Currently, almost all nations worldwide are faced with biological invasions of alien species. Alien species are diverse, and their number is significant. Their share worldwide is steadily increasing, and the speed of this process is increasing. 

They threaten natural and anthropogenic ecosystems and negatively affect the state of forestry and the economic efficiency of agro-industrial and fish-breeding enterprises. 

New, previously unknown species of animals constantly appear, the potential harm of which requires assessment. Often, the appearance of alien species in a particular area has undesirable consequences for their new home and neighbors. Today we will list the 10 most harmful, invasive species from around the world.

1. Kudzu

Kudzu
Kudzu | Image Credit – Flickr
Scientific NamePueraria Montana
Native toJapan and China
Introduced in the USA1876
Reason for IntroductionAs an ornament and prevention of soil erosion 
Current StatusFound in the Caucasus, southern Africa, Switzerland, Italy, or Canada
Invasive BecauseInvades entire Ecosystem, can grow to 30 meters

Kudzu, aka Pueraria Montana, is a liana-like plant with leaves similar to wild grapes native to Japan and Southeast Asia. 

This plant was introduced to the south of the United States (to Philadelphia) in 1876. It was presented to the local population as a fast-growing plant that effectively curbs the development of soil erosion. 

Kudzu became so popular that it spread throughout the United States as part of the Civil Conservation Corps, which was a component of the New Deal that promoted governance and job creation in the United States. 

The idea was that because the kudzu takes root rather quickly, it fixes the dirt in place so you can feed it to your livestock afterward.

Thus, it is clear that mature kudzu does grow about 12 inches per day. Mature plants are believed to grow approximately 60 feet in one season under optimal conditions. It is speedy for the plant.

The plant is considered invasive because it invades entire ecosystems quickly. It is due to many factors. Since the people removed the plant from its natural range and environment, it kept its growth within ecological limits. 

Here in the US, the only pest that eats the plant in ample quantities is the Kudzu beetle. This particular insect is a versatile legume, so it also feeds on soybeans, which is a crop.

Another reason why this plant is doing so well is because the southern winter doesn’t kill the plant. Frosts and a cold front can kill the leaves and stem, but the root portion remains intact and may grow back next year.

In 2010, the Forest Service estimated that Kudzu covered 227,000 acres. It is more than twice the size of the city of Atlanta.

This plant in the United States began to be called “the vine that swallowed the south.”

Already in the second year, under favorable climatic conditions and the presence of the necessary support, this plant species can grow and reach a height of 30 meters. In the absence of support, it spreads horizontally, absorbing everything in its path: abandoned houses, cars, power lines, other trees, and shrubs.

This Japanese climbing plant, considered by the International Union for Conservation of Nature (IUCN) as one of the most harmful invasive species, is found in places as diverse as the Caucasus, southern Africa, Switzerland, Italy, and Canada.

2. Beaver

Beaver
Beaver | Image Credit – Flickr
Scientific NameCastor Canadensis
Native toCanada
Introduced in Argentina1946
Reason for IntroductionFor fur 
Current StatusFound in Argentina, Chile, Alaska, Arctic
Invasive BecauseBuilds dams originating new lakes

In 1946, Argentina introduced beavers from Canada to start a promising fur industry in Tierra del Fuego that soon proved unsuccessful. 

It was then that these sympathetic-looking animals were released into the wild and began a history of colonization, radically changing the island’s physiognomy and ecosystems. 

But in the absence of natural predators – such as bears and coyotes – these semi-aquatic rodents have reproduced rapidly, and their population is estimated to exceed 100,000 individuals. 

The beaver gnaws the trees to the ground and chops them down to build his burrow and dams, which change the course of the streams and consequently the surrounding vegetation and soil. 

For example, flooded areas that form around dams become grasslands with levels of humidity, light, and vegetation where native species cannot survive. 

Due to these significant environmental impacts, beavers are considered one of the leading agents for transforming Tierra del Fuego ecosystems. Their invasion constitutes one of the greatest threats to nature conservation.

It is admirable how cleverly they build their huts, how complex they build dams, how competently they repair them. It is fascinating to observe beavers in wildlife. 

However, in areas with active agricultural activity, beavers are indeed the cause of many troubles. 

