In an oak woodland a single oak tree is fed on by very large numbers of caterpillars. The caterpillars are in turn eaten by blue tits, and the blue tits are...
Question 1 Report
In an oak woodland a single oak tree is fed on by very large numbers of caterpillars. The caterpillars are in turn eaten by blue tits, and the blue tits are eaten by a sparrowhawk. This simple food chain is shown in the diagram below. Ecologists working in one study area counted the number of individual organisms present at each stage of the chain, and their results are given in the table.
| Organism | Number counted |
|---|---|
| Oak tree | 1 |
| Caterpillars | 3000 |
| Blue tits | 40 |
| Sparrowhawks | 2 |
(a) State what information a pyramid of numbers is designed to show. [1]
(b) State which organism in this food chain is the producer. [1]
(c) Using the figures in the table, describe the shape of the pyramid of numbers that would be drawn from these data. [2]
(d) Explain why this pyramid of numbers does not form a regular pyramid shape. [3]
(e) Calculate the mean number of caterpillars available to each blue tit, assuming that all of the caterpillars are eaten. [2]
(f) Suggest why a pyramid of biomass would give a more regular shape than the pyramid of numbers for this food chain. [1]
Answer Details
This question is about pyramids of numbers, using the oak tree to caterpillars to blue tits to sparrowhawk food chain.
- What a pyramid of numbers shows [1]. The number of organisms at each trophic level of a food chain [1].
- The producer [1]. The oak tree [1], because it makes its own food by photosynthesis.
- Shape of the pyramid [2]. The bar for the oak tree (producer) is very narrow because there is only one tree [1]; the caterpillar bar is by far the widest, and the bars then become smaller for the blue tits and smallest for the sparrowhawks [1].
- Why it is not a regular pyramid [3]. A single very large producer (the oak) supports a very large number of small primary consumers (caterpillars) [1]; a pyramid of numbers counts organisms and ignores their size or mass [1]; so the producer bar is narrower than the level above it and the pyramid is irregular (inverted at its base) [1].
- Mean caterpillars per blue tit [2]. \(3000 \div 40\) [1] \(= 75\) caterpillars per blue tit [1].
- Why a pyramid of biomass would be more regular [1]. The single oak has a very large biomass, so its bar would become the largest at the base, giving the normal pyramid shape [1].
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