Parts of the Amazon have been rainforest for tens of millions of years. Nothing external has been propping it up for all that time, which means the system must be doing something right internally. This page is about what that something is – the four things an ecosystem needs in order to keep going on its own.
📚 What you need to know
Stable ecosystems show efficient nutrient cycling, high biodiversity, high rates of photosynthesis and resistance to change.
Four requirements for stability: a supply of energy, recycling of nutrients, genetic diversity, and climatic variables staying within tolerance limits.
Genetic diversity is the number of different alleles of genes present in a population.
Stable does not mean static – natural selection is always acting.
Tropical rainforests such as the Amazon and the Congo are the standard example of a long-lived stable ecosystem.
What a stable ecosystem looks like
Stable ecosystems share a set of features, and they are all connected to each other rather than being a random list.
Efficient nutrient cycling, which allows the system to be self-supporting. Nothing has to be delivered from outside.
High biodiversity.
Stability itself, meaning resistance to change. For example, consumer population sizes do not change significantly, so resources are not overused.
High levels of photosynthesis.
The definition to useStability in an ecological context means resistance to change – the ecosystem carries on functioning in much the same way even when conditions vary.
The rainforest example
Some tropical rainforests, such as the Amazon in South America and the Congo in Africa, have remained in their current state for tens of millions of years. What keeps them going is worth breaking down, because it is really the four requirements in action:
They are highly diverse. The Amazon is thought to contain millions of invertebrate species, tens of thousands of plant species, thousands of bird and fish species, and hundreds of mammal species.
High levels of light and moisture mean photosynthesis rates are high.
Organic matter is cycled by detritivores such as termites, slugs and worms, and by decomposers such as fungi. The nutrients released are then taken up again by the trees.
Water is cycled within the ecosystem: it is lost from trees by transpiration, condenses, and falls again as rain.
Both loops close inside the forest. That is the whole reason a rainforest can sit in the same place for millions of years.
Healthy ecosystems are highly stable, but they are not entirely static. Natural selection is always acting on the species within them, so evolutionary change continues even in an ecosystem that looks unchanged from the outside. Stability is about the system, not about the individual species in it.
The four requirements for stability
An ecosystem needs all four of these. Take one away and the whole thing starts to unravel, which is why exam questions often give you a scenario and ask which requirement has been broken.
When a question describes a damaged ecosystem, work out which of these four boxes has been broken. That is usually the mark scheme.
1. Supply of energy
A reliable source of energy must be present, and for most ecosystems that source is sunlight. Light energy is converted into chemical energy by photosynthesis, which means plenty of photosynthetic organisms – plants or algae – have to be present. The stored chemical energy is then passed up food chains through consumption.
2. Recycling of nutrients
For an ecosystem to support itself, nutrient cycling is essential. If nutrients are not recycled, the supply simply runs out.
Nutrients are cycled when decomposers such as bacteria and fungi break down the carbon compounds – proteins, nucleic acids and the rest – in the tissues of dead organisms and in waste matter. Two things are released:
Carbon goes back into the atmosphere as carbon dioxide.
Minerals such as nitrates and phosphates go back into the soil.
Producers then take these up again and they re-enter the food chain. Note the condition attached: the ecosystem must have conditions that suit the decomposers. There needs to be enough oxygen and moisture, and the temperature must be suitable. An ecosystem that is too hot or too dry will have reduced nutrient cycling and will therefore be less productive.
Where humans break this one. If nutrients are physically removed from an ecosystem, the cycle is interrupted and productivity falls. Two everyday examples: fallen trees taken away for timber instead of being left to decompose, and crops being harvested. In both cases the nutrients leave the system in the back of a lorry and never come back.
3. Genetic diversity
Genetic diversity is one aspect of diversity, and it has a specific definition.
DefinitionGenetic diversity is the number of different alleles of genes present in a population.
High genetic diversity means natural selection has favourable alleles to act on, which gives a population the potential to adapt when the environment changes. The reverse is the point worth writing down: if a population has only a limited number of alleles, the chance that one of them happens to be favourable in the new conditions is reduced, and the population will be unable to adapt.
That is why genetic diversity allows populations to resist the effects of change in their environment – which is exactly what stability means.
4. Climatic variables within tolerance limits
Genetic variation only takes a population so far. If extreme environmental changes occur, conditions may move outside the tolerance limits of a species altogether. The climatic variables in question are things like temperature and rainfall.
Once conditions are outside its tolerance limits, a species must migrate or face extinction – the same two options that appeared in the climate change unit. And this is where the two topics join up: human activities are causing climate change at such a rapid rate that climatic variables in some ecosystems are already changing beyond tolerance levels.
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Energy in, nutrients round, alleles ready, climate steady
Four requirements, four short phrases. Energy in – a reliable input. Nutrients round – cycled, not imported. Alleles ready – enough genetic diversity to adapt. Climate steady – variables inside tolerance limits.
Worked examples
WE 1
Outline the features of a stable ecosystem
State three features that are typical of a stable ecosystem. (3 marks)
Any three of the following
Efficient nutrient cycling, so the system is self-supporting; high biodiversity; resistance to change, for example consumer population sizes staying roughly constant; high levels of photosynthesis.
Cycling, diversity, resistance, photosynthesis“it does not change” on its own is not enough – say what stays constant and why
WE 2
Explain the role of decomposers
Explain why the activity of decomposers is essential for the stability of an ecosystem. (4 marks)
Point 1: what they do
Decomposers break down the carbon compounds in dead tissue and waste matter.
Point 2: what is released
Carbon is released as carbon dioxide and minerals such as nitrates and phosphates are released into the soil.
Point 3: the return
Producers take these nutrients up again, so they re-enter the food chain rather than being lost.
Point 4: why it matters
Without this recycling the supply of nutrients would run out and the ecosystem could not support itself.
Decomposers are what make the system self-supportingyou can add that decomposers need suitable oxygen, moisture and temperature to work
WE 3
Apply the idea of genetic diversity
A population of plants has very low genetic diversity. Suggest why this population is less likely to survive a change in climate. (3 marks)
Point 1: the definition
Genetic diversity is the number of different alleles present in the population.
Point 2: the consequence
With few alleles, the chance that any one of them is favourable under the new conditions is reduced.
Point 3: the outcome
Natural selection therefore has little to act on, so the population cannot adapt and may not survive if conditions move outside its tolerance limits.
No variation means nothing for selection to work withselection does not create useful alleles – it can only favour ones that are already there
💡 Exam tips
Define stability as resistance to change, not as “nothing happens”.
Learn the definition of genetic diversity word for word.
Name the specific minerals released by decomposers: nitrates and phosphates.
Use the Amazon or Congo rainforest as your named example.
Remember that stable ecosystems are still subject to evolutionary change.
Link tolerance limits back to migrate or face extinction.
⚠ Common mistakes
Saying a stable ecosystem never changes. Species within it are always evolving.
Confusing genetic diversity with species diversity. Genetic diversity is about alleles within a population.
Saying decomposers “eat” the nutrients. They release them back into the soil and atmosphere.
Forgetting that harvesting removes nutrients. Taking crops or timber away interrupts the cycle.
Writing that populations adapt in response to change. Selection acts on alleles that already exist.
Ignoring the conditions decomposers need. Too hot or too dry means slower cycling and lower productivity.
Up next: Ecosystem Stability (Skills) – the two practical skills attached to this topic. How to build and run a mesocosm, and how to calculate percentage loss when an ecosystem is being cleared.
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