The introduction of non-native species represents one of the most widespread and complex ecological challenges facing the United States today. These organisms, transported across boundaries where they do not naturally occur, arrive with human assistance and alter ecosystems. Understanding the sheer scale of their presence is a necessary first step in addressing the significant environmental and economic impacts they generate.
Defining Exotic and Invasive Species
The terminology surrounding non-native organisms is distinct and carries specific regulatory weight in the US. An exotic, or alien, species is defined as any species that is not native to a particular ecosystem. This broad category encompasses all organisms moved beyond their historical range due to human activity, such as livestock, common garden vegetables, and ornamental plants. Many thousands of these non-native species exist within the country without causing measurable harm.
A species is designated as invasive only when it meets a second, more severe criterion. According to the definition established by US Executive Order 13112, an invasive species must not only be non-native to the ecosystem but must also be one whose introduction causes, or is likely to cause, economic or environmental harm, or harm to human health. While all invasive species are non-native, only a small fraction of non-native species are officially classified as invasive.
The Current Count of Invasive Species in the US
The total number of non-native species introduced to the US is estimated to be over 50,000, but the count of those that have become established is more specific. The United States Geological Survey (USGS) maintains the United States Register of Introduced and Invasive Species (US-RIIS), which catalogs organisms established and reproducing in the country. The US-RIIS, Version 2.0, details approximately 12,571 unique species names that are established non-natives across the conterminous US, Alaska, and Hawaii.
Of these established non-native species, only a fraction are actively classified as invasive and causing documented harm. For instance, in the conterminous 48 states, about 2,169 species are documented as invasive, representing roughly 25% of the established non-native species in that region. The count is not static, as species are continually introduced, and environmental changes can trigger a previously benign non-native species to become an active invader. The largest taxonomic groups represented in the established count are flowering plants and insects.
Primary Pathways of Introduction
Non-native species arrive in the US through two main mechanisms: intentional and unintentional introductions. Intentional introductions occur when people deliberately move a species for a perceived benefit, such as for the horticultural trade, where non-native plants are sold as ornamentals. The pet trade is another common source, with organisms like exotic fish or reptiles sometimes escaping or being released into local environments. Furthermore, some non-native species were introduced historically for erosion control or as game animals for hunting.
Unintentional introductions represent the accidental movement of organisms as a byproduct of global commerce and travel. A major pathway for aquatic species is the discharge of ship ballast water, which can release organisms like the zebra mussel into new harbors and waterways. Terrestrial pests often hitchhike in contaminated goods, packaging materials, or on vehicles and watercraft. Wood-boring insects, such as the emerald ash borer, are frequently transported in untreated wood packaging or in firewood moved by travelers.
Ecological and Financial Impact
Ecologically, invasive species are a primary driver of biodiversity loss, as they often outcompete native plants and animals for limited resources like food and light. They can alter entire habitats, such as changing soil chemistry or increasing the frequency and intensity of wildfires. The disruption of native food webs and the introduction of new diseases further compound the damage to natural ecosystems.
The financial burden associated with these invaders is immense and consistently increasing. The estimated annual cost to the US economy from invasive species damage and control efforts is frequently cited at over $120 billion. This figure accounts for losses in agricultural productivity, damage to infrastructure, and the expenditures required for monitoring, containment, and eradication programs. Since 1960, the total economic impact of biological invasions in the country has exceeded $1.2 trillion.