# Theme C: Interactions and Interdependencies

> IB Biology SL · IB SL Biology
> Source: https://www.owlsprep.com/study/ib-biology-sl-u3-overview/
> Weight: 23-27% of overall IB Biology SL exam

This unit explores how organisms interact with each other and their physical environments, from individual species to entire global ecosystems. You will learn how energy flows, nutrients cycle, and human activity impacts species survival and ecosystem stability.

**Prerequisites:** [Core IB Biology SL cell and molecular biology concepts](https://www.owlsprep.com/study/ib-biology-sl-u2-overview/)

## Learning objectives

- Explain how biotic and abiotic factors interact to shape community and ecosystem structure
- Calculate and interpret population growth, biodiversity, and energy flow metrics
- Analyze the structure and function of major global biogeochemical cycles
- Evaluate human impacts on ecosystems and evaluate evidence-based conservation strategies
- Use modern taxonomic systems to classify and compare groups of organisms

## Unit at a Glance

This unit follows a logical progression from small-scale interactions between species to large-scale global biogeochemical cycles and real-world conservation action. You will first build foundational knowledge of species interactions and community structure, then expand to whole ecosystem function, before exploring how we classify, measure, and protect global biodiversity.

This theme is heavily applied, with frequent exam questions linking theoretical concepts to real-world conservation issues and climate change impacts. Expect to analyze data sets and evaluate claims about ecosystem health in both multiple choice and extended response questions.

This unit covers the following core sub-topics:
- [Species and communities](https://www.owlsprep.com/study/ib-biology-sl-u3-species-and-communities/) — Covers niches, species interactions, community structure, and the role of keystone species.
- [Ecosystems and carbon cycling](https://www.owlsprep.com/study/ib-biology-sl-u3-ecosystems-and-carbon-cycling/) — Explores energy flow, trophic levels, and the global carbon cycle and its link to climate change.
- [Classification and biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u3-classification-and-biodiversity/) — Introduces modern taxonomic classification and methods for measuring ecosystem biodiversity.
- [Conservation of biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u3-conservation-of-biodiversity/) — Discusses threats to biodiversity and evidence-based conservation and restoration strategies.
- [Population dynamics](https://www.owlsprep.com/study/ib-biology-sl-u3-population-dynamics/) — Teaches exponential and logistic growth models, limiting factors, and carrying capacity.
- [Nitrogen and nutrient cycling](https://www.owlsprep.com/study/ib-biology-sl-u3-nitrogen-and-nutrient-cycling/) — Covers the nitrogen cycle and other key nutrient cycles in terrestrial and aquatic ecosystems.

## Common pitfalls

- **Wrong:** Confusing abiotic and biotic factors when describing ecosystem interactions.
  - Why it fails: IB examiners frequently penalize answers that mix these two factor categories.
  - Correct: Always explicitly label factors as living (biotic) or non-living (abiotic) in explanations.
- **Wrong:** Reversing the roles of carbon sinks and carbon sources in the carbon cycle.
  - Why it fails: Many students mix up the direction of carbon flow for key processes like photosynthesis and respiration.
  - Correct: Remember: photosynthesis removes CO₂ from the atmosphere (sink), respiration adds CO₂ to the atmosphere (source).
- **Wrong:** Assuming all introduced species do not impact native biodiversity.
  - Why it fails: IB expects recognition of invasive species as a major threat to native ecosystems.
  - Correct: Classify non-native invasive species as a key threatening process for native biodiversity in conservation questions.

## Cheatsheet

| Concept / Formula | Key Unit Summary |
| --- | --- |
| Simpson's Diversity Index | $D = \frac{N(N-1)}{\sum n(n-1)}$, higher $D$ = more diverse ecosystem |
| Logistic population growth | $\frac{dN}{dt} = rN \frac{(K-N)}{K}$, S-shaped curve approaching carrying capacity |
| Trophic energy transfer | ~10% of energy transfers between successive trophic levels in an ecosystem |
| Carrying Capacity ($K$) | Maximum stable population size an environment can support indefinitely |
| Nitrification | Conversion of ammonia to nitrates by nitrifying bacteria in the nitrogen cycle |
| Carbon sink | A reservoir that stores more carbon than it releases, e.g. old-growth forests, deep oceans |
| Endemism | Species that are native to and found only in one specific geographic location |

## What's next

Start this unit by learning the foundational concepts of species interactions and community structure to build your understanding of core ecological relationships. Once you complete all sub-topics in this unit, you will move on to the next theme covering human physiology.

- [Species and communities](https://www.owlsprep.com/study/ib-biology-sl-u3-species-and-communities/)
- [Theme D: Human Physiology](https://www.owlsprep.com/study/ib-biology-sl-u4-overview/)
- [Ecosystems and carbon cycling](https://www.owlsprep.com/study/ib-biology-sl-u3-ecosystems-and-carbon-cycling/)

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