Ecology and Ecosystems: Question 8
Syllabus 19.2
Ecologists studying a garden pond recorded the estimated numbers of organisms in one food chain:
- Duckweed (a tiny floating producer): 8000 plants
- Water fleas, which feed on the duckweed: 400
- Small fish, which feed on the water fleas: 25
- Herons, which feed on the small fish: 2
(a) State the trophic level of the water flea and of the small fish in this food chain. [2]
(b) Using the numbers given, describe the shape of a pyramid of numbers for this food chain, and explain why it has this shape. [3]
(c) A student claims that a pyramid of numbers is always a perfectly regular, upright triangle for any food chain. Using an example, explain why this claim is not always true. [2]
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Worked solution
Part (a): Trophic levels
A primary consumer feeds directly on a producer, and a secondary consumer feeds on a primary consumer.
- The water flea feeds on the duckweed (the producer), so it is a primary consumer.
- The small fish feed on the water fleas (the primary consumers), so they are secondary consumers.
Part (b): The shape of this pyramid of numbers
Listing the counts from producer to top predator:
- Duckweed (producer): 8000
- Water fleas (primary consumer): 400
- Small fish (secondary consumer): 25
- Herons (tertiary consumer): 2
Each count is smaller than the one below it, so plotting these as bars gives a pyramid that steadily narrows from a wide base (duckweed) up to a narrow top (herons), a regular, upright pyramid. This shape occurs because energy is lost at each transfer along the food chain, so each trophic level can only support a smaller number of organisms than the level it feeds on.
Part (c): Why a pyramid of numbers is not always upright
Although this pond food chain gives a regular, upright pyramid, this is not always the case. A pyramid of numbers only counts individual organisms, without taking their size into account. For example, a single very large tree, if it were the producer in a food chain, would be counted as just one individual, even though it is large enough to support hundreds of smaller insects feeding on its leaves. In that situation, the bar for the producer (1) would be narrower than the bar for the primary consumers above it (many hundreds), so the pyramid of numbers would not be a regular, upright triangle.
Final answers
- (a) Water flea: primary consumer. Small fish: secondary consumer.
- (b) A regular, upright pyramid, narrowing steadily from 8000 duckweed plants at the base to 2 herons at the top, because the number of organisms that can be supported decreases at each higher trophic level.
- (c) A pyramid of numbers is not always upright, for example when a single large producer (such as one large tree) supports many more, much smaller consumers, making the producer’s bar narrower than the level above it.