IB ESS HL3.1 Biodiversity & EvolutionHL only~13 min read
The Five Mass Extinctions
Five times, life on Earth has come close to ending. The worst of them killed 96% of marine species. Each one also cleared the way for whatever came next — which is why mammals, and eventually us, exist at all.
📘 What you need to know
A mass extinction is a widespread, rapid decrease in the diversity of life, wiping out at least 75% of species.
There have been five major mass extinctions in Earth’s history.
The Permian-Triassic (252 Ma) was the worst — the “Great Dying”, killing 96% of marine species.
The Cretaceous-Paleogene (66 Ma) ended the non-avian dinosaurs.
Rapid speciation follows a mass extinction, as survivors evolve to fill empty niches.
A sixth, anthropogenic mass extinction is believed to have begun.
What counts as a mass extinction
DefinitionMass extinction — a widespread and rapid decrease in the diversity of life on Earth, in which a significant proportion of species die off in a relatively short period. At least 75% of species are wiped out.
Each of the five has significantly reshaped life on the planet.
Bars are positioned by date along the 500-million-year axis. The Permian-Triassic stands out on both measures: worst losses, and the boundary between two eras.
The five events
Event
When
Losses
Possible causes
1. Ordovician-Silurian
approx. 444 Ma
Around 85% of marine species. The first mass extinction, mainly affecting marine life since most species lived in the oceans
Climate change (glaciation and global cooling); sea level drop reducing habitat space for marine species
2. Late Devonian
approx. 375 Ma
75% of all species, including many corals and fish. Took place over millions of years, impacting marine ecosystems
Volcanic activity leading to climate change; decreased oxygen levels in the oceans (hypoxia)
3. Permian-Triassic the “Great Dying”
approx. 252 Ma
The most severe: 96% of marine species and around 70% of land species
Massive volcanic eruptions in what is now Siberia, releasing huge amounts of carbon dioxide causing global warming and ocean acidification; possibly worsened by methane released from melting ice
4. Triassic-Jurassic
approx. 201 Ma
Around 80% of all species. Followed by the dominance of dinosaurs
Volcanic activity linked to the breakup of the supercontinent Pangaea, and the resulting climate change and sea-level rise
5. Cretaceous-Paleogene
approx. 66 Ma
Around 75% of species, including all non-avian dinosaurs, pterosaurs and ammonites
A large meteorite impact at the Chicxulub crater, Yucatán Peninsula, Mexico, causing fires, tsunamis and dust clouds that blocked sunlight; massive volcanic activity in India may have contributed
What causes mass extinctions
Explaining this shared mechanism turns a list of causes into a proper explanation, which is what the higher mark bands ask for.
Tectonic plate movements
Tectonic plates are large slabs of Earth’s crust that move over time, reshaping continents and oceans. Their movement causes habitat loss by altering land masses and changing ecosystems. The formation of supercontinents such as Pangaea combined previously separated land masses, reducing coastal areas and shallow seas — which are rich in biodiversity — and causing widespread species loss, particularly in marine environments. Plate movements can also trigger volcanic activity and earthquakes.
Super-volcanic eruptions
Super-volcanoes release massive amounts of ash, lava and gases such as sulfur dioxide and carbon dioxide. This can block sunlight, producing a temporary cooling known as a volcanic winter. Reduced sunlight kills plants, disrupting food chains and collapsing entire ecosystems. The Siberian Traps eruptions at the end of the Permian released enough gas and ash to make global temperatures plummet and to acidify the oceans.
Climate change
Mass extinctions can be caused by both global cooling and global warming.
Cooling reduces biodiversity by shrinking marine habitats and lowering temperatures beyond many species’ tolerance. The Ordovician-Silurian extinction was triggered by an ice age that reduced shallow marine habitats.
Warming brings increased temperatures, habitat destruction and ocean acidification, and melts polar ice, raising sea levels and altering ecosystems.
Sea-level changes
Fluctuating sea levels drastically reduce or expand habitats, particularly for marine species. A decline exposes shallow seas and coastal habitats, reducing space for organisms that depend on them — especially destructive for coral reefs and mangroves. A rise floods terrestrial areas, displacing land species.
