Communities and ecosystems at HL: A community and an ecosystem mean the same thing for SL and HL. At HL you return to these building blocks when you study the more detailed nutrient cycles and energy flows.
No extra HL content here — just get the key terms clear.
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Communities & ecosystems
Big idea: Living things are never alone! They form communities and, together with their surroundings, create ecosystems.
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What is a community?
A community is like all the students, teachers, and staff in your school — everyone living and working together, but only the living things count!
- Made up of many populations (like different classes or clubs)
- Includes producers (plants), consumers (animals), and decomposers (fungi, bacteria)
- All live in the same place at the same time
- Species interact: some help each other, some compete, some eat others
Community = living things only. No rocks, water, or sunlight included!
What is an ecosystem?
An ecosystem is like your school plus everything in it — people, buildings, playground, air, and even the weather!
- Includes a community (all living things)
- Also includes non-living things: sunlight, water, temperature, soil, rocks
- Energy flows through (like food chains and webs)
- Matter is recycled (like composting or water cycles)
Ecosystem = community + non-living environment. Both are needed!
Community vs ecosystem (exam favourite)
Community
- Only living things (plants, animals, fungi, bacteria)
- Made of different populations
- Focuses on how species interact (like food chains, competition, cooperation)
Ecosystem
- Living + non-living things (add water, sunlight, soil, etc.)
- Includes abiotic (non-living) factors
- Focuses on energy flow (who eats whom) and matter cycling (nutrients, water)
Exam tip: If a question mentions abiotic factors (like temperature, water, or sunlight), it’s about an ecosystem, not just a community!
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See exactly what to write to score full marks. Our AI shows you model answers and the key phrases examiners look for.
Habitats
A habitat is like your home — it’s the special place where a species lives and finds what it needs to survive.
- One habitat can be home to many different species (like a park with birds, insects, and trees)
- Different habitats support different communities (desert vs. rainforest)
- The type of habitat decides which species can survive there (fish need water, cacti need dry land)
Open ecosystems
Most ecosystems are open systems — like a classroom with open doors and windows.
- Energy comes in as sunlight (like light entering a room)
- Matter (water, nutrients, animals) moves in and out (like students and supplies entering and leaving)
- Ecosystems are connected to their surroundings (like classrooms in a school)
Ecosystems are dynamic — always changing because things can enter and leave.
Scale of ecosystems
Ecosystems come in all sizes — from tiny ponds to the whole planet! The scale you choose changes what you see.
- Small scale: a puddle, a garden, a single tree
- Medium scale: a forest, a lake, a coral reef
- Large scale: a whole biome (like the Sahara Desert), or even the entire Earth
Zooming in or out (changing scale) helps you focus on different interactions — like looking at a leaf vs. the whole forest.
IB-style question — Describe biotic and abiotic components of a named ecosystem [4]
Using a named ecosystem of your choice, describe what is meant by abiotic factors and biotic factors, giving one contextualised example of each. [4]
How to answer it, step by step
- Name the ecosystem first
• e.g. 'a temperate deciduous forest in central Slovakia'. Without this you lose the contextualised example marks (max 3/4).
• Abiotic = non-living physical/chemical components. Example: 'Seasonal temperature (−10 °C to 25 °C) determines which tree species survive.' - Biotic definition + contextualised example
• Biotic = living components / interactions between organisms.
• Example: 'Herbivory by roe deer limits beech seedling regeneration in the understorey.'
Final answer
Four marks = abiotic definition (1) + contextualised abiotic example (1) + biotic definition (1) + contextualised biotic example (1). Listing factors without linking them to a named ecosystem earns at most 2.
IB-style question — Suggest an ecological reason for a population change in a community [1]
Two diagrams show the same grassland ecosystem before and after invasion by a non-native grass species. In the invaded area, native hare populations are noticeably lower. Suggest one ecological reason for this decline. [1]
How to answer it, step by step
- Name the mechanism (food, shelter, or competition)
• Food: invasive grass replaced native wildflowers/shrubs that hares eat → less food.
• Cover: invasive grass provides less shelter from predators → higher predation. - Write one complete cause-and-effect sentence
• 'The invasive grass replaced native plants, reducing food variety and quantity for hares.'
• State WHAT changed AND HOW it affects the hare.
Final answer
One clear cause-and-effect sentence is all you need. Vague answers ('the ecosystem is disturbed') earn zero — you must name the ecological mechanism.