3. Lion Fish

Lion Fish
Lion Fish | Image Credit – Flickr
Scientific NamePterois volitans
Native toIndonesia
Introduced in the USA1985
Reason for IntroductionAquarium Fish
Current StatusFound in Atlantic Ocean, USA, Mediterranean, Gulf Coast
Invasive BecauseVoracious appetite decreasing number of local fish and aquatic creatures

The emblematic case is the lionfish (Pterois volitans), introduced as an aquarium fish and released in the Caribbean, and is currently dispersing to South America. 

It is a voracious predator and challenging to control due to its outer spines. Originally from Indonesia, these flashy, ominous-looking fish have lived outside their natural habitat since about 1985. It has been spreading and causing enormous environmental and economic damage ever since.

Experts believe that the appearance of lionfish in the Atlantic Ocean on the United States’ east coast occurred when:

  • only one or a few aquarium owners were either tired of their domestic lionfish, 
  • or could no longer afford them, 
  • or were simply couldn’t care for them properly and released them into the nearest ocean. 

Since then, lionfish populations have outgrown and spread rapidly over large areas of the east and south coasts of the United States. 

Nowadays, this invasive species has also established significant populations in the Caribbean and has spread to several locations along the coast of Central and South America.

The warming oceans and the expansion of the Suez Canal have caused the lionfish to reach the Red and Mediterranean Seas. They are spreading rapidly, and some have already reached Tunisia and Italy.

The primary method by which these species were able to spread over such vast areas is their capability to lay eggs that float freely on the surface of groundwater.

Eggs laid by females in the same region can be dispersed by ocean currents and gyres, traveling to distant places and being fertilized by lionfish males during their journey. 

It allows lionfish populations to settle in areas where other lionfish previously could not reach by simple migration. Combined with the fact that lionfish mature very quickly, their ability to spread becomes even faster. It is just one of the reasons that the number of invasive lionfish is tough to control.

Invasive lionfish are the most harmful to the environment with their voracious appetite. The lionfish anatomy allows these species to consume small fish whole by opening their mouths, disengaging their jaws, and creating a kind of vacuum that attracts their target without any problem. 

In addition, the lionfish’s stomach can expand to accommodate as much food as possible. It means that even one non-native lionfish can swallow large numbers of small local fish and juveniles of other larger predators such as perch and sea bass. 

The voracious eating habits of lionfish can cause severe disruption, especially in already vulnerable coral reef ecosystems, which cascade up the food chain and affect several trophic levels. 

Without a natural predator to keep them at bay, the impact of lionfish on populations of native fish species and the communities that depend on them has been devastating.

4. Yellow Jacket Wasp

Yellow Jacket Wasp
Yellow Jacket Wasp | Image Credit – Flickr
Scientific NameVespula Germánica
Native toEurope, Asia, and Africa
Current StatusFound all over the world
Invasive BecauseKill bees, disrupt pollination of plants and crops

The yellow jacket wasp is native to northern Europe, Eurasia, and North Africa and is present in almost all regions of the planet. When they attack a hive searching for honey and pollen, these wasps manage to displace the bees that inhabit it, leaving it empty. 

According to the FAO,” three out of four crops on the planet” need pollination. Therefore, it is so problematic that a species like the yellow jacket wasp threatens the extinction of bees. 

The yellow jacket wasp, also known as Vespula Germánica, makes its appearance in late winter. It is a carnivorous predator, and its diet consists mainly of insects. However, they are also aggressive and can attack adult humans and children.

One of its nests can host up to 20,000 worker wasps. The danger is that each one can hunt up to 50 bees a day.

They also feed on fruits, thus damaging apple plantations and other species. And they are also carnivorous, so they attack livestock and even people. Its sting is dangerous because the venom can cause an allergic reaction. 

  

5. Bullfrog

Bullfrog
Bullfrog | Image Credit – Flickr
Scientific NameLithobates catesbeianus
Native toNorth America
Introduced in Europe1990s
Reason for IntroductionTrade
Current StatusFound in Europe in Large Numbers
Invasive BecauseCan eat anything on its way, transmits pathogens such as Batrachochytrium dendrobatidis that has caused the decline of amphibians on a global scale

Various amphibians have also become invasive, competing with native species and preying on native fauna. 

The bullfrog (Lithobates catesbeianus) is one of the most harmful species for several reasons:

  • It has a high reproductive potential.
  • The larvae produce toxic secretions and therefore lack natural predators outside their area of origin.
  • It can transmit pathogens. 

