Endangered Species Endangered species are living things whose population is so reduced that they are threatened with extinction. Thousands of species are included in this category. The International Union for the Conservation of nature and Natural Resources publishes a list of threatened mammals, birds, reptiles, amphibians, and (many people do not consider them) plants .CAUSES OF EXTINCTION Millions of years before humans, extinction of living things was linked to geological and climate, the effects of which were translated into major alternation of the environment. Environmental change is still the primary cause of the extinction of animals, but now the changes