The World’s Worst Invasive Mammals

Prized for its “medicinal” properties in parts of Asia and as a trophy species by South American hunters, the red deer (Cervus elaphus) has spread from its native Eurasia to the Americas, New Zealand and Australia. These deer aren’t picky when it comes to choosing a home—they inhabit temperate rain forests, mountain ridges, open grasslands and man-made clearings meant for livestock or agriculture. They do, however, devour specific plants, especially thick, moist grasses. This often leads to severe overgrazing and soil erosion, which disrupts the natural balance of the ecosystem and squeezes out smaller species with a similar diet. In Australia’s Royal National Park just outside Sydney, for example, patches of forest with higher deer densities have 30 to 70 percent fewer plant species than nearby areas with fewer deer.

In northern Chile and Argentina, red deer out-compete the Hippocamelus bisulcus, an endangered deer, and the guanaco, a South American llama. Red deer also spread bovine tuberculosis to co-habiting livestock. Their only natural predator is the puma, so humans are forced to control the deer population through hunting.

Ever since farmers in the mountains of western Iran domesticated the goat (Capra hircus) more than 10,000 years ago, populations have spread and thrived all over the world. Goats travel mostly in herds that can cover areas up to 12 miles across. Notoriously tough, they can survive in the harshest of environments, from isolated islands to steep mountain faces.

These scruffy herbivores will eat any plant they find; their four-chambered stomachs can digest almost any tough plant matter. Their eating habits can alter the composition of vegetation and quash biodiversity, particularly on isolated islands that have a delicate ecological balance. In recent years, aerial hunting, hunting dogs and GPS technology have been used to effectively control goat populations. But as domestic goats are the most widely consumed meat and milk source in the world, feral goats (which are domestic goats that become established in the wild) aren’t likely to disappear anytime soon.

There are an estimated 60 million feral cats (Felis catus) in the United States alone. Together, they kill around 480 million birds every year. Cats were domesticated (or perhaps domesticated themselves, according to some scientists) in the Fertile Crescent region of the Mediterranean in the early days of human civilization; the cats eradicated mice and rats before they could get to grain reserves. The cats did their job and proliferated throughout the world, thanks in large part to their popularity as human pets.

When house cats are allowed free range outdoors by their owners, however, or simply don’t have owners, they not only wreak havoc as opportunistic hunters, they can also spread disease. In addition to carrying rabies, 62 to 82 percent of cats in a recent study tested positive for toxoplasmosis, a parasite that has been shown to cause neurological damage to sea otters and other marine mammals that are exposed when heavy rainfall washes infected cat feces into the water. Cats have also hurt populations of birds, reptiles and other creatures. The black stilt of New Zealand (a seabird), the Okinawa woodpecker and the Cayman Island ground iguana are just a few of the dozens of endangered species at risk due to the proliferation of feral cats.

The long-tailed macaque (Macaca irus), a native of Southeast Asia, has been introduced into Mauritius, Palau, Hong Kong and parts of Indonesia. Identifiable by their extended tails—which are often longer than both head and body combined—this primate competes with birds for native fruits and vegetation, which make up 60 to 90 percent of their diet. Macaques have also been known to prey on the eggs and chicks of endangered birds. They give birth only to one offspring every couple of years, but that’s quite enough for scientists, who are currently investigating a vaccine to render females infertile in order to aid in population control.

They may look like furry friends, but short-tailed weasels (Mustela erminea, also known as stoats or ermine) can decimate mammal and bird populations. These intelligent, versatile predators can swim up to a mile in the ocean and can roam 40 miles at a time. They fearlessly attack larger animals and kill more than they can eat in one sitting, bringing in as much food as they can get their paws on.

In select cases like that of New Zealand, the weasel, a native of Eurasia and North America, has been introduced to exterminate smaller invasive mammals like rabbits.  “They haven’t really killed the rabbits, but what they have done is become a major predator of native wildlife, particularly birds,” says Mick Clout, a conservation ecologist at the University of Auckland in New Zealand.

The weasels feast on baby kiwis, New Zealand’s iconic bird, and they have contributed to the extinction of several other bird species. In response, Operation Nest Egg has set up kiwi nurseries that protect the chicks until they get big enough to protect themselves.

Popular pets and a source of meat, rabbits (Oryctolagus cuniculus) are native to the Iberian Peninsula. Phoenician traders introduced them to the wider Mediterranean, and rabbit populations have since become established in Africa, Australia and the Americas, where their persistent burrowing and overgrazing erodes soils and threatens native species. Rabbits are especially a problem on the 800 islands where they were introduced as food sources or tourist attractions, such as New Zealand’s South Island. If populations aren’t controlled on these islands, the rabbits have the power to wipe out every last bit of vegetation.

Rattus rattus originated in India and has spread like wildfire throughout the world, leaving no continent untouched. The rodents are scavengers and eat anything that is, was or ever will be edible and have contributed to the extinction of many birds, small mammals, reptiles, invertebrates and plant species. The bird decline has been the worst—it is now accepted that rats, not disease, were responsible for the disappearance of many native bird species in the 19th century, such as the Tahitian sandpiper. Rats are mostly nocturnal, which is why they can be seen scuttling around in the shadows; they carry pathogens, including bubonic plague, typhus, toxoplasmosis and trichinosis; and they breed frequently, giving birth to litters of three to ten with as few as 27 days in between.

