Nature does it without trying: Sustainability is a natural property of ecosystems: left alone, an ecosystem is sustainable.
The points to remember
- Left alone, most ecosystems are sustainable: they can keep going indefinitely.
- They run on sunlight, an energy input that does not run out.
- They recycle matter: nutrients go round and round, so nothing is used up.
- Their inputs balance their outputs, so their storages stay about the same.
- People try to copy this when they use resources sustainably (1.3).
Remember it as: Sunlight in, matter recycled, inputs = outputs: it can go on for ever.
Real example: the Amazon rainforest has grown in the same place for tens of millions of years, powered by sunlight and recycling its own nutrients.
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Inputs balanced by outputs: A sustainable ecosystem is in a steady state: what comes in each year equals what goes out.
The points to remember: reading a flow diagram
- In a steady-state ecosystem, inputs are balanced by outputs.
- Flows keep going, but the storages stay about the same over the years.
- On a flow diagram: add up the inputs, add up the outputs, then compare.
- Equal: a steady state, so the ecosystem is sustainable.
- Outputs bigger than inputs: a storage is running down, which cannot last.
Remember it as: Add the ins, add the outs, compare.
The same for energy: the sunlight the Amazon's plants capture is lost again as heat, so the energy stored in its living things stays about the same.
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How do we know some ecosystems really can last? The best evidence comes from tropical rainforests.
The points to remember: the evidence
- Fossils, such as pollen and leaves, show some tropical rainforests have lasted millions of years.
- They lasted through climate changes, fires and storms, recovering each time.
- Long-term measurements of undisturbed forests show inputs about equal to outputs.
Rainforest fossils: Fossil leaves about 58 million years old, found in a coal mine at Cerrejón, Colombia, show that rainforest grew in South America long ago, with plant families still found in the Amazon today.
So: the rainforest has survived many changes in climate, yet it is still there. That is sustainability as a natural property.
Why natural ecosystems keep going: Four things work together. Resilience, diversity and storages were met in 1.2.16 to 1.2.18.
The points to remember
- Energy: a steady supply of sunlight powers the producers.
- Matter: decomposers recycle nutrients, so nothing runs out.
- Negative feedback keeps populations in check (1.2.9).
- Diversity and large storages help it recover after a disturbance (1.2.17).
Remember it as: Sunlight, recycling, feedback, diversity.
Real example: fallen leaves on the Amazon floor rot within weeks, and tree roots near the surface take the nutrients straight back up.
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People can break the natural balance by taking more out than comes in.
The points to remember: breaking the balance
- Humans can make outputs bigger than inputs: harvesting, clearing, overfishing.
- Then storages run down: soil nutrients, water, fish stocks.
- Pushed far enough, the ecosystem may pass a tipping point into a new state (2.1.22).
Real example: When Amazon forest is cleared for cattle, rain washes the thin soil's nutrients away, and there are no roots to take them back up: outputs now exceed inputs.
Remember: a steady state is sustainable only while the inputs keep up with the outputs.
How this comes up: Paper 2, Section B (a) [4]: four linked points with a named example. Or a flow diagram in Section A.
Some ecosystems, such as tropical rainforests, can keep going for millions of years without running down.
Outline how the inputs and outputs of a named ecosystem are balanced in a steady state.
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