Some of the Worst Invasive Species Humans Carried Around Earth

From ant megacolonies to aggressive mosquitoes with a thirst for human blood, these are definitely bad omens.
Christopher McFadden

Our species has managed to do some incredible things in the short time we have spent on this planet. We've spread to every continent on the planet, and made wonderful art, culture, and many wonderful machines

But, either through accident or on purpose, we have also helped spread some particularly nasty plants, animals, and microorganisms to places they don't belong. Some of these have proved to be devastating to long-established ecosystems. 

But one could argue that we shouldn't beat ourselves up too much. It's just nature doing its thing by taking advantage of our success.

What is an invasive species? 

The most generally accepted definition of an invasive species is any organism that causes ecological and, in some circumstances, economic harm, in an environment in which it is not native.

The most common definition in the United States is that they are "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health."

invasive species fire ant
The European fire ant is widely considered to be an invasive species. Source: Thompson Rivers University/Flickr

The International Union for Conservation of Nature (IUCN) defines invasive species as "animals, plants or other organisms introduced by [humans] into places out of their natural range of distribution, where they become established and disperse, generating a negative impact on the local ecosystem and species."

As you can see, the main qualifier for meeting this definition is the fact that the species, whether it be a plant, animal, or microscopic organism, is in some way better or more efficient at competing in a particular niche, or niches, than native species. 

Invasive species also tend to have the following characteristics:

  • Large population growth rate and fast maturity rate
  • High dispersal ability
  • Tend to not have any natural predators in their new habitat
  • Highly adaptable 
  • Tend to be generalists (can eat a wide range of foods)

All well and good, but what exactly qualifies a species to be considered "native" to a region? 

For anyone with even a modicum of interest in natural history, speciation is the formation of distinct new species which can sometimes push existing "established" species out of particular niches and areas — perhaps even driving them to extinction. If aggressive enough (literally and figurately), these species can expand over large geographical areas given enough time.

invasive species nature
Source: Marcel van Schooten/Flickr

This is a story as old as life itself.  

The very idea that an ecosystem is stable and fixed (with regards to competing species) is nonsensical when viewed over evolutionary history. So what makes invasive species so special in the long history of life on this planet?

The process of speciation, competition, and expansion is usually very slow and limited geographically (e.g. mountain ranges and large waterways may impede the process). The main problem associated with invasive species is the speed at which transportation of new exotic species around the world has occurred as a result of human activities. 

Even our species took tens or hundreds of thousands of years to spread around the planet (including likely engaging in some subspecies mixing along the way). 

So, you might ask, is the term "invasive" just a bit subjective? Is a species only invasive from a human perspective?  

By way of example, let's take a quick look at the evolution of modern horses, Equus ferus. It is thought that their ancestors actually evolved in what is today North America, and later radiated out into South America, Eurasia, and Africa before becoming extinct in North America. Horses were later reintroduced to the continent in the 15th-century through human activities, and flourished. 

invasive species horses
Source: Karsun Designs/Flickr

Should horses be considered an "invasive species" in this context? We'll let you decide. 

Are humans an invasive species?

So, you might be wondering to yourself, could our species also be considered invasive

Despite a single, broadly accepted definition of the term invasive species, we can extrapolate from other invasive species to critically analyze whether our own species should also be considered invasive, and what this might mean.

We humans can now be found on every continent of the planet, even floating between continents and flying above them. We've even made it into space. For this reason, with regards to invasiveness, we certainly fit the bill.

To be considered invasive, a species also has to be non-native. Most of our global migration occurred tens to hundreds of thousands of years ago. While there is s no time limit for what defines a species to be native or not, most would consider humans to be native to many parts of the world by now. At one time, humans would have been invasive, but that would have been quite a long time ago now.

