Human activity and tipping points at Higher Level: The same ideas as SL. HL's real case is the northern cod off Newfoundland, which collapsed in 1992 and has not come back.
Practise this as you read
- Trace the Amazon loop step by step.
- Explain why a tipped ecosystem is hard to restore.
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When an ecosystem tips: Human activity can push an ecosystem to a tipping point. The original ecosystem collapses and a new equilibrium develops.
The points to remember
- At a tipping point, a small extra change makes the ecosystem collapse.
- The original ecosystem is replaced by a new equilibrium: a different stable state.
- Human activity often pushes it there: clearing, overfishing, pollution, taking water.
- Past the tipping point, positive feedback takes over and change speeds up.
- Going back is hard: the new state holds itself in place.
Remember it as: Push, tip, collapse, new state, hard to undo.
Tipping points and feedback loops in any system were met in 1.2.11 and 1.2.12. Here they happen to whole ecosystems, because of people.
Collapse, then a new equilibrium: Say both halves: the old ecosystem collapses, and a different one takes its place and stays.
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The Amazon rainforest: Amazon trees transpire, which cools the land and falls again as rain. The forest makes much of its own rain.
The points to remember: the Amazon loop
- Deforestation removes trees that transpire water vapour.
- Less water vapour means less cooling of the land and less rain.
- The remaining forest becomes hotter and drier, with a longer dry season.
- More trees die and burn, so even less vapour: a reinforcing loop.
- Past the tipping point, large areas could turn to savanna: the new equilibrium.
Real data: air samples show the heavily cleared south-east Amazon now gives off more carbon dioxide than it takes in, a sign that this part of the forest is losing its balance.
The northern cod: an ecosystem that tipped: For about 500 years, huge numbers of cod were caught off Newfoundland, Canada. Then factory trawlers came.
The points to remember: the northern cod
- Human activity: overfishing removed cod faster than they could breed.
- In 1992 the stock had fallen by about 99%; the fishery closed and about 30,000 people lost jobs.
- Cod were the top predator; without them, shrimp and snow crab increased.
- New equilibrium: thirty years on, cod have not fully recovered.
Hard to undo: the shrimp and crab, and the fish that eat young cod, now hold the new state in place.
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Some ecosystems resist being tipped far better than others. Resilience was met in 1.2.16 to 1.2.18.
The points to remember: more or less likely to tip
- More species diversity and complex food webs: more resilient.
- A keystone species present: more resilient.
- Large storages, plenty of nutrients, water or biomass: more resilient.
- A larger system and strong negative feedback: more resilient.
- A steady state, inputs balanced by outputs, as in a climax community: more resilient.
- Strong positive feedback or human damage to diversity and storages: less resilient.
- Already close to a tipping point: less resilient.
Say which way, and say 'large': Each factor needs its effect: more or less resilient. 'Storages' alone is not enough: say large or abundant storages.
Real example: overfishing took the cod, a top predator, out of the food web, so the ecosystem lost the negative feedback that had kept shrimp and crab in check.
How this comes up: Paper 2, Section B (a) [4]: four factors and their effect on resilience. Section B (c) [9]: tipping points and people.
Coral reefs around the world are under growing pressure from warming seas and overfishing.
Outline how four different factors influence the resilience of an ecosystem.
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