Habitat Destruction

1. Deforestation.

• Trees cleared for: timber + paper, palm oil plantations, soya farming, cattle ranching, urbanisation.
• ~10 million hectares of forest lost annually.
• Particularly catastrophic in tropical rainforests (highest biodiversity).

2. Urbanisation.

• Cities expand → habitat replaced with concrete + buildings.
• Roads fragment habitats → animals can't move between patches.

3. Agriculture.

• Wild lands converted to fields + pasture.
• Pasture for cattle is one of the BIGGEST drivers of Amazon deforestation.

4. Mining.

• Strip mining destroys surface land entirely.
• Tailings (waste) pollute soil and water.
• Coal mining + oil drilling fragment habitats.

5. Dams + flooding.

• Hydroelectric dams flood vast areas → upstream habitat destroyed.
• Three Gorges Dam (China) displaced 1.4 million people + flooded huge area.

6. Coastal development.

• Mangroves cleared for shrimp farming and tourism.
• Coral reefs damaged by anchors, sediment, pollution.

Worked qualitative. Why is the AMAZON RAINFOREST particularly important?

• Highest biodiversity on Earth — ~10% of all known species.
• Massive carbon sink (absorbs ~5% of human CO₂ emissions).
• 'Rivers in the sky' — pumps moisture across South America.
• Once cleared, doesn't easily return — soil quality degrades.

Cambridge tip. When asked causes of habitat destruction, give 4-5 different ones. Cambridge marks each.

1. Loss of biodiversity → extinction.

• Specialised species lose their only habitat.
• Slow-reproducing species (large mammals, big trees) can't recover.
• Once extinct, gone forever.
• Examples: Sumatran rhino (down to ~80 individuals), countless insects + plants undescribed before extinction.

2. Soil erosion.

• Tree roots hold soil in place.
• Without forests, RAIN washes soil away → topsoil lost.
• Soil ends up in rivers as sediment → kills aquatic life.
• Farmland left becomes infertile after a few seasons.

3. Climate change.

• Trees absorb CO₂ via photosynthesis.
• Deforestation + burning RELEASE stored carbon to atmosphere.
• Less forest = more CO₂ = more global warming.
• Tropical deforestation accounts for ~15% of global emissions.

4. Flooding.

• Forests act like sponges — soak up rain, release slowly.
• Without forests, rain runs off rapidly → flash floods downstream.
• Loss of life + infrastructure damage.

5. Reduced local rainfall.

• Trees transpire huge amounts of water → contribute to local rainfall.
• Removing forests reduces rainfall, sometimes causing droughts.

6. Ecosystem services lost.

• Pollination (by insects + birds living in forests).
• Pest control (predators of crop pests).
• Water purification.
• All ecological 'free services' lost when habitats destroyed.

Worked qualitative. Why might cutting forests for agriculture eventually REDUCE agricultural productivity?

• Soil erosion (no tree roots to hold soil).
• Loss of nutrient cycling.
• Reduced rainfall locally.
• Loss of pollinators (some crops rely on insects).
• Short-term gain, long-term loss — sustainability problem.

Cambridge tip. Always link causes to multiple consequences. 'Deforestation causes climate change' is partial; 'deforestation releases CO₂ which contributes to climate change' is full.

Background extinction rate.

• Natural rate of species extinction over geological time: ~1 species per million species per year.
• Most extinctions happen in mass extinctions (5 in Earth's history).

Current rate.

• 100-1000× higher than background.
• Many scientists call current era 'sixth mass extinction'.
• Driven primarily by HUMAN ACTIVITY: habitat loss, climate change, hunting, invasive species.

Why humans are particularly bad for biodiversity:

• Vast scale of land use (~50% of Earth's habitable land used by humans).
• Rapid speed (decades vs millions of years).
• Multiple simultaneous pressures.

Why extinction matters:

1. Medical. Many medicines come from natural compounds. Quinine (malaria) from Cinchona bark; aspirin from willow; many cancer drugs from plants. Extinct species = lost medicines.

2. Ecological. Species are interconnected. Extinction can cascade through food webs.

3. Ethical. Humans cause the harm; many feel responsible to prevent it.

4. Economic. Tourism (safaris, reef diving), fisheries, pollination — all depend on biodiversity.

5. Aesthetic + cultural. Many cultures value certain species; biodiversity is part of natural beauty.

Worked qualitative. Why is the GIANT PANDA's situation so concerning?

• Specialised diet (bamboo) — needs specific habitats.
• Slow reproduction (one cub every few years).
• Habitat fragmented by human activity.
• Population recovering somewhat with conservation, but still vulnerable.
• Symbol of biodiversity conservation.

Cambridge tip. When asked why we should preserve species, give 3-4 reasons (medical, ecological, ethical, economic). Cambridge marks the diversity of reasons.