Once transported in this manner, they become removed from the predators, parasites, and diseases that kept them in balance in their native environments. As a result of losing these controls, they often become pests in the areas into which they are introduced.
Alien species threaten our environment and livelihoods in many ways.
They can reduce water quality and quantity by increasing runoff and erosion and this, in turn, degrades near-shore marine ecosystems like coral reefs. But it has suffered the highest rates of extinction of any area of the United States and one of the highest rates anywhere in the world, with hundreds or possibly thousands of unique species already extinct. The primary agent of this remarkable loss of native biodiversity has been the massive introduction by humans of alien species to the Hawaiian Islands. Alien species constitute the primary cause of endangerment for virtually all of these endangered plants and animals. Also, the sheer volume of introductions overwhelms native species.