4b Feeding Relationships¶
Part of 4 Ecology and the Environment.
Feeding relationships show how matter and energy move through ecosystems. The main emphasis is on trophic levels, the meaning of food chains and food webs, and why only a small fraction of energy is transferred onwards.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
4b-lo-1 |
4.6 understand the names given to different trophic levels, including producers, primary, secondary and tertiary consumers and decomposers | Trophic Levels |
4b-lo-2 |
4.7 understand the concepts of food chains, food webs, pyramids of number, pyramids of biomass and pyramids of energy transfer 4.8 understand the transfer of substances and energy along a food chain |
Food Chains, Webs and Pyramids |
4b-lo-3 |
4.9 understand why only about 10% of energy is transferred from one trophic level to the next | Why Transfer Is Inefficient |
Trophic Levels¶
Each position in a food chain is called a trophic level:
| Trophic level | Name | Description |
|---|---|---|
| 1st | Producers | Make their own food by photosynthesis (e.g. plants and algae). They form the base of all food chains. |
| 2nd | Primary consumers | Herbivores that eat producers. |
| 3rd | Secondary consumers | Carnivores that eat primary consumers. |
| 4th | Tertiary consumers | Carnivores that eat secondary consumers. At the top of a food chain, they are called apex predators. |
| — | Decomposers | Bacteria and fungi that break down the dead bodies and waste products of all other organisms, releasing nutrients back into 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 shows one linear pathway of feeding relationships, with arrows indicating the direction of energy transfer (from eaten to eater). Chains are organised by trophic level.
A food web is a network of interconnected food chains showing how all organisms in a habitat feed. Because most organisms eat more than one type of prey, a food web is a more realistic model of the real ecosystem. Changes to one population can have knock-on effects throughout the web — for example, if one prey species declines, a predator may switch to an alternative prey.
Pyramids of numbers show the number of organisms at each trophic level. Usually each level is smaller than the one below, but exceptions occur — a single large tree (producer) can support many insect consumers, so the numbers pyramid may be inverted at the base.
Pyramids of biomass show the dry mass of material at each trophic level. Unlike number pyramids, these are almost always wider at the base and narrower at the top, because each trophic level has less biomass than the one below.
Why Transfer Is Inefficient¶
Producers capture approximately 1% of incident light energy for photosynthesis, as not all light falls on photosynthesising parts of the plant.
Of the energy taken in at each trophic level, only about 10% is transferred to the next level. The remainder is lost because: - Not all biomass is eaten — carnivores cannot consume bones, hooves, claws or teeth. - Not all food eaten is digested — indigestible material (such as cellulose in plant cell walls) is egested as faeces, never entering the consumer's biomass. - Respiration — a large proportion of absorbed energy is used in aerobic respiration, which releases heat and carbon dioxide as waste. - Excretion — nitrogen-containing wastes such as urea are lost as urine.
The formula for calculating the efficiency of biomass transfer is:
efficiency (%) = (biomass transferred to the next level ÷ biomass available at the previous level) × 100
Because so little energy passes on at each step, food chains are usually short (typically 3–5 levels). Fewer organisms can be supported at higher trophic levels, which is why apex predators are always rarer than their prey.
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 of ecosystem feeding relationships.
- Pyramid of numbers vs pyramid of biomass: A number pyramid can be inverted (e.g. one tree feeding many insects); a biomass pyramid is almost always wider at the base.
- Egestion vs excretion: Egested material (faeces) was never absorbed by the consumer. Excreted material (urine, CO₂) was once part of the organism's metabolism.
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; an apex predator.
- Decomposer: an organism (bacterium or fungus) that breaks down dead organic material and returns nutrients to the soil.
- 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 at one trophic level that is passed to the next (approximately 10%).