4BI1

Ecology & Food Chains

Ecology and the Environment · 8 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 12% of your exam marks.

stable
High
Stable12%

Food chains, energy transfer, and ecological definitions are regularly tested, often as short-answer questions.

Food chains can be drawn as ecological pyramids, where each trophic level is shown as a horizontal bar. The width of the bar represents the amount of something at that level. There are three types of pyramid.

Pyramid of numbers

A pyramid of numbers shows how many organisms are at each trophic level. The width of each bar is proportional to the number of individuals.

For most food chains, this gives a true pyramid shape:

grass (millions of plants) → vole (thousands) → owl (few)

But pyramids of numbers are not always pyramid-shaped. If many small organisms feed on one large organism, the producer level is narrower than the consumer level:

oak tree (1) → caterpillar (thousands) → robin (a few) → flea (many)

This gives an awkward shape with a narrow base.

A pyramid of numbers for a grassland food chain: a wide bar of lots of grass plants at the base, a narrower bar of some voles above it, and a single narrow bar of one owl at the top
Source: Food Chains, Webs & Environmental Pyramids by Save My Exams

Pyramid of biomass

A pyramid of shows the total mass of living matter at each trophic level (in g/m² or kg per area).

Pyramids of biomass are almost always pyramid-shaped, because the biomass at each level is always less than the level below. The example above looks normal in biomass terms:

oak tree (huge mass) → caterpillars (small mass) → robins (smaller mass) → fleas (tiny mass)

A pyramid of biomass showing biomass decreasing up the trophic levels: producers (5000–10,000 g/m² of oak tree) at the wide base, primary consumers (50–100 g/m² of caterpillar), secondary consumers (5–10 g/m² of robin), and tertiary consumers (0.5–1 g/m² of woodpecker) as the narrow top
Source: Food Chains, Webs & Environmental Pyramids by Save My Exams

Pyramid of energy

A pyramid of energy shows the energy stored in the biomass at each trophic level (in kJ/m²/year).

Pyramids of energy are always pyramid-shaped. Energy is lost at every level, so there is always less stored energy at higher trophic levels than at lower ones. This is true for every food chain without exception.

A pyramid of energy in kJ per square metre per year: 1,000,000 J of sunlight at the base, producers with 10,000 J, primary consumers with 1000 J, secondary consumers with 100 J, and tertiary consumers with 10 J, each level about a tenth of the one below
Source: Food Chains, Webs & Environmental Pyramids by Save My Exams

Why bigger animals are rarer

The pattern of "many small organisms supporting fewer large ones" appears everywhere because of the energy loss between levels. A square kilometre of grassland can feed thousands of rabbits, but only a few foxes can live off the rabbits. This is why top predators are rare in most ecosystems.