Core: Ecosystems
IB Biology HLΒ· 5 min read
1. Key Components of Ecosystemsβ β ββββ± 15 min
Ecosystem
A community of interdependent living organisms (biotic) interacting with their non-living (abiotic) physical environment, forming a stable self-sustaining system
Example:
A small pond or a large temperate forest are both examples of ecosystems
Ecosystems can range dramatically in size, from a tiny patch of soil to an entire biome. All ecosystems require a constant input of external energy (almost always sunlight) to maintain function, while nutrients are continuously recycled within the system.
Distinguish between biotic and abiotic factors in a rocky intertidal zone, giving two examples of each.
- 1
First, define the two categories:
- 2
Biotic factors = all living components of the ecosystem; abiotic factors = all non-living physical/chemical components.
- 3
Two examples of biotic factors: mussel population, sea anemone population
- 4
Two examples of abiotic factors: seawater salinity, rock surface temperature
Exam tip:
Always include specific examples when asked to distinguish between terms in Paper 2 to earn full marks.
2. Feeding Relationships & Trophic Levelsβ β ββββ± 20 min
Trophic Level
The position an organism occupies in a food chain, defined by its feeding relationship to other organisms in the ecosystem
Organisms are grouped by feeding mode: Producers are autotrophs that make their own organic molecules via photosynthesis. Consumers are heterotrophs that feed on other living organisms. Detritivores and saprotrophs are decomposers that break down dead organic matter to recycle nutrients.
Classify the following organisms by trophic level and feeding mode: oak tree, fox, mushroom, earthworm.
- 1
Oak tree: Producer (autotroph), trophic level 1. Produces organic molecules via photosynthesis.
- 2
Fox: Secondary/tertiary consumer (heterotroph), feeds on other consumers, trophic level 3 or 4.
- 3
Mushroom: Saprotrophic decomposer, externally digests dead organic matter to recycle nutrients.
- 4
Earthworm: Detritivorous decomposer, internally digests dead organic matter from leaf litter.
3. Energy Flow & Ecological Pyramidsβ β β βββ± 20 min
Energy flows through ecosystems in one direction only, unlike nutrients which are recycled continuously. Only ~10% of energy is transferred from one trophic level to the next, because most energy is lost as heat from cellular respiration, or remains unused in waste.
If producers in an ecosystem capture 10,000 kJ of energy from sunlight, how much energy would you expect at the tertiary consumer level?
- 1
Apply the 10% energy transfer rule between trophic levels. Start with producer level (trophic 1): 10,000 kJ.
- 2
Primary consumer (trophic 2): 10% of 10,000 = 1000 kJ.
- 3
Secondary consumer (trophic 3): 10% of 1000 = 100 kJ.
- 4
Tertiary consumer (trophic 4): 10% of 100 = 10 kJ. Final answer = 10 kJ.
Ecological pyramids represent the number of organisms, biomass, or energy at each trophic level. Pyramids of energy are always upright because energy always decreases at higher trophic levels. Pyramids of numbers and biomass can be inverted, for example when one large producer (a tree) supports many small herbivores.
4. Nutrient Cyclingβ β β βββ± 15 min
Unlike energy, nutrients such as carbon, nitrogen, and water are continuously recycled within ecosystems. Decomposers play an essential role in breaking down dead organic matter and releasing inorganic nutrients back into the soil and atmosphere for reuse by producers.
Explain the role of saprotrophs in the carbon cycle.
- 1
Saprotrophs (fungi and bacteria) secrete digestive enzymes onto dead organic matter from dead plants and animals.
- 2
Enzymes break down complex organic carbon compounds into smaller molecules that the saprotroph absorbs for growth.
- 3
Respiration by saprotrophs releases carbon dioxide back into the atmosphere, which producers can fix during photosynthesis, completing the cycle.
5. Common Pitfalls
Wrong move:
Claiming nutrients flow one-way through ecosystems like energy
Why:
Energy is continuously added from the sun and lost as heat, so it flows one way. Nutrients are finite on Earth and must be recycled.
Correct move:
State that energy flows one-way, while nutrients are continuously recycled via decomposers.
Wrong move:
Confusing detritivores and saprotrophs, classifying fungi as detritivores
Why:
Detritivores ingest dead organic matter internally, while saprotrophs absorb nutrients after external digestion.
Correct move:
Classify fungi/bacteria as saprotrophs, and earthworms/dung beetles as detritivores.
Wrong move:
Claiming all ecological pyramids are always upright
Why:
Only pyramids of energy follow this rule; pyramids of numbers and biomass can be inverted.
Correct move:
Only state that pyramids of energy are always upright; other pyramid types can be inverted.
Wrong move:
Claiming 10% of incoming solar energy is captured by producers
Why:
Only ~1% of incoming solar energy is captured by photosynthesis; 10% is the transfer rate between trophic levels.
Correct move:
Remember 1% capture by producers, 10% transfer between trophic levels.
Wrong move:
Classifying producers as heterotrophic
Why:
Producers make their own organic molecules, so they are autotrophic, not heterotrophic.
Correct move:
Classify producers as autotrophs; all consumers and decomposers are heterotrophs.
6. Quick Reference Cheatsheet
Component | Definition | Key Fact |
|---|---|---|
Ecosystem | Community + abiotic environment | Requires constant energy input |
Biotic | Living components | E.g. plants, animals, bacteria |
Abiotic | Non-living components | E.g. temperature, pH, water |
Producer | Autotroph, trophic level 1 | 1% solar energy captured |
10% Rule | Energy transfer between trophic levels | 90% lost as heat |
Pyramid of Energy | Energy per trophic level | Always upright |
Detritivore | Ingests dead organic matter | E.g. earthworm |
Saprotroph | Absorbs digested dead matter | E.g. fungi, bacteria |
Energy | Movement through ecosystem | One-way flow, not recycled |
Nutrients | Elements for growth | Recycled via decomposers |
7. Frequently Asked
Is the difference between detritivores and saprotrophs examinable?
Yes, this distinction is tested frequently in both Paper 1 and Paper 2. Remember: detritivores ingest dead organic matter internally, while saprotrophs absorb nutrients after external digestion.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· P1
Distinguish detritivores vs saprotrophs
- 2024 Β· P2
Energy flow calculation question
- 2023 Β· P1
Identify upright vs inverted pyramids
Going deeper
What's Next
Understanding the core structure of ecosystems is the foundation for all further ecological topics in IB Biology HL. This knowledge directly supports your understanding of climate change impacts, species interactions, and conservation biology, which are all frequently tested in both Paper 1 and Paper 2. Next, you will build on this core knowledge to explore specific nutrient cycles in more detail, including the carbon cycle and its connection to climate change. You will also learn about population dynamics, carrying capacity, and human impacts on ecosystems, which are common extended response topics in Paper 2. Mastering the foundational terms and concepts covered here will make these more advanced topics significantly easier to understand.