Study Guide

Core: Ecosystems

IB Biology HLΒ· 6 min read

1. Biotic vs Abiotic Ecosystem Componentsβ˜…β˜…β˜†β˜†β˜†β± 15 min

πŸ“˜ Definition

Ecosystem

A community of living organisms interacting with their non-living physical environment as a single system that exchanges energy and matter with the outside environment.

Example:

A coastal mangrove swamp, including trees, fish, sediment, salinity, and sunlight.

Biotic factors include all components derived from living organisms, even if the organism is no longer alive. Abiotic factors are non-living physical and chemical properties of the environment that shape where organisms can survive.

πŸ“ Worked Example

Classify each of the following as biotic or abiotic: (a) Dead leaf litter, (b) Ocean water temperature, (c) Bacteria in soil, (d) Carbon dioxide dissolved in lake water

  1. 1

    Step 1: Recall that any material originating from a once-living organism is biotic. Dead leaf litter comes from a living plant, so (a) is biotic.

  2. 2

    Step 2: Water temperature is a non-living physical property, so (b) ocean temperature is abiotic.

  3. 3

    Step 3: Bacteria are living organisms, so (c) is biotic.

  4. 4

    Step 4: Dissolved carbon dioxide is a non-living chemical compound in the environment, so (d) is abiotic.

Exam tip:

IB exam questions often test if you know dead organic matter counts as biotic, not abiotic.

2. Trophic Levels and Energy Flowβ˜…β˜…β˜…β˜†β˜†β± 20 min

πŸ“˜ Definition

Trophic Level

T1=primaryproducers,T2=primaryconsumers,T3=secondaryconsumers,T4=tertiaryconsumersT1 = primary producers, T2 = primary consumers, T3 = secondary consumers, T4 = tertiary consumers

The position an organism occupies in a food chain, defined by how it obtains energy and nutrients.

Energy flows in one direction through ecosystems: it enters as sunlight (or chemical energy for chemoautotrophs) and is lost as heat at every trophic level via cellular respiration. On average, only ~10% of energy is transferred from one trophic level to the next, a rule of thumb called the 10% rule. This low transfer efficiency limits the number of trophic levels most ecosystems can support.

πŸ“ Worked Example

A savanna ecosystem has 15,000 kJ of energy stored in primary producers. Calculate the approximate energy available to secondary consumers.

  1. 1

    Step 1: Convert 10% transfer efficiency to a decimal: 10% = 0.1

  2. 2

    Step 2: Energy in primary producers (T1) = 15,000 kJ

  3. 3

    Energy in primary consumers (T2):

  4. 4
    15000Γ—0.1=1500 kJ15000 \times 0.1 = 1500 \text{ kJ}
  5. 5

    Step 3: Energy in secondary consumers (T3):

  6. 6
    1500Γ—0.1=150 kJ1500 \times 0.1 = 150 \text{ kJ}

Exam tip:

If the question does not specify a transfer percentage, use 10% as the default.

3. Matter Cycling in Ecosystemsβ˜…β˜…β˜…β˜†β˜†β± 20 min

Unlike energy, matter (including carbon, nitrogen, and water) cycles continuously between biotic and abiotic components of ecosystems. Decomposers (fungi and bacteria) are critical to this process: they break down dead organic matter and release nutrients back into the soil or atmosphere for reuse by producers.

πŸ“ Worked Example

Outline how nitrogen from the atmosphere becomes available to plant cells for making proteins.

  1. 1

    Step 1: Inert nitrogen gas (Nβ‚‚) from the atmosphere is fixed into ammonia (NH₃) by nitrogen-fixing bacteria, which live either in root nodules of legumes or free in soil.

  2. 2

    Step 2: Nitrifying bacteria convert ammonia first into nitrites (NO₂⁻) then into nitrates (NO₃⁻), the form of nitrogen that plants can absorb.

  3. 3

    Step 3: Plants absorb nitrates via their roots and use them to make nitrogen-containing organic molecules like amino acids, which are assembled into proteins.

  4. 4

    Step 4: When the plant dies, decomposers break down the organic nitrogen back into ammonia to repeat the cycle.

4. Food Chains and Food Websβ˜…β˜…β˜†β˜†β˜†β± 15 min

πŸ“˜ Definition

Food Web

A complex, realistic network of interconnected food chains that shows all feeding relationships in an ecosystem, accounting for organisms that feed at multiple trophic levels.

Example:

In a forest food web, a bear can be both a primary consumer (when eating berries) and a tertiary consumer (when eating fish).

Food chains are simplified linear models of feeding relationships, while food webs capture the complexity of real ecosystems. More complex food webs are generally more stable, because the loss of one species can be compensated for by other feeding connections.

πŸ“ Worked Example

What trophic level does the fox occupy in this chain: Grass β†’ Grasshopper β†’ Frog β†’ Fox

  1. 1

    Step 1: Primary producers are always trophic level 1, so grass = T1.

  2. 2

    Step 2: Count up each level: Grasshopper (eats grass) = T2 (primary consumer), Frog (eats grasshopper) = T3 (secondary consumer), Fox (eats frog) = T4 (tertiary consumer).

  3. 3

    Conclusion: The fox is a tertiary consumer at trophic level 4.

5. Common Pitfalls

Wrong move:

Classifying dead organic matter as an abiotic factor

Why:

While the organism is no longer alive, the material is derived from a once-living organism, so it is classified as biotic

Correct move:

Classify all organic matter from living organisms as biotic, regardless of whether the organism is still alive

Wrong move:

Stating that energy cycles through ecosystems like matter does

Why:

Energy enters as sunlight and is permanently lost as heat, so it cannot be recycled

Correct move:

Remember the mnemonic: energy flows, matter cycles

Wrong move:

Multiplying by 10 instead of 0.1 when applying the 10% energy rule

Why:

This leads to an answer that is 100 times larger than the correct value, a common calculation error in exams

Correct move:

Always convert the percentage to a decimal before multiplying: 10% = 0.1, 15% = 0.15

Wrong move:

Assuming a species always occupies the same trophic level

Why:

Many omnivorous species feed at multiple trophic levels depending on what they are eating

Correct move:

Always determine trophic level based on the specific interaction in the given food chain or web, not the species as a whole

6. Quick Reference Cheatsheet

Key Concept

Definition

Key Rule/Fact

Ecosystem

Biotic community + interacting abiotic environment

Exchanges energy and matter with surroundings

Biotic factor

Component from living organisms

Includes dead organic matter

Abiotic factor

Non-living physical/chemical component

E.g. temperature, pH, sunlight

Energy flow

One-way movement through trophic levels

~10% transfer efficiency between levels

Matter cycling

Continuous recycling of elements

Decomposers are required for nutrient recycling

Trophic level 1

Primary producers

Autotrophs that make their own food

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· Paper 1

    Classify biotic/abiotic components

  • 2024 Β· Paper 2

    Explain energy flow in ecosystems

  • 2023 Β· Paper 1

    Identify trophic level in food chain

  • 2022 Β· Paper 2

    Compare energy flow and nutrient cycling

What's Next

Understanding core ecosystem structure and function is the foundation for all advanced ecological topics in IB Biology HL. This sub-topic appears regularly in both multiple-choice and extended-response questions, and is a common source of calculation and data analysis marks. Mastery of core concepts here will make it much easier to understand how ecosystems respond to change, including human-caused disturbances. Next, you will explore more specific topics including nutrient cycles, population dynamics, and human impacts on global ecosystems, which make up a large portion of the ecology section of the IB Biology HL exam.