Sustainability and resilience at HL: Resilience and sustainability work the same way for SL and HL. It's a concept HL students reuse heavily — both when judging human impacts and in the HL law-and-economics questions around conservation.
Nothing extra to memorise here yet — just understand resilience and sustainability.
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🌱 Sustainability & Resilience
The Big Idea: Nature is like a sports team: Sustainability means not exhausting all your players so the team can keep winning. Resilience is how quickly your team recovers after a tough loss!
♻️ What is Sustainability?
Pizza Party Analogy 🍕: Imagine you're at a party with 12 slices of pizza and 12 friends. Sustainability = everyone takes ONE slice, so everyone gets fed. Unsustainable = one person grabs 6 slices and others go hungry!
- Use resources at a rate they can be replaced (like fishing only as many fish as can be born each year)
- The ecosystem keeps working year after year (like a garden that produces vegetables every summer)
- Future generations get their fair share too (your grandkids can enjoy forests and clean water)
Sustainable = Take only what can grow back! Like picking apples but leaving the tree healthy.
🛡️ What is Resilience?
Bounce-Back Power: Resilience = How well an ecosystem can recover after something bad happens. Think of it like a rubber ball — drop it, and it bounces back up!
Real Example: Yellowstone After Fire 🔥: In 1988, huge wildfires burned 36% of Yellowstone National Park. But within 10 years, new trees grew back, animals returned, and the ecosystem recovered. That's resilience!
- Disturbance happens (fire, flood, disease, pollution)
- Ecosystem changes for a while (fewer animals, less plant cover)
- Ecosystem recovers and keeps going (new growth, species return)
Resilient = Bends, but doesn't break! 🌳 Like a palm tree in a hurricane — it sways but doesn't snap.
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🌪️ Disturbance & Recovery
What's a Disturbance?: A disturbance is any event that disrupts the ecosystem — like a surprise fire drill at school. Everyone scatters, but a resilient school gets back to class quickly!
- Natural disturbances: forest fires, hurricanes, volcanic eruptions, floods
- Human disturbances: oil spills, deforestation, overfishing, urban sprawl
- High resilience = quick recovery (healthy coral reefs bounce back faster than damaged ones)
IB loves this chain: Disturbance → Resilience → Recovery. Always use it in essay answers!
🦁 Why Biodiversity Matters
Biodiversity = Backup Players: Imagine a football team with only one striker. If they get injured, you're in trouble! But a team with 5 strikers can substitute and keep playing. More species = more backup!
- More species = more jobs get done! (Different pollinators, different decomposers, different predators)
- If one species disappears, others can step in and fill the role
- Complex food webs = more stability (like a net with many strings — cut one, the net still holds)
Real Example: Bees vs. Other Pollinators 🐝: If bees decline, butterflies, moths, flies, and beetles can still pollinate many plants. High biodiversity = backup pollinators!
High biodiversity = high resilience! 🦋 More species = stronger safety net.
🛢️ Storages Help Absorb Change
What's a Storage?: Storages are like savings accounts for nature — places where resources (water, carbon, nutrients) are kept for tough times. A wetland stores water like your piggy bank stores coins!
- Forests store carbon in trees and soil (releasing it slowly, not all at once)
- Lakes & wetlands store water (preventing floods and providing water during droughts)
- Soil stores nutrients (feeding plants over many years)
Big storages = nature's emergency fund! 💰 They cushion the ecosystem when bad things happen.
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🔁 Redundancy = Backup Systems
Redundancy means many species do the same job. Like having 3 goalkeepers on a team — if one is sick, another steps in!
- Several species do the same job (bees, butterflies, flies, and beetles ALL pollinate flowers)
- Losing one species doesn't break the whole system
- Common in healthy, diverse ecosystems
Real Example: Decomposers 🍂: Dead leaves are broken down by bacteria, fungi, earthworms, and beetles. If one type disappears, the others keep the job going!
⚠️ Low Resilience = High Risk
Danger Zone: Low resilience = more likely to collapse when disturbed. Like a Jenga tower with half the blocks missing — one shake and it falls!
- Low biodiversity (few species, few backup players)
- Small storages (no savings to draw from)
- Heavy human pressure (pollution, overexploitation, habitat destruction)
Real Example: Coral Reefs 🪸: Coral reefs with low diversity and bleaching stress have LOW resilience. One marine heatwave can push them past the tipping point — permanent damage they can't recover from.
Low resilience = higher risk of tipping points — changes so big the ecosystem can't bounce back.
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IB-style question — Resilience: diversity and population size
Explain the ways in which greater species diversity and larger population sizes each contribute to making an ecosystem more resilient to disturbance. [7]
How to answer it, step by step
- Species diversity → resilience
• Functional redundancy: if one species is lost, another fills its niche — ecosystem keeps functioning.
• Complex food webs create more negative feedback loops, dampening population swings. - Population size → resilience
• Larger populations carry more genetic diversity → more individuals survive novel disease or climate shifts.
• Small populations risk extinction from random events, so larger = safer.
Final answer
Full marks require BOTH diversity AND abundance discussed separately with specific mechanisms — not just 'more species = more stable'. Classic error: writing only about diversity and ignoring population size.
IB-style question — Why monocultures are fragile
Explain why a single-species pine plantation is less resilient to disturbance than a natural mixed woodland. [3]
How to answer it, step by step
- No functional redundancy + no genetic diversity
• All trees are the same species, so one pathogen can kill the entire stand with nothing to replace it.
• Plantation trees are often near-identical genotypes — one disease sweeps through with no resistance. - Simple food web = weak feedback
• Fewer species means fewer feeding relationships and fewer negative feedback pathways.
• Population swings are larger and recovery after disturbance is slower.
Final answer
Three distinct points earn three marks. Name a different mechanism for each — redundancy, genetic diversity, food-web complexity. Repeating the same idea scores only 1.