What is an Invasive Species?

Japanese barberry, an invasive species, can out-compete native undergrowth and prevent forest regeneration.
Photo courtesy of NYS DEC.

An invasive species does not naturally occur in a specific area: it is non-native to the ecosystem. The introduction of an invasive species can cause harm to the environment, to the economy, or to human health.

Since invasive species are in a new environment, free from natural predators, parasites, or competitors, they often develop large population sizes very rapidly. These large populations can out-compete, displace or kill native species. They can reduce wildlife food and habitat. Some also have the potential to disrupt vital ecosystem functions, such as water flow, nutrient cycling, or soil decomposition. Other invasive species cause massive amounts of economic damage to the agricultural business by destroying crops and contaminating produce. Some invasive species can cause direct harm to humans or domestic animals.

How do they arrive?

Invasive species arrive through many means. People may intentionally introduce an invasive species for an agricultural or ornamental purpose. Once introduced, some of these species escape their enclosures or cultivation and establish as a viable population. Accidental introductions usually result from contaminated freight, or from the movement of contaminated wood products (including shipping pallets, bracing and other dunnage), plants, or food products. The individuals or propagules (seeds, eggs, spores, or other biological material capable of propagation) of invasive species hitchhike in these shipments.

Red-eared slider turtles, commonly sold as pets, are highly adaptable and are able to hibernate over the winter to survive in the wild. Because they are aggressive and can be meat-eaters throughout their lives, they have caused imbalances in the ecology of their new habitats. Also, they are able to hybridize with native turtles, damaging the integrity of the native species and introducing new pathogens to native animals.
The Burmese python is a popular snake to have as a pet, but some people release the snake into the wild when it outgrows its welcome. They have been found to feed on a wide variety of birds and mammals in the Everglades, even an occasional alligator, impacting the natural order of the ecological community.
Photo courtesy of: Skip Snow, National Park Service, Bugwood.org

Invasive species move from one place to another using a variety of pathways. Some pathways can be natural, such as when a species is carried by wind or ocean currents. Other pathways are human induced and can be intentional or unintentional.

Some common pathways include:

  • Abandoned pets and ornamental plants;
  • Ballast water discharged from ships;
  • Importation of seeds, plants, fruits, and vegetables; and
  • Soil brought in with nursery stock.




Spotted knapweed is an allelopathic species. It releases a chemical into the soil which inhibits the germination and growth of native species. Photo courtesy of: University of Idaho Archive, University of Idaho, Bugwood.org

Japanese knotweed can grow from seeds, which are produced in large numbers and dispersed by wind and water. Seed viability is high, and seed
bank densities have been measured at 220 – 1758 seeds per square meter. Photo courtesy of: Chris Evans, River to River CWMA, Bugwood.org

Why are invasive species successful in our environment? They:

  • Lack predators, pathogens, and diseases to keep population numbers in check.
  • Produce copious quantities of seed with high viability.
  • Have successful dispersal mechanisms that attract wildlife.
  • Thrive on disturbance, and are very opportunistic
  • Are fast growing.
  • Are habitat generalists and do not have specific or narrow growth requirements.
  • Some demonstrate allelopathy – they produce chemicals that inhibit the growth of other plants nearby.
  • Have longer photosynthetic periods – they are first to leaf out in the spring and last to drop leaves in autumn.
  • Alter soil and habitat conditions where they grow to better suit their own survival and expansion.




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