Invasive species are plants or animals introduced to new environments where they cause environmental, economic, or health harm. Unlike exotic species that coexist peacefully, invasives thrive without natural predators, reproduce rapidly, and disrupt native ecosystems. They're the second-leading cause of species extinctions globally, costing an estimated $120 billion annually in the United States alone.
One consequence of globalization is that wildlife has traveled across the globe alongside people and products. This practice of transporting animals from their native regions to new areas dates back thousands of years. The Roman Empire frequently imported animals from foreign lands for entertainment in the Colosseum or for military purposes. European explorers continued this tradition, bringing novel specimens back from the New World to arouse interest in future expeditions and advance zoological knowledge.
In many cases, transplanted species don't survive in their new environment. A lack of proper food sources, combined with unsuitable climate conditions, leads to short lifespans. However, when a species thrives and successfully reproduces, it can disrupt its new habitat and become what ecologists call an invasive species.
What Makes a Species Invasive?
Introducing a species into a new environment may have various outcomes. An exotic or alien species is one that's been introduced to a new place but doesn't necessarily have negative consequences. For example, many fish species have been introduced into the Great Lakes for sport fishing. They have no documented negative impacts and provide recreational opportunities and a food source.
However, when these alien species begin to cause negative consequences in their new habitats, they become invasive. Invasive species may cause environmental harm, economic damage, or impact human health. Understanding what makes a species invasive rather than simply exotic is crucial for wildlife biologists and conservation specialists working to protect native ecosystems.
A key factor contributing to the invasiveness of many species is the absence of predators in their new environment. This is complex and results from thousands of years of evolution in different places. Predators and prey often coevolve through a phenomenon known as the coevolutionary arms race. This means that as prey evolve better defenses, predators evolve better ways to exploit them. The classic example comes from cheetahs and antelopes. Faster antelopes survive better because they can escape cheetahs more effectively. The fastest cheetahs then survive better because they can catch the faster antelope. Neither species ultimately gains an advantage because they continually evolve in response to one another.
However, when a plant or animal enters a new environment, they'll likely encounter predators who haven't been evolving alongside them. These predators can't successfully exploit the new prey. Defense mechanisms such as venom, size, or speed that have been matched by adaptations in their native predators are suddenly without match in the new environment. This allows the species to proliferate rapidly as it no longer faces effective predation. Many insects or fungi that are invasive in the United States come from regions where native trees have evolved resistance to their effects. When these species enter the US, they encounter trees with no resistance and can rapidly decimate forests.
Invasive species may also exploit resources that native species can't use, allowing them to establish strongholds in new environments. Introduced into the Western United States, barbed goatgrass thrives in serpentine soils where native plants don't normally grow. This has given them a solid foothold in the area. Combined with the fact that grazing animals don't like the taste, the grass has spread rapidly throughout California.
Some species alter the environment in ways that favor them but harm native species, a process known as ecological facilitation. Yellow starthistle, also introduced to the West Coast, secretes the chemical compound 8-hydroxyquinoline from its roots. This chemical harms native plants, thereby allowing starthistle to expand its range by eliminating native competitors.
Where Do Invasive Species Come From?
Invasive species may enter new environments through many routes. Understanding these pathways is essential for developing prevention strategies and helps guide the work of professionals in fish and wildlife management programs.
| Introduction Method | Examples | Prevention Approach |
|---|---|---|
| Intentional Introduction | Beach vitex (ornamental), mongoose (pest control) | Risk assessment before introduction, ecological studies |
| Ballast Water Discharge | Zebra mussels, sea lampreys (60+ species in Great Lakes) | Mandatory ballast water treatment, monitoring protocols |
| Cargo/Shipping | Asian tiger mosquito (tires), chestnut blight (trees), fire ants (soil) | Cargo inspection, quarantine procedures |
| Pet Release | Burmese pythons (Everglades), lionfish (Caribbean) | Import restrictions, public education, surrender programs |
Intentional Introduction Gone Wrong
Some species are transported to new locations and intentionally established, but with unforeseen consequences. Beach vitex was planted in coastal North Carolina in the 1980s as an ornamental plant for coastal homes. However, the plant began overtaking native species after it became established. The plant can contribute to dune instability by displacing deeper-rooted native species that stabilize coastal sands, although the causal relationship remains under investigation by coastal ecologists.
