# Core: Ecosystems

> IB Biology HL · IB Biology HL 2025+
> Source: https://www.owlsprep.com/study/ib-biology-hl-u2-core-ecosystems/

This subtopic covers the structure and key processes of ecosystems, including interactions between biotic and abiotic components, one-way energy flow between trophic levels, and closed nutrient cycling.

**Prerequisites:** [Cell respiration](https://www.owlsprep.com/study/ib-biology-hl-u1-cell-respiration/); [Photosynthesis](https://www.owlsprep.com/study/ib-biology-hl-u1-photosynthesis/)

## Learning objectives

- Distinguish between biotic and abiotic components of an ecosystem
- Describe energy flow and nutrient cycling in ecosystems
- Identify multiple trophic levels for organisms in food webs
- Explain the role of decomposers in nutrient recycling

## Ecosystem Structure: Biotic vs Abiotic Components

**Ecosystem** — A community of living organisms (biotic) interacting with their non-living (abiotic) physical environment as a single integrated system. Ecosystems range in size from small (a puddle) to large (a whole rainforest).

*Example:* A rocky shore ecosystem includes seaweed and crabs (biotic) plus water temperature, rock pH, and salinity (abiotic).

All ecosystem factors are split into two core groups: biotic components include all living organisms and biological interactions between them, while abiotic components include all non-living physical and chemical factors that shape the environment.

- Common abiotic factors: temperature, water availability, sunlight intensity, soil pH, nutrient concentration, salinity
- Common biotic factors: predation, competition, symbiosis, disease, decomposition

**Worked example:** A student lists these features of a temperate forest: 1. Oak tree, 2. Soil pH 5.5, 3. Decomposition rate of leaf litter, 4. Winter temperature 2°C. Classify each as biotic or abiotic.

1. Recall the definition of each: biotic = related to living organisms/their activities; abiotic = non-living physical/chemical properties.
2. 1. Oak tree: Biotic (it is a living organism)
3. 2. Soil pH 5.5: Abiotic (non-living chemical property of soil)
4. 3. Decomposition rate of leaf litter: Biotic (decomposition is carried out by living organisms, it is a biological interaction)
5. 4. Winter temperature 2°C: Abiotic (non-living physical environmental factor)

> **Exam tip:** Always remember that biological processes like decomposition or competition are classified as biotic, not abiotic, factors.

## Trophic Levels and Feeding Relationships

**Trophic Level** — The position an organism occupies in a food chain or food web, defined by how it obtains energy and nutrients from other organisms.

Producers (autotrophs) occupy the first trophic level, making their own organic molecules via photosynthesis. Consumers (heterotrophs) occupy higher levels, obtaining energy by feeding on other organisms. Decomposers break down dead organic matter to recycle nutrients.

**Worked example:** A gray wolf eats both deer (primary consumers that eat grass) and beavers (secondary consumers that eat herbivorous fish). Identify all trophic levels the gray wolf occupies.

1. Map the first feeding sequence: Grass (T1: producer) → Deer (T2: primary consumer) → Gray Wolf. In this chain, the wolf is at T3 (secondary consumer).
2. Map the second feeding sequence: Algae (T1: producer) → Herbivorous fish (T2: primary consumer) → Beaver (T3: secondary consumer) → Gray Wolf. In this chain, the wolf is at T4 (tertiary consumer).
3. Conclusion: The gray wolf occupies two trophic levels: 3 (secondary consumer) and 4 (tertiary consumer).

> **tip**
>
> Most omnivores and generalist predators occupy multiple trophic levels, depending on what prey they are eating. Never assume an organism only has one fixed trophic level in a food web.

## Energy Flow in Ecosystems

Energy flows in one direction through ecosystems, and is not recycled. Only approximately 10% of energy is transferred from one trophic level to the next. The remaining 90% is lost as heat from cell respiration, used for the organism's own growth and maintenance, or remains undigested and unavailable to the next level.

**Worked example:** If producers in a grassland fix 10,000 kJ of energy via photosynthesis, how much energy is available to secondary consumers?

