An invasive species is an introduced, nonnative organism (disease, parasite, plant, or animal) that begins to spread or expand its range from the site of its original introduction and that has the potential to cause harm to the environment, the economy, or to human health.
A few well-known examples include the unintentional introduction of the West Nile virus, chestnut blight, the South American fire ant, zebra mussels, Burmese pythons, and sea lamprey. These are in addition to the intentional introductions of salt cedar (Tamarisk), kudzu vine, house sparrows, starlings, and nutria.
Harmful, non-native species can be found in all ecosystems across the United States. These species can cause costly economic and ecological damage each year including crop decimation, clogging of water facilities and waterways, wildlife and human disease transmission, threats to fisheries, increased fire vulnerability, and adverse effects for ranchers and farmers.
Learn more: USGS Invasive Species Program