Sometimes, alien plants or animals become established in their new environment and spread unchecked, threatening native biodiversity. Invasive alien species have been identified as the second greatest threat to biodiversity after habitat loss. All forms of life evolve in close interaction with their immediate environment.
Species develop mechanisms to protect themselves from predators.
The result can be catastrophic. For millennia, oceans, mountains, rivers and deserts served as natural barriers to the movement of certain plants and animals, providing the isolation essential for unique species and ecosystems to evolve. In just a few hundred years, however, international trade and the expansion of global travel, accompanied by intentional introductions, have inadvertently ended millions of years of biological isolation. This has created major problems that affect developed and developing countries alike. Hundreds, possibly thousands of extinctions have been caused by alien invasive species.