Another trait of invasive species is the changes they bring to the new habitats they move into or the microbial, plant, or animal populations found there. Since whenever humans enter a new environment they tend to bring large changes to that environment, and are often not the best custodians of the environment, we probably meet these criteria.

invasive species humans
Source: James Cridland/Flickr

In conclusion, while we do share some characteristics of the general definitions of invasive species, we cannot really be considered invasive species today. However, humans are a main driver for invasive species — we are often the means by which other species are able to invade new ranges.

Why are invasive species a problem?

Whatever the reason for its introduction, an invasive species can cause harm to both the life and natural resources in an ecosystem, and impact the livelihoods and lives of human populations that rely on the affected resources, whether that is directly (i.e. killing a food source) or indirectly. In some circumstances, invasive species can also impact human health directly — for example, when a disease-carrying organism extends its range to new areas.

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According to the National Wildlife Federation (NHF), invasive species are among the leading threats to native wildlife:

"Approximately 42 percent of threatened or endangered species are at risk due to invasive species."

In fact, the term is often used to describe species introduced through human activities (whether it be consciously or not) but does extend to any introduction of a non-native species through natural means. Contrary to popular belief, an invasive species doesn't necessarily have to come from a foreign country either. 

"For example, lake trout are native to the Great Lakes, but are considered to be an invasive species in Yellowstone Lake in Wyoming because they compete with native cutthroat trout for habitat," explains the NHF. 

invasive species crops
Source: United Nations Photo/Flickr

With regards to the human-related introduction, an invasive species can be exported to a new environment through a variety of means, for example, in water or other materials used as ballast in ocean-going ships; transporting a non-native species to raise in a new area; killing off a part of the food chain entirely through over-hunting, allowing other species to alter their range or niche; or through human-made changes to the climate that allow species to extend or alter their range — to name just a few methods.

In the worst-case scenario, the introduction of new species can cause localized extinction events within an ecosystem. This can include local plants, animals, or a combination of the two, ultimately leading to a reduction in biodiversity by directly competing with native organisms or altering habitats in such a way that stresses native species beyond their tolerance. 

As previously mentioned, and to put things into perspective, expansions, and migration of species across geographical regions is a completely natural phenomenon. To this end, invasion of long-established ecosystems by "alien" or novel species has long been the norm throughout the history of life on this planet. 

However, as our species becomes ever more technologically advanced, human-facilitated introduction and spread of species around the planet have been accelerated beyond what would "normally" be possible with completely natural processes. 

Human activity, over the course of our history, has also greatly increased the rate, scale, and geographical range of potentially invasive species. Depending on your definition of "natural", with regards to human activities, it is widely agreed that our activities have greatly accelerated the introduction of invasive species, especially since the "Age of Discovery", and the development of widescale international trade and travel. 

invasive species infected plants
Aphid-infested African nightshade. Source: ICIPE/Flickr

So what's the takeaway here? 

Most organisms will tend to expand beyond their initial geographical range, if given the chance. If humans can provide them with a means of accelerating that process, they will definitely take us up on the opportunity.

We'll let you decide if, ultimately, that is a good or a bad thing.  

What are some of the most damaging human-introduced invasive species?

Here we've selected some of the most widely considered "worst" invasive species. We've restricted the selection to macroscopic organisms for simplicity rather than anything else. 

Given the hundreds, if not thousands of invasive species identified you shouldn't be shocked to find out that this list is far from exhaustive. It is also in no particular order.

1. The Asian carp (common carp) is a pretty aggressive invasive species

invasive species common carp
Source: David Adam Kess/Wikimedia Commons

Native to Eastern Russia and China, the Asian carp is one of the world's most destructive invasive species. The term 'Asian carp' actually encompasses a number of large cyprinid fish (like the common carp), rather than being specific to one species.

Brought to the Americas and Europe as a foodstuff, as pets, or for sport fishing, these species of carp have proved devastating to many local fish stocks. 

