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