Subtitles section Play video Print subtitles Massive vines that blanket the southern United States, climbing as high as 100 feet as they uproot trees and swallow buildings. A ravenous snake that is capable of devouring an alligator. Rabbit populations that eat themselves into starvation. These aren't horror movie concepts. They're real stories, but how could such situations exist in nature? All three are examples of invasive species, organisms harmful not because of what they are, but where they happen to be. The kudzu vine, for example, had grown quality in its native east Asia, eaten by various insects and dying off during the cold winters. But its fortunes changed when it was imported into the southeastern United States for porch decoration and cattle feed. Its planting was even subsidized by the government to fight soil erosion. With sunny fields, a mild climate, and no natural predators in its new home, the vine grew uncontrollably until it became known as the plant that ate the South. Meanwhile in Florida's Everglades, Burmese pythons, thought to have been released by pet owners, are the cause of decreasing populations of organisms. They're successfully outcompeting top predators, such as the alligator and panther, causing a significant reduction in their food sources. They're not a problem in their native Asia because diseases, parasites, and predators help to control their population size. And in Australia, European rabbits eat so many plants that they wipe out the food supply for themselves and other herbivores. They're a pretty recent addition, intentionally introduced to the continent because one man enjoyed hunting them. Like the Burmese pythons, various factors in their native habitat keep their numbers in control. But in Australia, the lack of predators and a climate perfect for year-long reproduction allows their populations to skyrocket. So why does this keep happening? Most of the world's ecosystems are the result of millennia of coevolution by organisms, adapting to their environment and each other until a stable balance is reached. Healthy ecosystems maintain this balance via limiting factors, environmental conditions that restrict the size or range of a species. These include things like natural geography and climate, food availability, and the presence or absence of predators. For example, plant growth depends on levels of sunlight and soil nutrients. The amount of edible plants affects the population of herbivores, which in turn impacts the carnivores that feed on them. And a healthy predator population keeps the herbivores from becoming too numerous and devouring all the plants. But even minor changes in one factor can upset this balance, and the sudden introduction of non-native organisms can be a pretty major change. A species that is evolved in a separate habitat will be susceptible to different limiting factors, different predators, different energy sources, and different climates. If the new habitat's limiting factors fail to restrict the species growth, it will continue to multiply, out-competing native organisms for resources and disrupting the entire ecosystem. Species are sometimes introduced into new habitats through natural factors, like storms, ocean currents, or climate shifts. The majority of invasive species, though, are introduced by humans. Often this happens unintentionally, as when the zebra mussel was accidentally brought to Lake Erie by cargo ships. But as people migrate around the world, we have also deliberately brought our plants and animals along, rarely considering the consequences. But now that we're learning more about the effects of invasive species on ecosystems, many governments closely monitor the transport of plants and animals, and ban the imports of certain organisms. But could the species with the most drastic environmental impact be a group of primates who emerged from Africa to cover most of the world? Are we an invasive species?