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.