An invasive species is defined as a non-native organism whose introduction to a new environment causes, or is likely to cause, environmental or economic harm, or harm to human health. The problem posed by the truly invasive subset is enormous, representing one of the top global threats to biodiversity, economies, and food security. Globally, the annual costs associated with these invaders are estimated to exceed $423 billion.
Disrupting Native Species Populations
Invasive species often succeed in new habitats because they arrive without the natural predators, parasites, or diseases that regulated their populations in their native range. This lack of biological control allows their numbers to swell rapidly, leading to intense competition with native fauna and flora for limited resources. For example, the invasive lionfish in the Atlantic Ocean aggressively outcompetes native fish for food and space, leading to significant declines in local fish populations that cannot compete as effectively.
Another highly destructive mechanism is direct predation, particularly in vulnerable island ecosystems where native species have not evolved defenses against the new threat. The Burmese python, introduced to the Florida Everglades, has established itself as an apex predator, causing steep declines in native mammal populations, including raccoons, opossums, and bobcats. Similarly, the accidental introduction of the brown tree snake to Guam resulted in the near-total elimination of native forest bird species, illustrating the devastating impact of a novel predator.
Invasive species can also threaten native populations through hybridization. This occurs when an invasive species breeds with a closely related native species, producing hybrid offspring that dilute the native gene pool. If the hybrid offspring are numerous, the distinct genetic identity and unique adaptations of the native species can be lost forever, effectively leading to extinction.
Altering Ecosystem Structure and Function
Beyond direct biological threats, invasive species fundamentally change the physical environment, creating conditions that further favor their dominance and disadvantage native life. One of the most significant alterations involves changes to the natural fire regime. Invasive grasses like cheatgrass in western North America can establish dense, continuous layers of fine fuel, dramatically increasing the frequency and intensity of wildfires.
These new “grass/fire cycles” shorten the interval between burns, preventing slower-growing native shrubs and trees from reaching maturity, which ultimately transforms shrublands into non-native grasslands. Invasive plants also manipulate the chemical composition of the soil, often increasing available nitrogen, which then favors the growth of other fast-growing non-native species over native flora.
Invasive plants can also drastically modify regional water cycles, affecting the availability of water for all other life forms. Certain deep-rooted non-native trees and shrubs, such as saltcedar, consume excessive amounts of groundwater, which can lower water tables and dry up streams and wetlands. This modification of hydrology eliminates the necessary aquatic and riparian habitats required by native species, leading to cascading effects throughout the entire local food web.
Financial Costs and Economic Damage
The ecological damage caused by invasive species translates directly into immense financial costs and economic damage across multiple sectors of human society. Global economic losses are estimated to have reached at least $1.28 trillion between 1970 and 2017, with annual costs continuing to rise as global trade intensifies. In the United States alone, the annual cost of invasive species is estimated to be over $21 billion, with the majority of this expense representing direct damages and losses.
Agriculture is one of the hardest-hit sectors, facing substantial expenses from crop loss, reduced yields, and the continuous need for pesticide application to control invasive insects and weeds. For instance, invasive pests and plant diseases can wipe out harvests, directly impacting the livelihoods of farmers and national food security. Furthermore, invasive species cause widespread damage to critical infrastructure, necessitating expensive repair and maintenance.
Aquatic invaders like the zebra mussel, for example, colonize and clog water intake pipes, power plant cooling systems, and municipal water treatment facilities. Beyond direct damages, billions of dollars are spent annually on monitoring, research, eradication, and long-term control programs, diverting funds from other societal needs.
Risks to Human and Animal Health
Invasive species pose direct threats to human and domestic animal health by acting as carriers for diseases or by introducing new toxins and physical hazards. Many non-native species serve as disease vectors, transporting pathogens to human populations that have little or no natural immunity. The Asian tiger mosquito, one of the most widespread invasive mosquitoes globally, is a known vector for dangerous viruses like West Nile and Dengue fever in new geographic areas.
Additionally, invasive plants can introduce severe allergens or physical irritants into the environment. Giant hogweed, a plant that has spread widely outside its native range, contains a toxic sap that causes severe skin inflammation and blistering when exposed to sunlight. Aquatic invasive species can also facilitate harmful algal blooms that produce potent toxins, contaminating drinking water and impacting seafood safety.
Invasive species also introduce novel pathogens that can jump to new hosts, including livestock and humans. This creates the potential for outbreaks of diseases previously unknown to the local ecosystem. This makes the management of invasive species a significant public health concern.