Key Idea: Topic 2.2 explains how ecosystems are organised, and why some ecosystems can recover (resilience) while others cross tipping points and collapse.
✅ Core definitions (exam must-know)
- Community
- All populations of different species living and interacting in the same area (living things only).
- Ecosystem
- A community plus the abiotic environment (sunlight, water, soil, temperature).
- Habitat
- The place where an organism lives.
- Resilience
- How well and how quickly an ecosystem recovers after disturbance.
- Disturbance
- An event that disrupts an ecosystem (fire, flood, pollution, deforestation).
If a question mentions abiotic factors, it is about an ecosystem, not just a community.
🌍 Ecosystems as systems & scale
- Most ecosystems are open systems (energy and matter enter and leave)
- Scale matters: puddle → pond → forest → biome → Earth
- Ecosystems involve energy flow (food chains/webs) and matter cycling
🛡️ What increases resilience?
- High biodiversity = more resilience (backup species)
- Storages (carbon, water, nutrients) buffer change
- Redundancy = many species perform the same role
Memorise the chain: Disturbance → resilience → recovery.
🦸 Keystone species
- Keystone species have a disproportionately large impact
- Often top predators controlling prey populations
- Ecosystem engineers (e.g. beavers) physically modify habitats
- Removal can trigger a trophic cascade
⚠️ Low resilience & tipping points
- Low biodiversity, small storages, high human pressure
- Tipping point = threshold after which change is rapid and often irreversible
- Low resilience = ecosystem closer to collapse
High marks come from linking ideas: biodiversity ↓ → resilience ↓ → tipping point risk ↑.
Community vs ecosystem: A community includes only the living organisms in an area, while an ecosystem includes the community plus abiotic factors such as soil, water, and climate. Three factors that increase ecosystem resilience: • High biodiversity (more species = more alternative pathways) • Complex food webs (less dependence on one species) • Genetic diversity within populations (greater adaptability) Keystone species & trophic cascade: A keystone species has a disproportionately large effect on its ecosystem. Removing it can cause a trophic cascade, where changes at one trophic level affect multiple other levels.