The introduction of non-native species into the United States has been a continuous process since the first colonial settlements. Of the estimated 50,000 non-native species that have established themselves in the country, a significant minority have become invasive, causing widespread ecological and economic harm. Understanding the origin and means of transport of these introduced species is a major focus for containment, informing where to target prevention efforts and develop effective biosecurity strategies.
Establishing the Scale of the Problem
An invasive species is defined as a non-native organism whose introduction causes or is likely to cause economic or environmental harm, or harm to human health. Invasive species outcompete native plants and animals for limited resources, prey upon vulnerable populations, and introduce new diseases to which local species have no resistance. This ecological pressure contributes to the decline of native biodiversity. The financial toll associated with these species is substantial, with estimates of the annual cost in the United States ranging from over $20 billion to $120 billion per year in damages and control efforts.
Geographical Origins of Major Invasive Species
The vast majority of the most damaging invasive species in the U.S. trace their origins back to two primary global regions: Europe and Asia. This geographical pattern is a direct result of historical human activity, particularly the established trade routes and agricultural exchange. Species from these regions often possess traits that allow them to thrive in temperate climates similar to those found across much of the continental United States.
Europe: Early and Consistent Source
Europe was the earliest and most consistent source of introduced species, driven by early colonization and the deliberate introduction of familiar animals and plants. The European starling, for example, was intentionally released in New York’s Central Park in the 1890s. Another well-known European invasive is the rock pigeon, which was brought over by early colonists and now inhabits nearly every U.S. city.
Asia: Increasing Dominance
Asia has become an increasingly dominant source, especially since the mid-19th century when expanded trade connections facilitated the movement of goods and organisms. The Asian longhorned beetle has caused millions of dollars in damage to hardwood trees across the Northeast. This insect arrived accidentally, likely concealed within untreated wooden packing materials shipped from Asia. Similarly, the fast-growing Kudzu vine was intentionally imported from Japan in the late 1800s to control soil erosion.
Tropical and Subtropical Regions
While Europe and Asia are the largest contributors, other tropical and subtropical regions are sources for invasives in warmer U.S. states like Florida and Hawaii. The water hyacinth, a plant that chokes waterways, was introduced from South America for its ornamental flowers. The brown tree snake, responsible for devastating bird populations on Guam, is a recent example of species originating in the Western Pacific that pose a threat to tropical U.S. territories.
Primary Introduction Pathways
The movement of species from their native origins to the United States occurs through identifiable pathways largely tied to global commerce and human migration. These pathways are generally categorized as either intentional or unintentional, explaining the physical mechanism by which organisms successfully cross geographical barriers.
Unintentional Pathways: Shipping and Trade
Commercial shipping and trade represent a major unintentional pathway for both marine and terrestrial invaders. Aquatic species, such as the Zebra mussel, are often transported across oceans in the ballast water tanks of large cargo ships, which are discharged into new ports. Terrestrial pests, including wood-boring insects, frequently enter the country hidden within wooden crates, pallets, or other packaging materials used during transport.
Intentional Pathways: Horticulture and Control
Intentional introductions, often for specific human uses, are also a significant source of invasive species. The horticulture and ornamental plant trade is responsible for introducing a large percentage of invasive woody plants, many chosen for their hardiness and aesthetic appeal. Additionally, some species were brought in as a form of biological control to manage other pests, such as the Harlequin lady beetle, which was introduced from Asia to control aphids but displaced native lady beetles.
Pet and Aquarium Trade
The pet and aquarium trade provides another vector for invasion through the accidental or intentional release of exotic animals. The Burmese python in the Florida Everglades and the Red-eared slider turtle are examples of pets released by owners that successfully established invasive populations. Even items like aquarium moss balls have been found to carry hitchhiking species, such as microscopic zebra mussel larvae, highlighting the difficulty of regulating every point of entry.