Trophic levels and pyramids of biomass
Biology · GCSE · The School
Every organism sits at a trophic level
Producers (usually plants, via photosynthesis) form level 1. Primary consumers, which eat producers, form level 2. Secondary consumers, which eat primary consumers, form level 3. Tertiary consumers, which eat secondary consumers, form level 4. Energy — as biomass — flows up through these levels, from level to level, in one direction.
Only a fraction of biomass transfers upward
When one organism eats another, only around 10% of the biomass in the organism eaten is converted into the biomass of the organism eating it. The rest is lost: as waste that is not digested, as heat from respiration, and in movement and other life processes — none of which becomes new biomass for the next level.
A worked calculation
A field of grass has a biomass of 10,000 kg. Rabbits eating that grass gain roughly 10%: 1,000 kg of rabbit biomass. A fox eating those rabbits gains roughly 10% of that: 100 kg of fox biomass. From 10,000 kg of grass, only 100 kg ends up as fox — a 100-fold reduction across just two transfers, purely from the 10% rule applied twice.
Why this limits how long a food chain can be
Each trophic level starts with roughly a tenth of the biomass of the one below it. By the fifth or sixth level, so little biomass remains that it can no longer support a viable population — there simply is not enough available. This is the biological reason food chains rarely extend beyond four or five levels, not a coincidence of what happens to eat what.
Pyramids of biomass are drawn to scale
A pyramid of biomass draws each trophic level as a bar, with its WIDTH proportional to the biomass at that level. Because roughly 90% is lost at each step, the pyramid narrows sharply moving up — level 1 is always the widest bar, and by level 4 the bar is a fraction of level 1's width, drawn to scale rather than just getting generally smaller.
Where the 10% actually goes
A full-mark answer names where the missing 90% is lost, not just that it is lost. Respiration releases energy as heat, which cannot become biomass. Movement uses energy without adding mass. Egestion removes undigested material that was never absorbed into the body at all. Naming all three, rather than one vague "energy is lost", is what separates a partial answer from a complete one.