Unit Overview

Theme C: Interactions and Interdependencies

IB Biology SL· 5 min read 📊 23-27% of overall IB Biology SL exam

1. 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

Covers niches, species interactions, community structure, and the role of keystone species.

★★⏱ 10 min

Ecosystems and carbon cycling

Explores energy flow, trophic levels, and the global carbon cycle and its link to climate change.

★★⏱ 12 min

Classification and biodiversity

Introduces modern taxonomic classification and methods for measuring ecosystem biodiversity.

★★★⏱ 10 min

Conservation of biodiversity

Discusses threats to biodiversity and evidence-based conservation and restoration strategies.

★★★⏱ 11 min

Population dynamics

Teaches exponential and logistic growth models, limiting factors, and carrying capacity.

★★★★⏱ 9 min

Nitrogen and nutrient cycling

Covers the nitrogen cycle and other key nutrient cycles in terrestrial and aquatic ecosystems.

★★★⏱ 8 min

2. Common Pitfalls

Wrong move:

Confusing abiotic and biotic factors when describing ecosystem interactions.

Why:

IB examiners frequently penalize answers that mix these two factor categories.

Correct move:

Always explicitly label factors as living (biotic) or non-living (abiotic) in explanations.

Wrong move:

Reversing the roles of carbon sinks and carbon sources in the carbon cycle.

Why:

Many students mix up the direction of carbon flow for key processes like photosynthesis and respiration.

Correct move:

Remember: photosynthesis removes CO₂ from the atmosphere (sink), respiration adds CO₂ to the atmosphere (source).

Wrong move:

Assuming all introduced species do not impact native biodiversity.

Why:

IB expects recognition of invasive species as a major threat to native ecosystems.

Correct move:

Classify non-native invasive species as a key threatening process for native biodiversity in conservation questions.

3. Quick Reference 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	$\frac{d N}{d t} = r N \frac{( K - N )}{K}$ , 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.