Ecosystem

About this unit

An ecosystem is a functional unit of nature in which living organisms interact among themselves and with their physical environment. This unit explains the structure of an ecosystem (its biotic and abiotic components and their vertical stratification) and how it functions: how producers fix solar energy (productivity), how dead matter is broken down (decomposition), how energy flows through trophic levels in food chains and food webs, and how these relationships are shown as ecological pyramids.

Key ideas

• Ecosystems may be terrestrial (forest, grassland, desert) or aquatic (pond, lake, sea), natural or man-made.
• Gross primary productivity minus respiration losses gives net primary productivity.
• Decomposition involves fragmentation, leaching, catabolism, humification and mineralisation.
• Energy flow is unidirectional, from the sun to producers to consumers.
• Roughly 10 per cent of energy passes from one trophic level to the next.
• Pyramids of number and biomass can be inverted, but the pyramid of energy is always upright.

How to prepare

Learn definitions precisely (standing crop, GPP, NPP, secondary productivity, detritus). Practise 10 per cent law calculations and draw examples of upright and inverted pyramids. Note the factors that speed up or slow down decomposition, and the difference between grazing and detritus food chains.