An invasive species is a type of organism that is brought to a new place and causes harm to that environment. These species can damage habitats and areas, leading to problems for nature, the environment, and the economy. Since the 20th century, invasive species have become major issues for the economy, people, and the environment around the world.
While the spread of organisms into established ecosystems is a natural process, human actions have greatly increased how fast, how much, and how far invasive species spread. For thousands of years, humans have helped spread organisms, starting with early movements, increasing during the Age of Discovery, and increasing again with international trade.
Examples of invasive plant species found in many parts of the world include the kudzu vine (Pueraria), water hyacinth (Pontederia crassipes), common ragweed (Ambrosia artemisiifolia), and yellow starthistle (Centaurea solstitialis). Examples of invasive animal species include European rabbits (Oryctolagus cuniculus), domestic cats (Felis catus), and red swamp crayfish (Procambarus clarkii).
Terminology
Invasive species are a group of non-native species that have settled in a new area and may harm native species or reduce biodiversity. The word "invasive" is not clearly defined and can depend on people's opinions. Invasive species can include plants, animals, fungi, and microbes. Some people also consider native species that move into human areas, like farms or gardens, as invasive. Others may include native species that spread into natural areas. A few sources even suggest that humans, like Homo sapiens, could be called invasive, but this idea is debated because humans have the ability to learn and adapt in many ways.
The term "native" can also be unclear. For example, the ancestors of today's horses (Equus ferus) first lived in North America and later spread to Eurasia. After becoming extinct in North America, horses were brought back by Spanish explorers in the 1500s. This led to a debate in 1943 about whether these horses were native to North America, where their ancestors originally lived.
Studying invasive species involves many areas of science, but most research has focused on ecology and biogeography. Much of the early work was influenced by a 1958 book by Charles Elton called The Ecology of Invasion by Animals and Plants, which described general patterns of biological invasions. Research on this topic was limited until the 1990s, when more studies were done, mostly on land plants. As the field grew, scientists realized they needed clearer terms to describe invasive species and events. However, no official system of terms exists today. The field is often called "invasion ecology" or "invasion biology," but it lacks standard language because it draws ideas from many different areas, such as agriculture, zoology, and disease study.
To address unclear and negative language often used in discussions about invasive species, scientists Colautti and MacIsaac suggested a new naming system. Their approach focuses on where species live (biogeography) instead of their type (taxa). This system emphasizes how species interact with their environment rather than their classification, human health, or economic effects. It looks at individual populations, not whole species, and judges their success in a specific area. This model applies to both native and non-native species and does not automatically label successful introductions as harmful.
According to the USDA's National Invasive Species Information Center, the term "invasive species" is defined very specifically. As stated in Executive Order 13112, "Invasive species" refers to non-native species that cause or may cause harm to the economy, environment, or human health.
Causes
When a new species arrives in an area, it usually starts with a small population. At this stage, it may struggle to reproduce and grow enough to survive. Sometimes, the species needs to be introduced multiple times before it can establish itself. Human activities, like ships moving between ports or vehicles traveling on highways, can create repeated chances for the species to reach and settle in the new area. This repeated arrival is called high propagule pressure.
In ecosystems, the amount of available resources affects how much impact a new species can have. Ecosystems that are balanced have a steady amount of resources, but the arrival of an invasive species can change this balance. For example, after a forest fire, native grasses and plants often grow back first. If an introduced species spreads faster than these native plants, it can take over by using the same resources, pushing the native plants out. Nitrogen and phosphorus are often the main resources that limit growth in these situations. Every species has a role in its ecosystem, called an ecological niche. Some species fill many roles, while others are very specific. Invasive species might take over unused niches or create new ones. For example, when part of an ecosystem is disturbed, like when land is cleared for farming, the area between the disturbed land and the untouched land becomes a new habitat. This can lead to some species thriving while others decline.
In 1958, a scientist named Charles S. Elton suggested that ecosystems with more species are less likely to be invaded because there are fewer empty niches. However, other scientists later found examples of highly diverse ecosystems that were heavily invaded, arguing that diversity might not always prevent invasions. This debate depended on the size of the study areas. Smaller studies often showed that more diverse ecosystems were less likely to be invaded, while larger studies sometimes showed the opposite. This might be because invasive species can take advantage of more resources and weaker interactions between species in larger areas. However, this pattern does not seem to apply to invasive animals.
Island ecosystems may be more likely to be invaded because their native species often have few strong competitors or predators. Also, their distance from other populations makes it easier for invasive species to find open niches. For example, native birds in Guam have been greatly reduced by the invasive brown tree snake. However, the distance of islands and the open water around them can also slow the arrival of invasive species. For example, countries like Australia and the Galapagos Islands have many species that are found nowhere else because it is hard for other species to reach them.