Grey squirrels (Sciurus carolinensus) are native to the eastern United States and have spread to the western part of the country as well as to the Britain, Ireland, Italy and South Africa. Though not as widespread as rats, the grey squirrel has had marked impacts on its wooded habitat. When their usual diet of nuts, seeds, fruits and fungi is hard to come by, grey squirrels strip the bark off beech and sycamore trees. In areas where their counterpart, the red squirrel, is present, they outcompete them, causing red squirrel populations to dwindle.  Grey squirrels can also carry the parapoxvirus, which causes a debilitating and deadly disease in the native red squirrel.

The brushtail possum (Trichosurus vulpecula) lives only in its native Australia and nearby New Zealand, where it was introduced in 1837 to establish a fur trade. The nocturnal marsupial is about the size of a cat and lives mostly in trees. In Australia, dingoes and bush fires keep the population in check. But in New Zealand, an environment that evolved almost completely devoid of land mammals for 65 million years, until the arrival of the Maori around 1250 A.D., the possum invasion is a very different story.

Possums are now ten times more abundant in New Zealand than they’ve ever been in Australia. With an absence of predators, the possums are free to roam and graze on whatever’s palatable. Their feeding on eucalyptus leaves has created a large imbalance in the island forest vegetation, and the possum’s appetite for birds has depleted some species like the threatened kokako bird and the kereru, a native pigeon.

“The trouble is, they’re actually quite nice animals,” says Mick Clout, a conservation ecologist at the University of Auckland in New Zealand. “If you see them in their native Australia where they belong, they’re fantastic. But they don’t really belong here [in New Zealand].”

Of utmost economic concern is that possums are the main wild vector of bovine tuberculosis, which can devastate cattle. Though the animals are still trapped for their pelts, this does not completely control the population and wildlife authorities have been forced to use other, sometimes controversial, methods, such as aerial poisoning.

Herpestes javanicus is a small, agile creature with a slender body, short legs and a muscular tail. Hailing from Iran, India, Myanmar and the Thai Malay peninsula, the mongoose was introduced to islands including Mauritius, Fiji, the West Indies and Hawaii to control rat infestations on sugar cane plantations. But the mongoose soon found tastier morsels: native mammals, reptiles, amphibians, insects and birds. Now, both the rat and mongoose continue to degrade these island ecosystems. Fiji’s barred-wing rail and Hispaniola’s racer have both gone extinct because of the mongoose, and the critically endangered petrel of Jamaica may soon follow.

Editor's Note: This entry originally had a photo that was incorrectly identified as a mongoose. We have replaced that image with the one above. Thanks to our readers for catching our error.

Myocastor coypus, or the nutria, is a semi-aquatic rodent originally from South America. Despite their resemblance to rats, nutria were once cultivated for their soft fur. Large groups escaped from fur farms and bred larger feral populations that now inhabit parts of Europe, North America and Asia.

These rodents are accomplished burrowers; their tunnels run through the reed beds and marshlands where they live, eroding river banks and dykes and damaging irrigation facilities. In large numbers, nutria can eat so much vegetation that what began as marshland can quickly turn into open water. In Japan, nutria threaten the critically endangered dragonfly Libellula angelina and the deep-bodied bitterling fish. In Italy, nutria have destroyed the layer of water lilies that once allowed whiskered terns to breed.

Apart from humans, mice (Mus musculus) are thought to be the most widely distributed animal in the world. Humans and mice have carried on a somewhat imbalanced partnership over the past 8,000 years: mice take shelter in man-made structures like houses and pass on diseases such as bubonic plague and salmonella. Mice can devour crops and human food reserves. And perhaps second only to eating, the thing mice do best is breed. Females have five to ten litters per year of around six young each. Their numbers sometimes even reach plague status, with millions of mice yielding extensive economic damage by eating stored food or digging up crops. Mice have also been shown to prey on albatross chicks and cause breeding failures in albatross and petrel populations in places like Gough Island in the South Atlantic.

Known as wild or feral hogs, pigs or boars, wild pigs (Sus scrofa) once roamed European and Asian hillsides. The purebred pigs have now gone extinct in much of their native range, but they have spread to other parts of the world including New Zealand, Australia, Latin America and North America. Pigs root as deep as three feet below the soil’s surface using long, sharp tusks. This tears apart surface vegetation and alters the nitrogen content of the soil. Hunters appreciate pigs’ cunning and aggression, but these same traits cause pigs to outcompete native species. They have even been known to terrorize visitors to national parks. And the pigs can carry foot-and-mouth disease and an array of other unsavory illnesses that can devastate domestic animal populations. The United States has experienced a dramatic increase in wild pigs in the past 30 years, especially in Texas, where damages are estimated to cost $400 million each year.

The Vulpus vulpus, or red fox, is native to Eurasia, North Africa, Central America and the Arctic. In its native habitat, the fox is at times considered a vital check on small mammals and rodents. But in areas where the fox has invaded, its presence can be detrimental. Since being introduced to Australia for hunting purposes in the mid-1800s, foxes have contributed to the decline of dozens of native animals, including newborn lambs. The fox roams expansive distances of up to 190 miles, which makes it a dangerous carrier of diseases like rabies. Traditional means of fox control—poison, hunting and fencing—are in place, though hunting has declined in recent years due to decreased demand for furs.