Mass extinctions are intervals in Earth history when a very large fraction of species disappear in a geologically short time. They matter because they reshape ecosystems, remove dominant groups, and open ecological space for surviving lineages to diversify. The fossil record shows five especially severe events, often called the Big Five, spread across the last 541 million years.
Studying them helps scientists understand how climate, oceans, volcanoes, impacts, and life interact on a planetary scale.
Each mass extinction had a different mix of causes, such as rapid climate change, ocean anoxia, sea level shifts, massive volcanism, or asteroid impact. After extinction, recovery usually took millions of years, followed by adaptive radiations as surviving organisms filled newly available niches. Today, habitat destruction, overharvesting, invasive species, pollution, and human driven climate change are causing biodiversity loss at unusually high rates.
Many biologists compare the modern crisis to past mass extinctions and study whether Earth is entering a sixth one.
Key Facts
- A mass extinction is usually defined as the loss of at least 75% of species over a geologically short interval.
- The Big Five occurred near 444 Ma, 372 to 359 Ma, 252 Ma, 201 Ma, and 66 Ma, where Ma means million years ago.
- The end Permian extinction, about 252 Ma, was the most severe and eliminated roughly 90% of marine species.
- The end Cretaceous extinction, 66 Ma, is linked to the Chicxulub impact and led to the loss of nonavian dinosaurs.
- Biodiversity change can be described as net diversification = speciation rate - extinction rate.
- Extinction risk rises when environmental change is faster than populations can migrate, adapt, or recover.
Vocabulary
- Mass extinction
- A mass extinction is a global event in which many species from many habitats disappear in a relatively short span of geological time.
- Fossil record
- The fossil record is the preserved evidence of past life used to reconstruct when organisms lived and when they disappeared.
- Adaptive radiation
- Adaptive radiation is the rapid diversification of a lineage into many species that occupy different ecological niches.
- Ocean anoxia
- Ocean anoxia is a condition in which seawater has too little dissolved oxygen to support many marine animals.
- Biodiversity
- Biodiversity is the variety of life at the levels of genes, species, ecosystems, and ecological interactions.
Common Mistakes to Avoid
- Thinking a mass extinction happens overnight, which is wrong because most unfold over thousands to millions of years even though that is short on a geological timeline.
- Assuming one cause explains every extinction, which is wrong because many events involved interacting stresses such as volcanism, warming, acidification, and oxygen loss.
- Confusing extinction with evolution, which is wrong because extinction removes lineages while evolution changes surviving lineages over generations.
- Interpreting recovery as a quick return to the same ecosystem, which is wrong because recovery can take millions of years and often produces new communities with different dominant groups.
Practice Questions
- 1 A fossil survey finds 1,200 species before a crisis and 270 species after it. What percent of species went extinct, and does this meet the 75% threshold often used for mass extinction?
- 2 The end Permian extinction occurred about 252 Ma and the end Cretaceous extinction occurred about 66 Ma. How many million years passed between these two events?
- 3 Explain why the extinction of dominant species can lead to adaptive radiation among surviving groups during the recovery period.