It is native to North America and has spread due to captive breeding for human consumption and use as a pet. It has now spread o Northern Italy, western France to all over Europe. 

Crabs, ducklings, snails, tree frogs, even grass snakes and sand lizards – the stomachs of bullfrogs contain almost everything that the local fauna has to offer. 

The frog can grow up to 45 centimeters in size and up to 2.5 kilograms in weight. It does not have natural enemies in local latitudes such as snakes, turtles, or particular predatory fish. That is why it is spreading at high speed along the Rhine.

Its only enemies are the environmentalists, who have been fighting the bullfrogs for over a year. 

It is still relatively easy to catch adult animals because they usually give themselves away with their inimitable roaring calls. Hunting for the young animals and the countless tadpoles is significantly more difficult. Bullfrogs lay spawning clumps of up to 25,000 eggs at a time. 

It has a severe impact on ecosystems since it reduces the populations of native species and transmits pathogens such as Batrachochytrium dendrobatidis. This fungus has caused the decline of amphibians on a global scale. The consumption of specimens born in polluted waters can affect human health.

6. Water Hyacinths

Water Hyacinths
Water Hyacinths | Image Credit – Flickr
Scientific NameEichhornia crassipes
Native toSouth America
Introduced in Africa1937
Reason for IntroductionGarden Ornament
Current StatusFound in Africa, Europe, Asia, And Australia
Invasive BecauseDeath of local aquatic creatures due to lack of oxygen, mosquito-spread

The warm weather-loving aquatic plants from South America and their beautiful flowers made people make a gross mistake – bringing them home, where they quickly covered the water surface and thereby blocked the access of sunlight to other plants. 

As a result, this reduced oxygen in the water, causing damage to wildlife, which caused the death of other plants that provided food and shelter for fish and other aquatic animals. 

Water hyacinth ( Eichhornia crassipes ) is characterized by rapid growth, can migrate over long distances, has high productivity (1 plant gives more than 3000 seeds per year), and can exist in a wide range of environmental conditions. The seeds remain viable for 20 years.

An overgrowth of water hyacinth has also caused mosquitoes to spread. Hyacinths are now a problem in Australia and Asia by drowning in waterways in tropical and subtropical Africa and the southern United States and Mexico. 

Brought to the United States in 1884 during the New Orleans World’s Fair, they spread so quickly that they began to block shipping channels.

It was first registered in Zimbabwe in 1937, then in Mozambique in 1946. At the 20th century’s end and the beginning of the 21st century, it spread to large lakes – Tanganyika and Victoria. 

Water hyacinth blocks waterways, clog water pipes and canals, and damage to rice growing in Southeast Asia has been reported. Blocking irrigation canals leads to flooding (Africa, Asia). 

Cases of power system failures due to exposure to water hyacinth in hydroelectric power plants (e.g., Lake Victoria) have been reported.

7. Goldenrod 

Goldenrod 
Goldenrod | Image Credit – Flickr
Scientific NameSolidago
Native toNorth America
Introduced in Europe1850s
Reason for IntroductionGarden Ornament and Medicinal purpose
Current StatusFound in Europe and America
Invasive BecauseDon’t allow the other local plants to grow

This herbaceous perennial of the Asteraceae family is indisputably familiar to all summer residents. 

Tall plants with narrow leaves and caps of golden flowers were at one time actively promoted as an ideal ornamental crop for a summer cottage. 

Goldenrod is very beautiful, blooms until the very cold, overgrows, quickly reproduces even on its own, is unpretentious to soils, does not require any maintenance, endures strong shading, and is an excellent honey plant pollinated by a wide range of insects, is exceptionally winter-hardy. 

In addition, it is a relatively valuable medicinal plant. It would seem – what could go wrong? Goldenrod began to displace the local not-so-adaptive flora thanks to its incredible unpretentiousness and uncontrolled reproduction. 

Goldenrod has a powerful, fast-growing horizontal rhizome and many tiny, light seeds easily carried by the wind. 

Forming dense, tall, dense thickets, producing inhibitor substances that have a depressing effect on the root system of neighboring plants, and actively spreading by self-seeding, this native of distant North America very quickly becomes dominant in any territory. 

They leave no chance for not-so-large and aggressive native species and are a mortal threat. The intensive spread of goldenrod in Europe began only after the 1850s.

If this plant is present in your area, the only way to combat its uncontrolled spread is to cut off the inflorescences immediately after wilting, preventing the formation of seeds. 

