5 Invasive Species That Could Overrun the World

Humans often believe they're the masters of the Earth, but we do make some questionable decisions. One example: kudzu. Introduced to the U.S. during the 1876 Centennial Exposition, Americans were so charmed by this fragrant, leafy plant that they began using it for decoration. Later, they even used it as livestock feed. The government got involved too, urging the Civilian Conservation Corps to plant it for erosion control. But not a single thought was given to how this foreign species would behave in a new environment. A critical oversight.

Kudzu thrived in the Southern U.S., growing up to 60 feet (18.3 meters) annually, overtaking buildings and utility poles, and suffocating cars and homes. It's still a persistent problem to this day.

Sadly, this isn’t a rare occurrence. Over the years, people have introduced non-native species to their countries for their beauty or potential problem-solving abilities. For example, amphibians or birds were brought in to combat insect pests harming crops. However, these species often lacked natural predators in their new homes, and without checks on their growth, they flourished and became significant problems—sometimes even much larger ones.

Invasive species are now widespread across the globe, causing significant damage to ecosystems and threatening biodiversity. Their presence outside of native regions is costing billions of dollars in remediation, removal efforts, and prevention strategies [source: EarthTrends]. Despite many governments tightening regulations on travel, imports, and plant nurseries, non-native species continue to be unintentionally transported across the world—hidden in shoes, luggage, or even hitchhiking on boat hulls. Some of these species—such as the five on this list—seem nearly unstoppable.

5: Asian Carp

Imagine you're peacefully drifting down a river or across a lake when suddenly, the water erupts with chaos. Enormous, prehistoric-looking fish start leaping and thrashing around. One of them smacks you in the face, leaving a painful bruise. Does that sound like a serene moment? Hardly. These ‘attacks’ are just one of the many problems posed by Asian carp.

Asian carp, originally from China and Southeast Asia, were brought to the southeastern U.S. over 20 years ago to help clear algae from catfish ponds. Since then, these voracious fish have made their way up the Mississippi and Illinois Rivers, consuming so much plankton and other organisms that there's almost nothing left for native species. With their rapid reproduction rate and the lack of natural predators, they’re quickly overwhelming native fish populations. Now, they are preparing to invade the Great Lakes—a region already struggling with invasive sea lampreys, zebra, and quagga mussels—putting the $7 billion fishing and tourism industries at risk [source: Harrison].

However, the Great Lakes are not the only places under threat. Asian carp have also made their way into the Kansas River and are now threatening to spread into the Arkansas River. They're causing similar disruptions in Eastern Europe as well [source: Pearce]. And who can say where these giant fish—some weighing over 100 pounds (45.4 kilograms)—might appear next? [source: WebEcoist].

A glimmer of hope: In China, Asian carp are considered a prized delicacy, though they are becoming increasingly rare due to overfishing. Some fishermen in the Midwest are now catching these fish and exporting them back to China [source: WebEcoist].

4: Golden Bamboo

Bamboo is widely admired for its tall, sturdy structure, graceful green leaves, and soothing, exotic appearance. Its popularity is so great that hundreds of bamboo species have been brought to the U.S. for use as ornamental plants. However, some bamboo varieties, particularly the 24 species within the Phyllostachys genus [source: Brown], are a bit more troublesome. Among them, Phyllostachys aurea, or golden bamboo, is particularly problematic.

Golden bamboo was introduced to Alabama from China in 1882, originally intended to serve as privacy screens, both visually and acoustically. A fast-growing, aggressive plant, it can grow up to 30-40 feet (9-12 meters) tall, and it quickly spreads, overpowering everything in its path. It damages native plants and the habitats they support, offering no benefits in return. Today, golden bamboo is a major issue in the Southeastern U.S., from Maryland to Arkansas, though it also poses a threat in Oregon and other Western states. The financial burden of controlling its spread is a staggering $138 billion annually for U.S. taxpayers [source: Brown].

But it's not just America that's fighting golden bamboo. The plant is cultivated worldwide as an ornamental in tropical to temperate areas, and other countries -- such as Australia -- have problems controlling it, too [source: U.S. Forest Service, Bamboo Wholesale].

3: European Rabbits

Ah, bunnies. They're so adorable, aren't they? Unless you're a homeowner or farmer, that is. Those cute, fuzzy little critters are ruining land all over the world, causing soil erosion through their overgrazing and burrowing. They also nibble on people's landscaping and flowers, and negatively impact native species by damaging fragile ecosystems.

European rabbits are native only to Southern Europe and Northern Africa. But over time, they've been introduced to almost every continent. And wherever they've been introduced, they quickly proceeded to, well, breed like rabbits. For example, a mere 24 were released in Australia in 1859 by an English farmer who thought they'd provide "a touch of home, in addition to a spot of hunting" [source: WebEcoist]. Today, rabbits have contributed to the extinction of nearly an eighth of Australia's mammal species, ruined the country's soil and caused millions of dollars annually in agricultural damage [source: Environmental Graffiti].

Australians did try to eradicate their rabbit population in 1950 by introducing the Myxoma virus to their mainland. A biological control agent, this virus causes Myxomatosis, a disease fatal in nearly every rabbit that contracts it. Five hundred million rabbits died, but the 100 million that remained developed a resistance to the disease [source: WebEcoist]. And now, the bunnies' numbers are again on the rise [source: Zukerman].

2: European Starlings

The European starling, known for its noisy and aggressive behavior, has spread across nearly every part of the world, often because of its striking appearance [source: Columbia]. In the U.S., the birds were introduced in the late 19th century, around 1890, when a group of Shakespeare enthusiasts released 100 starlings into Central Park, hoping to introduce all the birds mentioned in the Bard’s plays to North America. Today, more than 200 million of these starlings inhabit the continent [source: OMAFRA].

Beyond their striking appearance, which includes glossy black feathers with iridescent green and purple specks, starlings are omnivores. They form enormous flocks, sometimes containing over a million individuals, and these masses wreak havoc on agricultural crops. They are particularly fond of grapes, olives, cherries, and grains, often attacking fields at the early stages of crop growth and uprooting young plants to consume the seeds. Starlings also push out native bird species by competing for food and nesting sites. Additionally, they have been known to invade livestock and poultry areas, contaminating feed and water sources while devouring the food. Their flocks have even been linked to deadly collisions with aircraft [source: WebEcoist, Columbia].

While some defend European starlings for their appetite for insects, which is why they were introduced to countries like New Zealand in the first place, the overwhelming consensus is that the harm they cause far outweighs any potential benefits from bug control [source: Columbia].

1: Cane Toads

The cane toad, a species originally from Venezuela and Guyana, has been introduced into many countries. Known for devouring insects that threaten sugarcane and other crops, these toads have become infamous for their aggressive nature. Reaching lengths of up to 15 inches (38.1 centimeters), they consume nearly any terrestrial creature they come across. Competing with native amphibians for resources and breeding areas, they pose a serious threat. To make matters worse, their skin secretes a powerful toxin, which is deadly to both domestic animals and wildlife, and in some cases, even humans. Fatalities have been reported from consuming these toads or their eggs [source: Butler].

Cane toads are especially problematic in places like the U.S. and Australia. In Australia, the situation is dire, with many believing that eradication is no longer feasible due to the overwhelming numbers of toads. However, one researcher from Queensland is exploring a solution: genetically modified cane toads that can only produce male offspring. This approach may eventually lead to their extinction if these modified toads interbreed with wild populations [source: IMB - Institute for Molecular Bioscience]. Only time will tell whether the toads or humans will prevail in this battle, and whether we have learned from our past mistakes of introducing non-native species into new environments.