Basically, American bullfrogs, eastern gray squirrels, and American minks, which have been introduced to other parts of the world, either accidentally or intentionally, are doing the same thing that the Burmese pythons and feral pigs are doing in the United States―disrupting the ecological balance. When we talk of the world's most invasive species, the American bullfrog (Rana catesbeiana), which happens to be the largest frog in North America, is one species that cannot be ignored. Native to the eastern half of the United States, the bullfrog is now found in more than 40 countries across 4 continents. While its presence in countries like Mexico and Canada, or even in other parts of North America, may not come across as a surprise, its presence in Europe, i.e., in Italy, France, and the Netherlands, and even farther in China definitely does. Even more surprising is the fact that the species was introduced to most of these countries as a food source. In its newfound habitat, the American bullfrog poses a serious threat to the ecosystem, predating on native species of insects, fish, snakes, and other frogs, and competing with other species for food. If the American bullfrog escaped from frog farms, eastern gray squirrels (Sciurus carolinensis) escaped from zoos and other captive habitats to establish themselves in different parts of the world. Far from its native habitat in the eastern and midwestern United States, the eastern gray squirrel is now found in England, Ireland, Italy, and even South Africa. It may come across as a harmless squirrel at first, but its voracious feeding habit makes it one of the most unwanted species of Europe. Additionally, the species is also known to cause harm to the local ecosystem by displacing the native species, like the red squirrel. It is one of the 100 alien species that made it to the list of 100 of the World's Worst Invasive Alien Species compiled by the World Conservation Union's Invasive Species Specialist Group (ISSG). The North American raccoon (Procyon lotor), also known as the common raccoon, northern raccoon, and simply raccoon at times, is an omnivorous mammal native to the deciduous and mixed forests of North and Central America. Raccoons were introduced to the USSR, France, and Germany intentionally so as to establish a population that could be hunted for their fur. The species did more than what was expected of it, and didn't just establish itself, but literally invaded the new habitat. Its exceptional adaptability had a crucial role to play in its invasion of Europe and Asia. Additionally, the wild population was also fueled with people irresponsibly releasing their pet raccoons in the wild after realizing that they didn't make good pets. The American mink (Neovison vison), a mustelid native to the United States and Canada, was introduced to Europe and South America in 1920 with the aim of promoting fur production. The introduced range of this animal spans Iceland, Scandinavia, Russia, Germany, France, Spain, Italy, and the British Isles in Europe, as well as Argentina in South America. The intentional release of the species in Europe turned out to be a nightmare for the native European mink whose population was affected by the intruder. Interestingly, the case of American minks has a striking resemblance to that of nutria, which were introduced to the United States for the very same reason. Yet another species native to the United States which has been wreaking havoc in the foreign waters for quite some time now is the American lobster (Homarus americanus). While its native habitat spans the Atlantic waters along the coast of North America, this lobster species is now found in the coastal waters of Europe as well. The species either escaped whilst being transported from North America to Europe, where it was in high demand as food, or was intentionally introduced in these waters to establish its population. The American lobster is also known to be a carrier of the red tail disease, which is taking its toll on the European lobster population. The red swamp crayfish (Procambarus clarkii), also known as the Louisiana crayfish, is a freshwater crayfish which was restricted to the southeastern United States at one point of time, but has now established itself in parts of Europe, Africa, Asia, and South America. It was either introduced in these countries as a culinary species or for its use as a bait. Interestingly, the red swamp crayfish has invaded parts of North America as well. The biggest problem with this species is the alarming rate at which they breed and their ability to adapt to harsh conditions. According to a study published in the journal PLOS ONE, the red swamp crayfish is known to come out of water and feed on terrestrial plants in times of food scarcity. The Colorado beetle (Leptinotarsa decemlineata), ten-striped spearman, ten-lined potato beetle, or simply the potato bug―call it whatever you want, you can't deny the fact that it is one of the most loathed pests in various parts of the world. Native to Colorado (United States) and Mexico, the species has hitchhiked its way to the UK, Ireland, Sweden, Finland, Cuba, and Costa Rica. In most of the places, the Colorado potato beetle is considered a regulated pest. The spread of potato beetles in Europe and North America was partly assisted by the fact that it developed resistance to DDT in the mid-1950s. You will hardly come across someone who would need an introduction to largemouth bass (Micropterus salmoides) in the United States, or most of North America for that matter, where it happens to be a fairly common freshwater gamefish. At the same time, it is also considered a delicacy, which further adds to its popularity in its native habitat. It was precisely because of these two reasons that the largemouth bass was introduced in various countries of the world, where it eventually established itself. Owing to its tendency of native species, the largemouth bass is considered an invasive alien in countries, like Poland and South Korea. The rosy wolfsnail (Euglandina rosea) is also called the cannibal snail, owing to its tendency to feed on other snails. It is precisely because of this reason that its spread in various parts of the world has become a major concern today. The terrestrial snail, indigenous to tropical North America, has spread to nearly all the islands in the vicinity, including the Solomon Islands, Hawaii, Guam, Vanuatu, as well as far-off countries, like Japan, India, Sri Lanka, Hong Kong, Mauritius, and a whole lot of other countries in Southeast Asia. Ironically, the rosy wolfsnail was introduced in most of these regions as a part of the ambitious biological pest control program to curb the spread of invasive giant African land snail. The Leidy's comb jelly (Mnemiopsis leidyi), also known as warty comb jelly or sea walnut, is a comb jelly species that has earned notoriety for invading the major seas of Europe. A species native to the western Atlantic coastal waters, the Leidy's comb jelly has now established itself in the Black Sea, Caspian Sea, Mediterranean Sea, and more recently in the North Sea and Baltic Sea. The Leidy's comb jelly is believed to have made it to the European waters through the ballast water of merchant's ships. The species is known to be a voracious feeder of zooplankton, which is why its introduction in new habitats is considered a threat for the ecosystem. Like we said in the beginning, some of the species mentioned here were exported by the United States accidentally, while some were introduced deliberately. Incidentally, the odds of an alien species establishing itself in a new region are more in case of deliberate introductions as the number of specimen released is considerably large. Among America's most invasive, for instance, we have an apt example in form of nutria, which were introduced intentionally to promote fur production. All this, however, doesn't really matter when we take into consideration the damage caused by these animals; both, on the economic and ecological fronts. Such is the impact of these species, that even the World Trade Organization (WTO) has raised concerns about the threat they pose and put its weight behind stringent trade norms to curb their spread.