10 Animals That Are Bad for the Environment

When an imbalance occurs in an ecosystem, humans aren't the only ones to blame.

cattle graze in deforested Amazon land

Brasil2 / Getty Images

Animals play an important role in keeping ecosystems balanced, healthy, and strong. And, like humans, animals can respond to an imbalance with extreme behavior that can be harmful to the environment. Many of these animals are invasive, and only cause problems once introduced to an area with no natural predators. Others find that being confined to certain areas causes them to increase their amount of destruction.

Here are 10 animals that can be bad for the Earth when nature's balance gets disrupted.

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Elephant in Kenya pulling down a tree

Doug Woods / Flickr / CC BY 2.0

Elephants are the world's largest and most powerful land animals, so it's not surprising they have a profound impact on the ecosystem. To reach food, elephants regularly break branches, uproot bushes, and push down whole trees—sometimes several trees next to each other.

Elephants prefer to roam across a vast territory, so forests can usually recover from the damage they cause. But when fences, farmland, and human encroachment shrink the range of these magnificent beasts and their actions become concentrated in a smaller region, elephant behavior radically alters the environment.

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Thick swarm of plague locusts filling the skies and ground

CSIRO / scienceimage.csiro.au / CC BY 3.0

The locust swarm of lore is one life phase of a type of short-horned grasshopper. It becomes a plague under the right conditions. The swarms can cover hundreds of square miles and consist of many billions of locusts. They are highly migratory and can quickly strip whole fields of vegetation. Each insect consume 2 grams of vegetation daily, which means that a "swarm of 80 million can consume food equivalent to that eaten by 35,000 people a day."

The swarm is initiated by a population burst caused by rain followed by drought, pushing greater numbers of the insects into a smaller area. Scientists in England and Australia say these close quarters trigger a chemical response. Unfortunately, the change also creates a domino effect—swarming locusts are driven to breed and eat at an increased rate.

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Crown-of-Thorns Sea Star

large thorny sea star on dead coral

Johan J.Ingles-Le Nobel / Flickr / CC BY-ND 2.0

This large starfish gets its name from the venomous, long spines that cover its body. They live among and feed on coral polyps. When the species becomes overpopulated, it can destroy vast coral reef ecosystems. In fact, widespread destruction of the Great Barrier Reef is partly blamed on these sea stars, which have experienced a population explosion over the last decade or so. They are especially harmful at times when the coral is already weakened due to bleaching incidents.

The outbreaks are likely due to pollution from agricultural runoff, creating algae blooms that allow the crown-of-thorns' natural predators—including its main one, the giant triton—to get a less thorny and easier meal elsewhere. During outbreaks, the starfish feed on adult corals and prevent the maturation of young corals.

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Beef Cattle in overgrazed area

Mangiwau / Getty Images

According to the U.N. Food and Agriculture Organization, cattle farming is responsible for 14.5% of greenhouse gases. Cattle emit a large volume of methane through burping and flatulence, a more powerful greenhouse gas than carbon dioxide.

Cattle ranching is also a major source of deforestation worldwide, most notably in South America's Amazon rain forest, which release yet more carbon into the atmosphere. Driven by the growing food demands of an expanding human population, cattle in many regions of the world are overgrazing, reducing the ecosystem's biodiversity in the process.

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Common Carp

invasive carp in river

mark6mauno / Flickr / CC BY-SA 2.0

The common carp, native to Asia, is a true bottom-feeder, uprooting and disturbing submerged vegetation. These fish are notorious for altering their environment. After they disturb the vegetation, they release phosphorous through their droppings. The combined effect is reduced food for other animals and plants in the waterway.

These fish grow fast and aren't picky eaters. They are most dangerous when introduced into an alien habitat and become an invasive species. There are invasive carp in every U.S. state and throughout the Great Lakes. Natural resource agencies in the United States and Australia spend millions annually to control the common carp.

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herd of goats of all colors on overgrazed pasture

Oregon State University / Flickr / CC BY-SA 2.0

Goats can have profound negative effects on habitats that are not adapted to them. They can be voracious grazers, often with a taste for native scrub, trees, and other vegetation, turning whole woodlands into deserts if left unchecked. They compete for pasture with domesticated animals. Feral goats are particularly bad in places like Australia and on isolated islands worldwide where human populations have attempted to establish a settlement. Goats are rugged animals that can easily revert to a feral existence if allowed to do so.

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Cane Toads

Greyish brown cane toad with many warts standing on grass

Florida Fish and Wildlife / Flickr / CC BY-ND 2.0

Cane toads have become wildly successful as an invasive species in Oceania, the Caribbean, and the United States. National Geographic describes them as "one of the worst invasive species in the world." Ironically, cane toads were purposely introduced to foreign habitats to eradicate agricultural pests, and in the process, they became pests themselves.

These highly adaptable South American natives, which eat almost anything and breed year-round, are most dangerous to native wildlife because their poison glands are toxic to birds, mammals, fish and reptiles—and anything else that attempts to eat them.

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Bark Beetles

lesser larch bark beetles and their galleries under the bark of a black pine tree

Gilles San Martin / Flickr / CC BY-SA 2.0

Many bark beetle species choose dead or rotting wood to reproduce in, but several species (including the mountain pine beetle of western North America) are known to attack and kill live trees. Whole stands of forest can be destroyed if bark beetle numbers get out of control. The bugs can also be carriers of disease, as is the case with the American elm bark beetle, which transmits Dutch elm disease. They have high tolerance for overwintering and adapt well to warming temperatures.

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large group of grey rats on ground stripped of vegetation
John Downer / Getty Images

Rats are wildly successful animals wherever they live—a characteristic that makes them dangerous when introduced to nonnative areas. One primary example has been introducing black rats onto Lord Howe Island, a small habitat in the Tasman Sea where much of the island's unique native wildlife has been wiped out by invading rats. Rats also bear disease, and rat population outbreaks can cause substantial food losses, especially in developing countries.

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Climate change protest
In Pictures Ltd. / Corbis via Getty Images

Of all the animals on Earth, humans are the most environmentally destructive. Humans cause major imbalances—global warming, the extinction crisis, over-harvesting of land and sea, damaging resource extraction, pollution, overpopulation, and industry. Some of these impacts are just now beginning to be recognized. For example, plastic pollution is not only a visible nuisance; it creates long-lasting health issues. Fortunately, humans are capable of rapid cultural change. They always have a choice—and a chance—for change.

View Article Sources
  1. Spinage, C.A., and F.E. Guinness. "Tree Survival in the Absence of Elephants in the Akagera National Park, Rwanda." Journal of Applied Ecology, vol. 8, no. 3, Dec. 1971, pp. 723-728, doi:10.2307/2402679

  2. Njagi, David. "The Biblical locust plagues of 2020." BBC: Future Planet.

  3. "Crown-of-thorns starfish." Oceana.

  4. FAO and UNEP. "The State of the World's Forests 2020."

  5. "Cyprinus Carpio Linnaeus, 1758." USGS.

  6. "Cane Toad." USDA National Invasive Species Information Center.

  7. "Lord Howe Island - Rodent Eradication Project." Lord Howe Island Rodent Eradication Project.