Mass Extinctions

Mass extinctions are geological events in Earth’s history characterized by a rapid and significant decline in the diversity and abundance of life forms. Read here to learn more about them.

These events result in the extinction of a large number of species across various taxonomic groups.

Throughout the Earth’s history, there have been several mass extinctions, each with distinct causes and consequences.

There have been five big mass extinctions in Earth’s history – these are called the “Big Five”. Understanding the reasons and timelines of these events is important to understand the speed and scale of species extinctions today.

What are mass extinctions?

A mass extinction is an event in Earth’s history characterized by a significant, rapid, and widespread decline in the diversity and abundance of life forms on the planet.

During a mass extinction, a substantial proportion of Earth’s species, both on land and in the oceans, goes extinct within a relatively short geological time frame.

These events have had profound and lasting effects on the course of evolution, shaping the composition of life on Earth.

Significant characteristics of mass extinctions include:

• Rapid Extinction Rates: Mass extinctions involve a rapid and high rate of species loss compared to the background rate of extinction, which is the normal rate of species loss between extinction events.
• Global Impact: Mass extinctions affect many habitats and ecosystems globally, including terrestrial and marine environments. The impact is not limited to a specific region.
• Diversity Loss: A large percentage of Earth’s species, spanning various taxonomic groups, experience extinction during a mass extinction event. This loss of diversity has long-term consequences for ecosystems and the evolutionary trajectory of life.
• Ecological Disruption: Mass extinctions often lead to significant disruptions in ecological relationships and food webs. The loss of key species can have cascading effects throughout ecosystems.
• Opportunities for Evolution: While mass extinctions result in widespread species loss, they also create ecological opportunities for the evolution and diversification of surviving species. New ecological niches become available as a result of the extinction event.
• Environmental Changes: Mass extinctions are often associated with significant environmental changes, including alterations in climate, sea levels, and atmospheric composition. These changes can be triggered by various factors such as asteroid impacts, volcanic activity, or long-term geological processes.
• Geological Significance: Mass extinctions are marked by distinctive geological signatures in the fossil record. The boundary between rock layers associated with the extinction event and those above it often reveals evidence of abrupt changes.

The history of life on Earth is punctuated by several recognized mass extinction events, each associated with different causes. Notable examples include the Permian-Triassic extinction (often referred to as the “Great Dying”) and the Cretaceous-Paleogene extinction, which led to the demise of the dinosaurs.

Big Five mass extinctions

1. Ordovician-Silurian Extinction (End-Ordovician, around 443 million years ago):

• Causes: Possible causes include glaciation, sea-level changes, and decreased oxygen levels in the oceans.
• The tectonic uplift of the Appalachian mountains created lots of weathering, sequestration of CO2, and with it, changes in climate and ocean chemistry.
• Impact: Approximately 85% of marine species were eliminated, particularly brachiopods and conodonts.

2. Late Devonian Extinction (Late Devonian, around 360 million years ago):

• Causes: Possible causes include climate change, sea-level fluctuations, and the development of terrestrial ecosystems.
• Impact: About 75% of species, including many marine organisms, were affected.

3. Permian-Triassic Extinction (End-Permian, around 252 million years ago):

• Causes: Likely a combination of volcanic activity, climate change, and the release of methane hydrates.
• Intense volcanic activity in Siberia caused global warming. Elevated CO2 and sulfur (H2S) levels from volcanoes caused ocean acidification, acid rain, and other changes in ocean and land chemistry.
• Impact: Often referred to as the “Great Dying,” it resulted in the loss of around 96% of marine species and 70% of terrestrial vertebrate species.

4. Triassic-Jurassic Extinction (End-Triassic, around 201 million years ago):

• Causes: Possibly related to volcanic activity in the Central Atlantic Magmatic Province (CAMP) and climate change.
• Impact: Approximately 80% of species in the oceans were affected, including many marine reptiles.

5. Cretaceous-Paleogene Extinction (End-Cretaceous, around 66 million years ago):

• Causes: Often associated with the impact of a large asteroid or comet, leading to a sudden environmental disruption.
• Impact: About 75% of Earth’s species, including non-avian dinosaurs, became extinct. Mammals, birds, and certain reptiles survived and diversified.

6. Holocene Extinction (Ongoing, from approximately 11,700 years ago to the present):

• Causes: Primarily driven by human activities, including habitat destruction, overexploitation, pollution, and climate change.
• Impact: Ongoing loss of biodiversity, affecting various species across the globe.

Read: Holocene Extinction

Why do we need to understand these events?

Understanding mass extinctions is crucial for scientists studying Earth’s history, evolution, and the factors that influence biodiversity.

• The ongoing concern about human-induced environmental changes and the loss of biodiversity has led to increased awareness of the potential for a contemporary mass extinction, often referred to as the Holocene extinction or Anthropocene extinction.
• This ongoing extinction event is driven by human activities such as habitat destruction, climate change, and overexploitation of resources.

There’s a natural background rate to the timing and frequency of extinctions: 10% of species are lost every million years, 30% every 10 million years, and 65% every 100 million years.

• It would be wrong to assume that species going extinct is out of line with what we would expect.
• Evolution occurs through the balance of extinction – the end of species – and speciation – the creation of new ones.

However, mass extinctions are periods with much higher extinction rates than normal. They are defined by both magnitude and rate. Magnitude is the percentage of species that are lost. Rate is how quickly this happens. These metrics are inevitably linked, but we need both to qualify as a mass extinction.

Also read: Fossils: Everything you need to know

Conclusion

These mass extinctions have shaped evolution by opening ecological niches for new species to evolve. They are marked by profound changes in the composition of life on Earth and have been crucial in determining the trajectory of biological diversity.

While natural events such as asteroid impacts and volcanic eruptions have played roles in some mass extinctions, human activities are increasingly recognized as significant contributors to the current ongoing extinction crisis, making conservation efforts crucial for preserving biodiversity.

Read: Geologic Time Scale

-Article by Swathi Satish