Mass extinction

Mass extinction is distinguished from ordinary background extinction by its speed, global reach, and taxonomic breadth. Paleontologists compare fossil occurrences through successive rock layers, correcting where possible for uneven sampling, reworking, and the Signor–Lipps effect, in which an extinction appears gradual because the last fossils are unlikely to represent the final survivors.

There is no single universally binding percentage threshold, but the concept generally denotes losses far above normal extinction rates across multiple lineages. Extinction intensity can also vary between marine and terrestrial environments, regions, and ecological roles. Consequently, a brief disappearance in one local fossil deposit is not by itself a mass extinction.

The five conventionally recognized Phanerozoic mass extinctions occurred at the end of the Ordovician, Late Devonian, end of the Permian, end of the Triassic, and Cretaceous–Paleogene boundaries. The end-Permian event was the most severe known, eliminating most marine species and many terrestrial groups; its causes are strongly associated with Siberian Traps volcanism, greenhouse warming, ocean deoxygenation, and acidification.

The Cretaceous–Paleogene event removed non-avian dinosaurs and many marine organisms. Evidence from the Chicxulub impact structure links it to a large asteroid impact, although environmental stresses before the impact remain part of scientific discussion.¹ Other events had different combinations of glaciation, sea-level change, volcanism, anoxia, and climate disruption.

Mass extinctions usually arise from interacting environmental shocks rather than one universal mechanism. Rapid climate change, ocean chemistry shifts, habitat loss, volcanic emissions, asteroid impacts, and food-web collapse can each reduce survivorship; their effects are amplified when ecosystems lose redundancy and species cannot migrate or adapt quickly.

Recovery is also uneven. Surviving lineages may diversify into ecological vacancies, but rebuilding taxonomic richness and complex habitats commonly takes millions of years. The fossil record therefore distinguishes the initial extinction pulse from a prolonged interval of ecological reorganization. Some groups recover numerically while remaining functionally or morphologically different from their predecessors, and evolutionary recovery does not restore the exact species that disappeared.

The fossil record can conceal important details because extinction intensity differs sharply among environments and because rare species disappear from samples before they become globally extinct. Some events were protracted or pulsed rather than instantaneous, and the label “Big Five” simplifies a much larger history of severe regional and taxonomic crises.

Modern biodiversity loss is not identical to any one ancient event: its principal drivers include land and sea-use change, direct exploitation, climate change, pollution, and invasive species.³ Habitat fragmentation can create an extinction debt, in which populations persist temporarily but are already unlikely to survive. Researchers therefore track both recorded extinctions and declines in abundance, range, and genetic diversity. Whether current losses meet a geological definition of a sixth mass extinction depends on thresholds and future rates, but contemporary extinction rates are already unusually high compared with long-term background estimates.²

Background extinction

The comparatively low, ongoing rate at which species disappear between major extinction episodes.

Extinction pulse

A relatively brief interval of unusually elevated extinction within a longer environmental crisis.

Signor–Lipps effect

The statistical appearance of a gradual extinction caused by incomplete fossil sampling near the true last occurrence.

Extinction debt

The delayed loss of species after environmental deterioration has made their eventual extinction likely.