Asian carp are pretty large fish, have big appetites, and breed very quickly. This combination of traits has seen them outcompete a wide number of native fish for food wherever they are introduced. Asian carp are even known to feed on the eggs of native fish. 

They also tend to be messy eaters. When feeding, Asian carp routinely stir up sediment and microorganisms from the bottom of lakes and river beds. This lowers the water quality, which can kill off organisms like native freshwater mussels

2. The cane toad got a little out of control

invasive species cane toad
Source: Benjamint444/Wikimedia Commons

Native to South and Central America, this diminutive little amphibian happens to be pretty good at out-competing other species anywhere it's introduced. It was imported into a number of countries with similar habitats to its home continents (such as Australia) as a natural form of pest control for cane sugar crops.

However, lacking predators in its new environment, the cane toad populations soon exploded and toads began devastating native ecosystems. The toads also produce a highly toxic ooze to ward off predators. While its native predators are immune, predators in its new environments are not — at least not yet. Those predators that do attempt to eat the toad never live to tell the tale. 

With no natural predators able to keep their populations under control, the cane toad population exploded, putting immense pressure on native animals and plant species. This is mainly because these toads will eat almost anything, including small birds, other reptiles and amphibians, and small mammals, out-competing native species for food, as well as breeding habitat.

3. The zebra mussel might be the world's most dangerous stowaway

invasive species zebra mussels
Source: Neil DeMaster/Flickr

The zebra mussel is a freshwater species native to the lakes of southern Russia and Ukraine, but has since expanded its territory to many parts of the world. Now found in many other parts of Russia, Europe, and North America, this invasive species is certainly something of a bully — if you are another shellfish. 

Inadvertently transported to other parts of the world in the ballast of ships, these mussels are also able to attach themselves to the outsides of boats, or simply hitch a ride on floating vegetation. 

Zebra mussels are able to quickly expand their numbers, and attach themselves to anything they can — including native species of mussels. Their abundance can clog intake pipes of water treatment and power plants, and encrust boat motors and hulls. The mussels' presence, and the way they filter water also lead to changes in nutrient cycles and the reduction of native plankton populations, which in turn leads to a reduction in zooplankton, meaning less food for native mussels to live on. 

Not the best guests to have at your dinner table. 

4. The small Indian mongoose really punches above its weight

invasive species small indian mongoose
Don't let its appearance fool you, this is a deadly killer. Source: Minakksi/Wikimedia Commons

Another invasive species introduced for pest control, the small Indian mongoose has proved devastating to several native species. A native of Southeast Asia, they were introduced to Asia, Central, and South America to help hunt rodents to protect sugar plantations. 

Despite their cute appearance, these animals are actually very aggressive predators. So much so, they are have been linked to the extinction of the bar-winged rail.

They have also hunted other animals to near extinction, like the Jamaica petrel, hawksbill turtle, pink pigeon, Amami rabbit, and many other natives birds, reptiles, and small mammals. 

Our own species is also not immune to these incredible animals. While they don't actively hunt us (thank goodness), they do act as carriers for some nasty diseases, like rabies and the and human Leptospira bacterium.

5. The Asian long-horned beetle is not the best guest either

invasive species long horned beetle
Source: P.S. Meng et al/Wikimedia Commons

Originating from China, Japan, and Korea, the Asian long-horned beetle is another of the world's worst invasive species. Imported to many parts of North America and Europe in shipments of wooden packaging and trees, these beetles have since devasted many other ecosystems.

The problem with these beetles is that they tend to lay their eggs in any deciduous tree (trees that seasonally shed leaves) they can find. When the eggs hatch, the larvae feed on the soft bark of the trees, making it increasingly harder for the trees to transport nutrients to where they are needed.

Eventually, the larvae will burrow into the middle of the tree, leaving large tunnels behind them. This severely weakens the tree in the process. 

The adults eventually emerge from the perforated tree, breaking through the bark and leaving even bigger holes. Sadly, many trees cannot easily survive this kind of treatment, and will die and collapse if the infestation was serious enough. 