Some invasive species were introduced as unsuccessful attempts to control other invasive species. In the 1800s, rats that arrived on ships to the Caribbean islands infested sugar cane fields, causing substantial crop damage. Farmers introduced mongooses as a predator control for rats across many islands, including the Virgin Islands. However, rats are nocturnal and sleep in trees, whereas mongooses are diurnal and can't climb trees, so they weren't successful at eradicating the rats. As a result, these islands now have two invasive species to contend with.
Accidental Escapes and Pet Releases
Other species are intentionally transported to different locations but are released accidentally, as sometimes happens with animals in zoos and aquariums. Lionfish are thought to have been introduced to the Caribbean when several escaped from a broken beachfront aquarium during Hurricane Andrew in 1992. Sometimes people buy exotic pets and release them when they no longer want to care for them.
This has been the case with Burmese pythons that are invasive in the Everglades. These were once popular pets because they're brightly colored and have an easy-going demeanor. However, they can grow up to 6 feet in the first year and live more than 20 years. A full-grown Burmese python can reach 13 feet long and requires a specially-made enclosure and large amounts of food. This quickly becomes too burdensome for many owners, who then release them into the wild. As a result, a large population of these snakes now inhabits South Florida, where zoologists and wildlife managers are working to track and control their spread.
Transportation Through Shipping
A large number of invasive species have been transported incidentally through shipping. The Great Lakes provide a compelling example. For millennia, the Great Lakes remained separated from other major bodies of water. When the St. Lawrence Seaway, a system of canals and dams, was built to connect the Great Lakes to the Atlantic Ocean, this waterway provided a conduit for invasive species to enter the area.
This mainly happened through the discharge of ballast water. Ballast water is used to weigh down empty ships and then discharged when ships enter a port to make weight available for cargo. The water is typically from the previous port where the ship was docked and often contains living organisms from that area. By discharging ballast water into the Great Lakes when they arrive, ships have introduced at least 60 invasive species into the area, according to EPA data.
In the 16th century, Spanish galleons also transported invasive species, but did so via ballast. They would load ships with soil rather than water, but the soil also contained fire ants. As ships stopped at various ports along trade routes, dumping out the soil released fire ants into these areas, which were quickly able to colonize new places.
Many other examples exist of invasive species hitching rides on cargo to enter new habitats. The fungus known as chestnut blight was introduced to chestnut trees imported from Japan in the late 19th century. The Asian tiger mosquito was introduced accidentally in tires shipped into the United States from Asia. Naval shipworms entered the San Francisco Bay on cargo ships in the mid-19th century and caused significant damage to piers and harbors.
Once they enter a new habitat, multiple components may facilitate their spread. Roads, for example, provide pathways for invasive species to move into new areas. Habitats along roads are devoid of native vegetation, facilitating fire ant mound formation in these areas. When areas are cleared for grazing, the absence of native vegetation reduces competition with native species and can facilitate the establishment of invasive species.
What Are the Consequences of Invasive Species?
Invasive species can have numerous negative impacts on the areas they invade. These consequences span environmental, economic, and health dimensions, making invasive species management a critical focus for environmental biology professionals.
| Invasive Species | Origin | Primary Impact | Annual Cost (US) |
|---|---|---|---|
| Zebra Mussels | Eurasia | Clog water intakes, alter ecosystems, accumulate toxins | $500 million (Great Lakes region) |
| Kudzu Vine | Japan | Overgrow forests, block sunlight, topple trees | $1.5 million (power line control) |
| Burmese Python | Southeast Asia | Decimate mammal and bird populations in Everglades | Millions (control efforts) |
| Hemlock Woolly Adelgid | Asia | Kill hemlock trees (up to 80% mortality in affected areas) | Millions (treatment and replacement) |
| Asian Carp | Asia | Outcompete native fish, disrupt commercial fisheries | Billions in potential losses if established in Great Lakes (USGS models) |
Habitat Loss and Ecosystem Disruption
Perhaps the most significant consequence is widespread habitat loss. The hemlock woolly adelgid is an invasive insect from Asia that rapidly kills infested hemlock trees. In some parts of the Eastern United States, it's estimated that up to 80% of hemlock trees have been killed. These forests provide critical habitat for many animal species, and the loss of this habitat may drive some species to extinction.
