IGCSE Biology 0610: Pyramids of Numbers and Biomass in a Mangrove Food Chain

Ecologists studying a single, very large mangrove tree growing at the edge of a tidal creek recorded the organisms living on and around it. Among the tree's roots they counted 850 mangrove tree crabs, which feed only on the tree's fallen leaves and bark. Over the wider creek, they also recorded 15 collared kingfishers, which hunt over several mangrove trees and prey mainly on the mangrove tree crabs.

(a) State the trophic level of the mangrove tree crab and of the collared kingfisher in this food chain. [2]

(b) The mangrove tree itself is counted as a single producer, yet it supports 850 mangrove tree crabs. Describe the shape of a pyramid of numbers drawn using these figures, and explain why it does not look like a typical, upright pyramid. [3]

(c) State one advantage of drawing a pyramid of biomass, rather than a pyramid of numbers, to represent this food chain. [2]

(d) The collared kingfishers were also seen occasionally catching small fish from tidal pools nearby. Use this observation to explain the difference between a food chain and a food web. [2]

Part (a): Trophic levels

A primary consumer feeds directly on a producer, and a secondary consumer feeds on a primary consumer.

The mangrove tree crab feeds only on the mangrove tree (the producer), so it is a primary consumer.
The collared kingfisher feeds on the mangrove tree crabs (the primary consumers), so it is a secondary consumer.

Part (b): An unusual pyramid of numbers

A pyramid of numbers is normally built by counting the individual organisms at each trophic level. Here:

Producer (mangrove tree): 1
Primary consumer (mangrove tree crab): 850

Plotted as bars, the producer’s bar (1) would be far narrower than the primary consumer’s bar (850) above it, so the pyramid of numbers does not taper upward as a regular pyramid. Its base is narrower than the level above it.

This happens because a pyramid of numbers only counts individuals, and it does not take into account the size of each organism. A single mangrove tree is enormous compared with a single crab, so counting it as just “1” badly understates how much living material it actually represents.

Part (c): Why use a pyramid of biomass instead

A pyramid of biomass measures the total mass of living material at each trophic level, rather than simply counting individuals. Because the one mangrove tree has an enormous mass compared with the combined mass of the 850 crabs, a pyramid of biomass would correctly show the tree’s bar as the widest, giving a normal, upright pyramid shape. This makes the pyramid of biomass a more realistic representation of the food chain than the pyramid of numbers, which is distorted by counting one very large organism as a single unit.

Part (d): Food chain versus food web

A single food chain, such as mangrove tree → mangrove tree crab → collared kingfisher, shows only one feeding pathway. But the collared kingfisher also feeds on small fish, which belong to a different food chain entirely (for example, algae → small fish → collared kingfisher). Because the kingfisher appears in both of these food chains, they are linked together. A food web is a network showing all of these interconnected food chains, rather than the single, isolated pathway shown by any one food chain alone.

Final answers

(a) Mangrove tree crab: primary consumer. Collared kingfisher: secondary consumer.
(b) The pyramid of numbers has a narrower base (1) than the bar above it (850), because counting one enormous tree as a single individual does not reflect how much living material it represents.
(c) A pyramid of biomass would show the mangrove tree’s true mass, giving a normal, upright pyramid shape instead of a distorted one.
(d) The kingfisher links two different food chains (crabs and small fish) together, and a food web shows these interconnected food chains, unlike a single food chain.