4b Feeding Relationships¶

Part of 4 Ecology and the Environment.

Energy enters most ecosystems as light, is captured by producers, and passes along food chains as one organism eats another. At every step most of it is lost, which is why food chains are short and why each level of a pyramid is smaller than the one below.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

The trophic levels in a food chain, how food chains, food webs and pyramids represent feeding relationships, and why only a small fraction of energy passes from one level to the next.

Trophic Levels¶

Each feeding position in a food chain is a trophic level. In the chain grass → rabbit → fox, grass is the producer, the rabbit is the primary consumer and the fox is the secondary consumer.

Trophic level	Name	Description
1st	Producers	Make their own food by photosynthesis, such as plants and algae. They form the base of every food chain
2nd	Primary consumers	Herbivores that eat producers
3rd	Secondary consumers	Carnivores that eat primary consumers
4th	Tertiary consumers	Carnivores that eat secondary consumers. A consumer at the top of a chain is an apex predator
—	Decomposers	Bacteria and fungi that break down the dead bodies and waste of all other organisms and return nutrients to the soil. They use extracellular enzymes to digest material outside their cells and then absorb the soluble products

Food Chains, Webs and Pyramids¶

A food chain is one pathway of feeding relationships. The arrows show the direction of energy transfer, from the organism that is eaten to the organism that eats it, which is the reverse of the way you might say "the fox eats the rabbit".

A food web joins many food chains together. Most organisms eat more than one kind of food, so a web is a more realistic model of an ecosystem. It also shows knock-on effects. If one prey species declines, a predator may switch to another prey, and a species that depended only on the lost prey will decline too.

Pyramids of numbers show the number of organisms at each trophic level. Each level is usually smaller than the one below, but not always. One oak tree supports thousands of insects, so the pyramid is inverted at the base.

Pyramids of biomass show the dry mass of material at each trophic level. Dry mass is used because water content varies. These pyramids are almost always widest at the base and narrower at each step up, because each level contains less biomass than the one below.

Exam technique

In a food chain or web, the arrow always points towards the consumer. A question that asks what happens when a species is removed wants a chain of consequences: what the predator of that species eats instead, and what the species itself used to eat. Say whether each population rises or falls and give the reason.

Why Transfer Is Inefficient¶

Producers capture only about 1% of the light energy that reaches them, because not all of it falls on photosynthesising parts of the plant.

Of the energy taken in at each trophic level, only about 10% passes to the next. The rest is lost for four reasons:

Not all biomass is eaten. Carnivores cannot eat bones, hooves, claws or teeth.
Not all food eaten is digested. Indigestible material, such as the cellulose in plant cell walls, is egested as faeces and never becomes part of the consumer's biomass.
Respiration. A large share of the energy absorbed is used in respiration, which releases heat and carbon dioxide.
Excretion. Nitrogen-containing wastes such as urea are lost in urine.

The efficiency of transfer is calculated as:

efficiency (%) = (biomass transferred to the next level ÷ biomass available at the previous level) × 100

Because so little passes on at each step, food chains are usually short, with three to five levels. Fewer organisms can be supported at higher levels, which is why apex predators are rarer than their prey.

Worked example: efficiency of transfer

Cattle in a field eat 800 kg of grass. Over the same period the cattle gain 72 kg of new biomass.

Efficiency = (72 ÷ 800) × 100 = 9%

So 91% of the grass biomass was lost as faeces, as waste, or as the carbon dioxide and heat of respiration, and did not become new cattle biomass.

Common Confusions¶

Food chain vs food web: A food chain is one pathway of energy flow. A food web is a network of interconnected chains and is a more realistic model.
Pyramid of numbers vs biomass: A pyramid of numbers can be inverted, as when one tree supports many insects. A pyramid of biomass is almost always widest at the base.
Egestion vs excretion: Egested material, such as faeces, was never absorbed into the consumer. Excreted material, such as urine and carbon dioxide, was made by the consumer's own metabolism.
Arrow direction: The arrow points from the organism that is eaten to the one that eats it.

Check Yourself¶

In the food chain algae → water fleas → minnows → pike, name the producer and the tertiary consumer.
State the role of decomposers in an ecosystem.
Explain why a pyramid of numbers can be inverted but a pyramid of biomass almost never is.
A rabbit eats 500 g of grass and gains 40 g of biomass. Calculate the efficiency of transfer.
Give three reasons why only about 10% of the energy at one trophic level passes to the next.
Explain why food chains rarely have more than five trophic levels.

Answers

The producer is algae and the tertiary consumer is pike.
Decomposers break down dead organisms and waste, so nutrients are returned to the soil for plants to use again.
One large organism, such as a tree, can support many small consumers, so the numbers can be inverted. Biomass measures the dry mass of each level, and a consumer level always has less biomass than the level it eats, so the pyramid narrows.
Efficiency = (40 ÷ 500) × 100 = 8%.
Any three of: not all biomass is eaten; not all food eaten is digested and some is egested; energy is lost in respiration as heat; energy is lost in excretion of waste such as urea.
Only about 10% of the energy passes on at each step, so very little remains after four or five transfers. There is not enough energy to support another level of consumers.

Key Terms¶

Trophic level: a feeding position in a food chain or food web.
Producer: an organism that makes its own food by photosynthesis, forming the base of a food chain.
Primary consumer: a herbivore that feeds on producers.
Secondary consumer: a carnivore that feeds on primary consumers.
Tertiary consumer: a carnivore that feeds on secondary consumers.
Decomposer: an organism that breaks down dead organic material.
Food web: a network of interconnected food chains showing feeding relationships in an ecosystem.
Pyramid of biomass: a diagram showing the relative dry mass of material at each trophic level.
Pyramid of numbers: a diagram showing the number of organisms at each trophic level.
Energy transfer efficiency: the proportion of energy passed to the next trophic level.