Invasive species facts for kids
An invasive species is a type of plant, animal, or other living thing that moves to a new place and causes harm. These species are not native to their new environment. They can damage habitats and natural areas, leading to problems for the environment, other living things, and even the economy. Sometimes, even native species can become harmful if human actions change their food web. Since the 1900s, invasive species have become a big problem around the world.
It's natural for living things to spread to new places. However, humans have greatly sped up this process. For thousands of years, people have accidentally or purposefully moved species. This started with early human travels and increased with global trade. Famous invasive plants include the kudzu vine and Japanese knotweed. Well-known invasive animals are European rabbits, domestic cats, and carp.
Contents
What Are Invasive Species?
An invasive species is a non-native species that has settled in a new area and threatens local species and biodiversity. The term "invasive" can sometimes be confusing. Invasive species can be plants, animals, fungi, or tiny microbes. Some people even include native species that spread into human areas like farms.
Scientists who study invasive species often work in ecology (how living things interact with their environment) and biogeography (where living things are found). A key book on this topic is The Ecology of Invasion by Animals and Plants by Charles Sutherland Elton from 1958. This book helped create a general idea of how biological invasions happen.
The United States Department of Agriculture (USDA) defines an invasive species as a non-native species that is likely to cause harm. This harm can be to the economy, the environment, or human health.
Why Do Species Become Invasive?
For an introduced species to become invasive, it usually needs to survive and reproduce in its new home, even when there are only a few of them. It can be hard for a new species to get started. But if humans keep moving the species to the new place, like ships repeatedly sailing to a port, it gives the species many chances to settle.
How Ecosystems Change
In natural ecosystems, there's a balance of resources. Invasive species can upset this balance. For example, after a forest fire, native plants usually grow back. But an introduced species that spreads quickly can take over. It might use up food and nutrients like nitrogen and phosphorus faster than native plants.
Every species has a special role, or ecological niche, in its native ecosystem. Some species have many roles, while others are very specialized. Invading species might fill empty roles or create new ones. For instance, when land is cleared for farming, the edge between the forest and the cleared land creates a new habitat. This new habitat can help some species thrive that wouldn't normally do well in the forest.
Some scientists once thought that ecosystems with many different species were harder for invaders to take over. They believed there were fewer empty roles. However, other scientists found that some very diverse ecosystems were still heavily invaded. This debate often depends on how big the study area is. Small studies might show that diversity helps prevent invasion, while large studies might show the opposite.
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Island ecosystems are often more likely to be invaded. This is because island species often have fewer strong competitors or predators. Also, islands are far from other populations, so they might have more "open" roles for new species. For example, the native birds on Guam were greatly harmed by the invasive brown tree snake.
In New Zealand, the first invasive species were dogs and rats brought by early settlers. These and other introduced species caused huge damage to New Zealand's unique native species. The same happened in Madagascar. Cutting down trees not only destroys habitats but also lets non-native plants like prickly pear and silver wattle invade. The water hyacinth forms thick mats on water, blocking sunlight and harming aquatic life. The shrub lantana (Lantana camara) is now invasive in over 60 countries.
Invasive plants can change the land itself. For example, kudzu (Pueraria montana), a vine from Asia, was brought to the southeastern United States to stop soil erosion. Invasive animals can also change the land. The Chinese mitten crab (Eriocheir sinensis) can dig and disturb the soil more.
Sometimes, a native species can become harmful if human actions change its food web. For example, the purple sea urchin (Strongylocentrotus purpuratus) has destroyed kelp forests in California. This happened because its natural predator, the California sea otter (Enhydra lutris), was overhunted.
How Species Traits Help Them Invade
Invasive species often have special traits that help them outcompete native species. These traits can include fast growth and quick reproduction, like how some plants spread using their roots. They might also be good at living near humans or have successfully invaded other places before. Domestic cats, for example, are very good hunters. They have become wild and invasive in places like the Florida Keys.
An introduced species might become invasive if it can take resources away from native species. If these invaders evolved where there was a lot of competition or predation, their new environment might have fewer strong competitors. This allows the invader to spread quickly.
An invasive species might also use resources that native species can't reach. For example, some plants have long roots that can get deep water. Others can live in soil types that native plants avoid. Barbed goatgrass, for instance, can grow thickly on serpentine soils in California, crowding out native species.
