Analyze an environmental problem doing calculations

Energy flow in a prairie

Units 1 and 5
10 points
About 24 minutes

You can use a calculator on this question, just like on exam day.

You work through an environmental scenario using numbers: set up and solve calculations (showing your work), then explain the science and propose solutions. Several of the 10 points come from the calculations, usually one for the setup and one for the answer. On the exam: Question 3 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question

Ecologists measured energy flow in a 50-hectare restored tallgrass prairie. They found that the prairie's producers capture 24,000 kJ of energy per square meter per year through photosynthesis (gross primary productivity), and use 9,600 kJ per square meter per year for their own cellular respiration. (1 hectare = 10,000 m²)

Suggested time: 24 minutes

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Part (a)

2 points

Calculate the net primary productivity (NPP) of the prairie in kJ/m²/yr. Show your work.

Your answer to part (a)

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Part (b)

2 points

Using the 10% rule, calculate the amount of energy available to secondary consumers in the prairie, in kJ/m²/yr. Show your work.

Your answer to part (b)

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Part (c)

2 points

Calculate the total net primary productivity of the entire 50-hectare prairie for one year. Show your work.

Your answer to part (c)

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Part (d)

1 point

Explain why only about 10% of the energy at one trophic level is available to the next trophic level.

Your answer to part (d)

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Part (e)

1 point

A rancher plans to graze cattle on part of the prairie. Describe one effect of overgrazing on the prairie's soil.

Your answer to part (e)

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Part (f)

1 point

Propose one grazing practice the rancher could use to prevent overgrazing.

Your answer to part (f)

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Part (g)

1 point

Using your answer to part (b) or the 10% rule, explain why a diet based on plants can feed more people from the same area of land than a diet based on beef.

Your answer to part (g)

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