Trophic Levels in an Ecosystem¶
Part of 4.7 Ecology.
Trophic levels organise feeding relationships and help explain why food chains rarely have many steps. At each transfer, only some biomass becomes part of the next level.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
4.7.4-lo-1 |
4.7.4.1 Students should be able to describe the differences between the trophic levels of organisms within an ecosystem. 4.7.4.1 Trophic levels can be represented by numbers, starting at level 1 with plants and algae. Further trophic levels are numbered subsequently according to how far the organism is along the food chain. 4.7.4.1 Level 1: Plants and algae make their own food and are called producers. 4.7.4.1 Level 2: Herbivores eat plants/algae and are called primary consumers. 4.7.4.1 Level 3: Carnivores that eat herbivores are called secondary consumers. 4.7.4.1 Level 4: Carnivores that eat other carnivores are called tertiary consumers. Apex predators are carnivores with no predators. 4.7.4.1 Decomposers break down dead plant and animal matter by secreting enzymes into the environment. Small soluble food molecules then diffuse into the microorganism. |
Trophic Levels and Feeding Relationships |
4.7.4-lo-2 |
4.7.4.2 Pyramids of biomass can be constructed to represent the relative amount of biomass in each level of a food chain. Trophic level 1 is at the bottom of the pyramid. 4.7.4.2 Students should be able to construct accurate pyramids of biomass from appropriate data. 4.7.4.3 Students should be able to: 4.7.4.3 • describe pyramids of biomass 4.7.4.3 • explain how biomass is lost between the different trophic levels. 4.7.4.3 Producers are mostly plants and algae which transfer about 1 % of the incident energy from light for photosynthesis. 4.7.4.3 Only approximately 10 % of the biomass from each trophic level is transferred to the level above it. 4.7.4.3 Losses of biomass are due to: 4.7.4.3 • not all the ingested material is absorbed, some is egested as faeces |
Pyramids of Biomass and Numbers, Biomass Transfer and Efficiency |
4.7.4-lo-3 |
4.7.4.3 • some absorbed material is lost as waste, such as carbon dioxide and water in respiration and water and urea in urine. 4.7.4.3 Large amounts of glucose are used in respiration. 4.7.4.3 Students should be able to calculate the efficiency of biomass transfers between trophic levels by percentages or fractions of mass. 4.7.4.3 Students should be able to explain how this affects the number of organisms at each trophic level. |
Biomass Transfer and Efficiency |
Trophic Levels and Feeding Relationships¶
- Producers such as green plants form the first trophic level because they make their own food by photosynthesis. They convert light energy into chemical energy stored in biomass.
- Primary consumers feed on producers, secondary consumers feed on primary consumers, and tertiary consumers feed on secondary consumers.
- Food chains show one linear pathway of energy transfer. Food webs show a more realistic picture because most organisms feed on or are eaten by more than one species.
- Decomposers break down dead organisms and waste, returning materials to the environment.
Pyramids of Biomass and Numbers¶
- A pyramid of biomass shows the mass of living material at each trophic level, drawn to scale so that the width of each bar represents the relative amount of biomass.
- The producer bar is always at the bottom; each bar above it is usually narrower because biomass decreases at every step.
- Pyramids of numbers show the count of organisms at each level instead. Unlike biomass pyramids, these can produce unusual shapes because one large organism can support many smaller ones.
- Biomass pyramids are more useful for showing how much living material is available to the next trophic level.
Biomass Transfer and Efficiency¶
- Only around 10% of biomass is typically transferred from one trophic level to the next. The rest is lost because not all biomass is eaten, not all eaten material is digested, and organisms use much of it in respiration.
- Respiration accounts for a large part of the loss, because organisms release energy to stay alive rather than turning all food into new body tissue.
- This inefficiency explains why food chains rarely extend beyond four or five trophic levels.
- The efficiency of biomass transfer between two levels can be calculated as: (biomass of higher trophic level ÷ biomass of lower trophic level) × 100.
Common Confusions¶
- Pyramid of biomass vs pyramid of numbers: a pyramid of numbers can be inverted if one large organism supports many smaller ones, but biomass still decreases up the chain.
- Food chain vs food web: a food chain is a single path; a food web shows the network of linked feeding relationships.
- Why energy is lost, not just biomass: organisms do not pass on everything they eat because much of it is used in respiration and released to the surroundings.
Key Terms¶
- Trophic level: a feeding position in a food chain.
- Producer: an organism that makes its own food, usually by photosynthesis.
- Consumer: an organism that gets food by eating other organisms.
- Biomass: the mass of living material in organisms.
- Pyramid of biomass: a diagram showing the biomass present at each trophic level, drawn to scale.
- Pyramid of numbers: a diagram showing the number of organisms at each trophic level.