Ecosystems and carbon cycling

IB Biology SL· Theme C.2· 40 min read

1. Ecosystem Structure★★☆☆☆⏱ 10 min

📘 Definition

Ecosystem

A community of living organisms (biotic) interacting with each other and their non-living (abiotic) physical environment. Ecosystems can range from small puddles to the entire biosphere.

Example:

A coral reef ecosystem includes coral, fish, algae (biotic) plus water temperature, dissolved minerals, sunlight (abiotic).

All ecosystems rely on two core processes: one-way energy flow and nutrient cycling. Unlike energy (which is lost as heat and cannot be reused), nutrients like carbon are constantly recycled between biotic and abiotic components.

📐 Worked Example

Sort the following components of a temperate forest into biotic and abiotic factors: fallen oak leaf, soil pH, wolf, mushroom, rainfall, sunlight.

1
Step 1: Recall the definition of biotic (any living/once-living material) and abiotic (any non-living physical/chemical component).
2
Step 2: Sort each component:
- Biotic: fallen oak leaf (dead organic matter from a living tree), wolf, mushroom
- Abiotic: soil pH, rainfall, sunlight

Exam tip:

Dead organic matter (detritus, fallen leaves, dead organisms) is always classified as biotic, even though it is no longer alive.

2. Carbon Cycle Pools and Processes★★★☆☆⏱ 15 min

📘 Definition

Carbon Flux

The rate of movement of carbon between different carbon pools (reservoirs) in the carbon cycle. Fluxes are measured in mass of carbon per unit time.

Carbon is stored in four major global pools, with different processes moving carbon between them:

Atmosphere: Carbon stored as carbon dioxide ( $C O_{2}$ ) and methane ( $C H_{4}$ )
Biosphere: Carbon stored as organic matter in living and dead organisms
Hydrosphere: Carbon stored as dissolved inorganic carbon in oceans
Lithosphere: Carbon stored as fossil fuels and carbonate rock (long-term storage)

Photosynthesis: $C O_{2}$ moves from atmosphere/hydrosphere → biosphere
Cell respiration: Carbon moves from biosphere → atmosphere as $C O_{2}$
Decomposition: Decomposers break down organic matter, release $C O_{2}$ via respiration
Combustion: Burning of biomass/fossil fuels releases stored carbon to the atmosphere
Fossilization: Dead organic matter becomes fossil fuel, carbon moves from biosphere → lithosphere

📐 Worked Example

Identify the direction of the carbon flux for photosynthesis, and explain its role in the carbon cycle.

1
Step 1: Photosynthesis is carried out by autotrophs (plants, algae, cyanobacteria) to make organic glucose for growth.
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Step 2: The flux of carbon is from the atmosphere (or hydrosphere for aquatic autotrophs) to the biosphere.
3
Step 3: This process removes carbon dioxide from the atmosphere and fixes it into organic carbon that can be used by other organisms in the ecosystem.

3. Human Impacts on Carbon Cycling★★★★☆⏱ 15 min

Human activity has dramatically altered natural carbon fluxes over the last 200 years, leading to a net increase in atmospheric greenhouse gas concentrations and global climate change.

Combustion of fossil fuels: releases carbon stored in the lithosphere (fossil fuels) to the atmosphere, causing a large net increase in atmospheric $C O_{2}$
Deforestation: reduces the amount of $C O_{2}$ fixed by photosynthesis, leaving more carbon in the atmosphere
Permafrost thaw: rising temperatures thaw frozen Arctic permafrost, allowing decomposition of stored organic matter, releasing $C O_{2}$ and methane, creating a positive feedback loop

📐 Worked Example

Explain why permafrost thaw is a positive feedback loop for climate change.

