Study Guide

Core: Ecosystems

IB Biology HLΒ· Theme C: Interaction and Interdependence, Unit 3Β· 25 min read

1. 1. Core Hierarchical Terminologyβ˜…β˜…β˜†β˜†β˜†β± 7 min

πŸ“˜ Definition

Ecosystem

A dynamic system consisting of a community of living organisms (biotic components) interacting with each other and their non-living (abiotic) physical environment

Example:

A small pond, tropical rainforest, or arctic tundra are all distinct ecosystems

Ecosystems are organized hierarchically from individual organisms up to the entire biosphere. Distinguishing between these levels is a common exam assessment objective.

  • Individual: A single organism of a given species

  • Population: A group of individuals of the same species living in the same area at the same time

  • Community: All populations of different species living and interacting in the same area

  • Habitat: The physical environment where a species normally lives

  • Niche: The functional role of a species within its ecosystem

πŸ“ Worked Example

A biologist studies a group of grey wolves (Canis lupus) living in Yellowstone National Park. Identify whether this group is a species, population, or community, and name their habitat.

  1. 1

    Step 1: Recall definitions: A population is a group of the same species living in the same place at the same time.

  2. 2

    Step 2: The group is all the same species (Canis lupus) in the same geographic area, so this matches the definition of a population.

  3. 3

    Step 3: The habitat of these wolves is the Yellowstone National Park temperate forest ecosystem.

Exam tip:

IB questions often ask you to distinguish between these hierarchical terms, always confirm if the question refers to one species or multiple species.

2. 2. Abiotic vs Biotic Ecosystem Componentsβ˜…β˜…β˜†β˜†β˜†β± 6 min

All components of an ecosystem are divided into abiotic (non-living) and biotic (living/derived from living) factors. Both shape the distribution and abundance of species in an ecosystem.

πŸ“˜ Definition

Abiotic Factor

A non-living physical or chemical factor that influences organisms and ecosystem function

Example:

Temperature, water availability, sunlight, soil pH, and dissolved oxygen

πŸ“ Worked Example

Classify each of the following as abiotic or biotic: (a) soil pH, (b) a fallen log, (c) competition between two plants, (d) average annual temperature

  1. 1

    (a) Soil pH is a chemical property of non-living soil, so this is an abiotic factor.

  2. 2

    (b) A fallen log originates from a once-living tree, so it is a biotic component of the ecosystem.

  3. 3

    (c) Competition is an interaction between two living organisms, so this is a biotic factor.

  4. 4

    (d) Average annual temperature is a non-living physical condition, so this is an abiotic factor.

3. 3. Energy Flow Through Trophic Levelsβ˜…β˜…β˜…β˜†β˜†β± 8 min

Energy flows through ecosystems in one direction, from producer autotrophs up through consumer trophic levels. Unlike nutrients, energy is not recycled: most energy is lost as heat from cellular respiration at each trophic level.

πŸ“˜ Definition

Trophic Level

The feeding position an organism occupies in a food chain, starting from producers at level 1

Only approximately 10% of energy is transferred from one trophic level to the next, a rule widely tested in calculation and explanation questions.

πŸ“ Worked Example

If producers in a grassland ecosystem have a total energy store of 12,000 kJ m⁻² yr⁻¹, calculate the energy stored in secondary consumers.

  1. 1

    Step 1: Recall the 10% rule: 10% (0.1) of energy is passed between each trophic level. Producers are trophic level 1.

  2. 2
    Energy (primary consumers, level 2)=12,000Γ—0.1=1,200 kJ mβˆ’2 yrβˆ’1\text{Energy (primary consumers, level 2)} = 12,000 \times 0.1 = 1,200 \text{ kJ m}^{-2} \text{ yr}^{-1}
  3. 3
    Energy (secondary consumers, level 3)=1,200Γ—0.1=120 kJ mβˆ’2 yrβˆ’1\text{Energy (secondary consumers, level 3)} = 1,200 \times 0.1 = 120 \text{ kJ m}^{-2} \text{ yr}^{-1}
  4. 4

    Final answer: Secondary consumers store 120 kJ m⁻² yr⁻¹ of energy.

