# Ecosystems and carbon cycling

> IB Biology SL · Theme C: Interactions and Interdependencies
> Source: https://www.owlsprep.com/study/ib-biology-sl-u3-ecosystems-and-carbon-cycling/

This sub-topic explains ecosystem structure, including the difference between biotic and abiotic components, then breaks down the movement of carbon through Earth’s spheres. We cover key carbon cycle processes, human impacts, and common exam requirements.

**Prerequisites:** [Cell respiration](https://www.owlsprep.com/study/ib-biology-sl-u2-cellular-respiration/); [Photosynthesis](https://www.owlsprep.com/study/ib-biology-sl-u2-photosynthesis/)

## Learning objectives

- Distinguish between biotic and abiotic components of an ecosystem
- Outline the main pools and processes of the global carbon cycle
- Explain how human activity alters carbon fluxes and impacts climate
- Analyze common exam questions about carbon cycling and ecosystem structure

## Ecosystem Structure

**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.

## Carbon Cycle Pools and Processes

**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 ($CO_2$) and methane ($CH_4$)
- **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: $CO_2$ moves from atmosphere/hydrosphere → biosphere
- Cell respiration: Carbon moves from biosphere → atmosphere as $CO_2$
- Decomposition: Decomposers break down organic matter, release $CO_2$ 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.
2. 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.

## Human Impacts on Carbon Cycling

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.

> **info**
>
> IB examiners frequently ask about the link between rising atmospheric $CO_2$ and ocean acidification: increasing dissolved $CO_2$ lowers ocean pH, which harms calcifying organisms like coral and shellfish.

- Combustion of fossil fuels: releases carbon stored in the lithosphere (fossil fuels) to the atmosphere, causing a large net increase in atmospheric $CO_2$
- Deforestation: reduces the amount of $CO_2$ fixed by photosynthesis, leaving more carbon in the atmosphere
- Permafrost thaw: rising temperatures thaw frozen Arctic permafrost, allowing decomposition of stored organic matter, releasing $CO_2$ 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 $CO_2$ 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.

## Exam Skills for Carbon Cycling Questions

**Exam command terms**

Common command terms for this topic have specific exam expectations:

- **Outline** — Give a brief clear description, no detailed cause-and-effect explanation required *(Outline the role of decomposers in the carbon cycle)*

- **Explain** — Give a detailed cause-and-effect account, link processes to outcomes *(Explain how human activity alters carbon fluxes)*

- **Draw/Label** — Marks are awarded for each correctly labeled pool or flux, keep it simple *(Draw and label the carbon cycle showing four key pools)*

**Check your understanding**

1. Which of the following is an abiotic factor in a wetland ecosystem?

   - Dead cattail stem
   - Water pH
   - Dragonfly
   - Bacteria

   *Why:* Correct! Water pH is a non-living chemical property, so it is abiotic. All other options are biotic components.

2. 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

   *Why:* Correct! Deforestation removes trees that fix carbon via photosynthesis, and decomposition of dead plant material releases carbon to the atmosphere.

## Common pitfalls

- **Wrong:** Classifying dead organic matter as an abiotic component
  - Why it fails: Dead organic matter originates from living organisms, so it is always classified as biotic
  - Correct: Group all living and dead biological material as biotic, and only non-living physical/chemical factors as abiotic
- **Wrong:** Stating that carbon flows one-way through ecosystems like energy
  - Why it fails: Energy is lost as heat and cannot be recycled, but carbon is constantly reused and cycled between pools
  - Correct: Remember: energy flows one-way, nutrients like carbon cycle
- **Wrong:** Confusing carbon pools with carbon fluxes
  - Why it fails: Pools are stores of carbon, while fluxes are processes that move carbon between pools
  - Correct: When asked to identify fluxes, always select processes, not locations where carbon is stored
- **Wrong:** Assuming all human activities increase atmospheric carbon
  - Why it fails: Some human activities increase carbon removal from the atmosphere, such as reforestation and carbon sequestration
  - Correct: Analyze each activity separately to determine the direction of the flux change
- **Wrong:** Forgetting methane is a carbon-containing greenhouse gas
  - Why it fails: Most students only mention carbon dioxide, but methane is a critical part of the carbon cycle and climate change
  - Correct: Include methane when discussing permafrost thaw and anaerobic decomposition

## 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 | $CO_2$, $CH_4$ | 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 $CO_2$ |
| Flux: Lithosphere → Atmosphere | Fossil fuel combustion | Human-caused $CO_2$ increase |
| Positive Feedback Loop | Permafrost thaw | Accelerates climate change |

## 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.

- [Classification and Biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u3-classification-and-biodiversity/)
- [Conservation of biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u3-conservation-of-biodiversity/)
- [Population dynamics](https://www.owlsprep.com/study/ib-biology-sl-u3-population-dynamics/)

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