Two forests, the same storm. One is back to normal in a decade. The other becomes grassland and stays that way. The difference is not luck — it comes down to diversity and resilience, and to how hard the system was pushed. This page is where succession meets human impact, and it is a favourite for extended-response questions.
📚 What you need to know
An ecosystem’s ability to tolerate disturbance and stay in equilibrium depends on its diversity and its resilience.
Diversity covers the variety of species, genetic variation, habitats and ecological functions in the system.
Resilience is the ability to recover after a disturbance. High resilience means a quick return to the original state; low resilience means slow or incomplete recovery.
Greater diversity usually means greater resilience, for two reasons: species redundancy and genetic variation.
Human activities — burning, agriculture, grazing pressure, deforestation, mining — can divert succession.
If the disturbance is mild and resilience is high, the system recovers. If it is severe and resilience is low, it shifts to an alternative stable state.
Deforested rainforest turning into savanna or grassland is the standard example, and the new state has lower diversity, productivity and resilience.
Diversity and resilience
These are two separate ideas that get treated as one. Diversity is about what is in the system. Resilience is about what the system does when something goes wrong.
Two definitions to learndiversity = the variety of species, genes, habitats and functions present resilience = the ability of an ecosystem to recover after a disturbance
Both lines start at the same place and take the same hit. Everything that matters happens after the drop — which is why resilience is defined by recovery, not by how hard something is to knock down.
Why diversity gives resilience
Species redundancy. Several species do similar jobs. If one is lost, the others can fill its ecological role, so the function of the ecosystem keeps going.
Genetic variation. More genetic diversity within a species means a better chance that some individuals can cope with the new conditions, so the population adapts instead of collapsing.
Other things push resilience up or down too:
Size — a larger ecosystem has more room for species to recolonise from.
Climate — warm, wet conditions allow faster regrowth than cold or dry ones, which is why tundra recovers so slowly.
Speed of reproduction — systems built from fast-breeding organisms bounce back quicker than one built from slow-growing trees.
Existing human pressure — a system already stressed by pollution or fragmentation has far less capacity left to absorb the next shock.
Think of a football team. Diversity is having a squad of players who can each cover more than one position; resilience is still winning after two of them get injured. A team of eleven specialists with no substitutes is a monoculture.
How humans divert succession
Left alone, succession runs towards a climax community. Human activity can stop it, reverse it, or push it somewhere else entirely. The main ones to name:
Burning — controlled fires clear land for farming or manage wildfire risk, but fire kills plants, lowers soil fertility and alters nutrient cycles.
Agriculture — ploughing and cropping hold the land permanently at an early seral stage.
Grazing pressure — constant grazing removes seedlings, so shrubs and trees never establish.
Resource use, especially deforestation — removes the climax vegetation outright.
Mining — strips off topsoil and vegetation, causes erosion and landslides, and the chemicals used can pollute water and damage aquatic life.
These have direct effects (a tree is cut down) and indirect effects (soil erodes, nutrient cycling changes, the water cycle shifts). Both alter the biotic and abiotic components, and between them they change the course of succession.
The nasty part. Agriculture and grazing cause soil erosion and loss of vegetation cover, which changes nutrient cycling, which changes which species can grow. The community that comes back is not the one that was removed.
Tipping points and alternative stable states
Whether the damage is temporary or permanent depends on how hard the push was and how resilient the system is.
Mild disturbance, high resilience — the ecosystem recovers and returns to something close to its original state.
Severe disturbance, low resilience — the ecosystem is permanently changed. It settles into a new stable state with a different set of species and interactions.
The point of no return is the tipping point: past it, the system does not slide back, it slides forward into the new state.
The depth of the valley is the resilience. Once the ball is over the hill, removing the pressure does not undo anything — the system now sits happily in the second valley, and returning it means pushing it back up a slope all over again.
Case study: rainforest to savanna
Tropical rainforest is highly resilient. Under enough human pressure, even it can shift.
Deforestation and agriculture. Trees are removed for grazing land and crops. This reduces habitat complexity and causes biodiversity loss, disrupts nutrient cycling, changes the water cycle, causes soil erosion and loss of topsoil, lowers primary productivity, and can eventually trigger desertification.
Mining. Topsoil and vegetation are stripped away, erosion and landslides follow, and mining chemicals pollute watercourses, which damages aquatic life downstream.
The new state. The forest is replaced by savanna or grassland with lower biodiversity, lower productivity and different abiotic and biotic conditions.
Why it sticks. That new system has lower resilience and is far less able to recover to forest, partly because the soil and the local rainfall that supported the forest have both changed.
This is the argument for considering environmental impact before, not after. Protecting a natural process like succession is cheaper and more reliable than trying to reverse a system that has already tipped.
Worked examples
EXAM Q1
Explain why an ecosystem with high biodiversity is usually more resilient. [3]
Point 1: species redundancy
Several species carry out similar ecological roles, so losing one does not stop that function.
Point 2: genetic variation
More variation within populations means some individuals are likely to tolerate the new conditions and survive to breed.
Point 3: food web structure
More species means more feeding routes, so a broken link does not collapse the whole web.
Redundancy and variation give the system alternatives when something is lost“redundancy” is the technical term the mark scheme wants
EXAM Q2
Outline how human activity can divert succession to an alternative stable state. [3]
Step 1: name an activityDeforestation for grazing land removes the climax vegetation.
Step 2: describe the knock-on effects
Soil erodes, nutrient cycling and the water cycle change, and productivity falls.
Step 3: the new state
Conditions no longer support forest regrowth, so savanna or grassland establishes and persists.
Past the tipping point the system stabilises in a new, less diverse statesay why it does not simply grow back — the soil and rainfall have changed
EXAM Q3
Two grasslands are burnt. One recovers within five years; the other is still bare after twenty. Suggest two reasons for the difference. [2]
Reason 1: diversity and seed sources
The recovering site probably had higher diversity and surviving seed banks or nearby populations to recolonise from.
Reason 2: severity and abiotic damage
The other fire may have been more severe, destroying the topsoil, or the site may be drier and colder so regrowth is slow.
Resilience depends on both what survives and how badly the abiotic conditions were damaged“suggest” means apply what you know to unfamiliar data — no single right answer
💡 Exam tip
Keep diversity and resilience as separate defined terms, then link them. Do not treat them as the same word.
Define resilience as recovery after a disturbance — not resistance to being disturbed in the first place.
Name species redundancy and genetic variation explicitly. They are the two named reasons on the syllabus.
Use the phrase alternative stable state when a system does not return to what it was. It is a mark-scheme term.
For “evaluate” questions, weigh both sides: human activity can also be managed to help recovery, for example through replanting or protected areas.
Link back to succession: a diverted system is being held at an earlier seral stage or pushed into a different one.
⚠ Common mix-up
Saying stability means nothing changes. A stable ecosystem changes constantly; it just stays in the same overall state.
Confusing resilience with resistance. Resistance is not being knocked down; resilience is getting back up.
Assuming all human disturbance is permanent. If the ecosystem is resilient and the disturbance is mild, it can recover fully.
Thinking an alternative stable state is unstable. It is stable — that is precisely the problem, because it will not drift back on its own.
Forgetting the abiotic damage. Losing topsoil or changing local rainfall is often what makes recovery impossible, not the loss of species alone.
Treating “more species” as automatically more resilient. It usually is, but a diverse system already under heavy pressure can still be fragile.
Up next: Measuring Abiotic and Biotic Components of Ecosystems — the practical toolkit behind every graph you have read in this sub-topic.
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