Ecological Pyramids and Energy Transfer

Pyramids of numbers

A pyramid of numbers represents the number of organisms at each trophic level of a food chain. For grass → rabbit → fox, there are typically many grass plants, fewer rabbits, and fewer foxes still, which produces the familiar tapering shape the name suggests.

To construct one, place the producer trophic level at the bottom, draw a horizontal bar for each trophic level above it, and make the width of each bar represent the number of organisms present at that level, stacking the consumer levels in feeding order.

The limitation is equally important: a pyramid of numbers takes no account of organism size. In one tree → many insects → fewer birds, the producer level may contain only a single individual, even though that one tree contains far more biological material than any of the insects feeding on it — so the shape can come out distorted, sometimes not resembling a pyramid at all, purely because it counts individuals and nothing else.

Pyramids of biomass

A pyramid of biomass represents the total biomass — the mass of biological material present — at each trophic level, rather than a headcount of organisms. To construct one, place producers at the base, add the consumer levels above them in the same feeding order, and let the width of each bar represent biomass rather than number of organisms; to interpret it, compare the widths and relate them to the amount of biological material each level actually holds.

This is why a pyramid of biomass generally gives a better representation of a food chain than a pyramid of numbers: it represents the amount of living material present, not just how many individuals happen to be there. In tree → insects → birds, a pyramid of numbers may show only one producer, but a pyramid of biomass reflects that this single tree still holds vastly more biological material than the insects feeding on it — avoiding the distortion that happens whenever a small number of very large organisms supports a large number of much smaller ones.

Pyramids of energy — Supplement

A pyramid of energy represents the energy transferred through each trophic level, with producers again forming the base and successive consumer levels stacked above them. The width of each level represents the amount of energy transferred at that trophic level, and because some energy is always lost between one level and the next, less energy is available — and so the bars get narrower — the further up the pyramid you go.

Each type of pyramid measures something different: numbers measure how many organisms there are, biomass measures how much biological material there is, and energy measures how much energy is actually being transferred through the system. A pyramid of numbers can be distorted by large differences in organism size, and a pyramid of biomass takes size into account but still only captures material present at one moment, rather than the ongoing transfer of energy. A pyramid of energy represents that transfer directly, which is why it gives a particularly strong picture of how available energy decreases through a food chain — and why Extended candidates are expected to be able to draw, describe and interpret it, and to compare its usefulness against pyramids of numbers and biomass.

Why energy transfer between trophic levels is inefficient — Supplement

Only part of the energy available at one trophic level ever becomes chemical energy in the organisms at the next level, and three things explain why.

Not every organism or body part is eaten

A predator may consume only part of its prey — bones, roots, shells or woody tissue can go uneaten, and entire organisms within a population may never be eaten at all. The chemical energy locked in that material is simply never transferred to the next consumer.

Not everything eaten is absorbed

Some ingested material can’t be digested and absorbed, so it leaves the body as waste — its chemical energy never becomes part of the consumer’s own tissues in the first place.

Organisms use energy themselves

Every organism respires, and the energy released during respiration is used for its own biological processes before eventually being transferred to the environment, mostly as heat. Put together, only a fraction of the chemical energy an organism obtains from feeding ends up stored in new biomass that could later be eaten by the next consumer — which is exactly the pattern a pyramid of energy is drawn to show.

Why food chains are usually short — Supplement

Because energy transfer between trophic levels is inefficient, less energy remains available at each successive level of a chain such as producer → primary consumer → secondary consumer → tertiary consumer. At every transfer, a substantial share of the energy already present has been lost to the environment or remains in material that was never passed on. Eventually too little energy is left to support another trophic level at all — which is why food chains usually contain fewer than five trophic levels, even though quaternary consumers do occur.

Why eating crop plants directly is more energy-efficient than eating livestock fed on crops — Supplement

Compare crop plant → human with crop plant → livestock → human. The second route adds an extra trophic transfer: the livestock respires, moves, produces waste and maintains its own body, and some of its tissue may never be eaten at all. As a result, only part of the chemical energy originally present in the crop ends up stored in livestock tissue that a human actually consumes. Eating the crop directly involves one fewer trophic transfer, and so avoids one entire stage of energy loss — which is why it is more energy-efficient for humans to eat crop plants than to eat livestock that were themselves fed on those crops. This is a statement about energy transfer through trophic levels, not a judgement about what any individual should choose to eat.