Extinction is the end of an organism or of a group of organisms (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point. A species becomes extinct when there are none left that can reproduce and create viable offspring. Extinction therefore becomes a certainty when all members of a species die without producing any offspring that survive to adulthood.
A species may become extinct for a number of reasons, including natural disaster, disease, climate change, human activity such as hunting or introduced predators or competitors, or changes in its environment (such as in habitat fragmentation). There is no single mechanism that always causes extinction. Some scientists estimate that up to 99 percent of all species that have ever lived on Earth are now extinct.
The rate of extinctions on Earth has accelerated since humans appeared on the planet; some experts estimate it is 100 times higher now than it was before humans arrived. Nevertheless, most biologists believe we are currently in only the sixth great mass extinction event in Earth’s history, often called the Holocene or Anthropogenic extinction event because it is largely caused by human activity.”