Non-native species are organisms introduced to an ecosystem where they did not naturally evolve, often due to human activity. An invasive species is a subset of these non-native organisms whose establishment causes or is likely to cause environmental, economic, or human health harm. These biological invaders are now considered one of the top drivers of global biodiversity loss, rapidly disrupting the delicate balance of established ecosystems worldwide. The financial impact alone is immense, posing a significant global burden.
Direct Biological Threats to Native Species
Invasive species pose a serious threat to native wildlife by directly altering species-to-species interactions within the food web. A primary mechanism of decline is competition for limited resources such as food, light, and space, which native species are often ill-equipped to handle. For example, the zebra mussel in the Great Lakes filters massive quantities of plankton, depleting the food supply for native mollusks and fish larvae. The invasive lionfish in the Caribbean also outcompetes native fish for prey, significantly reducing juvenile fish populations on coral reefs.
Direct predation by non-native species can be devastating, especially in isolated island ecosystems where native fauna have not evolved defenses. The brown tree snake, introduced to Guam, is responsible for the extirpation of nine species of native birds because local species lacked defense mechanisms. In the Florida Everglades, the Burmese python preys on native mammals and birds, causing significant population declines.
Non-native species can also destroy the genetic distinctiveness of native populations through hybridization. This occurs when an invader is closely related enough to interbreed, diluting the native gene pool. For instance, smooth cordgrass in San Francisco Bay hybridized with the native California cordgrass, threatening the indigenous plant through genetic swamping. The resulting hybrids often display enhanced vigor, allowing them to rapidly outcompete the original native species.
Alteration of Ecosystem Structure and Function
Beyond direct biological interactions, invasive species fundamentally change the physical environment, creating conditions unsuitable for native life. This is often seen in the alteration of natural disturbance regimes, such as fire frequency and intensity. Cheatgrass in the arid grasslands of the Western United States dries out earlier than native grasses, creating a continuous fuel bed that increases wildfire frequency. These hotter fires destroy native plants that are not fire-adapted, allowing the invasive grass to further dominate the landscape.
Invasive plants can also disrupt the chemical cycles that govern the health of an ecosystem. Some species fix atmospheric nitrogen into the soil, drastically increasing nutrient levels beyond the natural range. This change in soil chemistry favors the growth of other invasive species while making the habitat less habitable for native plants that thrive in nutrient-poor conditions. Plants like the saltcedar actively deposit large amounts of salt into the surrounding soil, making the ground inhospitable to salt-intolerant native vegetation.
The balance of water resources, or hydrology, is often upset by non-native invaders. Certain invasive plants consume excessive amounts of water compared to native species, which can lower water tables and reduce stream flows in arid regions. This leads to significant changes in water quality and availability for native aquatic organisms and terrestrial wildlife. Changes to the physical structure of an ecosystem, such as erosion caused by feral pigs rooting up soil, further destabilize the environment and make it vulnerable to further invasion.
Economic Damage and Resource Loss
The financial toll exacted by non-native species represents a profound burden on governments, industries, and private landowners. In the United States alone, the annual cost of managing and recovering from damage caused by invasive species is estimated to exceed $21 billion. This cost includes damage to resources, losses in productivity, and the expenses associated with control and eradication programs.
Agriculture and forestry are particularly vulnerable sectors, suffering billions in losses annually due to reduced crop yields and damaged timber. Introduced weeds cost the U.S. agricultural economy significantly each year through crop destruction and the expense of herbicide application. The grazing capacity of rangelands is also decreased by invasive plants like leafy spurge, which reduces the available forage for livestock.
Infrastructure faces direct damage from certain aquatic invaders, requiring expensive preventative and repair efforts. Zebra mussels, for example, clog the intake pipes for power generation plants and municipal water treatment facilities, necessitating continuous and costly removal. The impact also extends to recreational and tourism industries, as invasive species can degrade fishing grounds, reduce property values, and damage watercraft.
Risks to Human and Animal Health
Non-native species pose direct and indirect risks to the health of human and domestic animal populations. Many invasive organisms act as vectors, introducing or carrying pathogens that cause serious disease. The Asian tiger mosquito, which is highly invasive globally, is a known carrier for viruses such as West Nile Virus and Dengue fever, increasing the risk of transmission to humans.
Other invaders cause direct physical harm through aggressive behavior or toxins. Africanized honey bees lead to larger, more forceful attacks on humans and domestic animals than native species. The red imported fire ant also delivers a painful sting that can cause significant allergic reactions and injury.
Indirectly, some invasive plants facilitate the spread of disease by altering the environment to favor vectors. Water hyacinth, an aquatic plant, creates ideal habitats for snails that carry the parasite responsible for schistosomiasis, a disease affecting millions globally. Non-native animals like feral pigs can also spread diseases such as brucellosis to livestock and contaminate water sources with harmful bacteria like E. coli.