Conservation experts have found that in the United States, invasive alien plant infestations cover 100 million acres and are spreading at a rate of 14 percent per year an area twice the size of Delaware. Recent studies have also revealed that San Francisco Bay is invaded by a new exotic species on the average of once every 12 weeks. Naturally occurring movement of species into the United States is uncommon. Most invasive species arrive in association with human activities or transport. Species can be brought into the country and released intentionally, or their movement and release can be an unintentional byproduct of cultivation, commerce, tourism, or travel.
Many species enter the United States each year as contaminants of commodities.
Agricultural produce, nursery stock, cut flowers, and timber can harbor insects, plants pathogens, slugs, and snails. Weeds continue to enter the United States as seed contaminants. Plant pathogens sometimes arrive as unintended contaminants of plant materials. Fish and shrimp pathogens and parasites have been introduced into the United States on infected stock for aquaculture. Crates and containers can harbor snails, slugs, mollusks, beetles, and microorganisms.