In New Zealand, the first invasive species were dogs and rats brought by early settlers around 1300. These and other introductions caused harm to many native species. Similar problems happened in Madagascar. Logging harms ecosystems directly by destroying habitats and allows non-native plants, like prickly pear and silver wattle, to grow. The water hyacinth forms thick mats on water, blocking sunlight and harming aquatic life, which also increases the cost of managing the problem. The lantana plant is now invasive in over 60 countries and has led to large efforts to control it.
In addition to islands, areas that are heavily managed with fences are also more likely to be invaded. One reason is that species can enter through ways fences cannot stop. For example, many plants spread through wind, water, or birds that carry seeds. Small animals and insects can also get through fences. Another reason is that intensive management, like clearing plants or disturbing soil, reduces competition from native plants and makes it easier for invasive species to grow. Fences are often built to keep large animals out, but invasive plants face no threats in these areas and can grow freely. In general, invasive species have traits that help them survive, such as being very hardy. These traits, along with the disturbed environment of fenced areas, create conditions that allow invasive species to thrive.
Invasive plants can change the land through processes like building up soil or protecting it. For example, kudzu, a plant from Asia, was introduced to the southeastern United States to control soil erosion. Invasive animals can change the land through activities like digging, wearing down rock, or building structures. For example, the Chinese mitten crab has increased the amount of digging and erosion in its new environment.
A native species can become invasive if human changes to the food web disrupt its natural balance. For example, the purple sea urchin has grown too much in northern California because its natural predator, the sea otter, was overhunted.
Invasive species often have traits that help them outcompete native species. These traits might include fast growth and reproduction, or the ability to interact with native species in ways that give them an advantage. Studies have shown that many invasive species can be identified based on these traits alone. Some invasive species have only a few of these traits, and other non-invasive species may have them too. Common traits include fast growth, association with humans, and a history of successful invasions. For example, domestic cats have become invasive in places like the Florida Keys because they hunt wildlife effectively.
An introduced species might become invasive if it can outcompete native species for resources. If the species evolved in an environment with strong competition or predators, the new environment might have fewer competitors, allowing the invader to grow. Ecosystems that are fully used by native species can be thought of as systems where one species' gain is another's loss. However, this is not always the case.
An invasive species might use resources that native species cannot access, such as deep water from a long root or certain types of soil. For example, barbed goatgrass was introduced to California on serpentine soils, which have poor water retention, low nutrients, and high levels of magnesium and calcium.
Vectors
Non-native species often move through many ways, but most are linked to human actions. While natural movement can happen, humans usually move these species faster and farther than natural forces. One of the earliest human-related movements happened when early humans brought the Pacific rat (Rattus exulans) to Polynesia.
Humans have moved species both on purpose and by accident. During the colonial period, groups tried to introduce animals and plants that Europeans were familiar with, believing it would improve nature by adding species they found valuable or nostalgic. Another reason for intentional movement was to use certain species to control pests. For example, the cane toad was brought to Australia to help control cane beetles, which harm sugar cane crops. Although only 100 toads were first brought, there are now over 200 million in Australia. Another example is the Indian mongoose, which was introduced to Hawaii to control rats that damaged sugar cane. However, this did not work well because the mongoose is active during the day, while the rats are active at night. This introduction caused new problems for Hawaii’s ecosystems.
Plants and seeds brought for gardening or as pets can also move species across borders. These animals might escape and become invasive. Organisms can also travel on vehicles without being noticed. Accidental human help is the main way species are introduced, except in polar regions. Some invasive insects spread diseases, like the Asian citrus psyllid (Diaphorina citri), which carries a disease called citrus greening. The arrival of invasive species in a new area depends on how easy it is for them to take over there.
Once invasive species become strong in an area, they may become important to the local ecosystem, and removing them could harm the environment. Economics often plays a big role in introducing non-native species. For example, the high value of the Chinese mitten crab explains why it might have been intentionally released in other areas.
Marine trade has changed how ocean species move. New ways include organisms attaching to ship hulls or being carried in ballast water. Molnar et al. (2008) studied hundreds of marine invasive species and found that shipping is the main way these species spread.
Many marine organisms can stick to the outside of ship hulls. These organisms can travel between different bodies of water and increase the risk of biological invasions. Currently, there are no rules to control hull fouling, but California and New Zealand have stricter rules in their areas.
Another way non-native aquatic species move is through ballast water, which ships take in at sea and later release in ports. About 10,000 species are moved daily through ballast water. Many are harmful. For example, zebra mussels from Eurasia likely reached the Great Lakes through ballast water. These mussels compete with native species for food and oxygen, and they can survive in small puddles left in ballast tanks. Rules exist to reduce these risks, but they are not always effective.
