Where does the energy go?
335 words over 28 numbered lines, then 6 parts worth 12 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice unless a source is named.
Where does the energy go?
A barn owl hunts at dusk over rough grassland, listening for the
rustle of a field vole in the grass below. A single owl needs to catch
about four voles every night. A pair raising a family of chicks may
need more than a thousand voles in a year.
The voles eat grass, so a simple food chain on the grassland is grass,
then voles, then barn owls. Each stage in a food chain is a trophic
level. The grass is a producer, the vole is a primary consumer and the
owl is a secondary consumer. When organisms in any trophic level die,
decomposers feed on their remains.
Ecologists measured how much energy was transferred to each trophic
level on one area of grassland in a year. Their results are shown in
Figure 1. Only a small part of the energy in the grass ends up in the
bodies of voles. Much of the grass is never eaten, and some of the
grass that is eaten cannot be digested and passes out in the faeces.
Voles also use a lot of energy in respiration, which releases energy
for movement and to keep their bodies warm, and this energy is lost to
the surroundings as heat.
Owls swallow voles whole and later cough up pellets of fur and bone
that they cannot digest. Students can pull these pellets apart to find
out what the owl has been eating. One pellet may contain the skulls of
two or three small mammals.
Because so much energy is lost at each stage, food chains rarely have
more than four or five trophic levels. A pyramid of numbers for this
grassland would show many grass plants, fewer voles and very few owls.
In years when the number of voles falls, fewer owl chicks survive.
Leaving strips of rough grass at the edges of fields gives voles more
places to live. Barn owls can also be harmed when rat poison used on
farms passes along the food chain into their bodies.
Figure 1 Energy transferred to each trophic level on an area of grassland in one year
| Trophic level | Organism | Energy / kJ per m² per year |
|---|---|---|
| Producer | Grass | 21,000 |
| Primary consumer | Field vole | 2,100 |
| Secondary consumer | Barn owl | 42 |
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term producer.
(1)Use Figure 1 to calculate the percentage of the energy in the voles that is transferred to the barn owls.
(2)Explain why only a small part of the energy in the grass is transferred to the voles.
(3)Explain why food chains rarely have more than five trophic levels.
(2)Describe how a pyramid of biomass for this food chain would differ from a pyramid of numbers if the producer were a single large oak tree instead of grass.
(2)Suggest why fewer owl chicks survive in years when the number of voles falls.
(2)Total for this question: 12 marks
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