1. Recall the 10% transfer rule between successive trophic levels.
2. Calculate energy available to primary consumers (trophic level 2):
3. $$0.1 \times 10000 = 1000 \text{ kJ}$$
4. Calculate energy available to secondary consumers (trophic level 3):
5. $$0.1 \times 1000 = 100 \text{ kJ}$$
6. Final answer: 100 kJ of energy is available to secondary consumers.

> **info**
>
> The 10% energy loss as heat aligns with the second law of thermodynamics, which states that all energy conversions are inefficient, and some energy is always lost as disorder (heat).

## Nutrient Cycling in Ecosystems

Unlike energy, nutrients (carbon, nitrogen, phosphorus) are recycled within closed biogeochemical cycles in ecosystems. Decomposers (saprotrophic bacteria and fungi, detritivores) play a critical role, breaking down dead organic matter to release inorganic nutrients back into the environment for producers to reuse.

**Biogeochemical Cycle** — A closed cycle that moves nutrients between biotic (living) and abiotic (non-living) reservoirs of the Earth, with no net gain or loss of total nutrients over time.

**Worked example:** Explain why nutrient cycling is a closed system but energy flow is an open system in ecosystems.

1. Define open and closed systems: an open system exchanges energy/matter with its surroundings, while a closed system cycles matter internally with no net change.
2. For energy flow: Energy enters ecosystems as sunlight and is permanently lost as heat from cell respiration, so it is continuously exchanged with the surroundings, making it an open system.
3. For nutrient cycling: Nutrients are never lost from the ecosystem, they are continuously recycled between biotic and abiotic pools, making it a closed system for matter.

## Common pitfalls

- **Wrong:** Classifying decomposition as an abiotic factor
  - Why it fails: Decomposition is a biological process carried out entirely by living decomposers, so it relates to interactions between living organisms
  - Correct: Classify decomposition and all other biological processes (competition, predation) as biotic factors
- **Wrong:** Using 100% energy transfer between trophic levels for calculations
  - Why it fails: Students often forget that most energy is lost as heat during cellular respiration, and that only ~10% transfers between levels
  - Correct: Always use the 10% transfer rule for energy calculation questions, remember energy is not recycled
- **Wrong:** Assigning only one fixed trophic level to every organism in a food web
  - Why it fails: Omnivores and generalist predators feed at multiple levels depending on their prey, so they have multiple trophic positions
  - Correct: Identify all trophic levels an organism occupies based on every feeding relationship shown in the food web
- **Wrong:** Stating that nutrients are permanently lost from ecosystems during cycling
  - Why it fails: Students often mix up energy flow and nutrient cycling, incorrectly applying the one-way energy rule to nutrients
  - Correct: Remember that nutrients are continuously recycled and reused within ecosystems, only energy flows one-way and is lost as heat

## Cheatsheet

| Concept | Key Details | Core Rule |
| --- | --- | --- |
| Biotic factors | Living organisms, predation, decomposition | All related to living organism activity |
| Abiotic factors | Temperature, pH, salinity, sunlight | Non-living physical/chemical properties |
| Energy flow | One-way transfer from sun to heat | ~10% transfer between trophic levels |
| Nutrient cycling | Carbon, nitrogen, phosphorus cycles | Closed system, nutrients are recycled |
| Trophic levels | 1: Producer, 2: Primary, 3: Secondary, 4: Tertiary | Omnivores occupy multiple levels |

## What's next

Understanding core ecosystem structure is the foundation for deeper learning of more complex ecological topics in IB Biology HL. You will build on this knowledge to analyze climate change impacts on ecosystems, model population growth dynamics, and evaluate strategies for biodiversity conservation. This topic also connects core energy concepts from cell biology, linking molecular-level processes like photosynthesis and respiration to large-scale ecosystem function.

- [AHL: Protein structure and function](https://www.owlsprep.com/study/ib-biology-hl-u2-ahl-protein-structure-and-function/)
- [AHL: Membrane structure and transport](https://www.owlsprep.com/study/ib-biology-hl-u2-ahl-membrane-structure-and-transport/)
- [AHL: Vertebrate organ systems](https://www.owlsprep.com/study/ib-biology-hl-u2-ahl-vertebrate-organ-systems/)

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