In Earth’s long history estimated 99% of all species ever living here went extinct. Extinction is part of life’s history and extinction of single species happens all the time. Over time lost species are eventually replaced as natural selection acts on the survivors and new species evolve. Mass extinctions are defined by the loss of a large part of the biodiversity in a (geologically speaking) short span of time, when the evolution of new species can’t compensate the losses. This can be seen also in the geological record.
Five big mass extinction events are recognized by paleontologists. At the end of the Ordovician 443 million years ago, when an estimated 86% of all marine species disappeared. At the end of the Devonian 360 million years ago, when 75% of all species disappearance. At the end of the Permian 250 million years ago, the worst extinction event so far with 96% of all known species lost. At the end of the Triassic 201 million years ago, when 80% of all species disappeared and the famous mass extinction, including some of the dinosaurs, at the end of the Cretaceous 65 million years ago, when 76% of all species went extinct. Minor extinction events also happened, like the one at the end of the ice-age some 10,000 years ago.
Scientist long debated and still debate about the factors driving mass extinction. Factors contributing to the disappearance of a species can be geological catastrophes, like volcanism, impacts or climate change, but also biological factors, like competition, diseases or depletion of resources. The most likely explanation for the most recent mass extinction at the end of the Cretaceous, is the impact of a large asteroid, disrupting the ecosystems on a global scale. For the extinction of many large mammal species at the end of the last ice-age among climate change also the spread of hunter-gatherer societies may played a role.
In the last 400 years, a large number of mammal, bird, amphibian and reptile species went extinct. Especially species with a limited range, living for example only on islands, quickly disappeared after humans arrived there, hunting the small populations or destroying their natural habitat. Research published some years ago compared the recent extinction rate with the reconstructed rates during geologically quiet times and mass extinctions. At the time, the study confirmed that current extinction rates are higher than would be expected from the fossil record, approaching a mass extinction. Nowadays human activity is impacting the environment on a global scale and on an ever-accelerating rate.
According to a summary report from the United Nations, one in four known species are at risk of extinction. Amphibians are among the most vulnerable with 40% of the studied species at risk, followed by conifers with 34%, reef corals with 33%, cartilaginous fish like sharks and rays with 31%, mammals with 25% and birds with 14%. Among invertebrates, 27% of the studied crustaceans are at risk. Recent surveys have also shown a dramatic collapse in insect populations.
According to the report, only a quarter of Earth’s surface is substantively free of the impacts of human activities, especially in high latitudes, an area that will continue to shrink in the next decades. Habitat loss is a major threat to biodiversity, followed by over-exploitation of animal populations, the spread of invasive species and diseases by humans, pollution and climate change.