Some ecosystems have run for millions of years without anyone managing them. They do it by balancing what comes in against what goes out, and by recycling almost everything in between. Push one of those balances too far, though, and the system does not bend back — it flips into something else.
📚 What you need to know
Sustainability is an ecosystem’s ability to keep its balance and productivity going over time.
In a steady-state ecosystem, inputs of energy, nutrients and water are balanced by outputs.
Nutrients move between stores (biomass, litter, soil) along transfers (leaf fall, decomposition, uptake).
A tipping point is a threshold where a small extra change triggers a large, often irreversible shift.
Deforestation can drive a positive feedback loop: fewer trees, less rainfall, more fire, fewer trees still.
Keystone species have an effect on the ecosystem far larger than their numbers suggest.
Biosphere integrity is one of the nine planetary boundaries, and rising extinction rates show it has been crossed.
Steady state: balanced inputs and outputs
A steady-state ecosystem is not frozen. Individuals are born and die constantly, and nutrients move all the time. What stays roughly constant is the overall picture, because inputs match outputs.
Inputs: sunlight, rainfall, nutrients dissolved in rain, minerals released from weathered rock.
Outputs: heat from respiration, water and nutrients lost in run-off and leaching, animals migrating away.
That balance is what gives an ecosystem its resilience — the ability to absorb a disturbance and return to roughly where it was.
Draw this from memory as three circles and six arrows. The size of each store and the thickness of each arrow are what change between a rainforest, a desert and a tundra.
Evidence that ecosystems can last
Tropical rainforests have persisted for millions of years through changes in climate and sea level. Their resilience comes from two things: enormous biodiversity, and a dense web of interactions, so if one species declines others can take over its role. A system with many overlapping links absorbs shocks better than a simple one.
Tipping points and positive feedback
Resilience has a limit. A tipping point is the threshold beyond which the system cannot recover its original state. Past it, the ecosystem collapses into a different one — a new equilibrium with different species, lower biodiversity and often a different local climate.
Trees return water to the air through transpiration, and that water vapour becomes rain. Remove enough trees and the forest stops watering itself.
Positive feedback does not mean good news. It means the response pushes the change further in the same direction. In ecosystems it is almost always the dangerous kind, because it is what carries a system past a tipping point.
Keystone species: small numbers, big effect
A keystone species has an influence on the structure of its ecosystem that is far bigger than its abundance would suggest. Remove it and the effects cascade through the whole community.
Species
What it does
What happens if it goes
Purple sea star
Preys on mussels on rocky Pacific shores, keeping mussel numbers down.
Mussels spread and take over the rock, crowding out other species and cutting diversity.
African elephant
Feeds on shrubs and trees, opening gaps in vegetation and moving nutrients around.
Woody plants thicken up, grassland is lost, and habitat variety falls for many other species.
The wording that scores: a keystone species has a disproportionately large effect relative to its abundance. Saying “it is important” is not enough — the comparison with its numbers is the point.
Biosphere integrity and planetary boundaries
The planetary boundaries model sets out nine Earth processes that need to stay inside safe limits if the planet is to remain stable. Biosphere integrity — the overall health and diversity of life — is one of them.
The evidence that this boundary has been crossed is the extinction rate. Species are being lost far faster than the natural background rate, driven by habitat destruction, pollution and climate change. Because ecosystems and species diversity depend on each other, the loss runs both ways: damaged ecosystems lose species, and losing species leaves ecosystems less able to cope with the next disturbance.
Conservation strategy
What it protects
Habitat conservation
Protects whole areas from destruction and fragmentation, keeping the niches that species need.
Species conservation
Targets the most endangered species directly, through breeding programmes, protection and reintroduction.
Sustainable resource management
Allows resources to be used at a rate the ecosystem can replace, so the system keeps functioning.
Worked examples
WORKED EXAMPLE
Explain how deforestation can lead to a new equilibrium in the Amazon rainforest.
Step 1: the first change
Clearing trees reduces transpiration, so less water vapour enters the local atmosphere.
Step 2: the knock-on effect
Less water vapour means less cloud and less rainfall, so the remaining forest becomes drier.
Step 3: the feedback
Drier forest burns more easily and suffers more drought, killing more trees — which reduces transpiration further. This is positive feedback.
Step 4: the tipping point
Past a threshold the forest cannot recover. It settles into a new equilibrium with fewer species, lower biodiversity and a drier climate.
Positive feedback carries the system past a tipping pointUse the words transpiration, positive feedback, tipping point and new equilibrium. They are the marking points.
WORKED EXAMPLE
Sea stars are removed from a stretch of rocky shore. Predict what happens to species diversity and explain why.
Step 1: what the sea star was doing
It preyed on mussels, keeping the mussel population well below its carrying capacity.
Step 2: what changes
With the predator gone, mussel numbers rise quickly and they spread across the rock surface.
Step 3: the effect on other species
Mussels out-compete other shore organisms for space, so those species are displaced.
Species diversity fallsThe chain is predator removed → prey increases → prey out-competes others → diversity drops.
WORKED EXAMPLE
A forest is cleared and the soil is farmed. Yields are high for three years, then fall sharply. Explain why, using nutrient stores.
Step 1: where the nutrients were
In a rainforest, most nutrients are held in the living biomass, not in the soil.
Step 2: what clearing does
Removing the trees removes the largest store, and there is no more leaf fall to resupply the litter and soil.
Step 3: why the drop is fast
Heavy rain leaches the remaining soil nutrients away, and crops remove more at every harvest.
Nutrient inputs stop while outputs continue, so the soil store emptiesThe phrase “inputs no longer balance outputs” is the steady-state idea applied to a real case.
💡 Exam tip
Answer feedback questions as a numbered chain that returns to its start. If it does not loop back, it is not a feedback loop.
State clearly whether feedback is positive (speeds the change up) or negative (damps it down). Marks are given for the label.
For a nutrient flow diagram, name the stores and the transfers separately. They are different words and different marks.
Keep one worked example ready for each of: tipping point, keystone species, and conservation strategy.
Link biodiversity to resilience. Higher diversity means more overlapping roles, so the system copes better with disturbance.
⚠ Common mix-up
Positive feedback assumed to be good. Positive describes the direction of the response, not its value.
Steady state read as “nothing changes”. Plenty changes; the totals stay roughly level because inputs match outputs.
Keystone species confused with the most common species. Keystone species are often uncommon — that is the whole point.
Stores and transfers mixed up. Soil is a store; leaching is a transfer out of it.
Saying rainforest soils are rich. Most tropical rainforest soils are nutrient-poor because the nutrients sit in the trees.
Treating a tipping point as reversible. Beyond it the system moves to a new equilibrium and does not simply bounce back.
Up next: Following Energy Through an Ecosystem (Topic 2.2) — why energy flows one way while matter goes round, and what the two laws of thermodynamics mean for a food chain.
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