Exam tip:

Always count trophic levels starting from producers, not consumers. It is very common to miscount and get an incorrect final answer.

4. 4. Types of Ecological Pyramidsβ˜…β˜…β˜…β˜†β˜†β± 4 min

Ecological pyramids are graphical representations of ecosystem structure, showing relative values of energy, biomass, or organism count at each trophic level.

  • Pyramid of energy: Shows total energy at each trophic level over time, always pyramid-shaped

  • Pyramid of biomass: Shows total dry biomass at each trophic level at one moment, can be inverted for aquatic ecosystems

  • Pyramid of numbers: Counts individual organisms at each trophic level, can be inverted for large producers

πŸ“ Worked Example

Explain why a pyramid of energy is always upright, while a pyramid of biomass can be inverted.

  1. 1

    Step 1: Pyramids of energy measure total energy flow over a full period of time. Energy is always lost as heat at each trophic level per the second law of thermodynamics, so higher levels always have less energy.

  2. 2

    This means pyramids of energy can never be inverted.

  3. 3

    Step 2: Pyramids of biomass measure standing biomass (biomass at one single moment in time), not total energy over time.

  4. 4

    In aquatic ecosystems, phytoplankton producers reproduce quickly and are consumed immediately by zooplankton, so their standing biomass at any moment is lower than consumer biomass. This creates an inverted pyramid of biomass even though total energy flow follows the 10% rule.

5. Common Pitfalls

Wrong move:

Confusing habitat with niche

Why:

Students often mix up these two terms, which are frequently tested in multiple choice and short answer questions

Correct move:

Use the mnemonic: Habitat = your address, Niche = your job to easily distinguish the two terms

Wrong move:

Counting primary consumers as the first trophic level

Why:

This leads to incorrect energy calculation answers, a common mistake in exam papers

Correct move:

Always remember producers (autotrophs) are the first trophic level, so secondary consumers are level 3

Wrong move:

Claiming energy is recycled in ecosystems

Why:

Students confuse energy flow with nutrient cycling, leading to lost marks on explanation questions

Correct move:

Energy flows one way and is lost as heat, only nutrients (carbon, nitrogen) are recycled in ecosystems

Wrong move:

Classifying dead organic matter as an abiotic factor

Why:

Students assume non-alive = abiotic, which is incorrect for material derived from living organisms

Correct move:

All components originating from living organisms (even dead) are classified as biotic factors

6. Quick Reference Cheatsheet

Term

Definition

Key Exam Note

Ecosystem

Community + abiotic environment

Dynamic open system

Population

Same species, same area/time

Interbreeding group

Community

All species, same area

Multiple interacting populations

Habitat

Where an organism lives

"Address" of organism

Niche

Functional role of organism

"Job" of organism, no overlapping niches

10% rule

10% energy transferred between trophic levels

Rest lost as heat from respiration

Pyramid of energy

Total energy per trophic level

Always upright

Pyramid of biomass

Standing biomass per trophic level

Can be inverted in aquatic ecosystems

7. Frequently Asked

What is the difference between a habitat and a niche?

A habitat is the physical location where an organism lives, while a niche is the role and set of interactions the organism has within that habitat, including resource use and interactions with other species. Per the competitive exclusion principle, two species can share the same habitat but never the same exact niche.

When this came up on past exams

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

  • 2025 Β· 1

    Niche differentiation multiple choice

  • 2024 Β· 2

    Describe energy flow in ecosystems

  • 2023 Β· 1

    Identify abiotic vs biotic factors

Going deeper

What's Next

Understanding core ecosystem structure is the foundation for all further ecology topics in IB Biology HL. You will next build on this knowledge to explore species interactions, niche differentiation, and nutrient cycling in more depth, before moving on to applied topics like climate change impacts, ecosystem conservation, and ecological succession. Many IB exam questions integrate core ecosystem concepts with applied topics, so a solid grasp of terminology and energy flow rules here is critical for earning full marks on extended response and data analysis questions. Mastery of this sub-topic also makes interpreting experimental ecological data much easier for paper 2 and 3 assessments.