1
Step 1: Initial warming from increased atmospheric greenhouse gases raises Arctic temperatures, thawing permafrost.
2
Step 2: Permafrost stores thousands of years of frozen dead organic matter that has not decomposed.
3
Step 3: Once thawed, decomposers break down the organic matter, releasing $C O_{2}$ and methane, both greenhouse gases, into the atmosphere.
4
Step 4: Additional greenhouse gases trap more heat, causing further warming and more permafrost thaw, creating a self-reinforcing loop.

4. Exam Skills for Carbon Cycling Questions★★★☆☆⏱ 10 min

✓ Quick check

Which of the following is an abiotic factor in a wetland ecosystem?
- Dead cattail stem
- Water pH
- Dragonfly
- Bacteria
Reveal answer
Water pH —
Correct! Water pH is a non-living chemical property, so it is abiotic. All other options are biotic components.
Which carbon flux increases as a result of deforestation?
- Movement of carbon from atmosphere to biosphere
- Movement of carbon from biosphere to atmosphere
- Movement of carbon from lithosphere to biosphere
- Movement of carbon from atmosphere to hydrosphere
Reveal answer
Movement of carbon from biosphere to atmosphere —
Correct! Deforestation removes trees that fix carbon via photosynthesis, and decomposition of dead plant material releases carbon to the atmosphere.

5. Common Pitfalls

Wrong move:

Classifying dead organic matter as an abiotic component

Why:

Dead organic matter originates from living organisms, so it is always classified as biotic

Correct move:

Group all living and dead biological material as biotic, and only non-living physical/chemical factors as abiotic

Wrong move:

Stating that carbon flows one-way through ecosystems like energy

Why:

Energy is lost as heat and cannot be recycled, but carbon is constantly reused and cycled between pools

Correct move:

Remember: energy flows one-way, nutrients like carbon cycle

Wrong move:

Confusing carbon pools with carbon fluxes

Why:

Pools are stores of carbon, while fluxes are processes that move carbon between pools

Correct move:

When asked to identify fluxes, always select processes, not locations where carbon is stored

Wrong move:

Assuming all human activities increase atmospheric carbon

Why:

Some human activities increase carbon removal from the atmosphere, such as reforestation and carbon sequestration

Correct move:

Analyze each activity separately to determine the direction of the flux change

Wrong move:

Forgetting methane is a carbon-containing greenhouse gas

Why:

Most students only mention carbon dioxide, but methane is a critical part of the carbon cycle and climate change

Correct move:

Include methane when discussing permafrost thaw and anaerobic decomposition

6. Quick Reference Cheatsheet

Category	Example	Key Role
Biotic Component	Living organisms, dead organic matter	Part of the biosphere carbon pool
Abiotic Component	Temperature, pH, water, rock	Non-living ecosystem factor
Atmosphere Pool	$C O_{2}$ , $C H_{4}$	Short-term carbon storage, greenhouse effect
Biosphere Pool	Plants, animals, detritus	Short/medium term organic carbon storage
Lithosphere Pool	Fossil fuels, carbonate rock	Long-term carbon storage
Flux: Atmosphere → Biosphere	Photosynthesis	Fixes carbon into organic matter
Flux: Biosphere → Atmosphere	Respiration, decomposition	Releases $C O_{2}$
Flux: Lithosphere → Atmosphere	Fossil fuel combustion	Human-caused $C O_{2}$ increase
Positive Feedback Loop	Permafrost thaw	Accelerates climate change

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 1
Carbon flux direction identification
2024 · 2
Draw and label carbon cycle
2023 · 1
Classify ecosystem components

Going deeper

syllabusOfficial IB Biology SL SyllabusTheme C reference

What's Next

Understanding ecosystems and carbon cycling is foundational for studying all interactions in Theme C of IB Biology SL. This topic builds on core concepts of photosynthesis and cell respiration, and underpins all further study of energy flow in ecosystems, climate change, and human impacts on biodiversity. Mastery of carbon pools, fluxes, and human impacts is required for most extended response questions on Theme C, so reviewing this content is critical for exam preparation.