Similarly, the health of many forests is threatened by kudzu vines, introduced to the United States in 1876 and widely planted across the Southeastern United States during the 1930s-1950s for erosion control and as a food source for grazing animals. The vine soon became invasive, however, and can completely overgrow entire forests. In the process, it prevents sunlight from reaching trees, effectively killing the forest. Additionally, the weight of thick mats of vines on trees can cause trees to break and fall over. Its ability to quickly overgrow and destroy forests has earned it the nickname "the vine that ate the South."
Physical Habitat Alteration
Some invaders can physically alter the habitat in addition to causing destruction. Fifty beavers from Canada were relocated to Tierra del Fuego, an archipelago at the southern tip of South America, in 1946 to be hunted for their pelts. Since then, they've multiplied and now number in the hundreds of thousands. The trees in the region aren't adapted to beaver activity as they are in North America, and most don't grow back after being gnawed by beavers.
Portions of the formerly pristine forests now look like a bulldozer has plowed through them. Additionally, beaver activity creates ponds that flood portions of the forest. These bodies of stagnant water alter the nutrient cycle in forests, thereby facilitating the proliferation of invasive plants. Beavers also build dams in drainage ditches of grasslands, and livestock commonly fall into them, where they become stuck and die.
Direct Impacts on Wildlife Populations
Other invasive species may not destroy habitat, but can have severe impacts by killing large numbers of endemic species. Burmese pythons, for example, are top predators in the Everglades. As such, they've decimated local mammal and bird populations. Capable of consuming deer and even alligators, these creatures eat virtually any animal they encounter in the Everglades. A number of threatened and endangered bird species have been found in the digestive tracts of pythons, prompting concern that they could drive some species toward extinction.
Sea lampreys in the Great Lakes parasitize native fish. Because native species haven't evolved defenses to lampreys, they often die outright from wounds, or wounds become infected and eventually cause mortality. Invasives can also threaten native species by outcompeting them for resources. Asian carp introduced into the United States outcompete native fish for both food and space, leading to large declines in native fish populations. Invasive species are the second-largest cause of species extinctions globally.
Human Health and Economic Impacts
Invasive species can also impact human health. Invasive zebra mussels accumulate toxins in their tissues like PCBs and PAHs. When other organisms prey on these mussels, the toxins are passed up the food chain and can enter animals consumed by humans. Ballast water from ships may also contain harmful bacteria, such as Vibrio cholerae. Invasive animals can serve as vectors of disease.
In addition to these impacts, invasive species have enormous economic costs. Zebra mussels in the Great Lakes can rapidly cover submerged surfaces, clogging up water intakes at water treatment facilities and power plants. Removing this invasive species costs an estimated $500 million annually in the Great Lakes alone. Power companies spend an estimated $1.5 million each year to control kudzu vines growing on power lines. Lampreys in the Great Lakes have decimated many fishery stocks to the point that they're no longer profitable. In the United States, invasive species cost an estimated $120 billion annually in control efforts and loss of environmental resources, based on 2005 data. Actual current costs may be higher, but are not tracked precisely across all sectors.
What Can Be Done to Deter Invasive Species?
Many strategies have been developed to mitigate damage caused by invasive species and to prevent future invasions. Students pursuing conservation careers learn these management techniques as part of their environmental science education.
An important component is educating people about the dangers of transporting wildlife to new areas. Many laws and regulations have also been enacted to prevent the future spread of invasive species. Ballast water in tankers must be decontaminated before it can be discharged from the vessel. Laws have also been passed to restrict the exotic pet trade, such as banning the import of Burmese pythons in the United States.
Promoting the harvest of invasive species is another widely used technique, although it's been employed with limited success. A python hunt in Florida in 2013 provided cash awards to people for killing pythons. In Argentina, officials sought to promote a market for beaver pelts and encouraged hunters to pursue them. People have also used the woody vines of kudzu to craft baskets and other items. A problem encountered with this strategy is that demand is often insufficient to have a discernible impact on invasive populations. In the case of pythons, they're extremely secretive and elusive, which makes them difficult to find for eradication.
