Trophic Levels in an Ecosystem¶

Part of 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.

What You Need to Learn¶

Connect positions in a food chain to the amounts of living material available at each stage. Construct a biomass pyramid from data, explain why only a small share reaches the next level, and calculate transfer efficiency. Include decomposers in the explanation of what happens to dead material.

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.
An apex predator has no predators of its own in that food web.
Decomposers secrete enzymes onto dead organisms and waste. These break large molecules into small soluble molecules that diffuse into the microorganisms, 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 much less biomass is passed to the next level.
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. Use bars of equal height and one consistent width scale. For 1,000 g of producers, 100 g of primary consumers and 10 g of secondary consumers, the bar widths must be in the ratio 100:10:1, with producers at the bottom.

Biomass Transfer and Efficiency¶

Producers transfer roughly 1% of the light energy reaching them into chemical energy through photosynthesis. This light-energy percentage is different from the biomass-transfer percentage between organisms.
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 uses glucose and releases carbon dioxide and water, so material leaves the organism as well as energy being transferred to the surroundings. Faeces contain undigested material; urine carries water and urea. These losses mean absorbed food does not all become new biomass.
This inefficiency limits food-chain length and usually supports fewer organisms at higher levels, especially when the animals are larger.
The efficiency of biomass transfer between two levels can be calculated as: (biomass of higher trophic level ÷ biomass of lower trophic level) × 100. For 80 g passed on from 1,000 g, the fraction is 80/1,000 = 0.08 and the percentage is 8%. Use the same mass units and comparable measurements for both levels.

Common Confusions¶

Pyramid of biomass vs pyramid of numbers: a pyramid of numbers can be inverted if one large organism supports many smaller ones, because counts ignore individual size. Biomass transfer decreases along a chain, though a snapshot of standing biomass can have a different shape if producers replace their biomass rapidly.
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.