A vulnerable species is a species that is not currently endangered but which faces a high probability of becoming endangered in the near future, either due to a currently declining population or to threats to its natural habitat. An endangered species is a species that is likely to become extinct in the near future, either in a specific geographic area or worldwide.
The International Union for Conservation of Nature (IUCN) currently identifies more than 10,000 species as vulnerable, and the number has been growing rapidly. The criteria it uses to determine whether a species is vulnerable relate to the rates of decline in its population and in the numbers and sizes of the areas in which it lives as well as to the absolute number of individuals currently living. However, the true number of vulnerable species may be far greater because so many species have yet to be discovered and cataloged and because of the rapid rate of habitat destruction worldwide.