Mass Extinctions (HL) (DP IB Environmental Systems & Societies (ESS))

Mass Extinctions

What is a mass extinction?

• Mass extinction is a widespread and rapid decrease in the diversity of life on Earth

  • It happens when a significant proportion of Earth's species die off in a relatively short period of time

  • At least 75% of species are wiped out in a mass extinction event

• There have been five major mass extinctions in Earth's history

  • Each has significantly reshaped life on the planet

The five major mass extinctions

1. Ordovician-Silurian Extinction (approx. 444 million years ago)

• This was the first mass extinction

• It mainly affected marine life, as most species lived in the oceans at the time

  • Around 85% of marine species became extinct

  • Possible causes include:

    • Climate change (glaciation and global cooling)

    • Sea level drop, which reduced habitat space for marine species

2. Late Devonian Extinction (approx. 375 million years ago)

• This event took place over millions of years and impacted marine ecosystems

  • 75% of all species died out, including many species of corals and fish

  • Possible causes include:

    • Volcanic activity, leading to climate change

    • Decreased oxygen levels in oceans (hypoxia)

3. Permian-Triassic Extinction (approx. 252 million years ago)

• Known as the 'Great Dying', it was the most severe mass extinction.

  • 96% of marine species and around 70% of land species became extinct

  • Possible causes include:

    • Massive volcanic eruptions in what is now Siberia

    • The volcanic activity released huge amounts of carbon dioxide, causing global warming and ocean acidification

    • Some scientists suggest this event was made worse by methane gas being released from melting ice

4. Triassic-Jurassic Extinction (approx. 201 million years ago)

• This extinction was followed by the dominance of dinosaurs

  • Around 80% of all species died out

  • Possible causes include:

    • Volcanic activity linked to the breakup of the supercontinent Pangaea

    • Resulting climate changes and sea-level rises

5. Cretaceous-Paleogene Extinction (approx. 66 million years ago)

• This is the most famous mass extinction, known for the extinction of the dinosaurs

  • Around 75% of species became extinct, including all non-avian dinosaurs, pterosaurs, and ammonites

  • Possible causes include:

    • A large meteorite impact in what is now the Yucatán Peninsula in Mexico (Chicxulub crater)

    • This caused fires, tsunamis, and dust clouds that blocked out sunlight, disrupting the climate

    • Massive volcanic activity in India, causing short-term global cooling and acid rain, may have contributed

What causes mass extinctions?

Tectonic plate movements

• Tectonic plates are large slabs of Earth's crust that move over time, reshaping continents and oceans

• When plates move, they can cause habitat loss by altering land masses and changing ecosystems.

  • Formation of supercontinents like Pangaea combined previously separated land masses, reducing coastal areas and shallow seas, which are rich in biodiversity

  • This led to widespread species loss, particularly in marine environments

• Plate movements can also trigger volcanic activity and earthquakes

  • These events can significantly impact life by destroying habitats and releasing gases into the atmosphere

Super-volcanic eruptions

• Super-volcanoes release massive amounts of ash, lava, and gases (such as sulphur dioxide and carbon dioxide) into the atmosphere

• This can block sunlight, leading to a temporary cooling of the Earth known as a 'volcanic winter'

  • The reduced sunlight can cause plants to die off, disrupting food chains and leading to the collapse of entire ecosystems

  • The Siberian Traps eruptions at the end of the Permian period released enough gases and ash to cause global temperatures to plummet and acidify the oceans

Climate changes

• Mass extinctions can be caused by both global cooling (ice ages) and global warming (greenhouse effects)

• Global cooling can reduce biodiversity by shrinking marine habitats and lowering temperatures beyond the tolerance of many species

  • The Ordovician-Silurian extinction was triggered by an ice age, which reduced shallow marine habitats

• Global warming can lead to increased temperatures, habitat destruction, and ocean acidification

  • Warming can cause polar ice caps to melt, raising sea levels and altering ecosystems

Sea-level changes

• Fluctuating sea levels can drastically reduce or expand habitats, particularly for marine species

• Sea-level decline can expose shallow seas and coastal habitats, reducing the space available for organisms that rely on these environments

  • This is particularly destructive to coral reefs, mangroves, and other coastal ecosystems, which are biodiversity hotspots

• Sea-level rise can flood terrestrial areas, displacing land species and altering ecosystems

Meteorite impact

• Large meteorites or asteroids crashing into Earth can cause massive destruction on a global scale

• Upon impact, meteorites create shockwaves and fires, but the more long-term damage comes from dust and debris thrown into the atmosphere

  • This debris can block sunlight for months or even years

  • This cools the planet and disrupts photosynthesis, leading to a breakdown of food chains

Rapid speciation after mass extinctions

• Speciation is the process by which new species evolve

  • After a mass extinction, many ecological niches become available due to the extinction of species

  • Surviving species often undergo rapid evolution to fill these empty niches

  • This is the reason why periods of rapid speciation frequently follow mass extinctions

    • For example, after the Cretaceous-Paleogene extinction, mammals diversified and evolved into many new species, leading to the eventual rise of humans

The sixth mass extinction (anthropogenic extinction)

• Anthropogenic means caused by humans

• Scientists believe a sixth mass extinction has begun, driven by human activities including:

  • Habitat destruction, such as deforestation and urbanisation

  • Climate change caused by burning fossil fuels is altering habitats and weather patterns

  • Overfishing, hunting, and pollution (e.g. plastic pollution, pesticides) are further threats to biodiversity