10 Feral Animals Wreaking Environmental Havoc

front view of wallaby standing on ground with ears perked

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Zoo escapees, natural-disaster refugees, throwaway pets, runaway farm animals — however they end up in the wild, feral animals are everywhere, and it's more than just cats and dogs. Give any domesticated animal a hospitable environment in the great outdoors with opportunities for breeding and chances are they’ll find a way to thrive.

Some feral animals are relatively harmless — charming additions to their adopted ecosystems. Others, though, are more like invasive transplants that spread mayhem wherever they roam. Read on to learn about some of the planet’s most harmful feral animals.

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Nile Monitor

nile monitor lizard shows forked tongue near puddle

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Native to Africa, the Nile monitor is a cousin of the Komodo dragon. These creatures are considered an invasive species in South Florida, where escapees from exotic pet stores and homes have taken to the wild and multiplied over decades.

These lizards are intimidating, with razor-sharp fangs and claws and growing to a length of up to 6.5 feet. However, though it may be disconcerting to see them wandering across backyard patios, climbing on roofs, and sliding into swimming pools, they are typically not aggressive toward humans unless threatened. Their main reason for being troublesome is their consumption of native wildlife and fish.

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group of four burros stand near dry bushes

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If you drive through the desert landscape of Nevada’s Red Rock Canyon National Conservation Area (just a 30-minute trip from the Las Vegas strip), you can’t miss them: feral burros wandering as freely and abundantly as squirrels.

These small donkeys are descendants of the burros abandoned by Spanish explorers in the 1600s and miners in the 1800s. They now roam most of the western United States under the protection of the Bureau of Land Management.

Burros compete with native wildlife for limited resources. They are aggressive and territorial, which means they often win, restricting other animals from the food and resources they need.

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Red-Eared Slider

red-eared slider turtle perches on log above water

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Red-eared sliders are one of the most common turtles sold in pet stores. But these darlings of the domesticated reptile world are also flourishing in ponds and lakes in New York’s Central Park and Prospect Park, as well as in waterways in several other states.

Mostly escapees and discards from homes, these feral turtles have been multiplying since the 1930s. Like burros, they are more aggressive than the wildlife they share habitats with, so they can bully other species away from important resources.

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front view of single-hump camel

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Used in the 1800s by settlers in the Australian Outback, camels fell by the wayside once automobiles came along. However, as of 2010, there were over 1 million feral camels in Australia, eating their way through native vegetation and even terrorizing towns as they search for water in drought-stricken regions.

In January 2020, the Australian government conducted a five-day cull of feral camels because they became dangerous to nearby communities and infrastructure. Some camel critics even blame their intestinal gas (methane) for climate change.

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profile of tan and gray hog walking along beach

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Descended from escaped farm pigs, hogs have taken to the wild in several states, including Arkansas, Texas, Alabama, and Wisconsin. These millions of boarish bandits destroy agricultural crops, residential property, and wildlife habitats. They even attack humans and livestock in their search for food.

Many communities now encourage hunters and residents to shoot or trap them. This is not the case on the island of Big Major in the Bahamas, though, where beach-loving feral swines delight tourists and locals.

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Guinea Pig

wild guinea pug sits alert in short grass

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While they're not as intimidating as real pigs, guinea pigs are a major nuisance on the Hawaiian island of Oahu. Authorities speculate that most of the island’s guinea pigs are descended from fugitive pets or even just one pregnant runaway. Either way, these furry ferals devour residents’ shrubbery and ornamental plants at an alarming rate and take a significant toll on native plants and crops as well.

Typically, each female guinea pig gives birth twice a year with up to four pups per litter, so the invasion is not likely to be quelled anytime soon.

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wallaby with long tail stands in sunlight

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Deep in the forest of Rambouillet, west of Paris and thousands of miles away from their native Australia, wallabies are flourishing. These kangaroo-like creatures are escapees from a nearby wildlife park decades ago. While they don’t appear to do much harm to local ecosystems, they do occasionally startle unsuspecting drivers, often winding up as roadkill.

Several other colonies of feral wallabies exist around the world as well. There’s one on Lambay Island, off the east coast of Ireland; the Dublin Zoo released them there in the 1980s after experiencing a sudden wallaby population explosion. Another colony of farm escapees thrives in Cornwall in the U.K. There’s even a colony in the Kalihi Valley on Oahu, made of up descendants of runaways from a local zoo nearly 100 years ago.

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brightly colored wild chickens walk on grass

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Hurricane Katrina brought many problems to New Orleans, one of which was an explosion of chickens. Groups of feral chickens wander through many neighborhoods, particularly the city's histori Ninth Ward, pecking and squawking all the way. Authorities believe they are descended from backyard hens and roosters that survived the deluge.

Philadelphia, Miami, Los Angeles, and Key West have also had their own struggles with feral chickens. Animal control officers attempt to capture these poultry pests and transplant them to local farms.

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Cow and Water Buffalo

wild black and brown cows rest in open grass

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On Lantau Island, the largest in Hong Kong, cattle and water buffalo were once used to plow rice paddies. With the decline of rural life in the 1970s, the cattle were set free and now wander the island grazing in herds. Many people find them a picturesque, endearing part of the Lantau experience. Others, however, want them gone, claiming that the seemingly placid beasts destroy fences, eat crops, block traffic on local roads, and even attack people. These accusations are not unfounded — in 2011, a young water buffalo charged and gored a man, severely injuring him.

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Burmese Python

close up of burmese python coiled body

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Nile monitors aren’t the only feral foreigners plaguing Florida. The state is also being invaded by Burmese pythons, which were introduced to the wild by errant pet owners. Tens of thousands of these snakes — some growing up to 20 feet long — inhabit the state’s Everglades National Park. There, researchers suggest they may be responsible for a precipitous decline in the populations of birds, reptiles, and native mammals including opossums, bobcats, rabbits, and deer.