If it seems to you that one-seeded plant in your flower bed will not do much harm, think about how the wind and insects will blow many tenacious seeds to neighboring areas and the nearest underbrush.

One of the most potent ways to completely get rid of goldenrod is to mow twice to four times a year (in May and August) for several years, followed by mulching or digging up the soil during the summer in dry weather conditions. 

8. Burmese Python

Burmese Python
Burmese Python | Image Credit – Flickr
Scientific NamePython bivittatus
Native toAsia
Introduced in the USA1979
Reason for IntroductionReleased by the pet owners and Cyclone Andrew in 1992
Current StatusFound in Florida
Invasive BecauseLocal species are disappearing at an unexpected rate

Burmese pythons are 7-meter long snakes as thick as a telegraph pole. They can swallow a whole alligator or cougar, lay up to 100 eggs a year, and are capable of killing a person.

Burmese pythons were first discovered in 1979. People brought them to Florida from southeast Asia, where their length even reaches 8 meters. 

The Burmese python, a subspecies of the Indian python, became invasive after being introduced to its new environment released by owners of this new type of pet that had become too bulky. 

Another hypothesis is that the destruction of a “reptile farm” by Cyclone Andrew in 1992 released many snakes. 

Recent work has shown that 300,000 Indian pythons have been imported to the United States over the past 30 years! According to American naturalists, global warming will further promote these snakes to the country’s north.

The authorities implemented in 2017 a disposal program of Python bivittatus to cope with the problem.

Since then, 6,278 specimens have been captured, including nearly 2,000 during the first eight months of 2020. A record that surpasses that of 2019, according to the latest report from the government agencies involved.

It has become invasive because it attacks birds, some of which are endangered, and even alligators. It also feeds on mammals. 

Thus, before the 2000s, the raccoon, the opossum of Virginia, and the rabbits were the most common species seen on Everglades Park roads. 

New USGS counts have revealed that sighting frequencies have decreased for these four species by 99.3, 98.9, and 100 percent, respectively! In addition, these mammals are more numerous in areas where the Burmese python has been detected recently and even more in areas where the reptile is still absent. 

Other species have also become rare since the arrival of the reptile in Florida; this is the case of the bobcat, the fox, and the white-tailed deer. Thus, the decline of these mammals is indeed attributable to pythons.

9. Grey Squirrel

Grey Squirrel
Grey Squirrel | Image Credit – Flickr
Scientific NameSciurus carolinensis
Native toNorth America
Introduced in England1876
Current StatusFound all over in UK and Europe
Invasive BecauseRed squirrels are disappearing, the number of songbirds declining

Europe’s gray squirrels ( Sciurus carolinensis ) originated in North America, where they are known as eastern gray squirrels. They eat large seeds, flowers, buds, fruits, mushrooms, insects, and sometimes bird eggs. 

They were first introduced to England in a concerted manner in 1876. As a result of a rapidly growing population and further introductions, they spread to the rest of Britain by the early to the mid-20th century. 

The eastern gray squirrel was introduced to mainland Europe in 1948 and quickly took advantage of European food sources, habitat, and lack of predators for gray squirrels. 

The indigenous squirrel of Europe is known as the red squirrel ( Sciurus vulgaris ). Both indigenous and invasive species have similar diets, use similar locations in the environment, and have similar activity patterns and ways of living. However, there is no proof of noticeable aggression or violence between the two species. 

These eastern gray squirrels are considered an invasive species in Europe due to their presence displacing red squirrels. Nearly 80% of available gray squirrel introductions worldwide have resulted in the successful development of the new area. 

Usually, gray squirrels do not need large numbers to start a new population. The enemy liberation hypothesis explains this efficiency of the proliferation of gray squirrels when the invading species survive better because their natural predators are absent. 

In addition, naturally occurring parasites are usually not present in the area to which the species has been introduced or invaded. Since gray squirrel poses such a threat to trees and red squirrels in Europe, great efforts are being placed to stop the further spread of gray squirrels.

One of the biggest problems associated with a gray squirrel as an invasive species is the effect that introduction has on red squirrel populations. 

Gray and red squirrels compete for resources, while gray squirrels will steal food caches from red squirrels. The decreased individual growth of red squirrels, juvenile recruitment, and reproductive success have been associated with competition between gray and red squirrels. 