Invasive species can change their environment. They might release chemicals that stop other plants from growing. Or they might change how herbivores (plant-eaters) behave. Some plants, like Kalanchoe daigremontana, produce chemicals that stop competitors. Others, like Stapelia gigantea, can help other seedlings grow in dry areas by providing shade and protection.
Changes in fire regimes are another way invaders can take over. Bromus tectorum, a grass from Eurasia, is very good at surviving fires. It spreads quickly after a fire and makes fires happen more often and burn hotter. This changes the local fire pattern so much that native plants can't survive, allowing the grass to become dominant.
Sometimes, one species helps another. For example, zebra mussels create more complex habitats on lake bottoms. This provides hiding places for other small creatures. This, along with the food from the mussels' waste, increases the number and types of bottom-dwelling invertebrates.
Introduced species can spread very quickly and in unexpected ways. When a small group of individuals starts a new population, it can lead to rapid evolution. This means the species can quickly adapt to its new environment, becoming even better at spreading and surviving new challenges like different temperatures or predators.
The enemy release hypothesis suggests that in every ecosystem, there's a balance. No single species can take over because of competitors, predators, and diseases. When an introduced species moves to a new place, it might become invasive if these natural controls are missing.
How Invasive Species Spread
Non-native species spread in many ways, but most are linked to human activity. While species naturally spread over time, humans often carry them much faster and farther. An early example is when early humans brought the Pacific rat (Rattus exulans) to Polynesia.
Ways species are moved include:
- Plants or seeds imported for horticulture (gardening).
- The pet trade moving animals across borders, where they can escape.
- Organisms hiding on transport vehicles like ships or planes.
- Diseases carried by invasive insects, like the Asian citrus psyllid carrying citrus greening.
Sometimes, once an invasive species takes over an area, it becomes an important part of that ecosystem. Removing it could cause more harm. Money also plays a role in introducing species. For example, the high demand for the valuable Chinese mitten crab might explain why it was intentionally released in foreign waters.
In Water Environments
Shipping has greatly changed how marine organisms travel. New ways include organisms sticking to ship hulls and being carried in ballast water. In fact, shipping is the main way marine invasive species are moved around the world.
Many marine organisms can attach to ship hulls. These organisms are easily carried from one body of water to another, which is a big risk for biological invasions. There are currently no strict rules for controlling hull fouling, but some places like California and New Zealand are planning tougher controls.
Another way aquatic species spread is through ballast water. Ships take in ballast water in one port and release it in another. About 10,000 species are carried this way every day. Many are harmful. For example, freshwater zebra mussels from Eurasia likely reached the Great Lakes through ballast water. These mussels compete with native organisms for food and oxygen. Rules try to reduce these risks, but they don't always work perfectly.
Climate change is making ocean temperatures rise. This will cause species to move to new areas, which could harm the environment as new species interact. For example, organisms in a ship's ballast tank traveling from a cool zone to tropical waters might experience big temperature changes. Surviving these changes during travel can make the species tougher, helping them survive in their new non-native home.
Effects of Wildfire and Firefighting
Invasive species often take advantage of disturbances like wildfires, roads, or foot trails to move into an area. Large wildfires can make soils sterile but also add nutrients. Invasive plants that can regrow from their roots have an advantage over native plants that rely on seeds.
Negative Impacts of Invasive Species
Invasive species can harm the places they invade, causing damage to the environment, ecology, or economy.
Ecological Impacts
The European Union says "Invasive Alien Species" are those outside their natural area that threaten biological diversity. Biological invasion is one of the top five reasons for global biodiversity loss. This problem is growing because of tourism and globalization.
Invasive species can cause native species to disappear. They might outcompete them for food, take over their niche, or even hybridize (breed) with related native species. This can lead to big changes in the types of plants and animals in an area, making the world's living things more similar and causing a loss of biodiversity. It's hard to say for sure that an invasion caused an extinction, but there's strong evidence that a fungus spread by international trade caused about 90 amphibian species to die out.
Many different invasive species arriving one after another can make the total effect worse. For example, in California's Bodega Harbor, the amethyst gem clam was introduced a century ago. It didn't displace native clams on its own. But in the 1990s, the introduction of the European green crab led to the amethyst gem clam increasing and the native clams decreasing. In India, many invasive plants have taken over natural areas, reducing food for wild plant-eaters and threatening animals like the tiger.
