This deep dive covers Why Ecosystems Are Under Threat — The Cause Chain within Sustaining Ecosystems for GCSE Geography. Revise Sustaining Ecosystems in The Living World for GCSE Geography with 15 exam-style questions and 18 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 2 of 14 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
🔍 Why Ecosystems Are Under Threat — The Cause Chain
Ecosystems do not collapse all at once. They unravel gradually, under pressure from several directions at the same time. Understanding the pressures matters because different management strategies address different ones — and an answer that simply says "deforestation is bad" without explaining the mechanism will never reach the top mark band.
Here is what the pressures look like in practice, and how they connect:
The Amazon has lost 17% of its forest cover in the last 50 years — an area the size of France. In the Congo Basin, 90% of the population relies on charcoal for cooking fuel, and trees are felled daily to produce it. When forest goes, the species that depend on it go too: habitats are fragmented into small, isolated patches where populations cannot survive long-term. The soil, no longer held by tree roots, washes away in the rain. The water cycle breaks down. And the carbon stored in the trees is released as CO₂.
Every ecosystem evolved over thousands of years to suit its specific climate. Ocean warming of just 1–2°C is enough to cause coral bleaching — the coral ejects the algae it depends on for food and turns white. Without the algae, the coral starves. In Arctic and sub-Arctic environments, rising temperatures are thawing permafrost, releasing methane (a greenhouse gas 80 times more potent than CO₂ over 20 years) and converting tundra into boggy wetland. In the UK, warmer winters allow pests and diseases to survive that would once have been killed by frost, threatening native woodland species.
The Great Pacific Garbage Patch is a floating mass of plastic waste covering an area roughly three times the size of France in the North Pacific Ocean. Marine animals mistake microplastics for food; plastic fragments move up the food chain from plankton to fish to seabirds to humans. In the Gulf of Mexico, agricultural run-off from the Mississippi River basin carries nitrate and phosphate fertilisers into the sea. These nutrients trigger explosive algae blooms that then die, decompose, and consume all the oxygen in the water — creating a dead zone the size of New Jersey where almost nothing can survive.
North Sea cod stocks collapsed by more than 90% between the 1970s and 1990s. Fishing fleets using ever-larger nets and more sophisticated fish-finding technology took far more fish than the population could replace. When a keystone species disappears from an ecosystem, the whole food web can shift: species that the cod ate explode in number; species that ate the cod starve. Recovery is slow — North Sea cod stocks are still at only a fraction of their historic levels even after decades of restrictions.
In the UK, grey squirrels introduced from North America in 1876 now outnumber red squirrels by 66:1. Grey squirrels carry a pox virus that doesn't harm them but is lethal to reds. They also outcompete reds for food. Red squirrels are now extinct across most of England. In Lake Victoria, East Africa, the Nile perch was introduced as a food fish in the 1950s. It was spectacularly successful — and catastrophically destructive. It ate the lake's 300+ species of cichlid fish, causing the fastest mass extinction ever recorded by biologists: around 200 species extinct within decades. Japanese knotweed, introduced to the UK as a garden plant in 1850, now costs £165 million a year to control and can crack concrete foundations.
All of these pressures feed into the same outcome: fewer species, less resilient ecosystems, declining services that humans depend on. This is not an abstract environmental problem. It is a practical problem for human survival.
Quick Check: Explain how agricultural run-off can create a "dead zone" in the ocean.
Agricultural fertilisers containing nitrates and phosphates are washed off farmland by rain and carried by rivers into the sea (e.g. via the Mississippi into the Gulf of Mexico). These nutrients trigger a rapid explosion in algae growth — an algae bloom. When the algae die, bacteria decompose them. The decomposition process consumes enormous amounts of dissolved oxygen in the water. Oxygen levels drop so low that fish, crustaceans and other marine life suffocate and die or flee the area, leaving a dead zone where almost nothing can survive. The Gulf of Mexico dead zone covers roughly 22,000 km² — the size of New Jersey.
Practice questions for Sustaining Ecosystems
Which of the following is a direct consequence of deforestation in tropical rainforests?
Explain the process of coral bleaching and why it threatens the Great Barrier Reef.