Resilience at HL: Resilience — a system's ability to absorb change and keep working — means the same thing for SL and HL. At HL you simply return to it more often, applying it to the extra material on ecosystems and human systems.
Nothing extra here yet — just focus on understanding what resilience means.
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Resilience
Big idea: Resilience is a system's ability to absorb disturbance and still keep functioning.
A resilient system can change in the short term but remain stable in the long term.
What does resilience look like?
- System is disturbed (fire, flood, disease, pollution)
- System changes temporarily
- System recovers and continues functioning
- Long-term stability is maintained
Resilient systems are dynamic, not static. Short-term change is normal.
Examples of resilient systems
- Human body fighting infection
- A forest recovering after a fire
- Predator–prey populations fluctuating but persisting
- A coral reef with high biodiversity
Resilient ecosystems may change in species numbers, but the system as a whole survives.
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What reduces resilience?
- Repeated or intense disturbances
- Loss of biodiversity
- Removal of key storages
- Human pressures (deforestation, pollution, overfishing)
Low resilience increases the risk of tipping points.
Factors that increase resilience
1. Diversity
Diversity makes systems more resilient.
- More species = more interactions
- If one species fails, another can replace its role (redundancy)
- Genetic diversity increases survival after disturbance
Example: If one prey species declines, predators may switch to another.
2. Size and number of storages
Large and multiple storages increase resilience.
- Water stored in lakes and reservoirs
- Biomass stored in forests
- Nutrients stored in soil
Large storages act as buffers — they slow down change.
Example: A lake dries out more slowly than a puddle because it has a larger storage.
Resilience and feedback
Resilient systems usually have strong negative feedback loops that counteract change.
- Negative feedback stabilises systems
- Positive feedback amplifies change
- Too much positive feedback reduces resilience
Resilience vs tipping points
When resilience is low, a system may cross a tipping point.
- Clear lake → nutrient input → algal bloom
- Forest → drought + deforestation → dieback
- Ice cover → warming → ice-free ocean
After a tipping point, the system settles into a new equilibrium.
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Human Impact on Ecosystem Resilience
Humans can affect ecosystem resilience in positive or negative ways. Our aim should be to reduce damage and strengthen systems so ecosystems can cope with change.
Negative human impacts on ecosystem resilience
Human activities often reduce resilience by lowering biodiversity and shrinking storages such as biomass, water, and nutrients.
- Deforestation reduces biomass and species diversity
- Monoculture farming replaces diverse ecosystems with a single crop
- Water extraction lowers lake and river storages
- Pollution harms sensitive species and food webs
When diversity and storages decrease, ecosystems become less able to recover and more likely to cross tipping points.
Damage is often delayed or hidden, meaning humans may not react until the ecosystem is already close to collapse.
Positive human impacts on ecosystem resilience
Humans can increase resilience by protecting ecosystems, restoring diversity, and increasing the size and number of storages.
- Protecting forests and stopping deforestation
- Planting mixed forests instead of single-species plantations
- Improving soil management to increase carbon storage
- Restoring wetlands to store water and reduce flooding
Resilience can also be improved in human systems, such as cities, by planting trees to reduce heat and store water.
Actions that increase diversity and storages usually increase ecosystem resilience.
Case study example: Freshwater lake ecosystem (eutrophication)
A freshwater lake is usually a stable system with clear water, aquatic plants, and healthy fish populations. Its resilience depends on balanced nutrient levels and sufficient oxygen.
Human activities such as agriculture and sewage discharge can add excess nutrients (nitrates and phosphates) to the lake.
- Extra nutrients enter the lake
- Algae grow rapidly (algal bloom)
- Algae block sunlight so plants die
- Decomposition uses up dissolved oxygen
- Fish and invertebrates die from low oxygen
If oxygen levels fall too low, the lake can cross a tipping point and shift from a clear, healthy state to a polluted, low-oxygen state.
After this shift, recovery is difficult because nutrients stored in sediments continue to feed algal growth even if pollution inputs are reduced.
This is a positive (reinforcing) feedback loop: more nutrients → more algae → less oxygen → ecosystem damage → conditions that favour more algae.
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IB-style question — what makes a system resilient
“Explain how greater diversity and larger storages make an ecosystem more resilient.” [3–4]
How to answer it, step by step
- Diversity
• more species = if one is lost, others fill its role
• system keeps functioning → resists hitting a tipping point - Larger storages
• bigger buffer = any loss is a smaller percentage
• less likely to pass the tipping point; recovers faster
Final answer
Every point must link the feature (diversity/size) to resilience — naming it without explaining the link loses marks.