Once a tree is infested, the only remedy is to chop it down, along with the other infested trees around it. The downed trees are then are burned or chipped to kill the larvae, eggs, and pupae. 

6. Japanese knotweed is tough to get rid of

worst invasive species japanese knotweed
Source: Jaap Tamminga/Wikimedia Commons

Reynoutria japonica, more commonly known as Japanese knotweed, is yet another of the world's worst invasive species. Native to Japan (hence the name) it was introduced into other parts of the world for ornamental purposes, as well as to act as a soil stabilizer — especially in coastal regions. 

This plant thrives in direct sunlight, and can be found in mainly moist habitats, but is also common on wasteland, and along roadways and other distributed areas. 

When the plant is fully established, it tends to form very dense stands that will shade and crowd out many other native plants. And this is bad news, for more than the local flora.

By out-competing native plants, the knotweed leads to a significant drop in biodiversity, with severe impacts on native food chains.

Once established, the plant is also notoriously tricky to eradicate. Its rhizomes can grow very rapidly, and cutting or pulling the adult plant out of the ground is usually not enough to get rid of it. Mowing infested areas every two weeks has been shown to kill off the infestation within a 2-year period. 

7. Whitefly can be devastating for crops

invasive species whitefly
Source: epitree/Flickr

Whitefly is the common name for a number of species of small insects of the family Aleyrodidae. These tiny bugs tend to feed on the undersides of plant leaves, and also tend to be pretty aggressive when introduced into new areas. 

Native to the Middle East and the Mediterranean, these little bugs were introduced to places like California in the 1980s through the transportation of infested plants When they first arrived, they had few natural predators, and rapidly spread and infected more than 40 species of plants in the state. 

If an infection becomes bad enough, the plant is literally starved to death. The ash whitefly, in particular, was a significant pest of fruit trees such as pear and pomegranate, and affected crop yields. Their presence also affected human activities in urban areas, due to large swarms of flying adults. Cars parked under infected trees could also have their paintwork ruined from dropping honeydew. 

Populations of the ash whitefly have since been controlled through the use of a small parasitic wasp called Encarsia inaron.

8. Lionfish is a beautiful, but deadly, invasive species

invasive species lionfish
Source: Alexander Vasenin/Wikimedia Commons

The Lionfish (Pterois voltans) is another of the world's worst invasive species. A native of the Red Sea, the Indian Ocean, and the Western Pacific, this fish has since been introduced to other parts of the world through the aquarium trade.

Thanks to this trade, the fish is now a common sighting in the Northwestern Atlantic and the Caribbean and is an example of one of the most rapid marine finfish invasions in history. 

In some places, its population growth is so impressive that it is feared it could replace native species of fish like the grouper. 

The fish has a series of elongated venomous dorsal and fin spines which it uses for protection. It is also a very aggressive ambush predator that preys on small and juvenile fish, crustaceans, mollusks, and isopods. With no native predators, these fish have the potential to devastate any local ecosystem they migrate into. 

Their behavior also impacts local human communities through the reduction in commercially important fish stocks. The lionfish's venom is also pretty nasty for any unfortunate diver, snorkeler, or aquarium owner who is accidentally stung. 

At present, the only effective way to manage this species is through culling. Other methods are also being explored, including the promotion of lionfish fishing industries. 

9. The brown marmorated stink bug is a real pest

invasive species stink bug
Source: Hectonichus/Wikimedia Commons

Another of the world's worst invasive species is the brown marmorated stink bug. A native of China, Japan, Korea, and other Asian regions, it appears to have been accidentally released in some parts of the U.S. in the late-1990s.

The nymphs and adults of the species are voracious eaters of plants, which, unfortunately, includes many important agricultural plants. By the end of the 2000s, the stink bug has become a seasonal pest in many orchards of the Eastern United States and Mid-Atlantic U.S.

