# Core: Ecosystems

> IB Biology HL · IB Biology HL 2025+
> Source: https://www.owlsprep.com/study/ib-biology-hl-u3-core-ecosystems/

This sub-topic introduces the core structure and function of ecosystems, covering key ecological terminology, abiotic/biotic interactions, energy flow between trophic levels, and ecological pyramids, all foundational for higher ecology topics in IB Biology HL.

**Prerequisites:** [Organism classification fundamentals](https://www.owlsprep.com/study/ib-biology-hl-u2-classification/); [Basic cell respiration and energy concepts](https://www.owlsprep.com/study/ib-biology-hl-u2-cell-respiration/)

## Learning objectives

- Define key ecosystem terminology including species, population, community, habitat and niche
- Explain the difference between abiotic and biotic factors in an ecosystem
- Describe energy flow through trophic levels in an ecosystem
- Distinguish between different types of ecological pyramids

## 1. Core Hierarchical Terminology

**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

**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. Step 1: Recall definitions: A population is a group of the same species living in the same place at the same time.
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. 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. Abiotic vs Biotic Ecosystem Components

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.

**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

> **info**
>
> Biotic factors include all interactions between living organisms (predation, competition, mutualism) and dead organic matter, even though it is no longer alive, it originates from living organisms.

**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. (a) Soil pH is a chemical property of non-living soil, so this is an abiotic factor.
2. (b) A fallen log originates from a once-living tree, so it is a biotic component of the ecosystem.
3. (c) Competition is an interaction between two living organisms, so this is a biotic factor.
4. (d) Average annual temperature is a non-living physical condition, so this is an abiotic factor.

## 3. Energy Flow Through Trophic Levels

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.

**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.

**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. Step 1: Recall the 10% rule: 10% (0.1) of energy is passed between each trophic level. Producers are trophic level 1.
2. $$\text{Energy (primary consumers, level 2)} = 12,000 \times 0.1 = 1,200 \text{ kJ m}^{-2} \text{ yr}^{-1}$$
3. $$\text{Energy (secondary consumers, level 3)} = 1,200 \times 0.1 = 120 \text{ kJ m}^{-2} \text{ yr}^{-1}$$
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. Types of Ecological Pyramids

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

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

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. This means pyramids of energy can never be inverted.
3. Step 2: Pyramids of biomass measure standing biomass (biomass at one single moment in time), not total energy over time.
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.

## Common pitfalls

- **Wrong:** Confusing habitat with niche
  - Why it fails: Students often mix up these two terms, which are frequently tested in multiple choice and short answer questions
  - Correct: Use the mnemonic: Habitat = your address, Niche = your job to easily distinguish the two terms
- **Wrong:** Counting primary consumers as the first trophic level
  - Why it fails: This leads to incorrect energy calculation answers, a common mistake in exam papers
  - Correct: Always remember producers (autotrophs) are the first trophic level, so secondary consumers are level 3
- **Wrong:** Claiming energy is recycled in ecosystems
  - Why it fails: Students confuse energy flow with nutrient cycling, leading to lost marks on explanation questions
  - Correct: Energy flows one way and is lost as heat, only nutrients (carbon, nitrogen) are recycled in ecosystems
- **Wrong:** Classifying dead organic matter as an abiotic factor
  - Why it fails: Students assume non-alive = abiotic, which is incorrect for material derived from living organisms
  - Correct: All components originating from living organisms (even dead) are classified as biotic factors

## 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 |

## 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.

- [AHL: Cell signaling](https://www.owlsprep.com/study/ib-biology-hl-u3-ahl-cell-signaling/)
- [AHL: Neurobiology](https://www.owlsprep.com/study/ib-biology-hl-u3-ahl-neurobiology/)
- [AHL: Plant hormone interactions](https://www.owlsprep.com/study/ib-biology-hl-u3-ahl-plant-hormone-interactions/)

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