referred to as an alien, exotic, or introduced species, is defined purely by its geographic origin. These are plants that occur in a specific area outside of their natural, historical range, and their presence is directly attributable to human activities, whether intentional or accidental. The classification relies on the concept of a geographical barrier, such as an ocean or continental boundary, which the species could not have crossed without human assistance.
The designation of a plant as non-native is specific to the ecosystem under consideration. A species can be native in one region and non-native just a few hundred miles away. For example, a plant native to the East Coast of North America is considered non-native when introduced to the West Coast. The defining feature is the absence of a long evolutionary history in the new location, meaning the plant has not co-evolved with the local climate, soil, or animal life.
Related Terminology and Classifications
Ecologists use further classifications to describe how well a non-native species survives and reproduces in its new environment.
Casual Plants
Casual Plants are species that can reproduce but fail to maintain their population over a long period. These plants must rely on continued human assistance or repeated introduction to persist. They are often seen as temporary populations that do not establish stable footholds.
Naturalized Plants
Naturalized Plants are non-native species that successfully reproduce and sustain populations over many life cycles without direct human intervention. These plants have overcome the environmental and reproductive barriers of the new region and are self-perpetuating. A naturalized plant typically integrates into the new location without causing widespread ecological disruption, and it does not become a native member of the plant community despite its establishment.
The Critical Distinction: Non-Native vs. Invasive
The distinction between non-native and invasive plants is based entirely on the plant’s impact on its new environment. All invasive plants are non-native, but the vast majority of non-native plants are not invasive. The official definition of an invasive species requires that its introduction causes, or is likely to cause, “economic or environmental harm or harm to human health.”
A non-native plant becomes invasive when it exhibits rapid reproduction and aggressive spread that displaces native species and disrupts ecosystem processes. This aggressive behavior is often attributed to the absence of the natural predators, pathogens, and herbivores that regulated its population in its original habitat. Without these ecological checks, the plant allocates more resources to growth and reproduction, allowing its population to grow unchecked.
The harm caused by invasive plants includes outcompeting native flora for light, water, and nutrients, which reduces local biodiversity. Some invasive species actively alter the physical environment by changing soil chemistry, hydrology, or the frequency and intensity of wildfires. For example, Tatarian honeysuckle can shade out native plants, and kudzu can blanket forest edges, strangling native trees.
Pathways of Introduction
Non-native plants are introduced to new regions through two main categories of human-assisted pathways: intentional and unintentional.
Intentional Pathways
Intentional introductions involve the deliberate movement of a species for a specific purpose. The ornamental plant trade is a major intentional pathway, as millions of plants are imported annually for use in landscaping and gardens. Other intentional introductions include using species for agriculture, such as new food crops, or for environmental enhancement, like using grasses for erosion control. This deliberate movement often occurs without full knowledge of a plant’s potential to become problematic in a new climate.
Unintentional Pathways
Unintentional introductions involve the accidental movement of plant material as a byproduct of human commerce and travel. Plant seeds or fragments can be transported as contaminants in shipments of agricultural products, such as grain or seed mixtures. Other accidental pathways include seeds hitchhiking on vehicles and machinery, or within packing materials used for international trade. Ballast water from cargo ships, while primarily responsible for aquatic introductions, can also carry plant fragments and seeds from distant ports.