Invasive species can change how ecosystems work. For example, invasive plants can change fire regimes (like cheatgrass), nutrient cycling (like smooth cordgrass), and water flow (like Tamarix) in native ecosystems. Invasive species that are closely related to rare native species can breed with them. This can harm or even cause the extinction of native species. For instance, breeding with introduced cordgrass threatens California cordgrass in San Francisco Bay. Invasive species compete with native species, putting 400 of the 958 endangered species in the U.S. at risk.
Accidental introductions of forest pests and plant diseases can change forest ecology and harm the timber industry. Forests in the U.S. are widely invaded by foreign pests, plants, and diseases.
The Asian long-horned beetle was first found in the U.S. in 1996. It was expected to damage millions of acres of hardwood trees. By 2005, millions of dollars had been spent trying to get rid of this pest. The woolly adelgid harms old-growth spruce, fir, and hemlock forests and the Christmas tree industry. Chestnut blight and Dutch elm disease are plant diseases with serious impacts. Garlic mustard is a big problem in eastern North American forests. It grows thickly in the understory, slowing the growth of tree seedlings and threatening to change the forest's tree types.
Native species can be threatened with extinction through "genetic pollution." This happens when invasive species accidentally breed with native species. This can lead to the loss or mixing of local genotypes (genetic makeup). This can happen if the introduced species is more numerous or fitter. Invading species have been shown to adapt to their new environments very quickly. An invading population might stay small for years and then suddenly explode in numbers. This is called "the lag effect."
Hybrids (offspring of two different species) from invasive and native species can mix their genes into the native gene pool over time. Even a small invading population can threaten much larger native populations. For example, Spartina alterniflora was introduced in San Francisco Bay and bred with native Spartina foliosa. The invading species had more pollen and better male fitness, which threatened the native populations.
Environmental Impacts
In South Africa's Cape Town area, removing thirsty invasive plants like Australian acacias and pines would save 50 billion liters of water each year within five years. This is like one-sixth of the city's current water needs. These water savings would double in 30 years. Removing these plants is much cheaper than other ways to get more water.
Human Health Impacts
Invasive species can affect human health. They can reduce available resources, spread human diseases, and impact recreational activities and tourism. Alien species have been linked to diseases like HIV, monkey pox, and severe acute respiratory syndrome (SARS).
Invasive species and efforts to control them can have long-term health effects. For example, pesticides used to treat a pest might pollute soil and water. Humans moving into previously remote ecosystems have been exposed to new diseases like HIV. Introduced birds (like pigeons), rodents, and insects (like mosquitoes and fleas) can carry and spread human diseases. Historically, epidemics like malaria and yellow fever spread this way. A recent example is the spread of West Nile virus, which killed humans, birds, and other animals. The introduced Chinese mitten crabs can carry Asian lung fluke. Waterborne disease agents, like cholera bacteria, are often carried in ballast water.
Economic Impacts
Globally, managing and controlling invasive species costs a lot of money. It's estimated to be around $1.4 trillion each year. The economic impact of invasive alien species alone was over $423 billion annually in 2019. This cost has grown quickly, quadrupling every decade since 1970.
Invasive species cause environmental damage, changing how ecosystems work and reducing the services they provide. This means more money has to be spent to control their spread, reduce further harm, and restore affected ecosystems. For example, the damage from 79 invasive species in the U.S. between 1906 and 1991 was estimated at $120 billion. In China, invasive species are reported to reduce the country's economic output by 1.36% each year.
Managing biological invasions can be very expensive. In Australia, for example, monitoring, controlling, managing, and researching invasive weeds costs about AU$116.4 million per year for government agencies alone.
While invasive species can sometimes offer economic benefits, like timber from invasive trees, these benefits are usually much smaller than the huge costs they create.
United States
In the Great Lakes region, the sea lamprey is an invasive species. In its original home, it was a parasite that didn't kill its host. But in the Great Lakes, it acts as a predator, eating up to 40 pounds of fish in 12–18 months. Sea lampreys prey on large fish like lake trout and salmon. Their destructive effects on large fish harm the fishing industry and have caused some fish populations to collapse.
Economic costs from invasive species include direct costs from lost production in farming and forestry, and management costs. Estimated damage and control costs in the U.S. are more than $138 billion annually. Money is also lost from recreation and tourism. If we put a price on the extinction of species, loss of biodiversity, and loss of ecosystem services, the costs would be much higher. It's often said that finding and responding to invasive species early is key. However, this only works if the species isn't reintroduced often and the cost of response is affordable.
