Invasive species are organisms introduced to an ecosystem outside their native range, whose presence causes harm to the environment, economy, or human health. This distinguishes them from non-native or exotic species, many of which coexist without adverse effects in their new environments. Their ability to spread and establish quickly makes understanding their effects important.
Ecological Disruption
Invasive species disrupt natural ecosystems, often leading to a reduction in native biodiversity. One primary mechanism is direct competition for resources such as food, water, light, and space. The kudzu vine (Pueraria montana), a fast-growing invasive plant in the southeastern United States, smothers native trees and vegetation by forming dense mats, blocking sunlight and limiting access to water and nutrients. This competition can lead to the decline or displacement of native plant communities, altering the ecosystem structure.
Invasive predators pose another threat, as native prey populations often lack evolved defenses against these new adversaries. The Burmese python (Python bivittatus) in the Florida Everglades illustrates this impact. These large constrictors prey on a wide array of native mammals and birds, including raccoons, opossums, bobcats, and even alligators. Road surveys in the Everglades have shown severe declines in mammal sightings, with raccoons decreasing by 99.3%, opossums by 98.9%, and bobcats by 87.5% in areas where pythons have proliferated.
Beyond direct competition and predation, some invasive species physically alter habitats, making them unsuitable for native life. Zebra mussels (Dreissena polymorpha) are filter feeders that consume large amounts of phytoplankton and suspended particles, increasing water clarity in infested lakes and rivers. While clearer water might seem beneficial, it allows sunlight to penetrate deeper, promoting the growth of benthic algae and aquatic plants that can outcompete native submerged vegetation and alter nutrient cycling. These changes can disrupt the base of the food web, affecting zooplankton and fish populations that rely on phytoplankton as a food source.
Economic Consequences
The financial burden of invasive species is substantial, impacting various sectors. In North America alone, economic costs attributed to invasive species have escalated from approximately $2 billion per year in the early 1960s to over $26 billion per year since 2010. Globally, the cumulative cost over the past 50 years is estimated to be $1.288 trillion.
Agriculture and forestry industries suffer damage from invasive insects, plants, and pathogens. The pink bollworm (Pectinophora gossypiella) caused economic losses to cotton farmers in Arizona and California through reduced yields and increased control expenses before its eradication from the U.S.. Similarly, invasive weeds like leafy spurge (Euphorbia esula) have decreased grazing capacity for livestock in the Great Plains Region.
Fisheries also face economic repercussions from aquatic invaders. The sea lamprey (Petromyzon marinus) in the Great Lakes preys on commercially important fish species, leading to population declines and impacting fishing industries. Beyond direct resource damage, invasive species incur high infrastructure and management costs. Zebra mussels, known for their ability to attach to surfaces, clog intake pipes of power generation facilities and water treatment plants, necessitating expensive cleaning and maintenance. Controlling and eradicating established invasive species requires ongoing investment.
Human Health and Social Repercussions
Invasive species can directly and indirectly affect human well-being. Some invasive species act as vectors for diseases. The Aedes aegypti mosquito, now widespread globally, transmits viruses such as dengue, chikungunya, and Zika. Its presence in new regions increases the risk of local disease outbreaks, as seen with dengue cases in California linked to travel-related infections.
Direct physical harm to humans is a concern. The red imported fire ant (Solenopsis invicta) inflicts painful stings that cause localized itching, swelling, and pustule formation. In some individuals, these stings can lead to severe allergic reactions, including life-threatening anaphylaxis. Approximately one-third of people in infested areas are stung each year, with 0.5% to 2% experiencing systemic allergic reactions.
Beyond health, invasive species degrade social amenities and recreational opportunities. Infestations of aquatic plants can choke waterways, preventing activities like swimming, boating, and fishing. Invasive plants can also limit access for hikers and other outdoor enthusiasts and diminish the aesthetic appeal of natural landscapes.
Alteration of Evolutionary Processes
Invasive species can alter the long-term evolutionary trajectories of native organisms, a more subtle but significant impact than immediate ecological disruption. One alteration is hybridization, where invasive species interbreed with closely related native species. This interbreeding can introduce foreign genes into the native gene pool, potentially diluting unique genetic traits or leading to the formation of hybrid offspring that may outcompete the pure native species. This “genetic pollution” can lead to the extinction of the original native species where the native population is overwhelmed by hybrids.
The presence of invasive species can also alter natural selection pressures on native organisms. Native prey may not recognize a newly introduced predator, lacking evolved behavioral or physical defenses to avoid it. This lack of recognition can lead to increased predation rates on native species, favoring individuals that rapidly adapt new avoidance behaviors or exhibit traits that offer protection. Similarly, invasive plants can change habitat structure, forcing native animals to alter their foraging or anti-predator behaviors, which can have cascading effects on food web dynamics and species coexistence.