Trophic levels: producer to top predator
Producers (level 1) are photosynthetic (plants, algae), making organic molecules from CO₂ and water using light. Primary consumers (2) are herbivores eating producers; secondary (3) eat primary; tertiary (4) eat secondary. Decomposers (bacteria, fungi) recycle dead matter from any level and are NOT a numbered trophic level. Every chain starts with a producer; the arrow shows energy passing prey → predator ("is eaten by", not "eats").
Number, biomass, energy pyramids compared
Pyramid of numbers counts individuals: usually pyramid-shaped but can invert (one oak feeds thousands of caterpillars). Pyramid of biomass shows total dry mass: nearly always upright, but can invert in aquatic systems where phytoplankton are eaten faster than they accumulate. Pyramid of energy is always a true pyramid — energy only decreases upwards, so it is the most reliable. Only ~10% passes to the next level.
About 10% energy transfer between levels
Only about 10% of the energy at one trophic level reaches the next. Losses: (1) heat from respiration (glucose broken down, much lost to the surroundings); (2) movement (also lost as heat); (3) egestion (undigested food leaves as faeces, its energy unavailable to the consumer). Some is also lost in excretion (urea). Each level multiplies the loss, so longer chains support fewer top predators — eating plant material directly is more energy-efficient.
Drawn from real examiner reports.
Food-chain arrows point prey → predator
The arrow shows energy flow ("is eaten by"): it points FROM the organism eaten TO the one eating — grass → rabbit → fox, never fox → rabbit → grass. Many candidates draw arrows the way an animal "eats" (backwards), reversing the energy flow. When drawing a chain from a food web, check every arrow points from prey to predator.
June 2024 Paper 1BR Q5(a)(ii)–(iii): candidates drew arrows the wrong way (predator → prey) and drew a pyramid of numbers instead of a food chain.
Mis-identifying tertiary consumers
A tertiary consumer eats secondary consumers (the fourth trophic level). In a branching web, one organism can be both a secondary and a tertiary consumer depending on the chain traced, so levels are easily miscounted. Method: for each named organism, trace a chain from the producer and count its position — a tertiary consumer sits fourth in at least one chain of the web.
June 2024 Paper 1BR Q5(a)(ii): mis-identifying a tertiary consumer was reported; some named a secondary consumer instead, showing incorrect trophic-level counting.
List loss routes AND link to chain length
Naming the loss routes (heat from respiration, egestion, movement) is only half the answer. For "why does a longer chain support fewer top predators?", the second element needs the losses linked to chain length: each extra level multiplies the ~10% loss, so further from the producer means less energy remains. Listing routes without the chain-length link caps the mark.
June 2023 Paper 1BR Q3(a)(iii): candidates listed energy-loss routes but failed to link them to the extra trophic level in the longer chain.
Longest chain means most trophic levels
When asked to draw the longest food chain from a web, choose the chain with the most links / trophic levels — not the chain containing the most organisms or the largest ones. A chain drawn through crowded levels can still be short. Count the arrows: the longest chain has the most arrows from producer to top predator.
June 2024 Paper 1BR Q5(a)(iii): some candidates picked a non-longest chain when asked to draw the longest food chain.
A producer is never a consumer
A producer makes its own food by photosynthesis (a plant or alga) and forms the base of every chain; a consumer must eat other organisms. Do not label a green plant as a primary consumer, and do not begin a chain with a herbivore. The first organism in any food chain is always a producer, occupying trophic level 1.
Only the energy pyramid is always upright
Pyramids of numbers and biomass can be inverted (e.g. one oak tree beneath thousands of caterpillars; phytoplankton eaten faster than they grow), but a pyramid of energy can never invert — energy only decreases up the chain. Also, decomposers are never drawn as a numbered pyramid bar; they act on dead matter from every level rather than at one position.
Trace every arrow through the web
Food-web questions are mostly retrieval; watch four traps: arrow direction (prey → predator); level counting (producer = 1, primary = 2, …); "longest chain" means most links, not most organisms; and for "explain the decrease in X", trace the effect along the arrows.
Energy-transfer calculations
To move UP a level, multiply the energy by 0.10 (10%); to work DOWN a level, divide by 0.10. Efficiency between two levels = energy at the higher level ÷ energy at the lower level × 100. Count the arrows: each is one 10% step, so a four-link chain applies 0.10 four times.
Give a direction of change
Examiners reward a stated direction, not a vague comment: write "less light REDUCES photosynthesis", not "light affects photosynthesis". Whenever a factor changes, say whether the process or population increases or decreases, and follow the effect through each trophic level.
Trophic levels are the feeding positions in an ecosystem. Every food chain begins with a producer (a photosynthetic organism) and ends with the top predator.
| Trophic level | Name | Example |
|---|---|---|
| 1st | Producer | Grass, algae, oak tree |
| 2nd | Primary consumer (herbivore) | Rabbit, caterpillar, aphid |
| 3rd | Secondary consumer (carnivore or omnivore) | Thrush, frog, fox |
| 4th | Tertiary consumer | Hawk, pike, orca |
| Any | Decomposer | Bacteria, fungi |
Key definitions (two-element, mark-scheme form):
Arrow convention: the arrow points in the direction of energy flow (from eaten to eater):
The arrow means "is eaten by" (energy passes to). It does NOT mean "eats".
Define the term producer in an ecological context.
The following food chain is found in a meadow ecosystem:
(a) State the trophic level of the shrew. (1 mark)
(b) Give the consumer name for the kestrel. (1 mark)
(c) State which organism is the producer and explain why. (2 marks)
