Transfers Along a Food Chain

A food chain is a simple way of showing what eats what in a habitat. The arrows always point in the direction the energy and substances flow — that is, from the organism being eaten to the organism eating it.

A typical food chain looks like this:

grass → grasshopper → frog → snake → hawk

Each organism in the chain is a feeding stage called a trophic level:

• Producers — green plants (e.g. grass). They make their own food by photosynthesis, using energy from the Sun. They are always the first trophic level.
• Primary consumers — animals that eat producers (herbivores, e.g. the grasshopper).
• Secondary consumers — animals that eat primary consumers (e.g. the frog).
• Tertiary consumers — animals that eat secondary consumers (e.g. the snake, then the hawk).

Two things are passed along the chain when one organism eats another:

1. Energy. The original source of energy is the Sun. Producers capture a small amount of the Sun's light energy during photosynthesis and store it as chemical energy in food. When an animal eats a plant (or another animal), some of this stored energy is transferred to it.
2. Substances. The materials that make up food — the carbon, nitrogen and other elements locked up in proteins, carbohydrates and fats — are also passed from one organism to the next. (Harmful substances, such as some pesticides, can be passed along a chain in the same way.)

So a food chain is really a flow of energy and a transfer of substances from the Sun → producers → consumers.

Exam tip. Remember the arrows in a food chain mean "is eaten by" / "energy flows to". A common error is to draw or read the arrows the wrong way round.

When an organism is eaten, not all of its stored energy passes to the next trophic level. In fact, only about 10% is transferred — roughly 90% is lost at each step. Where does all that energy go?

Energy is lost at each trophic level in several ways:

• Heat from respiration. Living things respire all the time to release energy from food. A lot of this energy ends up as heat, which is lost to the surroundings. This is the biggest loss, especially in warm-blooded animals (mammals and birds) that use energy to keep warm.
• Movement. Energy is used for moving — walking, running, flying, hunting. This energy is not stored in the body, so it is not available to the next level.
• Undigested waste (faeces / egestion). Not all of the food eaten can be digested. The parts that cannot be absorbed pass out of the body as faeces (egestion), taking their energy with them.
• Excretion. Waste products made inside the body, such as urea in urine, are removed by excretion. These also contain energy that is lost from the chain.

Only the energy that is stored in new body tissue (growth) is available to the next organism when it eats. That is the ~10% that gets passed on.

The diagram below shows what happens to the energy at one trophic level:

Exam tip. A reliable mark scheme lists the loss routes as heat / respiration, movement, faeces / undigested food (egestion) and excretion / urine. Naming at least three of these clearly is what scores the marks.

Because about 90% of the energy is lost at every trophic level, the amount of energy available gets smaller and smaller as you move up the chain.

Imagine a producer captures 10 000 units of energy. Following the ~10% rule:

Trophic level	Energy available
Producer (e.g. grass)	10 000 units
Primary consumer (e.g. grasshopper)	~1 000 units
Secondary consumer (e.g. frog)	~100 units
Tertiary consumer (e.g. snake)	~10 units
Top predator (e.g. hawk)	~1 unit

After just a few links, there is so little energy left that it could not support another animal above. There simply is not enough energy to feed a further trophic level.

This is exactly why food chains are usually short — about 4 or 5 links. The huge energy losses at each step put a natural limit on how long a chain can be.

It also explains why there are usually fewer organisms (or less biomass) at the top of a chain than at the bottom: a top predator needs to eat a great many animals lower down to get enough energy.

Exam tip. If a question asks why food chains are short, the key idea is: so much energy is lost at each trophic level (~90%, only ~10% passed on), so eventually there is not enough energy left to support another level.

The specification says we must understand the transfer of substances as well as energy along a food chain.

When one organism eats another, it takes in the materials that the food is made of:

• Carbohydrates, proteins and fats are digested and the building blocks (such as glucose and amino acids) are absorbed.
• The chemical elements locked in food — for example carbon and nitrogen — are passed on. The eater uses these substances to build its own body tissue, to grow and to repair itself.

So a food chain is a route by which materials move from organism to organism, not just energy. (These materials are eventually returned to the environment by decomposers — but for this subtopic you only need the idea that substances are passed along the chain.)

Harmful substances can travel the same way. If a producer takes up a harmful substance (such as a pesticide) that the body cannot break down or get rid of, it stays in the body. When that organism is eaten, the harmful substance is passed to the next level, and so on up the chain.

Exam tip. Keep the two ideas separate but linked: energy flows through a food chain (and most is lost), while substances/materials are transferred along it (and can build up in bodies further along the chain).

Let us tie the whole picture together with a single, clear sequence.

1. The Sun provides the original energy.
2. Producers (plants) trap a small part of the Sun's light energy by photosynthesis and store it as chemical energy in food.
3. A primary consumer eats the producer. It only gets about 10% of the producer's energy — the rest was lost as heat (respiration), in movement, in faeces (egestion) and in excretion.
4. A secondary consumer eats the primary consumer and again only gets about 10% of its energy.
5. This continues up the chain, with ~90% lost at every step, until there is too little energy left to support another level.

A simple way to hold the idea in your head:

Energy flows one way (Sun → plants → animals) and gets smaller at every step; substances are passed along too. Only ~10% is passed on, so food chains are short.

Exam tip. If asked to explain (not just state) why so little energy reaches the top of a chain, link the loss routes (heat/respiration, movement, faeces, excretion) to the ~10% transfer, and then to the chain being short. Showing the chain of reasoning is what earns the higher marks.