Climate change is increasing ocean temperatures, which causes species to move to new areas. These changes can create new interactions between species. For example, species in a ship’s ballast tank might experience temperature changes of up to 20°C during a trip. These temperature changes can help species survive in new areas by making them more resilient to future heat stress.
Invasive species often take advantage of ecosystem changes, such as wildfires, roads, or trails, to settle in new areas. Wildfires can destroy soil but add nutrients, giving invasive plants an advantage. These plants can regrow from their roots, while native plants depend on seeds to reproduce.
Adverse effects
Invasive species can harm the places they move into, causing problems for plants, animals, and the environment. They may also hurt the economy. The European Union says "Invasive Alien Species" are those that are not from their usual home and threaten the variety of living things. Biotic invasion is one of the top reasons for losing biodiversity around the world, and this problem is growing because of more travel and trade. This is especially true in freshwater areas that are not well protected, although rules about quarantines and ship ballast water have helped some.
Invasive species can push local plants and animals to extinction by competing for food, space, or by mixing their genes with related species. In addition to economic problems, these species can change the makeup of life in the areas they invade, leading to fewer unique plants and animals worldwide. It is hard to prove that a species invasion directly caused a species to go extinct, but strong evidence shows that about 90 types of frogs died out because of a fungus spread through international trade.
When many non-native species are introduced one after another, the problems can get worse. For example, the amethyst gem clam and the European green crab were both introduced to California. At first, the clam did not harm native clams, but when the crab arrived, the clam outcompeted the native species. In India, invasive plants have taken over 66% of natural areas, reducing food for animals and threatening species like tigers.
Invasive species can change how ecosystems work. For instance, some plants can increase the risk of fires, change how nutrients move in soil, or affect water flow. If invasive species are closely related to native species, they may mix their genes, which can harm or even kill native species. For example, a type of grass that was introduced to California has mixed with a native grass, making it harder for the native species to survive. About 400 out of 958 endangered species in the U.S. are at risk because of invasive species.
Unintended introductions of pests and diseases can damage forests and the wood industry. In the U.S., many forests are affected by foreign pests, plants, and diseases. The Asian long-horned beetle, introduced in 1996, could have harmed millions of acres of trees. By 2005, $30 million had been spent to try to stop it. Another example is the woolly adelgid, which harms forests and the Christmas tree business. Diseases like chestnut blight and Dutch elm disease also cause serious damage. Garlic mustard is a plant that spreads quickly in forests, harming tree growth and changing the types of trees that grow there.
Native species can be at risk when invasive species mix their genes with them. This mixing, called genetic pollution, can replace local genes with those from invasive species. This happens when invasive species are introduced or when habitat changes bring species together. Invasive species can quickly adapt to new environments, and their populations may grow rapidly after a slow start, a process called the "lag effect."
Hybrid offspring from invasive and native species can spread their genes over time, threatening native populations. For example, an invasive grass in San Francisco Bay mixed with a native grass, making the native species weaker. Sometimes, a small group of invasive species can harm much larger native groups. In some areas, coyotes have mixed with red wolves, reducing the number of red wolves.
In Cape Town, South Africa, removing invasive plants like Australian acacia and eucalyptus from water sources could save 50 billion liters of water each year. This is about one-sixth of the city's current needs, and the amount would double in 30 years. This method is much cheaper than other ways to get more water. A fund has been created to help remove these invasive plants.
Invasive species can also affect human health. When ecosystems change, it can reduce the availability of resources, spread diseases, and harm activities like tourism and education. Some invasive species have caused diseases like HIV, monkey pox, and SARS.
Efforts to control invasive species can also affect public health. For example, pesticides used to kill pests might pollute water and soil. People moving into remote areas have been exposed to diseases like HIV. Invasive animals like pigeons, rodents, and insects can carry diseases such as malaria, yellow fever, and bubonic plague. A recent example is the West Nile virus, which has killed many animals and people. Invasive crabs carry a lung disease, and waterborne diseases like cholera can also spread through ecosystems.
Favorable effects
Most scientists agree that invasive species usually harm biodiversity, although some scientists believe a few invasive species might help other species. Some invasive species can provide homes or food for other living things. In places where native species have disappeared or cannot be brought back, non-native species might take their place even if they cause problems elsewhere. For example, in the United States, the endangered southwestern willow flycatcher nests mainly in the non-native tamarisk plant. In India, the introduced mesquite plant is an aggressive invasive species but is used by native waterbirds in cities like Udaipur. Similarly, Ridgway's rail has adapted to a hybrid of two invasive Spartina plants that offer better hiding places and nesting areas. In Australia, saltwater crocodiles, which were once endangered, have recovered by eating introduced feral pigs.
