Extinction and Biodiversity

In this lesson we explore what happens when species disappear forever, the causes of extinction, the importance of biodiversity, and the methods used to conserve it.

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What Is Extinction?

Extinction occurs when there are no living individuals of a species remaining anywhere on Earth. The species is gone permanently — it cannot come back.

Extinction is a natural part of life's history. Over 99% of all species that have ever lived on Earth are now extinct. However, the current rate of extinction is estimated to be 100 to 1,000 times higher than the natural background rate, largely due to human activity.

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Causes of Extinction

Species can become extinct for many reasons, often a combination of several factors:

1. Environmental Change

Change	Effect
Climate change	Habitats become too hot, cold, wet or dry for a species to survive
Sea level change	Coastal and island habitats are flooded or exposed
Habitat destruction	Forests cleared, wetlands drained, grasslands converted to farmland

2. New Predators

When a new predator is introduced to an ecosystem (either naturally or by humans), prey species that have no defences against it may be driven to extinction.

Example: The introduction of rats, cats and stoats to New Zealand devastated native bird populations (e.g., the moa was hunted to extinction by arriving humans).

3. New Diseases

A new pathogen can spread rapidly through a population that has no immunity.

Example: Chytrid fungus (Batrachochytrium dendrobatidis) has caused dramatic declines and extinctions of amphibian species worldwide.

4. New Competitors

A new species that competes for the same resources (food, habitat, mates) can outcompete a native species.

Example: Grey squirrels, introduced to the UK from North America, outcompete native red squirrels for food and carry squirrelpox virus, to which red squirrels are vulnerable.

5. Catastrophic Events

Sudden, large-scale events can cause mass extinction:

• Asteroid impacts (e.g., the asteroid that struck the Yucatan Peninsula ~66 million years ago, contributing to the extinction of non-avian dinosaurs)
• Massive volcanic eruptions (e.g., the Siberian Traps eruptions ~252 million years ago, linked to the largest mass extinction in Earth's history — the end-Permian event)
• Sudden climate shifts caused by these events

6. Human Activity

Humans are currently the dominant cause of species extinction:

Human Activity	Impact
Habitat destruction	Deforestation, urbanisation, agriculture — by far the biggest threat
Pollution	Contaminates habitats (pesticides, plastics, oil spills, acid rain)
Overhunting / overfishing	Populations reduced below sustainable levels
Introduction of invasive species	Non-native species outcompete or prey on native species
Climate change (human-caused)	Global warming alters habitats faster than species can adapt

Exam tip: When asked about causes of extinction, give at least three different causes with a brief explanation for each. Include at least one cause related to human activity, as this is a key theme in GCSE Biology.

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Mass Extinction Events

A mass extinction is a period in Earth's history when a large proportion of species become extinct in a geologically short period of time. Scientists have identified five major mass extinctions:

Event	When	Estimated Species Lost	Likely Cause
End-Ordovician	~444 million years ago	~85%	Glaciation and sea level change
Late Devonian	~372 million years ago	~75%	Ocean anoxia, climate change
End-Permian ("The Great Dying")	~252 million years ago	~96%	Volcanic eruptions (Siberian Traps), climate change
End-Triassic	~201 million years ago	~80%	Volcanic eruptions, climate change
End-Cretaceous	~66 million years ago	~76%	Asteroid impact (and possibly volcanism)

Some scientists argue that we are currently in a sixth mass extinction, driven by human activity.

Exam tip: You do not need to memorise all five mass extinctions, but you should be able to describe the end-Cretaceous event (asteroid impact, ~66 million years ago, wiped out the non-avian dinosaurs) as a named example.

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What Is Biodiversity?

Biodiversity is the variety of all living organisms in an area or ecosystem. It has three main components:

Component	Description
Species diversity	The number of different species AND the abundance of each species in an area
Genetic diversity	The variety of genes (alleles) within a species
Ecosystem diversity	The variety of different habitats and ecosystems in a region

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Why Is Biodiversity Important?

1. Ecosystem Stability

Ecosystems with high biodiversity are more stable and resilient:

• More species means more complex food webs, so if one species declines, others can fill its role
• Diverse ecosystems are better able to recover from disturbances

2. Food Security

• We depend on a wide range of species for food
• Wild relatives of crop plants contain genetic diversity that can be used in selective breeding to develop new varieties (e.g., disease-resistant crops)
• Pollinators (bees, butterflies) are essential for crop production

3. Medical Resources

• Many medicines are derived from living organisms:

Medicine	Source
Aspirin	Originally from willow bark (Salix species)
Penicillin	From the mould Penicillium
Taxol (cancer treatment)	From Pacific yew tree bark
Artemisinin (malaria treatment)	From sweet wormwood (Artemisia annua)

• Undiscovered species may hold the key to future medicines
• Losing species means losing potential cures

4. Economic Value

• Ecotourism generates income for communities (e.g., coral reef tourism, safari tourism)
• Forests provide timber, non-timber products, and ecosystem services (water purification, flood prevention)

5. Ethical and Aesthetic Value

• Many people believe all species have an intrinsic right to exist, regardless of their usefulness to humans
• Natural environments have cultural, spiritual and recreational value

Exam tip: When answering questions about why biodiversity is important, aim to include at least three reasons from different categories (ecosystem stability, food, medicine, economics, ethics).

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Conservation Methods

Conservation aims to protect and maintain biodiversity. Key strategies include:

1. Protected Areas

• Nature reserves, national parks, marine protected areas (MPAs)
• Legally protect habitats from development, pollution and exploitation
• Examples: Great Barrier Reef Marine Park, Yellowstone National Park, UK Sites of Special Scientific Interest (SSSIs)

2. Seed Banks

• Stores of seeds from thousands of plant species, kept in controlled conditions (cool, dry, dark)
• Preserve genetic diversity of plant species, including those at risk of extinction
• Example: The Millennium Seed Bank at Kew's Wakehurst site in Sussex holds seeds from over 40,000 species

3. Captive Breeding Programmes