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: in parts of the Amazon, up to half of the rain comes from water vapour that the forest itself gave off, so every tree cleared means less rain for the rest.
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The Aral Sea: an ecosystem that tipped: In 1960 the Aral Sea, in Central Asia, was the world's fourth-largest lake. Its two rivers were then taken to water cotton fields.
The points to remember: the Aral Sea
- Human activity: river water taken for irrigation, so inputs fell below outputs.
- The water became saltier, and most native fish species died out.
- The fishing industry collapsed and towns were left far from the shore.
- New equilibrium: a salty desert, with dust storms, where the lake bed was.
Hard to undo: a dam built in 2005 let the small northern part refill and fish return, but the large southern part is still desert.
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: the Amazon has huge diversity, but clearing removes storages and diversity and brings it closer to its tipping point.
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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.
Parts of the Amazon rainforest are close to a tipping point.
Outline how four different factors influence the resilience of an ecosystem.
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