A threatened species is one likely to become endangered if limiting factors are not reversed. An endangered species faces imminent extinction.
A very large number of species of plants and animals are endangered due to the impact of man on the natural environment, whether direct or indirect. The survival of other species are threatened in four major ways: (1) by hunting, or otherwise exploiting them, for useful resources (skins, ivory, feathers, shells, horns, oils and meats), because they are pests, or simply for fun; (2) by interfering with ecosystems by introducing alien species, pollutants and diseases; (3) by destroying natural habitats, especially through urban expansion, agricultural and forestry development; and (4) by changing the climate. In addition to the loss of entire species, many species are losing whole populations at a rate that reduces their genetic variability and thus their ability to adapt the kind of environmental adversity with which they will increasingly be confronted. Within species, there is also a loss of races and varieties.
There are an estimated 5 to 30 million species on planet Earth but only 1.7 have been scientifically investigated. Most live in the canopies of remote rainforest. A species probably becomes extinct about every 9 hours; it may be as frequent as every three hours; by the end of the century it may be as frequent as every 20 minutes. In 1981 the official endangered species list stood at 230. By 1988 it was 35,000.
There are a large number of threatened and endangered species in China. In China, 15-20% of plant and animal species are threatened, well over the world's average of 10-15%. China is home to 156 of the 640 species listed in Appendix I of the International Convention on the Trade of Endangered Species of Flora and Fauna.
It was estimated in 1995 that 27 species were made extinct every hour of every day. Species were becoming extinct at 50 to 100 times the average expected natural rate. Only about 13 percent of the 13 or 14 million species on Earth had been scientifically identified.
The taxonomy of living organisms is intensively discussed and has been much influenced recently by genetic techniques. The "five kingdom" system has been modified and expanded into several "multiple kingdom" systems and the classification of groups is accordingly variable, especially above family level.
Recent studies indicate that many animal species, even when unperturbed by alterations to their environment or unusual weather, can undergo unpredictable major changes in their populations.