As they quietly expand their kingdoms, we can look in the mirror and ruefully shake our heads.
Over the past two centuries, amid growing trade and amid colonisation, plants, trees, fish and mammals have been transported from their home territories to new regions, for a range of reasons. Some shrubs were pretty, so they were planted as hedges (click here for a look at how much damage the lantana has done in India).
Some trees grew fast and seemed intriguing; now, it turns out there are few ways to stop them.
Fish, ants and other insects were moved along sea routes, both intentionally and unintentionally, and now suck oxygen and nutrients out of distant water bodies.
Invasive alien species crowd out natives, alter habitats, can change water and soil composition; they contribute to the ravaging of crops, destruction of houses, damage to streets, pavements and building foundations.
Over 3,500 of the most harmful such species were recognised in a first-of-its-kind 2023 global assessment report by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES).
The cost of controlling their spread can be so high that, in the UK, lawsuits have been won and damages paid after a particularly virulent Japanese weed was not reported by a seller to a home buyer.
Invasive aliens — contributing alongside other drivers such as climate change, pollution, land use change and the direct exploitation of species — are linked to 60% of recorded global extinctions, notes the IPBES assessment.
Meanwhile, in the absence of overarching frameworks and strict enforcement, new species continue to be introduced where they don’t belong.
“Take the Indian bullfrog, introduced to the Andaman Islands in the 2010s, during efforts to set up fish farms,” says KV Sankaran, former director of the Kerala Forest Research Institute, former coordinator of the Asia-Pacific Forest Invasive Species Network, and coordinating lead author of the IPBES assessment report. “It has become a threat to native amphibians, fish and small reptiles there.”
Beyond extinctions, these species disrupt habitats by acting as ecosystem engineers, says British ecologist Helen Roy, lead author of the IPBES assessment report and professor at University of Exeter. “They can affect pollination, decomposition, oxygen levels in water bodies and the availability of food, among other things. That ends up fundamentally altering the whole ecosystem.”
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Undoing the damage was never going to be easy. In most cases, though, we aren’t really trying.
“Currently, the cost of damages far exceeds the costs of management, which means we’re reacting rather than preventing, especially in India,” says Alok Bang, an ecologist, evolutionary biologist and professor at Azim Premji University in Bhopal.
With little data available on such damage, particularly here at home, it continues to be ignored by policymakers, the media and the public. Mustering public and political will is key in such situations (it’s how the whales were saved, and tigers brought back from the brink).
But that can take time, “and we don’t have decades to fix this,” Roy says. “We shouldn’t let gaps in data delay action. We already know enough to act, even as we improve our knowledge. The evidence is here, it’s clear, and action plans are available.”
Risk analysis, early detection and rapid response frameworks work. Stringent biosecurity laws can help, by monitoring what enters a country.
The governments of Australia and New Zealand, for instance, have moved from a blacklist of species to a “whitelist” approach. For over 25 years now, only species on their whitelists have been allowed into these countries; everything else is prohibited by default and requires risk-assessment.
“Blacklists leave space for unknown threats to enter,” as Sankaran points out. “A whitelist approach allows import only of material that has been identified as safe through rigorous risk assessment. This is crucial because there are thousands of invasive alien species that we know nothing about.”
As consumers, meanwhile, we can vote with our wallets. “Even small individual actions, such as responsible gardening and pet ownership, can go a long way,” Roy says.
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The alternative is to do nothing, and have another Aedes aegypti situation evolve.
This mosquito — carrier of diseases such as dengue, yellow fever, chikungunya and the Zika virus — is native to parts of Africa and South America. It spread around the world by breeding in water containers that were stowed on ships.
It now causes tens of thousands of deaths a year; has seen countries pour billions into disease-prevention and medical research. The mosquito, meanwhile, is everywhere (on every continent except Antarctica).
“We don’t notice gradual impacts until they become severe,” says Chilean ecologist Aníbal Pauchard of the Universidad de Concepción’s faculty of forest sciences. “This is one of the hidden burdens of the invasive alien species.” Take a look.
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PLECO FISH: The “vacuum cleaner” gone rogue
The Pleco family of fish are covered in rows of spiky armour, and can suck algae off rock. Many species can drag themselves out of water and “walk” on land with the help of their spines and fins.
Native to South America, they are now found in water bodies around the world.
It turned out they were useful in aquarium tanks, as a way to keep the water clean. And so they were bred, shipped around the world and spread. But they grow fast, and can reach 1.5 ft in length. When they grow too big for a tank, they are often released into a local water body or flushed into sewers.
They sometimes escape from fish farms too.
Out in the wild, this family of fish — made up of species such as suckermouth catfish, armoured sailfin catfish and vermiculated sailfin catfish — can wreak havoc.
Because of their spiky bodies, they have few natural predators. They reproduce quickly, then suck up algae and detritus, disrupting nutrient cycles.
They can survive in low-oxygen environments, boosting their numbers further.
They eat the eggs of native species, and burrow into river banks to lay their own eggs, in numbers so large that they have exacerbated flooding.
The problem has become so severe that, two years ago, the Selangor state government in Malaysia allocated about $ 11,000 in incentives, hoping to take about 50,000 kg of the fish out of the Langat River. That effort is still ongoing.
In India, these species are rampant along the Ganges and in rivers in the Eastern Ghats.
In Mexico, in an effort to crack down on their numbers, the pez diablo or devil fish is being turned into food for humans and pets.
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SNOWFLAKE PATTERNS: A coral that slays coral
One of the greatest threats to corals worldwide is humans. We damage reefs daily, via bottom trawlers, heightened tourism, pollution; not to mention human-induced rising temperatures.
In some parts of the world, we now have competition.
