Explain the flow of energy through an ecosystem and the role of decomposers in recycling nutrients

What this dot point is asking

SEAB wants you to explain how energy flows through an ecosystem, why energy is lost between trophic levels, why this makes food chains short, and how decomposers recycle nutrients back to the producers. You should understand that energy flows one way and is lost, while nutrients are recycled.

The answer

Where the energy comes from

Almost all the energy in an ecosystem comes from the Sun. Producers (green plants) capture a small part of this light energy by photosynthesis and store it as chemical energy in food. This is how energy enters the food chain.

How energy flows and is lost

Energy passes along the food chain as one organism eats another. But at each trophic level, most of the energy is lost, not passed on, through:

• Respiration, which releases energy (much of it as heat to the surroundings).
• Movement and other life processes.
• Undigested material lost in faeces, and waste such as urine.

Only a small fraction (often around a tenth) of the energy at one level is passed on to the next.

Why food chains are short

Because so much energy is lost at each level, there is less and less energy available higher up the chain. After about four or five trophic levels, too little energy remains to support another level of consumers. This is why food chains are usually short.

Pyramids of numbers

Because energy decreases up the chain, there are usually fewer organisms (or less total mass) at each higher level. A diagram of this is a pyramid of numbers (or biomass): many producers at the base, fewer primary consumers, and very few top predators.

Decomposers and recycling nutrients

Unlike energy, nutrients are recycled. Decomposers (bacteria and fungi) break down dead organisms and waste, releasing the mineral ions (such as nitrogen-containing ions) back into the soil. Producers then take up these ions to grow. So decomposers return nutrients to the start of the chain, keeping the ecosystem supplied. Without them, nutrients would stay locked in dead matter and the soil would run out.

Examples in context

Example 1. Why there are few lions. A grassland supports vast amounts of grass, many fewer grazing animals, and only a small number of lions. The falling energy up the chain means the top predators must be rare, illustrated by a pyramid of numbers.

Example 2. Compost in a garden. A compost heap is full of decomposers breaking down dead leaves and scraps. The rotted compost returns nutrients to the soil, which plants then use to grow, the nutrient cycle put to practical use by gardeners.

Try this

Q1. State two ways energy is lost between trophic levels. [2 marks]

• Cue. Any two of: respiration (heat), movement, undigested material in faeces, waste such as urine.

Q2. Explain why nutrients can be recycled but energy cannot. [2 marks]

• Cue. Nutrients are released from dead matter by decomposers and reused by producers; energy flows one way and is lost as heat, so it must keep being supplied by the Sun.

Q3. State the role of decomposers in an ecosystem. [2 marks]

• Cue. They break down dead organisms and waste, releasing mineral nutrients back into the soil for producers to take up.