# Core: Ecosystems

> IB Biology HL · IB DP Biology HL 2025
> Source: https://www.owlsprep.com/study/ib-biology-hl-u4-core-ecosystems/

This module covers the core structure and function of ecosystems, the fundamental unit of ecological study. You will learn how biotic and abiotic components interact, how energy flows, and how matter cycles to sustain life on Earth.

**Prerequisites:** [Basic ecology fundamentals](https://www.owlsprep.com/study/ib-biology-hl-introduction-to-ecology/); Organism classification by nutrition

## Learning objectives

- Distinguish between biotic and abiotic components of an ecosystem
- Explain one-way energy flow and cyclic matter movement in ecosystems
- Identify trophic levels and calculate energy transfer between levels
- Compare simplified food chains to complex real-world food webs

## Biotic vs Abiotic Ecosystem Components

**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.

**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. 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. Step 2: Water temperature is a non-living physical property, so (b) ocean temperature is abiotic.
3. Step 3: Bacteria are living organisms, so (c) is biotic.
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.

## Trophic Levels and Energy Flow

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

*Notation:* T1 = primary producers, T2 = primary consumers, T3 = secondary consumers, T4 = tertiary consumers

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.

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

1. Step 1: Convert 10% transfer efficiency to a decimal: 10% = 0.1
2. Step 2: Energy in primary producers (T1) = 15,000 kJ
3. Energy in primary consumers (T2):
4. $$15000 \times 0.1 = 1500 \text{ kJ}$$
5. Step 3: Energy in secondary consumers (T3):
6. $$1500 \times 0.1 = 150 \text{ kJ}$$

> **Exam tip:** If the question does not specify a transfer percentage, use 10% as the default.

## Matter Cycling in Ecosystems

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.

**Worked example:** Outline how nitrogen from the atmosphere becomes available to plant cells for making proteins.

1. Step 1: Inert nitrogen gas (N₂) 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. Step 2: Nitrifying bacteria convert ammonia first into nitrites (NO₂⁻) then into nitrates (NO₃⁻), the form of nitrogen that plants can absorb.
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. Step 4: When the plant dies, decomposers break down the organic nitrogen back into ammonia to repeat the cycle.

## Food Chains and Food Webs

**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.

**Worked example:** What trophic level does the fox occupy in this chain: Grass → Grasshopper → Frog → Fox

1. Step 1: Primary producers are always trophic level 1, so grass = T1.
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. Conclusion: The fox is a tertiary consumer at trophic level 4.

## Common pitfalls

- **Wrong:** Classifying dead organic matter as an abiotic factor
  - Why it fails: While the organism is no longer alive, the material is derived from a once-living organism, so it is classified as biotic
  - Correct: Classify all organic matter from living organisms as biotic, regardless of whether the organism is still alive
- **Wrong:** Stating that energy cycles through ecosystems like matter does
  - Why it fails: Energy enters as sunlight and is permanently lost as heat, so it cannot be recycled
  - Correct: Remember the mnemonic: energy flows, matter cycles
- **Wrong:** Multiplying by 10 instead of 0.1 when applying the 10% energy rule
  - Why it fails: This leads to an answer that is 100 times larger than the correct value, a common calculation error in exams
  - Correct: Always convert the percentage to a decimal before multiplying: 10% = 0.1, 15% = 0.15
- **Wrong:** Assuming a species always occupies the same trophic level
  - Why it fails: Many omnivorous species feed at multiple trophic levels depending on what they are eating
  - Correct: Always determine trophic level based on the specific interaction in the given food chain or web, not the species as a whole

## 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 |

## 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.

- [AHL: Nucleic acid structure](https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-nucleic-acid-structure/)
- [AHL: DNA replication extensions](https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-dna-replication-extensions/)
- [AHL: Transcription and translation extensions](https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-transcription-and-translation-extensions/)

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