Non-native species can sometimes help ecosystems by controlling harmful pests. Some invasive species have been in an area for so long that they are now considered part of the local environment. In the United States, the endangered Taylor's checkerspot butterfly now depends on the invasive ribwort plantain as the food source for its young.
Some invasive species have benefits for humans. For example, silver carp and common carp can be caught for food and sold to places where people already know about them, or used in pet food and mink feed. Water hyacinth can be used to make fuel through special machines called methane digesters. Other invasive plants can also be used to create energy.
Control, eradication, and study
Humans can fix the problems caused by invasive species. People care more about invasive species that affect their local areas. Controlling invasive species is important for protecting the variety of plants and animals in natural ecosystems. Studies and programs that target invasive species work best early in the invasion, such as when the first small group of the species arrives. One of the most promising ways to control invasive species is using genetic methods.
The original goal of these efforts was to protect crops from pests while still allowing food to be sold in other countries. In 1994, the first global rules were created, including the Agreement on Sanitary and Phytosanitary Measures (SPS Agreement). These rules are managed by the World Trade Organization. The International Maritime Organization oversees the Ballast Water Management Convention, which focuses on controlling water from ships. The Convention on Biological Diversity also includes steps to manage invasive species, though many are not required. This agreement is the most important international rule about the effects of invasive species.
Firefighters are now responsible for cleaning their equipment and public and private water systems to stop the spread of invasive aquatic species. In the United States, this is especially important in the American West, where invasive mussels and wildfires often happen at the same time.
Island restoration involves removing invasive species from islands. A 2019 study found that removing invasive animals from 169 islands could help 9.4% of the most endangered land animals on islands survive better.
Removing invasive animals from islands supports the United Nations Sustainable Development Goal 15, which focuses on protecting ecosystems.
Rodents were brought to South Georgia, an island in the southern Atlantic Ocean with no permanent people, by ships used for sealing and whaling in the 18th century. These rodents harmed the island's birds by eating their eggs and attacking their young. In 2018, South Georgia was declared free of invasive rodents after a long effort. Bird populations, including the South Georgia pipit and South Georgia pintail, have grown again.
Non-native species can be introduced to take over roles once filled by extinct native species. This process is called taxon substitution. On many islands, the extinction of tortoises led to problems with seed spread and plant eating. On islands near Mauritius, tortoises once helped balance the ecosystem by eating plants. In 2000 and 2007, non-native Aldabra giant tortoises were introduced to two islands. These tortoises now help spread seeds of native plants and eat invasive plants. Their grazing may replace the need for manual weeding, and they are already reproducing.
Eating invasive species to reduce their numbers has been tested. In 2005, Chef Bun Lai created the first menu at Miya's Sushi in Connecticut that included invasive species. At first, many menu items were not available because the species were not sold commercially. By 2013, the restaurant offered invasive fish like blue catfish and lionfish, as well as invasive plants like Japanese knotweed. Joe Roman, a biologist, runs a website called "Eat The Invaders." In the 21st century, groups like Reef Environmental Educational Foundation and the Institute for Applied Ecology have published cookbooks using invasive species. Invasive plants have also been studied for their potential to provide useful chemicals and food.
Supporters of eating invasive species say humans can eat any species they want, pointing to animals like the Caribbean monk seal and the passenger pigeon that were hunted to extinction. They also note Jamaica's success in reducing lionfish numbers by encouraging people to eat them. Critics argue that once a species is well-established, like the Indo-Pacific lionfish in the Atlantic Ocean, it is nearly impossible to remove. They worry that promoting consumption might spread invasive species further.
Pesticides are often used to control invasive species. Fungal herbicides are used against invasive plants. Even though invasive species often have smaller genetic diversity, some plants have developed resistance to these fungal treatments. For example, cheatgrass and Japanese stiltgrass have shown resistance to certain fungi. This is not unique to invasive plants, as wild plants like wild flax also have resistance to fungi. Crops have a disadvantage compared to wild plants because they are bred to take in more nutrients to grow more food.
A gene drive could be used to eliminate invasive species. For example, it has been suggested as a way to remove invasive mammals in New Zealand. A gene drive works by making a desired gene more likely to be passed on to offspring. Scientists are studying gene drives for conservation, as they may be safer for other species and less expensive than traditional methods. However, some scientists are concerned that gene drives might harm native species or cause unexpected problems if the gene changes or mixes with other species.
Predicting the effects of non-native plants can help manage them, as most non-native plants are introduced intentionally. Weed risk assessments try to predict whether a specific plant is likely to become a problem.