![]() (The mass species extinctions that occurred at the end of the Paleozoic and Mesozoic eras are noteworthy exceptions to this generalization. ![]() In modern times, however, species threatened by human activities are becoming extinct at a rate that far exceeds the pace of extinction throughout most of geologic history. In fact, according to the fossil record, only 2 –4% of the species that have existed on Earth exist today. Biological evolution, driven by natural climate change, catastrophic geologic events, and competition from better-adapted species, has resulted in the extinction of billions of species since the advent of life on Earth about three billion years ago. Species have become extinct throughout geological history. Many nations have their own version of the ESA and, like the United States, are members of the World Conservation Union (IUCN), and signatories of the Convention on International Trade In Endangered Species of Wild Fauna and Flora (CITES). In the United States, the Endangered Species Act (ESA) of 1973 protects threatened and endangered species that meet specified criteria. Threatened species are at risk of becoming endangered in the foreseeable future. ![]() Extirpated species have disappeared locally or regionally, but still survive in other regions or in captivity. Extinct species no longer exist anywhere on Earth, and once gone they are gone forever. Human causes of extinction and endangermeĪn endangered species of plant, animal, or microorganism is at risk of imminent extinction or extirpation in all or most of its range.
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