Weeds reduce crop yields in agriculture. Many weeds are accidentally brought in with commercial seeds and plants. Introduced weeds in pastures compete with native forage plants, threaten young cattle, or are unpleasant to eat because of thorns (like yellow starthistle). Lost forage from invasive weeds on pastures in the U.S. costs nearly $1 billion. A decline in pollinator services and fruit production has been caused by honey bees infected by the invasive varroa mite. Introduced rats have become serious pests on farms, destroying stored grains. The introduction of leaf miner flies to California has caused losses in the floriculture (flower growing) industry, as their larvae feed on ornamental plants.
Invasive plant diseases and insects that carry them can reduce farm yields and harm nursery plants. Citrus greening is a bacterial disease carried by the invasive Asian citrus psyllid. Because of this, citrus is under strict rules in areas where the psyllid is found.
Invasive species can impact outdoor activities like fishing, hunting, hiking, wildlife viewing, and water sports. They can damage environmental services like water quality, plant and animal diversity, and species abundance. Eurasian watermilfoil in parts of the U.S. fills lakes with plants, making fishing and boating difficult. The loud call of the introduced common coqui lowers real estate values in affected neighborhoods of Hawaii. The large webs of the orb-weaving spider Zygiella x-notata, invasive in California, disrupt garden work.
Europe
The total economic cost of invasive alien species in Europe between 1960 and 2020 is estimated to be around $140 billion. This estimate is very careful, meaning the actual cost might be much higher. Models suggest the true annual cost was around $140 billion in 2020.
Italy is one of the most invaded countries in Europe, with over 3,000 alien species. The economic impacts have been wide-ranging, from management costs to crop losses and damage to buildings. The total economic cost of invasions in Italy between 1990 and 2020 was estimated at $819.76 million. However, only 15 species have reliable cost estimates, so the real cost could be much larger.
France has at least 2,750 introduced and invasive alien species. A study found that they caused a total cost between $1.2 billion and $11.5 billion from 1993–2018. Damage costs were nearly eight times higher than management costs. Insects, especially the Asian tiger mosquito, caused very high economic costs. Over 90% of alien species in France had no costs reported in studies, but this doesn't mean they had no negative effects.
Positive Impacts of Invasive Species
Some scientists, like entomologist Chris D. Thomas, argue that most introduced species are neutral or even helpful to other species. However, most scientists agree that their effects on biodiversity are negative. Others point out that some conservation efforts can have extreme ideas. Some people believe that trying to stop nature from changing is wrong, as change is natural. Indigenous communities have often adopted introduced species, like the banana tree in the Americas, into their traditional diets.
Some invasive species can provide suitable homes or food for other organisms. In areas where a native species has died out or can't be brought back, non-native species can fill their role. For example, in the U.S., the endangered southwestern willow flycatcher mainly nests in the non-native tamarisk trees. The introduced mesquite is an aggressive invasive species in India, but it's the preferred nesting site for native waterbirds in some cities. Similarly, Ridgway's rail has adapted to an invasive hybrid cordgrass, which offers better cover and nesting habitat. In Australia, saltwater crocodiles, which were endangered, have recovered by eating introduced feral pigs.
Non-native species can also help restore an ecosystem. They can increase variety and biodiversity, creating small microclimates in sparse or eroded areas. This can help native species grow and reestablish. For example, in Kenya, guava trees in farmland attract many fruit-eating birds. These birds drop seeds from rainforest trees up to 2 kilometers away under the guavas, helping the forest regrow.
Non-native species can provide ecosystem services. For example, Asian oysters filter water pollutants better than native oysters in Chesapeake Bay. Some species have been in an area for so long that they are considered to have naturalized there. For instance, the bee Lasioglossum leucozonium, an invasive species in North America, has become an important pollinator for caneberry, cucurbit, apple trees, and blueberry bushes. In the U.S., the endangered Taylor's checkerspot butterfly now relies on invasive ribwort plantain as food for its caterpillars.
Some invasions offer possible commercial benefits. For example, silver carp and common carp can be harvested for human food or processed into pet foods. Water hyacinth can be turned into fuel using methane digesters, and other invasive plants can be used as a source of bioenergy.
