Understanding Extinction and Its Causes Revision Notes | GCSE Edexcel Higher Biology

1. What Is Extinction?

Imagine if every single dodo bird, every last one, disappeared from Earth forever. That's exactly what happened in 1681 - and it's what we call extinction.

This isn't the same as a species being rare or endangered. Extinction means zero individuals left - the species is gone forever. Once extinct, a species cannot come back naturally.

Think of it like this: if you had the last chocolate bar of a particular type in the world, and you ate it, that chocolate bar type would be 'extinct'. No more can be made because the recipe is lost forever.

2. The Main Causes of Extinction

Species don't just disappear randomly. There are specific factors that push them towards extinction. Understanding these causes helps us protect species that are currently at risk.

The main factors that contribute to extinction fall into several categories. Let's examine each one and see how they work in the real world.

When forests are cut down for farming, or wetlands are drained for building, the animals and plants that lived there lose their homes. No habitat means no food, no shelter, and no place to breed.

Picture a bird that only nests in old oak trees. If all the oak forests are cleared, where can it nest? Without nesting sites, the bird population crashes and may become extinct.

3. Human Activities and Extinction

Humans have become the biggest driver of extinctions in modern times. Our activities affect species in ways that natural changes rarely do - and much faster than species can adapt.

The dodo bird is a classic example. Sailors visiting Mauritius found these flightless birds more straightforward with practice to catch for food. They also brought rats and pigs that ate dodo eggs. Within 200 years of human arrival, every dodo was dead.

Modern examples include overfishing of bluefin tuna and illegal hunting of rhinos for their horns. When we take individuals faster than the species can replace them through reproduction, populations spiral towards extinction.

Pollution is another major human impact. Chemical pesticides, plastic waste, and oil spills poison ecosystems. Species that can't tolerate these pollutants die out, while those that survive may have reduced fertility or health problems.

4. Environmental Changes and Species Survival

Even without human interference, environmental changes can drive species extinct. The difference is that natural changes usually happen slowly, giving species time to adapt or migrate.

As temperatures rise, some habitats disappear entirely. Arctic species like polar bears lose sea ice hunting grounds. Mountain species run out of cooler areas to retreat to as temperatures climb.

Natural disasters can also cause extinctions, especially on islands. A volcanic eruption or hurricane might wipe out the entire population of a species that exists nowhere else.

Disease outbreaks can devastate species with no natural immunity. When a new pathogen spreads through a population, it can kill most or all individuals before they develop resistance.

Competition from invasive species creates another pressure. When humans accidentally or deliberately introduce species to new areas, these invaders may outcompete native species for food and habitat.

5. Real-World Examples and Case Studies

Looking at specific extinction cases helps us understand how these factors work in practice. Each story reveals different lessons about what makes species vulnerable.

Worked example: Case Study Analysis: Tasmanian Tiger Extinction

Analyse the factors that led to the extinction of the Tasmanian tiger (thylacine) in 1936.

1. Systematic hunting by humans — Government bounties encouraged killing thylacines as 'sheep killers'
2. Habitat loss through farming expansion — European settlement converted wilderness to farmland
3. Competition from introduced dogs — Domestic dogs competed for prey and territory
4. Disease transmission from domestic animals — Thylacines had no immunity to European animal diseases

Answer: Multiple factors combined: direct hunting, habitat destruction, competition, and disease - a pattern seen in many extinctions.

The passenger pigeon tells another story. Once the most abundant bird in North America with billions of individuals, it went extinct in 1914. Commercial hunting for food markets killed them faster than they could breed.

More recently, the northern white rhino became functionally extinct in 2018 when the last male died. Poaching for rhino horn, driven by demand in traditional medicine, reduced the population to unsustainable levels.

6. Exam Technique and Key Points

Extinction questions often ask you to describe factors or explain why certain species are at risk. Here's how to structure your answers for maximum marks.

Common exam mistakes to avoid: Don't confuse extinction with evolution - they're opposite processes. Don't say species 'choose' to become extinct - extinction happens TO them. Don't forget that extinction is permanent - once gone, species cannot return naturally.

When explaining extinction factors, use the structure: Factor → How it works → Effect on species → Example. This shows clear understanding and gets you the explanation marks.