Invasive species are organisms that are non-native to a given ecosystem and whose introduction causes or is likely to cause harm to the economy, the environment, or human health. These invaders, which include plants, animals, and pathogens, actively disrupt ecological balance and cost the United States billions of dollars annually. While precise quantification is difficult, establishing a baseline measure is necessary for effective management and public awareness. The sheer scale of the affected area underscores the need for a coordinated, national strategy.
Quantifying the National Footprint
The most authoritative estimates suggest that invasive species have impacted well over 100 million acres across the United States. This figure encompasses federal and non-federal lands, equating to an area roughly the size of the state of California. However, the total footprint is likely much larger, depending on how “affected” is defined and which species are included in the count. For example, the imported fire ant has alone infested an estimated 300 million acres in certain regions, primarily impacting agricultural and rangeland areas. Damages and control costs are estimated to be over $100 billion each year. Nearly half of the native species currently classified as endangered or threatened are impacted by invasive organisms, which fundamentally alter ecosystems and outcompete native life for vital resources.
Defining and Measuring Affected Land
The challenge in calculating a definitive total acreage stems from the diverse definitions and methodological difficulties inherent in large-scale ecological surveying. Federal and state agencies often use varying metrics to define an “affected acre,” creating discrepancies. One agency might count a minor presence of an invasive plant, while another only records dense infestations where the native ecosystem has been completely displaced.
Surveying technology also struggles with the landscape’s heterogeneity. Detecting invasive plants using aerial or satellite imagery is difficult because non-native species can be visually obscured or commingled with native vegetation. Furthermore, detecting invaders below dense forest canopies is practically impossible, meaning a significant portion of the affected area may be undercounted.
The impact of an invasive species is not always proportional to its density. Some species cause significant problems even at low population levels, while others only become problematic at high-density infestations. This variability makes it difficult to establish a consistent threshold for when an acre transitions from “unaffected” to “affected” for a national census. These complexities result in the reported acreage being a broad estimate, rather than a precise, universally agreed-upon figure.
Distribution Across Major Ecosystems
The impact of invasive species is not uniform across the landscape; instead, it concentrates heavily within specific ecosystem types. Forests, rangelands, and aquatic environments each face unique threats from specialized invaders that alter the fundamental ecological functions of the land.
Forests and Woodlands
In the nation’s forests and woodlands, non-native tree species are estimated to occupy approximately 18.8 million acres of the continental U.S. forest area, representing about 2.8% of the total. This figure does not fully account for the damage caused by invasive pests, such as the emerald ash borer or the hemlock woolly adelgid, which are actively wiping out millions of native trees across the country. The loss of these trees devastates the ecosystem and releases millions of tons of stored carbon into the atmosphere each year.
Rangelands and Grasslands
Rangelands and grasslands in the Western U.S. are severely impacted by invasive plants like cheatgrass, which has covered millions of acres and fundamentally altered the region’s fire regime. Cheatgrass is an annual plant that dries out earlier than native perennial grasses, creating a continuous bed of fine fuel that increases the frequency of wildfires. This cycle of fire and invasion prevents the reestablishment of native species, leading to the rapid conversion of native shrublands into simplified, invasive-dominated grasslands.
Aquatic and Wetlands
Aquatic and wetland ecosystems are threatened by invaders that change water quality and clog infrastructure. Aquatic organisms such as zebra mussels have caused extensive damage by colonizing lakes and rivers and clogging water delivery systems. Furthermore, animals like the nutria damage approximately 100,000 acres of coastal wetlands annually through their feeding habits, contributing significantly to coastal erosion.
The Annual Rate of Spread
The total affected acreage is not a fixed number, as the problem is characterized by rapid expansion and spread across previously unaffected areas. The dynamic nature of the invasion means that new land is continuously being lost to non-native species each year.
For invasive trees and plants alone, the total infested area on the continent is expanding at a rate of approximately 500,000 acres annually. This yearly loss of acreage is driven by various mechanisms, with human activity being the primary accelerator. The globalization of trade and increased human movement provide the vectors for species to travel far outside their native ranges.
A recent study showed that when factoring in human assistance—such as transport via shipping containers, vehicles, and ballast water—invasive species are expanding their ranges at an astronomical rate, about 1,000 times faster than native species. This rate can translate to a range expansion of 1,883 kilometers per year, far outpacing the ability of native plants and animals to adapt or migrate. The combination of human transport and the effects of climate change, which favor the establishment of many fast-growing non-native species, ensures that the national footprint will continue to grow rapidly unless aggressive control measures are implemented.