Branched pipe coral, also called snowflake coral because it is covered in intricate white patterns, is native to the Indo-West Pacific region, but has spread around the world, via the aquarium trade, souvenir trade, and as a result of hull fouling or the accumulation of marine life on submerged surfaces of ships and boats that then travel to new regions.
Snowflake corals are voracious feeders and consume large quantities of zooplankton, competing with slower-growing native species. They are highly aggressive and often take over their territory too. This disrupts ecological balances across the local food chain.
In addition, this species is hardier than most and can grow unaided on metal, concrete, wood, plastic — and on other corals and shellfish.
Countries such as Hawaii and Brazil are attempting to tackle the problem through manual removal by divers.
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TROPICAL FIRE ANTS: Peanut butter to the rescue
Tropical fire ants were dangerous enough when they were restricted to their native habitats in Central and South America.
The voracious eaters will attack anything with living tissue. Their venom is so powerful that they have been known to cause fatal reactions in humans. They were suitably respected, feared and avoided by the tribes of the Americas.
Then the first Spanish ships arrived, in the 16th century.
They began to dig up dirt and load it onto ships as ballast; then dump some of this dirt, as needed, at various pit stops; and sail home with the rest.
The earth often contained this hardy, fast-spreading omnivore.
Around the world, these insects now threaten tortoise species and iguanas; eat butterfly eggs, contributing to imbalances in their population; consume the natural predators of mealybugs and other pests, thus contributing to farm losses. They have even been known to chew through tubing, causing damage to irrigation systems.
Efforts to control the population include poison baits and nest sprays. In an interesting detail, they appear to find peanut butter irresistible. Fumigators often trace nests by leaving a peanut butter stick out where these insects have been seen, and then waiting for a trail of fire ants to appear.
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TREE OF HEAVEN: An unstoppable beanstalk
It’s a tall tree that grows fast, is incredibly hardy, and appears to repel insects.
Intrigued by these qualities, European explorers took saplings home, starting in the 1700s.
The Ailanthus altissima or Tree of Heaven is now among the 10 most widespread invasive alien species worldwide, according to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES).
The tree is more than somewhat unstoppable.
It produces hardy winged seeds — as many as 325,000 per mature tree annually — and can also reproduce asexually by cloning itself via a powerful root-sucker system. In this way, it forms dense thickets. It resprouts vigorously after cutting or a fire.
Around the world, it is crowding out native species, and damaging sewers, building foundations, railway tracks, roads and pavements.
It releases a chemical called ailanthone into soil that works to suppress fungi and thus starve or weaken nearby plants.
One thing it does nurture is the invasive spotted lanternfly. The tree provides egg-laying sites and nourishes this hardy insect’s nymphs, amplifying the crop-pest problem across regions where it now grows.
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CHYTRID FUNGUS: Invisible frog killer
The Chytrid fungus is so tiny, it can only be seen under a microscope. Yet it is deadly to amphibians such as frogs, toads and salamanders.
It slides into their skin, blocks water, salt and air uptake, and causes lethargy, weight loss, ulcers and eventually death. (It then bursts out of the slain creature, and moves on to infest others.)
Discovered in the late 1980s, the fungus is believed to have driven 90 amphibian species extinct over the past 40-odd years, and is at risk of wiping out others.
Genetic strains have been traced to the Korean peninsula. In 2018, an international study published in the journal Science traced its global spread to the expansion of commercial trade in amphibians. Localised infections became a global problem, as people began to seek out exotic frogs and salamanders as pets.
In an effort to address the problem, researchers at University of Melbourne and University of Rochester, in 2024, developed a way to test for these creatures, using a fluorescent chytrid infection model. Anti-fungal baths have also proven effective, saving frogs’ lives in laboratory settings. But this measure cannot, of course, solve the problem at scale.
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JAPANESE KNOTWEED: Home-wrecking tendrils
A perennial plant with small, cream flowers, native to Japan, China and Korea, it looks harmless enough. Yet the Japanese knotweed is currently devaluing land and property in the UK and Ireland. It is also rampant in parts of Europe, New Zealand and North America.
It was introduced on these continents in the 19th century, as an ornamental plant and because it had proven effective in fortifying embankments and preventing erosion.
Botanists were fascinated by its ability to withstand high temperatures and drought, thrive in different soil types and amid salinity. They studied its breeding patterns, hardy seeds and hardier roots.
These have since invaded building foundations, walls, drains, roads, pavements and retaining walls, driving home owners to despair amid mounting repair bills. Root it out and, even if tiny fragments remain, they will sprout new plants.
In the UK, failing to declare a Japanese knotweed infestation during real-estate sales has led to successful lawsuits.
In many countries, the weed must now be removed along with surrounding soil, by licensed contractors, who must then dispose of them at designated landfills.
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RED FOX: From hunted to hunter
The rather fairytale bushy-tailed red fox can thrive almost anywhere, from icy tundras to sandy deserts. It eats indiscriminately: rodents, birds, insects, fruit, garbage, smaller animals, other foxes. It is widely regarded as one of the world’s most versatile predators.
It is now also one of the world’s most prevalent, according to IUCN’s (the International Union for Conservation of Nature’s) Global Invasive Species Database.
It was taken from Europe to Australia and the US in the 19th century, to be added to parks and forests for recreational hunting.
In Australia, in the decades since, the fast-breeding fox has led to declines in the populations of ground-nesting birds and small- to medium-sized mammals such as the greater bilby (a creature about 2 ft long that looks like a cross between a mouse and a hare). It also preys on newborn lambs, kid goats and poultry.
While bounties were once paid for the slaying of this species, modern efforts include baiting programmes (the delivery of poisons or contraceptives via food) and targeted culling.
