Some ecosystems have run for millions of years without anyone managing them. That is not luck. It happens because inputs and outputs stay roughly balanced and because feedback keeps pulling the system back to a steady state. Push too hard, though, and an ecosystem can flip into something completely different.
📚 What you need to know
Sustainability is an ecosystem’s ability to keep its balance and productivity going over time.
In a steady state, inputs and outputs are roughly equal, so stores stay about the same size.
Flow diagrams show nutrient stores (biomass, litter, soil) and transfers between them.
Human activity can push a system past a tipping point into a new equilibrium.
Positive feedback amplifies a change; negative feedback resists it.
Keystone species have an effect far larger than their numbers suggest.
Biosphere integrity is one of the planetary boundaries, and extinction rates suggest we have crossed it.
Balanced inputs and outputs
A steady-state ecosystem is not a still one. Energy pours in as sunlight and out as heat every single day, and nutrients are constantly moving between soil, plants and dead material. What stays steady is the size of the stores, because what leaves each store is matched by what enters it.
Cut the trees down and the biomass store is removed, so the nutrients that were locked inside it are lost from the system for good.
Why rainforest soil is poor. Warm, wet conditions mean litter is decomposed very quickly and the nutrients are taken straight back up by plants. Almost nothing accumulates in the soil, which is why cleared rainforest land stops being fertile after only a few years.
Evidence that ecosystems are sustainable
Tropical rainforests have existed for tens of millions of years, through large swings in climate. That persistence is the evidence: a system that can absorb disturbance and carry on is a resilient one. Two things give it that resilience:
High biodiversity. If one species declines, others can do a similar job, so the system keeps functioning.
Complex interactions. Many overlapping food chains mean no single link is critical.
Resilience is not the same as being unchangeable. A resilient ecosystem is one that returns to something like its previous state after being disturbed.
Tipping points and positive feedback
Ecosystems can take a certain amount of pressure and recover. Past a certain point — the tipping point — a small extra push causes a large, often permanent change, and the system settles into a new equilibrium with different species and different conditions.
The engine of that change is positive feedback. Instead of resisting a change, the system speeds it up.
Trees recycle water back into the air, so removing them dries the region out, which kills more trees. Left running, the forest can flip to a drier, grassier system.
Keystone species
A keystone species has an effect on its ecosystem that is far bigger than its abundance would suggest. Remove it and the whole structure changes — which is exactly why the name comes from the stone at the top of an arch.
Species
What it does
What happens if it goes
Purple sea star
Preys on mussels on rocky shores, stopping them taking over.
Mussels spread and crowd out other shore species, so diversity falls sharply.
African elephant
Feeds on shrubs and small trees, keeping savannah open.
Woody plants take over, grassland is lost and many grassland species disappear.
Beaver
Dams streams and creates wetland.
Wetland habitat drains away, taking amphibians and water plants with it.
Notice the pattern: keystone species usually work by stopping one competitor dominating or by creating habitat that other species depend on.
Biosphere integrity and planetary boundaries
The planetary boundaries model sets out nine Earth processes that need to stay within safe limits if the planet is to stay stable. One of them is biosphere integrity — the health and diversity of life itself.
The evidence that we have crossed this boundary comes from extinction rates, which are now far above the natural background rate. Habitat destruction, pollution and climate change are the main drivers. Because ecosystems and diversity depend on each other, a fall in one drags down the other: fewer species means less resilient ecosystems, and damaged ecosystems support fewer species.
Conservation responses fall into three groups you can name:
Habitat conservation — protecting whole areas from destruction and fragmentation.
Species conservation — targeted protection of the most threatened species.
Sustainable resource management — using resources at a rate the ecosystem can replace.
Worked examples
WORKED EXAMPLE
Explaining a feedback loop
Explain how deforestation can act as a positive feedback mechanism and lead to a new equilibrium. [5]
Step 1: the first changeclearing trees reduces transpiration, so less water vapour enters the airStep 2: the knock-on effectless water vapour means less cloud and less rainfall over the regionStep 3: close the loopdrier forest burns more easily, so still more trees are lostStep 4: name itthe change amplifies itself, so this is positive feedback, not negativepast the tipping point the forest is replaced by drier grassland: a new equilibriuma loop answer must return to where it started — that is what makes it feedback
WORKED EXAMPLE
Predicting the loss of a keystone species
Sea stars are removed from a stretch of rocky shore. Predict and explain the effect on species diversity. [3]
Step 1: what the sea star was doingit preyed on mussels and kept mussel numbers downStep 2: what happens without itmussel numbers rise sharply and they take up most of the rock spaceStep 3: the consequenceother shore species are outcompeted for space and disappearspecies diversity falls, even though only one species was removed“predict and explain” needs the outcome AND the mechanism behind it
💡 Exam tip
Steady state means stores stay the same size, not that nothing is moving. Say so.
When drawing or reading flow diagrams, keep stores and transfers clearly separate — boxes or circles for stores, arrows for transfers.
Positive feedback amplifies, negative feedback stabilises. Learn those two verbs.
Use the Amazon as your named example for tipping points, and the sea star or elephant for keystone species.
“Relative to their abundance” is the phrase that earns the keystone definition mark.
Link biodiversity to resilience whenever a question asks why an ecosystem is stable.
⚠ Common mix-up
Thinking positive feedback is good. Positive means the change grows; it is usually the dangerous one.
Calling a common species a keystone species. The point is that its impact is large compared with how few of them there are.
Saying rainforest soil is rich. Most of the nutrients are in the living biomass, not in the soil.
Confusing a tipping point with any change. A tipping point causes a shift to a different, lasting state.
Treating sustainability as “not using anything”. It means using at a rate that can be replaced.
Listing all nine planetary boundaries. You only need biosphere integrity in detail for this topic.
Up next: Building a Modern Classification System — the first HL page, where DNA replaces appearance as the basis for grouping species.
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