As with many environmental problems, continued research yields insight into effective control measures. For example, studies have examined how effective traps are at capturing pythons. Genetic studies can also yield important information about how invasives have spread in an area and their potential to hybridize with native species. Predicting how the geographic range of an invasive species will expand is important for preparing for invasions into new areas.
Much research has also been devoted to determining the most effective methods for removing invasive plants, whether through herbicides or mechanical control. Using chemicals to control sea lampreys in the Great Lakes during their vulnerable larval stages has been shown to be effective without harming other wildlife. While many invasive species may never be fully eradicated, increased awareness and research offer methods to prevent their spread and to control the economic and environmental damage they can cause.
Frequently Asked Questions
What makes a species invasive versus exotic?
An exotic or alien species is simply one that's been introduced to a new environment but doesn't necessarily cause problems. Many exotic species coexist peacefully with native species and may even provide benefits such as sport-fishing opportunities. A species becomes invasive when it begins to cause environmental, economic, or health impacts. The key difference lies in the negative consequences: exotic species are neutral or beneficial, whereas invasive species actively harm their new ecosystems.
How do invasive species affect the environment?
Invasive species impact the environment in several ways. They destroy critical habitat by killing native plants and trees; alter physical landscapes through activities such as dam construction; kill or outcompete native wildlife for food and space; disrupt nutrient cycles and ecosystem processes; and can even change soil chemistry to favor their own growth. These cumulative effects make invasive species the second-leading cause of species extinctions globally, threatening biodiversity and ecosystem stability.
Where do most invasive species come from?
Most invasive species arrive via shipping and trade, either in ballast water discharged by cargo ships or concealed within cargo. At least 60 invasive species have entered the Great Lakes through ballast water alone. Other common sources include intentional introductions gone awry (e.g., ornamental plants that escape cultivation), pet releases (e.g., Burmese pythons in the Everglades), and failed biological control attempts (e.g., the mongoose introduced to control rats). Global trade and travel have dramatically increased the rate of species introductions worldwide.
What are the economic costs of invasive species?
Invasive species cost the United States a commonly cited $120 billion annually (based on 2005 data), though current costs may be higher and are not precisely tracked. Specific examples include $500 million per year to remove zebra mussels from water treatment facilities and power plants in the Great Lakes region, and $1.5 million annually for power companies to control kudzu vines on power lines. Beyond direct control costs, invasive species also destroy commercial fisheries, damage infrastructure, reduce agricultural productivity, and necessitate costly restoration efforts for damaged ecosystems.
Can invasive species be stopped or controlled?
While complete eradication is rarely possible, invasive species can be managed and controlled. Successful strategies include prevention through ballast water treatment and cargo inspection; early detection and rapid response programs; targeted removal using methods such as chemical treatments for sea lamprey larvae; habitat restoration to favor native species; public education about the risks of releasing pets; and ongoing research into biological control methods. The most effective approach is preventing introduction in the first place, as control becomes exponentially more difficult once species are established.
Key Takeaways
- Definition Matters: Invasive species cause environmental, economic, or health harm, unlike exotic species that integrate peacefully into new ecosystems.
- Predator-Prey Mismatch: The absence of natural predators allows invasive species to reproduce rapidly and dominate ecosystems, as native predators haven't evolved defenses against them.
- Multiple Introduction Pathways: Most invasives arrive through shipping (ballast water, cargo), pet releases, or intentional introductions gone wrong, with global trade accelerating the problem.
- Catastrophic Environmental Impact: Invasive species are the second-leading cause of extinctions globally, destroying critical habitat, altering landscapes, and decimating native populations.
- Enormous Economic Burden: The United States spends an estimated $120 billion annually on invasive species control (2005 estimate), with current costs likely higher across infrastructure, agriculture, and ecosystem restoration.
- Career Opportunities: Invasive species management creates demand for wildlife biologists, ecologists, and conservation professionals trained in ecology and botany.
Interested in protecting ecosystems from invasive species? Explore environmental science degree programs that prepare you for careers in conservation, wildlife management, and ecological restoration.
- Invasive Species: Causes, Impacts & Environmental Effects - February 23, 2015
- How Climate Change Affects Birds - February 11, 2015
- Environmental Consequences of Fishing Practices - February 6, 2015
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