Communities & ecosystems
Big idea: Living organisms do not exist alone — they form communities and interact with their physical environment to form ecosystems.
What is a community?
A community includes only living organisms.
- Made up of many populations
- Includes producers, consumers, and decomposers
- All organisms live in the same area at the same time
- Interactions between species are important
Community = living things only.
What is an ecosystem?
An ecosystem includes living and non-living components.
- Includes a community (biotic factors)
- Includes abiotic factors (light, water, temperature, soil)
- Energy flows through the system
- Matter is recycled
Ecosystem = community + abiotic environment.
Community vs ecosystem (exam favourite)
Community
- Only living organisms
- Different populations
- Focuses on interactions between species
Ecosystem
- Living + non-living components
- Includes abiotic factors
- Focuses on energy flow and matter cycling
If the question mentions abiotic factors, it is asking about an ecosystem, not just a community.
Habitats
A habitat is the physical environment where a species is found.
- One habitat can contain many species
- Different habitats support different communities
- Habitat determines which species can survive
Open ecosystems
Most ecosystems are open systems.
- Energy enters mainly as sunlight
- Matter enters and leaves as water, nutrients, and organisms
- Ecosystems interact with surrounding systems
Because ecosystems are open systems, they are dynamic and constantly changing.
Scale of ecosystems
Ecosystems exist at many different scales.
- Small scale: pond, field, garden
- Medium scale: forest, lake, coral reef
- Large scale: biome, Earth’s biosphere
Changing the scale changes what interactions you focus on.
Big exam takeaways
- A community includes only living organisms
- An ecosystem includes biotic and abiotic components
- Habitats are places where organisms live
- Most ecosystems are open systems
- Ecosystems exist at different scales
Sustainability & resilience
Big idea: Sustainability and resilience are essential for long-term ecosystem survival.
Sustainability in ecosystems
Sustainability means ecosystems can continue functioning over time.
- Resources are used at a rate they can be replaced
- Ecosystem processes continue long-term
- Future populations are not disadvantaged
Sustainable use = take no more than can be replaced.
What is resilience?
Resilience describes how well a system copes with change.
- Disturbance occurs (fire, flood, disease, drought)
- System changes temporarily
- System recovers and continues functioning
Resilient ecosystems bend — they do not break.
Disturbance and recovery
A disturbance can reduce population size or damage ecosystems.
- Natural disturbances: fires, storms, floods
- Human disturbances: pollution, deforestation, overfishing
- High resilience = faster recovery
IB questions often link disturbance → resilience → recovery.
Role of biodiversity
Biodiversity increases ecosystem resilience.
- More species = more roles in the ecosystem
- If one species declines, others can compensate
- Food webs are more stable than simple food chains
High biodiversity = high resilience.
Storages and resilience
Large storages help ecosystems absorb change.
- Biomass stored in forests
- Water stored in lakes and wetlands
- Nutrients stored in soil
Large storages act as buffers, slowing down change.
Redundancy
Redundancy increases resilience.
- Several species perform the same function
- Loss of one species does not collapse the system
- Common in diverse ecosystems
Think of redundancy like backup systems.
Low resilience and risk
Ecosystems with low resilience are more likely to collapse when disturbed.
- Low biodiversity
- Small storages
- Heavy human pressure
Low resilience increases the risk of tipping points.
Big exam takeaways
- Sustainability ensures long-term ecosystem use
- Resilience describes resistance and recovery
- High biodiversity increases resilience
- Large storages buffer change
- Low resilience increases collapse risk
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Keystone species
Big idea: A keystone species plays a critical role in maintaining ecosystem structure and stability.
What makes a species a keystone?
A keystone species is not always the most abundant species, but its removal causes major ecosystem changes.
- Has a strong effect on many other species
- Controls population sizes
- Helps maintain ecosystem balance
Small population — BIG impact.
Keystone species and food webs
Keystone species often sit at important positions in food webs.
- Many species depend on them directly or indirectly
- They help regulate energy flow
- They prevent dominance by one species
Removing a keystone species affects the entire food web.
Predators as keystone species
Top predators are common keystone species because they control prey populations.
- Predators limit herbivore numbers
- Vegetation is protected from overgrazing
- Habitats remain diverse
Example: Removing wolves can lead to too many deer, which overgraze vegetation and reduce biodiversity.
Ecosystem engineers
Some keystone species are ecosystem engineers.
- Change habitat structure
- Create new niches
- Support many other species
Example: Beavers create wetlands that support fish, birds, insects, and plants.
What happens if a keystone species is removed?
Removing a keystone species can trigger a trophic cascade.
- Population explosions or crashes
- Loss of biodiversity
- Ecosystem instability or collapse
Loss of keystone species can cause long-term or irreversible ecosystem damage.
Keystone species and resilience
Keystone species increase resilience by stabilising population interactions.
- Prevent dominance by one species
- Maintain biodiversity
- Support stable food webs
Keystone species help ecosystems bounce back after disturbance.
Big exam takeaways
- Keystone species have a disproportionate impact
- They are critical to food web stability
- Removal can cause trophic cascades
- They support biodiversity and resilience
- Protecting keystone species protects ecosystems
Human impact on ecosystems
Big idea: Human activities can push ecosystems beyond their natural limits, reducing biodiversity and threatening ecosystem stability.
Why humans have such a strong impact
Humans are a powerful ecological force because they change ecosystems faster and on a larger scale than most natural processes.
- Rapid population growth
- High resource consumption
- Advanced technology
- Global transport and trade
Human impacts are often fast, widespread, and long-lasting.
Major types of human impact
Human activities affect ecosystems in several key ways.
- Habitat destruction (deforestation, urbanisation)
- Pollution (air, water, soil pollution)
- Overexploitation (overfishing, hunting)
- Invasive species
- Climate change
Habitat destruction and fragmentation
When habitats are destroyed or broken into smaller pieces, species lose space, food, and breeding sites.
- Smaller populations
- Reduced genetic diversity
- Higher extinction risk
Example: Road building can fragment forests, isolating animal populations.
Pollution and ecosystem stress
Pollution introduces substances that organisms are not adapted to handle.
- Toxins accumulate in food chains
- Water pollution reduces oxygen levels
- Air pollution damages plants and soils
Pollution can reduce survival and reproduction across many trophic levels.
Overexploitation of species
When species are harvested faster than they can reproduce, populations decline.
- Collapse of fish stocks
- Loss of top predators
- Disrupted food webs
Removing one species can affect many others through the food web.
Tipping points in ecosystems
A tipping point is reached when pressure becomes too great.
- Gradual pressure builds up
- Sudden ecosystem shift occurs
- Recovery may be very slow or impossible
Example: Coral reefs shifting to algae-dominated systems.
Planetary boundaries and biosphere integrity
Human pressure can push Earth beyond safe limits known as planetary boundaries.
- Loss of biodiversity
- Disruption of ecosystems
- Reduced ability of Earth to support life
Protecting ecosystems helps protect global life-support systems.
Big exam takeaways
- Humans strongly affect ecosystems
- Habitat loss, pollution, and overuse reduce biodiversity
- Ecosystems can reach tipping points
- Human impacts can cause long-term instability
- Reducing pressure improves ecosystem survival