More recently, this pest has since spread to parts of Europe and South America. At present, there is no effective way to tackle this invasive species and controlling its spread has become a top priority for the U.S. Department of Agriculture. 

Some evidence exists that local predators, like wasps and birds, have begun to feed on them, which is encouraging. Other biological control methods are also under investigation, including the introduction of its natural predators from Asia. 

However, the North American wheel bug appears to be the most promising method of controlling stink bug populations. 

10. The European green crab has proved to be a very successful invader

invasive species european green crab
Source: Luis Miguel Bugallo Sanchez/Wikimedia Commons

Another devastating invasive species is the European green crab. Native to Europe and northern Africa, this crab has since been introduced to North America, Australia, parts of South America, and South Africa. 

The exact pathway of its introduction is not known, but it is likely a combination of natural dispersal, and hitching a ride in ships' ballast, hulls, and equipment, and in batches of commercial shellfish. 

This crab is not a picky eater by any means, and in some places has seriously damaged native crab and bivalve populations. The crab can also tolerate a wide range of salinity and temperature variations, allowing it to live in coastal regions, as well as estuaries.

This crab primarily preys on large and small snails and also tends to target soft-shelled clams and juvenile fish. 

11. The tiger mosquito is potentially a very dangerous invasive species

invasive species tiger mosquito
Source: frankieleon/Flickr

Native to subtropical areas of Southeast Asia, the tiger mosquito appears to have been accidentally introduced to other parts of the world through contaminated shipments of lucky bamboo. Transportation of this bamboo requires the plants to be shipped in standing water for preservation, which can also carry the mosquito larvae. 

Other transportation routes include the movement of used tires around the globe. Water trapped in the tires can also contain mosquito larvae. 

In their adult stage, they have a very distinctive striped appearance and have now spread to many parts of the world. 

Once adults, the females of the species are aggressive outdoor day biters and prey on humans, livestock, amphibians, and birds.  This mosquito, like its close relative Aedes aegypti, is a known vector for many diseases, including the dengue virus, yellow fever, and even the Zika virus. 

Further spread along trade routes is currently maintained through careful sterilization and/or quarantine of shipments. Specialized traps and other biological controls are also being investigated, with various levels of success. 

12. The Argentine ant has managed to establish a globe-spanning empire

invasive species argentine ant
Source: Matthew Townsend/Flickr

The Argentine ant might be the most impressive invasive species you have never heard of. Native to northern Argentina, Uruguay, Paraguay, Bolivia, and southern Brazil, it has since managed to forge a globe-spanning presence. 

Starting around the late-1890s, the ant has been inadvertently introduced to many parts of the world through international trade. 

The ant is relatively small, but its colonies (unlike many other ant colonies) tend to have a series of queens rather than just one. This gives them the ability to breed workers in huge numbers, allowing them to compete aggressively with other ant species through the weight of numbers. 

More importantly, is the fact that colonies of this ant tend to show a marked lack of aggression between its members. In fact, workers from different nests are often seen mingling with neighboring nests without being attacked. 

This may be due to the fact that they share a common heritage, with many colonies around the world showing little genetic variation. This may mean they all originate from the same original colony and researchers now believe they may even be considered one global mega colony. 

The ants are considered annoying pests, but can also prove to be devastating to agricultural plants. This is because the ants farm plant pest insects, such as mealybugs, scales, and aphids. In return for sweet honeydew secretions from the plant-feeding insects, the ants provide protection from natural enemies. But the larger number of plant-eating insects translates into more plants being eaten. 

And that, invasive species enthusiast, is your lot for today. Trust us when we say these are but a few of the many thousands of other invasive species around the world. 

If left uncontrolled, many of these will continue to pose very serious threats to human societies and local ecosystems. While it may be too late to control them once established, more care needs to be taken to prevent their spread to currently uncontaminated parts of the world. 

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