Organisation of an Ecosystem and Sampling

This lesson covers how ecosystems are organised, including food chains, food webs, trophic levels and the cycling of materials. It also covers sampling techniques for estimating population sizes. These are key topics in the AQA GCSE Combined Science Trilogy specification (8464).

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Levels of Organisation in an Ecosystem

All ecosystems have a hierarchical organisation:

Level	Definition	Example
Individual	A single organism	One rabbit
Population	All organisms of the same species in a habitat	All rabbits in a field
Community	All the populations of different species in a habitat	All plants, animals, fungi and bacteria in a field
Ecosystem	The community plus the abiotic (non-living) environment	The field including soil, water, climate

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Feeding Relationships

Food Chains

A food chain shows the transfer of energy from one organism to the next. Energy enters most ecosystems through photosynthesis by producers.

Example:

$\text{Grass} \xrightarrow{\text{eaten by}} \text{Rabbit} \xrightarrow{\text{eaten by}} \text{Fox} \xrightarrow{\text{eaten by}} \text{Eagle}$Grasseaten by​Rabbiteaten by​Foxeaten by​Eagle

Term	Definition	Example
Producer	An organism that makes its own food by photosynthesis	Grass, oak tree, phytoplankton
Primary consumer	An organism that eats producers (herbivore)	Rabbit, caterpillar, zooplankton
Secondary consumer	An organism that eats primary consumers (carnivore)	Fox, blue tit, small fish
Tertiary consumer	An organism that eats secondary consumers (top carnivore)	Eagle, shark, lion
Decomposer	An organism that breaks down dead matter	Bacteria, fungi

Trophic Levels

Each step in a food chain is called a trophic level:

Trophic Level	Organisms
Level 1	Producers (plants, algae)
Level 2	Primary consumers (herbivores)
Level 3	Secondary consumers (carnivores)
Level 4	Tertiary consumers (top predators)

Food Webs

A food web is a network of interconnected food chains in an ecosystem. It shows the complex feeding relationships more realistically than a single food chain.

graph TD
    A["Grass"] --> B["Rabbit"]
    A --> C["Grasshopper"]
    A --> D["Mouse"]
    C --> E["Frog"]
    C --> F["Robin"]
    D --> F
    D --> G["Owl"]
    B --> H["Fox"]
    E --> G
    F --> G
    H --> I["Eagle"]
    G --> I
    style A fill:#c8e6c9,stroke:#2e7d32
    style B fill:#fff9c4,stroke:#f9a825
    style C fill:#fff9c4,stroke:#f9a825
    style D fill:#fff9c4,stroke:#f9a825
    style E fill:#ffccbc,stroke:#d84315
    style F fill:#ffccbc,stroke:#d84315
    style G fill:#e1bee7,stroke:#6a1b9a
    style H fill:#e1bee7,stroke:#6a1b9a
    style I fill:#ef9a9a,stroke:#c62828

Exam Tip: AQA (8464) commonly asks you to predict what happens if one species in a food web is removed. Think through the knock-on effects: if foxes are removed, rabbits may increase (less predation), which means grass may decrease (more herbivory).

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Energy Transfer in Ecosystems

Energy is transferred through an ecosystem along food chains. However, not all energy is transferred to the next trophic level. Energy is lost at each stage:

How Energy Is Lost	Explanation
Respiration	All organisms use energy for life processes; this energy is released as heat
Excretion	Energy is lost in waste products (urine, faeces)
Not all parts eaten	Bones, fur and roots are often not consumed
Heat to surroundings	Energy dissipates as heat and cannot be reused

Typically, only about 10% of the energy at one trophic level is passed to the next.

$\text{Efficiency of transfer} = \frac{\text{Energy at next trophic level}}{\text{Energy at previous trophic level}} \times 100$Efficiency of transfer=Energy at previous trophic levelEnergy at next trophic level​×100

Worked Example

If producers contain 10,000 kJ of energy and primary consumers contain 1,000 kJ:

$\text{Efficiency} = \frac{1{,}000}{10{,}000} \times 100 = 10\%$Efficiency=10,0001,000​×100=10%

Exam Tip: AQA (8464) may ask you to calculate the efficiency of energy transfer between trophic levels. Always show your working and include the % symbol.

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The Carbon Cycle

Carbon is constantly recycled between the atmosphere and living organisms:

graph TD
    A["CO₂ in atmosphere"] -->|"Photosynthesis"| B["Carbon compounds<br/>in plants"]
    B -->|"Eaten"| C["Carbon compounds<br/>in animals"]
    B -->|"Respiration"| A
    C -->|"Respiration"| A
    B -->|"Death"| D["Dead organic matter"]
    C -->|"Death"| D
    D -->|"Decomposition<br/>(bacteria and fungi)"| A
    D -->|"Over millions<br/>of years"| E["Fossil fuels<br/>(coal, oil, gas)"]
    E -->|"Combustion"| A
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#c8e6c9,stroke:#2e7d32
    style C fill:#ffccbc,stroke:#d84315
    style D fill:#d7ccc8,stroke:#5d4037
    style E fill:#fff9c4,stroke:#f9a825

Key Processes in the Carbon Cycle

Process	What Happens to Carbon
Photosynthesis	CO₂ is removed from the atmosphere; carbon is incorporated into glucose in plants
Respiration	Carbon compounds are broken down; CO₂ is released back into the atmosphere
Combustion	Burning fossil fuels or wood releases stored carbon as CO₂
Decomposition	Decomposers (bacteria and fungi) break down dead organisms, releasing CO₂ through respiration
Fossilisation	Dead organisms may become fossil fuels over millions of years, locking carbon underground

Exam Tip: AQA (8464) frequently tests the carbon cycle. Make sure you can name the processes that REMOVE CO₂ from the atmosphere (photosynthesis) and those that ADD CO₂ (respiration, combustion, decomposition).

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Decomposition

Decomposers (bacteria and fungi) break down dead organisms and waste products, recycling nutrients back into the soil for plants to absorb.

Factors Affecting the Rate of Decomposition

Factor	Effect
Temperature	Higher temperature (up to a point) increases enzyme activity in decomposers, speeding up decomposition
Water (moisture)	Decomposers need water to survive and carry out chemical reactions
Oxygen	Aerobic decomposers work faster than anaerobic ones; waterlogged or compacted conditions slow decomposition
Number of decomposers	More decomposers means faster decomposition

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Sampling Techniques

To study ecosystems, ecologists need to estimate the size and distribution of populations. They use sampling techniques because counting every individual is usually impractical.

Quadrats

A quadrat is a square frame (usually 0.25 m² or 1 m²) placed on the ground to sample organisms in a defined area.

How to use quadrats for random sampling:

1. Lay out two long tape measures at right angles to create a coordinate grid
2. Generate random numbers to determine coordinates for placing quadrats
3. Place the quadrat at each coordinate and count the organisms inside
4. Calculate the mean number of organisms per quadrat
5. Scale up to estimate the total population in the habitat

$\text{Estimated population} = \text{Mean per quadrat} \times \frac{\text{Total area of habitat}}{\text{Area of one quadrat}}$Estimated population=Mean per quadrat×Area of one quadratTotal area of habitat​

Worked Example

A student places 10 quadrats (each 0.5 m²) randomly in a field of 2,000 m². They count daisies: