Describe the flow of energy through food chains and food webs, explain why food chains are short, and outline the carbon cycle

What this dot point is asking

SEAB wants you to describe how energy flows through food chains and food webs, to explain why energy loss limits the length of a food chain, and to outline the carbon cycle. The roles in a food chain, the reason food chains are short, and the carbon-cycle processes are common exam marks.

The answer

Food chains and the roles in them

A food chain shows the flow of energy from one organism to the next as food. The arrow points in the direction the energy flows (from the eaten to the eater). The roles are:

• Producers (green plants) make their own food by photosynthesis; they start every food chain.
• Consumers eat other organisms: a primary consumer (herbivore) eats producers, a secondary consumer eats the primary consumer, and so on.
• Decomposers (bacteria and fungi) break down dead organisms and waste, returning nutrients to the environment.

Food webs

In a real habitat, many food chains overlap because most animals eat more than one type of food. A food web shows these interconnected chains and gives a fuller picture of feeding relationships in an ecosystem.

Energy flow and its loss

Energy enters the chain when producers capture light in photosynthesis. At each stage, most of the energy is lost and does not pass to the next level, because it is used in:

• respiration (released as heat),
• movement,
• undigested waste (egestion).

Only about a tenth of the energy passes on at each step.

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. This is why food chains rarely have more than four or five levels: there is not enough energy left to support another level. It also explains why top predators are usually few in number.

The carbon cycle

Carbon is recycled between the air, living things and fuels:

• Photosynthesis removes carbon dioxide from the air, locking carbon into plants.
• Respiration by plants, animals and decomposers releases carbon dioxide back to the air.
• Decomposition of dead organisms releases carbon dioxide (through the decomposers' respiration).
• Combustion of wood and fossil fuels releases carbon dioxide to the air.

These processes keep carbon moving between the atmosphere and living organisms.

Examples in context

Example 1. Why eating plants feeds more people. Because energy is lost at each level, growing crops to eat directly feeds far more people than feeding those crops to animals and eating the meat. This is a direct, real-world consequence of the energy loss between trophic levels.

Example 2. Burning fossil fuels and the carbon balance. For millions of years carbon was locked away in fossil fuels. Burning them now releases that carbon as carbon dioxide far faster than photosynthesis removes it, upsetting the carbon cycle and raising atmospheric carbon dioxide, which links the cycle to climate change.

Try this

Q1. In the food chain lettuce to slug to hedgehog, name the producer and the primary consumer. [2 marks]

• Cue. Producer: lettuce; primary consumer: slug.

Q2. Explain why only a small amount of energy passes from one trophic level to the next. [2 marks]

• Cue. Most energy is lost at each level through respiration (as heat), movement and undigested waste, so only a small fraction is passed on.

Q3. Name the process that removes carbon dioxide from the atmosphere and two processes that return it. [2 marks]

• Cue. Photosynthesis removes carbon dioxide; respiration and combustion (and decomposition) return it.