In areas with considerable overlap of red and gray squirrels populations, a decrease in the bodyweight of red squirrels was observed in spring. Gray squirrels are partly responsible for the decline in select woodland and songbirds, as they sometimes feed on birds’ eggs and chicks.

The particular behavior of the gray squirrel, such as removing bark, also hurts trees and forest plantations.

10. Ash-leaved Maple 

Ash-leaved Maple 
Ash-leaved Maple | Image Credit – Flickr
Scientific NameAcer Negundo
Native toNorth America
Introduced in Europe1688
Reason for IntroductionAs an Ornament, a Tree for the park
Current StatusFound all over Europe
Invasive BecauseLocal plantations are disappearing, inhibits the growth of other species

This maple is native to North America but has long formed its vast range in Europe since 1688, actively displacing local species and even in some places becoming a threat to local ecosystems. 

Suppose its decorative garden forms delight breeders of this culture of incredible beauty with colored foliage (pink, bronze, green with rims, reddish-brown, light yellow). In that case, wild trees can become an absolute disaster for parks and forest plantations.

This maple is very winter-hardy and unpretentious to soil conditions. It has a very high growth rate and is resistant to air pollution. Coming out of cultural plantings and quickly penetrating local vegetation, the American maple poses a severe threat to biological diversity due to its very high ecological plasticity. 

The ability of this plant is much faster than other species to form multi-tiered thickets with the help of numerous and firm root and pneumatic shoots, and to release inhibitory substances (inhibiting the growth of other plants), hindering the renewal of native species. 

It makes the ash-leaved maple one of the most aggressive woody weeds in the forest zone Eurasia (parasitic weed). In addition, its pollen (and a lot of it is formed since wind-pollinated species) is a potent allergen for humans. In everyday life, botanists have even gotten accustomed to this tree’s unofficial nickname – the killer maple.

The growth of its population is already being curbed in some territories. It is gradually replaced with aboriginal tree and shrub species.

But be careful – since the ash-leaved maple communities are still legally classified as green spaces of the third category, their cutting down or damage without appropriate permits entails administrative responsibility. 

Conclusion

The fight against invasive alien species must be addressed and adapted to the characteristics of the individual species, also considering the territorial context. 

Citizens can carry out a vital containment action, reducing the pressure of invasive alien species on the natural environment by caring for the territory and its properties.

As regards public administration, since it is a transversal topic that involves the skills of many offices, a considerable organizational effort is required to plan interventions on a regional scale.

By carefully evaluating the species, situations, and available resources, it is possible to effectively carry out and achieve essential objectives to contain and, in some cases, eradicate invasive alien species, always in synergy with habitat restoration actions and other biodiversity erosion issues. 

From the highest international institutions to every citizen, everyone can do something. Among the many possible actions to preserve biodiversity, we indeed find actions to combat invasive alien species, but we must act urgently!

Comparison Table of Alien Species

NameSpeciesNative ToIntroduced ToIntroduced YearCurrent StatusInvasive Due To
KudzuPlantJapan and ChinaUSA1876Found in South Africa, Switzerland, Italy and CanadaInvades entire ecosystem
BeaverRodentCanadaArgentina1946Argentina, Chile,Alaska, ArcticBuild dams originating new lakes and destroying habitat
LionfishFishIndonesiaUSA1985Atlantic Ocean, Mediterranean, Gulf CoastVoracious Appetite, local aquatic creatures disappearing
Yellow Jacket WaspsInsectEurope, Asia, Africa N/AN/AAll over the worldKilling bees
BullfrogAmphibianNorth AmericaEurope1990All over EuropeSpread deadly fungus to other amphibians
Water HyacinthAquatic PlantSouth AmericaAfrica1937Africa, Asia, Europe, AustraliaLocal aquatic creatures are dying, spreading mosquito
GoldenrodPlantNorth AmericaEurope1850sEurope, AmericaInhibiting growth of local plants
Burmese PythonReptileAsiaUSA1979All over USA especially in FloridaLocal species are disappearing
Grey SquirrelRodentNorth AmericaEngland1876UK, EuropeRed Squirrel disappearing
Ash-leaved MaplePlantNorth AmericaEurope1688EuropeLocal Plantations disappearing

Ankur Pradhan holds a bachelor’s degree in education and health and three years of content writing experience. Addicted to online creative writing, she puts some of what she feels inside her stormy heart on paper. She loves nature, so she is trying to motivate people to switch to alternative energy sources through her articles.