Controlling and Studying Invasive Species
Humans can find ways to fix the problems caused by species invasions. People are often motivated to help with invasive species that affect their local area. Controlling alien species is important for protecting biodiversity in natural ecosystems. One promising method for control is using genetics.
Inspections and Quarantine
The main goal of inspections was to protect against agricultural pests while still allowing farm products to be exported. In 1994, the first global standards were agreed upon. These are overseen by the World Trade Organization. The International Maritime Organization oversees rules for ships' ballast water. The Convention on Biological Diversity also includes steps for its members to control invasive species.
Slowing the Spread
Firefighters are now responsible for cleaning their equipment and public water equipment. This is because of the risk of spreading aquatic invasive species. In the United States, this is a concern for wildland firefighters because quagga and zebra mussels and wildfires often happen in the same areas.
Reestablishing Native Species
Island restoration involves getting rid of invasive species. A 2019 study suggested that if invasive animals were removed from just 169 islands, it would greatly improve the survival chances of 9.4% of the Earth's most threatened island animals.
Removing invasive animals from islands fits with the United Nations Sustainable Development Goal 15.
Rodents were brought to South Georgia Island, an island in the southern Atlantic Ocean, in the 1700s by ships. They caused huge damage to the island's bird population, eating eggs and attacking chicks. In 2018, South Georgia Island was declared free of invasive rodents after a multi-year effort. Bird populations have bounced back, including the South Georgia pipit and South Georgia pintail, which are found only on that island.
Taxon Substitution
Non-native species can be introduced to fill an ecological role that a native species used to perform but is now extinct. This is called taxon substitution. On many islands, the extinction of tortoises led to problems with seed dispersal and plant eating. On the islets off Mauritius, extinct tortoises were key plant-eaters. Introducing non-native Aldabra giant tortoises in 2000 and 2007 has started to restore the ecological balance. The introduced tortoises are spreading seeds of native plants and eating invasive plants. They are also breeding.
Eating Invasive Species
The idea of eating invasive species to reduce their populations has been explored. In 2005, Chef Bun Lai of Miya's Sushi in New Haven, Connecticut created the first menu dedicated to invasive species. By 2013, Miya's offered invasive aquatic species like Chesapeake blue catfish and Florida lionfish. They also used invasive plants like Japanese knotweed. Joe Roman, a conservation biologist, runs a website called "Eat The Invaders." Organizations have published cookbooks using invasive species as ingredients. Invasive plants have also been looked at as a source of healthy chemicals and edible protein.
People who support eating invasive organisms say that humans can eat away any species they have an appetite for. They point to animals that humans have hunted to extinction, like the Caribbean monk seal. They also point to how Jamaica greatly reduced the lionfish population by encouraging people to eat them. Skeptics say that once a foreign species is firmly established, like the Indo-Pacific lionfish in the Western Atlantic, getting rid of it is almost impossible. Critics also worry that encouraging consumption might accidentally spread harmful species even more.
Pesticides and Herbicides
Pesticides are often used to control invasive species. Herbicides used against invasive plants include fungal herbicides. Even if an introduced population starts small, some genetic variation can give invasive plants resistance against these fungal bioherbicides. Invasive populations of Bromus tectorum have shown resistance to a biocontrol fungus.
Predicting Invasive Plants
Accurately predicting the impacts of non-native plants can be a very effective management option. This is because most non-native plant species are introduced on purpose. "Weed risk assessments" try to predict if a specific plant will have negative effects in a new environment. They often use a standard questionnaire. The score from this assessment helps decide if the plant should be prevented from being introduced. These assessments use information about the plant's biology, life cycle, native areas, and relationships to other species. The effectiveness of this approach is still debated.
Returning Invasive Species to Their Origin
In 2025, a project started in the Hebrides in Scotland to return an invasive species to its home country for the first time. Hedgehogs "native to the UK mainland" were brought to these islands decades ago to fight garden pests. But they caused serious harm to ground-nesting birds. The authorities decided to move them back to the mainland, helping the birds without harming the hedgehogs.
See Also
- Adventive plant
- Archaeophyte
- Climate change and invasive species
- Ecologically based invasive plant management
- Escaped plant
- Hemerochory
- Invasion genetics
- Lists of invasive species
- Naturalisation (biology)
- Neophyte (botany)
- Seed dispersal
- 100 of the World's Worst Invasive Alien Species
Images for kids
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Chinese mitten crab
