What is an Invasive Species?

People have been moving plants around the world for centuries. Most countries now rely on plants from other regions of the world for food, construction materials, ornamental plants and fibers. Organisms that have been moved from their native habitat to a new location are referred to as "non-native," "non-indigenous," "exotic," or "alien" to the new environment. Most U.S. food crops are non-native species and their beneficial value is obvious. A small percentage of non-natives, however, cause serious problems in their new environments and are collectively known as "invasive species.” knotweed infestation
How Did They Get Here?
Non-native species have been introduced into the U.S. in a variety of ways. Some non-native species, intentionally introduced for beneficial purposes, later turn out to be invasive. Examples include purple loosestrife, which was sold as an ornamental plant, as well as Japanese knotweed, which was introduced for erosion control. Yet many of the non-native species that later become invasive were unintentionally introduced; they move as unknown stowaways and hitchhikers when people and their products are transported by air, water, rail or road. Examples of invasive plant species unintentionally introduced into the U.S. include mile-a-minute vine and Japanese stiltgrass.
Why Are They a Problem?
The most important aspect of how a non-native plant does or does not become invasive is how it responds to a new environment. An invasive plant displays rapid growth and spread, allowing it to establish over large areas. Free from the vast and complex array of natural controls present in their native lands, including herbivores, parasites and diseases, invasive plants may experience rapid and unrestricted growth in new environments. Their phenomenal growth allows them to overwhelm and displace existing vegetation and form dense monocultures.
What Makes a Non-Native Plant Become Invasive?
  • Ability to grow in many conditions
  • Rapid growth
  • Ability to exploit and colonize disturbed ground
  • Ability to thrive in high nutrient conditions (i.e. excess fertilizers)
  • Reproduce rapidly by roots and shoots. If spread by seed, produce numerous seeds that disperse and sprout easily
  • Having roots and rhizomes with large food reserves
  • Ability to survive and reproduce under adverse conditions
  • Having high photosynthetic rates - "greening up" earlier in the spring than natives gives these plants an competitive advantage
  • Lack of natural predators, pathogens and parasites