Meteorite impact
Large meteorites cause massive destruction globally. The immediate impact creates shockwaves and fires, but the longer-term damage comes from dust and debris thrown into the atmosphere, which can block sunlight for months or years. This cools the planet and disrupts photosynthesis, breaking down food chains.
What happens afterwards
After a mass extinction, many ecological niches become available because the species that occupied them are gone. Surviving species then undergo rapid evolution to fill those empty niches, which is why periods of rapid speciation frequently follow mass extinctions.
The example that matters to us. After the Cretaceous-Paleogene extinction removed the non-avian dinosaurs, mammals diversified and evolved into many new species — leading eventually to the rise of humans. Without that extinction, the niches mammals expanded into would still have been occupied.
The sixth mass extinction
Anthropogenic means caused by humans. Scientists believe a sixth mass extinction has begun, driven by human activities:
Habitat destruction, such as deforestation and urbanisation.
Climate change caused by burning fossil fuels, altering habitats and weather patterns.
Overfishing, hunting and pollution, including plastic pollution and pesticides.
Make sure you can explain how each factor contributes, not just name it. The strongest answers show the mechanism: volcanic ash blocks sunlight, so plants die, so food chains collapse. Notice too that the sixth extinction differs from the previous five in one important respect — its cause is a species that could, in principle, choose to stop.
Worked examples
WE 1
Defining a mass extinction
Define mass extinction and state how many have occurred. (2 marks)
The definition
A widespread and rapid decrease in the diversity of life on Earth, in which a significant proportion of species die off in a relatively short period. At least 75% of species are wiped out.
The number
There have been five major mass extinctions in Earth’s history, each significantly reshaping life on the planet.
75% of species lost, five times overthe 75% threshold is the quantitative detail examiners look for
WE 2
Explaining a cause
Explain how super-volcanic eruptions can cause a mass extinction. (4 marks)
Step 1: what is released
Super-volcanoes release massive amounts of ash, lava and gases such as sulfur dioxide and carbon dioxide.
Step 2: the immediate effect
Ash and gases block sunlight, producing a temporary global cooling known as a volcanic winter.
Step 3: the ecological effect
Reduced sunlight causes plants to die off, disrupting food chains and leading to the collapse of entire ecosystems.
Step 4: the example
The Siberian Traps eruptions at the end of the Permian released enough gas and ash to make temperatures plummet and to acidify the oceans, contributing to the Great Dying.
Ash blocks light, producers die, food chains collapsethe mechanism, not the event, is what earns the marks
WE 3
After an extinction
Explain why periods of rapid speciation often follow mass extinctions. (3 marks)
Step 1: niches are emptied
A mass extinction removes a large proportion of species, so many ecological niches become available.
Step 2: surviving species expand
Surviving species face reduced competition and undergo rapid evolution to fill those empty niches.
Step 3: the example
After the Cretaceous-Paleogene extinction removed the non-avian dinosaurs, mammals diversified into many new species, leading eventually to the rise of humans.
Empty niches plus reduced competition equals rapid diversificationthe mammal example connects the whole page to why humans exist
💡 Exam tips
Learn the 75% threshold in the definition.
Know the Permian-Triassic (worst, 96% marine) and Cretaceous-Paleogene (dinosaurs) in detail.
Be able to explain the mechanism of each cause, not just name it.
Name Chicxulub and the Siberian Traps where relevant.
Remember climate change causes extinctions through both cooling and warming.
Always mention rapid speciation afterwards and the mammal example.
⚠ Common mistakes
Attributing all five to meteorites. Only the Cretaceous-Paleogene involved a major impact.
Saying the dinosaur extinction was the worst. The Permian-Triassic was far more severe.
Claiming all dinosaurs died out. Non-avian dinosaurs did; birds are surviving dinosaurs.
Naming a cause without a mechanism. Explain how it kills, not just what it is.
Assuming only warming causes extinction. The Ordovician-Silurian was triggered by an ice age.
Forgetting the recovery. Rapid speciation after an extinction is examinable content.
Up next: The Anthropocene. If a sixth mass extinction is under way and humans are causing it, the question becomes whether our impact is large enough to mark a new division of the geological timescale itself.
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