Invasive animal species have a bad reputation—from tree-killing insects to rampaging wild pigs, they often crowd out native fauna and alter the environment they invade. Though through no fault of their own—they are almost always introduced to new areas by humans.

What Is an Invasive Species?

Invasive species  are plants and animals that have been moved, typically over long distances, outside of their native habitat and into a new region, impacting the other species that live there. "Invasive" does not refer to a species as a whole, but rather to particular populations of that species based on location.

Often, invasive species proliferate rapidly because they lack natural predators that would keep their population in check. They range from tiny insects that harbor novel diseases to apex predators that can upend an entire food chain. 

Learn more about 10 invasive species that came to dominate new landscapes and changed the environment forever. 

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Earthworm

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Earthworms are considered one of the original invasive species. Given the ubiquity of earthworms, it only seems natural that they have existed underground across the world for millions of years. But in North America, native earthworms were largely wiped out by expanding glaciers during the Pleistocene ice age. Most of the earthworms in the United States, especially in northern states, are actually descended from  species that arrived in America with the first European settlers . 

While gardeners appreciate seeing  earthworms in the soil , worms have had a mixed effect in North American forests. Studies have shown that invasive earthworms can reduce ground cover, allow invasive plants to thrive, and reduce  populations of ground-nesting ovenbirds . 

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Cane Toad

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Cane toads are one of the most populous invasive species found in Australia. They were unleashed in 1935 to combat pests like cane beetles that were affecting sugarcane fields. However, with no natural predators, the toads proliferated rapidly and soon became a threat to native species. 

Cane toads prey on many smaller native animals, and would-be predators are not adapted to withstand their toxins. In some cases,  populations of native lizards and snakes dropped by 80 to 100%  after cane toads appeared. Cane toads now appear in most of northern and western Australia, and are spreading across the country at a  rate of about 30 miles per year . 

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Zebra Mussel

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Zebra mussels made their debut in North America in 1988, after being introduced by ships traveling from their native Russia. They have since spread across the Great Lakes, into the Mississippi River and its tributaries, and have been found in Colorado, Texas, Utah, Nevada, and California. Mussels may sound like one of the more nonthreatening creatures in the ocean, but the spread of zebra mussels has had broad repercussions. They edge out native clams and mussels, clog industrial intake valves, and  accumulate toxins that can affect waterfowl that prey on them .  

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Brown Rat

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Rats have a long, devastating history as an invasive species. They were the first invasive species to arrive on Australia's uninhabited Macquarie Island, soon after the island was discovered in the southern Pacific Ocean in 1810. The rats, along with introduced rabbits and cats, stripped the island of its natural vegetation and caused the extinction of two native bird species—the  Macquerie Island parakeet  and  Macquerie Island rail.  

In 2007, the Australian government  pledged $24.6 million dollars  to eradicate invasive species from the ecosystem by trapping, hunting, and monitoring. In 2014, they  announced that the project was a success.

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European Starling

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The European starling is native to Europe, Asia, and northern Africa but has been introduced to most of the world's habitats, except for tropical rainforests. In the United States, starlings were introduced as part of a plan to fill the American landscape with  all of the species referenced in works by Shakespeare . Starlings now exist in massive flocks that  outcompete native species, steal nests from other birds, and damage crops .

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Brown Tree Snake

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The brown tree snake decimated native bird populations in Guam after it was introduced to the Pacific island in the 1950s, probably via cargo ships or aircraft. The snakes spread rapidly across the entire island, and by the 1990s, some reports estimated about  30,000 snakes per square mile . They drastically reduced native animal populations and caused power outages by climbing electrical wires. 

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Mountain Pine Beetle

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Mountain pine beetles are only about one-fourth of an inch long, but these invasive pests have had an enormous impact on pine forests. They bore under a tree's bark, laying eggs and depositing a fungus that kills the tree . In the western United States and Canada, a 20-year outbreak that began in 1995 destroyed millions of acres of forest. The outbreak was particularly bad in British Columbia, where pine beetles killed  nearly 30% of all forests . Scientists believe that this outbreak was particularly extensive because  warmer winters due to climate change allow the beetles to expand their range . 

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Northern Pacific Seastar

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The northern Pacific seastar is an invasive species in Australia. It is a voracious predator that feeds on mollusks, crabs, dead fish, and other seastars. Female seastars can produce  10 to 25 million eggs per year , leading to rapid population growth. 

In Australia, it has contributed to the decline of the  spotted handfish , a unique fish that "walks" on the seafloor using highly adapted fins. The spotted handfish is now considered  critically endangered and is only found in the estuary of the Derwent River in Tasmania. 

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Wild Pig

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Wild pigs are one of the most prevalent invasive species in North America. They were brought to the Americas in the 1500s as domestic livestock. Escaped pigs soon turned into feral herds that live in the wild. In 2018, the population in the United States was estimated to be  6 million and growing, with feral pigs found in 35 states.

Controlling the feral pig population is mostly done by hunting, which is a monumental task. A study in Texas found that to keep the population from increasing, hunters would need to harvest 66% of the pig population every year due to its high reproductive rate. 

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Burmese Python

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Burmese pythons have supplanted alligators as the dominant apex predator in Florida, including in the protected ecosystem of the Everglades. They were  introduced in the area by the exotic pet trade , and found their way into the wild by escaping or being released intentionally by their owners.

Pythons are aggressive and capable hunters of many native species. A 2012 study found that pythons were responsible for massive declines in native mammal populations in South Florida, including the loss of 98.9% of opossums and 87.5% of bobcats.