When an invasive species is introduced to a new ecosystem, the fallout can be dire, but that hasn’t stopped hundreds of exotic animals and plants from finding their way to the U.S.

For as long as people have roamed the globe, animals have followed. Whether they stow away on an adventurer’s mode of transportation, find their way into imported food crates or are brought over purposely, these invasive species move into an ecosystem ill-prepared for their arrival.

Here are the most destructive invasive species in the U.S.

Rock Python

The Burmese python may be ravaging the Everglades, but its big, deadly cousin, the rock python, is even more destructive. The rock python’s origin is the grasslands of central and southern Africa. One of the half-dozen giant snakes in the world, it can grow to over 20 feet. Despite its slim form factor, it can stretch its jaws and skin to take in prey as large as pigs, antelopes and more. In Africa, the rock python eats everything from goats to crocodiles. To add more to its menacing reputation, it is a powerful swimmer and a prolific breeder. Naturalists fear that the rock python, which has infiltrated the Everglades, could mate with the Burmese python, forming a new “super snake.”

Nile Monitor Lizard

The Nile monitor lizard may be much less well-known than the pythons wreaking havoc in Florida, but it too is formidable – known as one of the most aggressive lizards to be found in the pet trade. Also called the water leguaan, or river leguaan, the giant representative of the monitor lizard family grows to about five feet, three inches in length, with the largest specimens reaching a petrifying eight feet. Nile monitor lizards have brawny bodies, strong legs and powerful jaws. Their teeth are sharp and pointed in juveniles, and blunt and peg-like in adults. The lizards boast sharp claws deployed for climbing, digging, defense and offense. In Florida, established breeding populations of Nile monitor lizards date back to the early 90s. The risk that they could derail native crocodilians – the American alligator and American crocodile – is huge given that they habitually raid crocodile nests, eat eggs and prey on small crocodiles in Africa. Additionally, many domestic pets and feral cats have gone missing in Cape Coral, Florida, and the Nile monitor lizard is the top suspect.

Snakehead

Dubbed the Frankenfish, the snakehead is a freshwater fish from Africa and Asia. The cylindrical invader stretches up to 36 inches. During the summer of 2002, several northern snakeheads turned up in a pond in Crofton, Maryland, about 20 miles northeast of Washington, D. C. Their threat was seen as so severe, that the incident made national headlines. Officials erected signs urging anglers to kill any snakeheads that they caught. The species is a devouring, top-tier predator. Unless you count anglers, it has no natural enemies, and could decimate native fish populations. Wiping out the invader looks to be an incredibly hard task because behaves unlike any other fish. It can breathe air and survive for up to four days out of water – longer when burrowed in the mud. It can even travel over land to new stretches of water by wriggling its body.

Brown Tree Snake

This invasive species is found on an organized, unincorporated territory of the United States in the western Pacific Ocean: Guam. There, you just might brush up against a brown tree snake. In Guam, the brown tree snake has reached densities of up to 100 per hectare (10,000 square meters). That is seriously bad news for local island wildlife. The invader has sparked the extinction of at least 12 bird species, making it as bad as any other North American invader, and it has a reputation as one of the world’s most aggressive invasive species. The brown tree snake arrived in Guam aboard cargo ships during World War II from Papua New Guinea, Indonesia, the Solomon Islands and both the northern and eastern regions of Australia. It grows up to six feet and is extremely venomous. It can decimate populations of small vertebrates in the areas it colonizes.

Africanized Bee

The Africanized bee has colonized the Southwestern U.S. and is a mutant hybrid of African and European honeybees. They arose in Brazil in the 50s, dubbed the “killer bee.” Fortunately, despite its nickname, the sting of the average Africanized bee is no worse than that delivered by the common honeybee. The Africanized bee, however, is much less sweet-natured. In fact, it is highly violent. Mass attacks and multiple stings that can be cumulatively fatal to pets and people are very common. The first Africanized bees in the United States were discovered in 1985, in the San Joaquin Valley of California. By 1990, they had spread to Texas from Mexico, and they are still on the march. They have become the dominant honey bee farming type in Central America because they outperform European subspecies and have improved productivity.