Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction.[233] Nearly all animal and plant species that have lived on Earth are now extinct,[234] and extinction appears to be the ultimate fate of all species.[235] These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass extinction events.[236] The Cretaceous–Paleogene extinction event, during which the non-avian dinosaurs went extinct, is the most well-known, but the earlier Permian–Triassic extinction event was even more severe, with approximately 96% of species driven to extinction.[236] The Holocene extinction event is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 100–1000 times greater than the background rate and up to 30% of current species may be extinct by the mid 21st century.[237] Human activities are now the primary cause of the ongoing extinction event;[238] global warming may further accelerate it in the future.[239] The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered.[236] The causes of the continuous "low-level" extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources (competitive exclusion).[47] If one species can out-compete another, this could produce species selection, with the fitter species surviving and the other species being driven to extinction.[106] The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of rapid evolution and speciation in survivors. In biology and ecology, extinction is the

end of an organism or of a group of organisms (taxon), normally 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. Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "re-appears" (typically in the fossil record) after a period of apparent absence. Through evolution, new species arise through the process of speciation—where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche—and species become extinct when they are no longer able to survive in changing conditions or against superior competition. The relationship between animals and their ecological niches has been firmly established.[2] A typical species becomes extinct within 10 million years of its first appearance,[3] although some species, called living fossils, survive virtually unchanged for hundreds of millions of years. Most extinctions have occurred naturally, prior to Homo sapiens walking on Earth: it is estimated that 99.9% of all species that have ever existed are now extinct.[3][4] Mass extinctions are relatively rare events; however, isolated extinctions are quite common. Only recently have extinctions been recorded and scientists have become alarmed at the high rates of recent extinctions.[5] Most species that become extinct are never scientifically documented. Some scientists estimate that up to half of presently existing species may become extinct by 2100.[6] It is difficult to estimate the trajectory that biodiversity might have taken without human impact but scientists at the University of Bristol estimate that biodiversity might increase exponentially without human influence.