mass extinction event

mass extinction event, any circumstance that results in the loss of a significant portion of Earth’s living species across a wide geographic area within a relatively short period of geologic time. Mass extinction events are extremely rare. They cause drastic changes to Earth’s biosphere, and in their aftermath they create opportunities for surviving species and their descendants as they adapt to the changed conditions, claiming abandoned ecological niches or filling newly created ones. Mass extinction events may be caused by comet and asteroid impacts, widespread volcanism, climatic changes, rapid changes in geography and ocean currents, or combinations of these factors.

(Read E.O. Wilson’s Britannica essay on mass extinction.)

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Extinction, which refers to the dying out of a single species, is a feature of Earth’s flora and fauna. The vast majority of species ever to have lived are extinct. The background extinction rate (the continual rate of species loss as they succumb to naturally occurring environmental and ecological forces) is quite low, amounting to roughly one to five species per year when the entire fossil record is considered.

There have been five unusually large extinction events in Earth’s history. Each one is known by a conspicuous decline in biodiversity that appears in the fossil record lasting up to tens of millions of years afterward. With the onset of each mass extinction event, the relatively sudden loss of vast numbers of species greatly simplified many of Earth’s biological communities or caused them to collapse. Over time, surviving species claimed ecological niches that had been occupied by other species (some of whom may have preyed upon or competed with them), and they filled new niches as ecosystems recovered and new biological communities established themselves. Ranked in descending order of severity, they are:

1. Permian extinction (about 265.1 million to about 251.9 million years ago), the most dramatic die-off, eliminating about half of all taxonomic families and about 90 percent of all species, which included some 95 percent of marine species (including all of the trilobites and nearly wiping out brachiopods and corals) and about 70 percent of land species (including plants, insects, and vertebrates).
2. Ordovician-Silurian extinction (about 443.8 million years ago), which eliminated about 25 percent of marine families and 85 percent of marine species.
3. Cretaceous-Tertiary (K-T), or Cretaceous-Paleogene (K-Pg), extinction (about 66 million years ago), involving about 80 percent of all animal species, including the dinosaurs and many species of plants. Although many scientists contend that this event was caused by one or more large comets or asteroids striking Earth, others maintain that it was caused by climatic changes associated with the substantial volcanic activity of the time.
4. End-Triassic extinction (about 201.3 million years ago), possibly caused by rapid climate change or by an asteroid striking Earth. This mass extinction event caused about 20 percent of marine families and some 76 percent of all extant species to die out, possibly within a span of about 10,000 years, thus opening up numerous ecological niches into which the dinosaurs evolved.
5. Devonian extinctions (407.6 million to about 358.9 million years ago), which eliminated 15–20 percent of marine families and 70–80 percent of all animal species. Roughly 86 percent of marine brachiopod species perished, along with many corals, conodonts, and trilobites.

A growing number of scientists note that Earth is in the midst of a sixth mass extinction, driven by human activities—namely, the conversion of natural areas to agriculture and urban uses, hunting and overfishing, pollution, and climate change. Researchers estimate that the current rate of species loss varies between 100 and 10,000 times the background extinction rate (see also biodiversity loss).