Practice questions · Topic 8.2

Unit 8 · Practice for the Topic 8.2 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: Energy flow through ecosystems, summed up

Video coming soon

What energy is spent on; endotherm and ectotherm; the levels up to the biome; energy flows through while matter cycles; one template for four cycles; autotrophs and heterotrophs; trophic levels, arrows the way the energy goes, the web from the table; about 10 % up a level and where the rest went; less sun, fewer hawks.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. The first free-response question walks you through one ecosystem’s energy figures one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it.

Question 1

An albatross drinks sea water. Glands above its eyes pump the extra salt out, so the salt in its blood stays at one level.

Which of the four things an organism uses energy for is the albatross doing?

A. Growing B. Reproducing C. Keeping its parts in order D. Keeping its inside steady

Question 2

A barn owl in a hard winter takes in 340 kJ per day from the voles it catches and uses 455 kJ per day.

Which of the following is the owl’s net energy?

A. −795 kJ per day B. −115 kJ per day C. +115 kJ per day D. +795 kJ per day

Question 3

A crocodile lies on a river bank in the morning sun with its mouth open. Its body warms from 24 °C to 32 °C in an hour.

Which of the following explains where the crocodile’s warmth came from?

A. Heat passed into it from the sun and the warm bank B. Its muscles made the heat by shivering C. Its cells respired extra food to make the heat D. Its open mouth took in warm air, which heated its blood

Question 4

How the rotifers reproduced at each amount of algae.

Rotifers are tiny animals that eat single-celled algae. A researcher keeps rotifers in tanks holding different amounts of algae and records how each tank’s rotifers reproduce, in the table below.

Which of the following strategies does the table show?

A. Timing mating so the young arrive when the algae do B. Switching from asexual to sexual reproduction as the food falls C. Pausing breeding while the algae are scarce, then breeding again D. Storing energy from the algae and breeding on it later

Question 5

A campfire burns 2 kg of dry wood down to 0.1 kg of ash.

Which of the following happened to the other 1.9 kg of the wood’s matter?

A. It was destroyed as the fire released the wood’s energy B. It left as heat C. It went into the air, mostly as carbon dioxide and water vapor D. It sank into the ground under the fire

Question 6

The water in one parcel of coastal air at 06:00 and at 12:00.

Meteorologists follow one parcel of coastal air from dawn to noon and measure the water in it, in the table below. No rain reached the ground.

Which of the following accounts for the change between 06:00 and 12:00?

A. The atmosphere lost 0.9 million kg of water, because the vapor reading fell between the two times B. The atmosphere gained 0.9 million kg of water, because the cloud grew between the two times C. The vapor evaporated out of the parcel into the sea beneath it, so the atmosphere lost water D. The atmosphere held 1.9 million kg of water at both times, because vapor condensed into cloud droplets

Question 7

For years a town’s washing powders carried phosphate, and the town’s waste water carried it into the town’s lake. A new law removes phosphate from every washing powder. No process takes phosphate out of the lake’s water any faster than before.

Over the following years, what happens to the lake’s store of phosphate?

A. It rises B. It stays the same C. It falls D. It falls to zero within the first year

Question 8

An orca hunts, kills and eats a seal.

Which kind of eater is the orca here?

A. A carnivore B. A herbivore C. A scavenger D. A decomposer

Question 9

On a heather moor, red grouse eat the heather’s shoots, and hen harriers eat the grouse.

Which trophic level are the hen harriers?

A. Producer B. Primary consumer C. Secondary consumer D. Tertiary consumer

Question 10

The alpine meadow’s food web, drawn from a survey.

The food web of an alpine meadow is drawn below. Each arrow runs from the eaten organism to the organism that eats it. Suppose a disease kills every marmot in the meadow.

Which of the following organisms loses a food source but still has food?

A. The ptarmigan B. The golden eagles C. The grasshoppers D. The alpine plants

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Analyze Data · 5 points

Ecologists survey a fjord. Its floating algae are eaten by small crustaceans, and the crustaceans are eaten by herring. The table gives what the algae turned into sugar in a year and what they respired, in kilojoules per square meter a year. A later survey measures the energy stored in the crustaceans’ level at 588 kJ per square meter a year.

What the fjord’s floating algae turned into sugar and what they respired, in kilojoules per square meter a year.

(a) Calculate the net primary productivity of the algae. (1 point)

Hint: Which of the two values in the table is taken away from the other?

kJ per square meter a year

Write down the values:

made = 6 800 kJ per square meter a year

respired = 2 600 kJ per square meter a year

Write down the equation and substitute:

net primary productivity = 6 800 - 2 600 = 4 200 kJ per square meter a year

A full-credit answer: Net primary productivity = made − respired = 4 200 kJ per square meter a year.

(b) By the 10 % rule, calculate the energy the crustaceans’ level would store. (1 point)

Hint: The crustaceans are one level above the algae. What do you multiply your answer to part (a) by?

kJ per square meter a year

Write down the equation and substitute:

crustaceans = 4 200 \times 0.10 = 420 kJ per square meter a year

A full-credit answer: The crustaceans’ level would store about 420 kJ per square meter a year.

(c) By the 10 % rule, calculate the energy the herring’s level would store. (1 point)

Hint: The herring are one level above the crustaceans. Start from your answer to part (b).

kJ per square meter a year

Write down the equation and substitute:

herring = 420 \times 0.10 = 42 kJ per square meter a year

A full-credit answer: The herring’s level would store about 42 kJ per square meter a year.

(d) Using the measured value of 588 kJ per square meter a year for the crustaceans’ level, calculate the transfer between the algae and the crustaceans as a percentage, to one decimal place. Answers within 0.05 % are accepted. (1 point)

Hint: The transfer is the level above divided by the level below, as a percentage. Which of your values is the level below?

Write down the equation and substitute:

transfer = \frac{588}{4 200} \times 100 = 14.0 %

A full-credit answer: Transfer = 588 ÷ 4 200 × 100 = 14.0 %.

(e) Determine whether the fjord’s measured transfer sits inside the range ecologists have measured, about 5 % to about 20 %, and whether it sits above or below the 10 % working rule. (1 point)

Hint: Compare your answer to part (d) with 5 %, 20 % and 10 %.

A full-credit answer: The transfer of 14.0 % sits inside the measured range and above the 10 % working rule, because 14.0 % lies between 5 % and 20 % and is larger than 10 %.

Check the box for each point your answer earns

inside the range (between 5 % and 20 %) AND above the 10 % rule, with the comparison stated.

Common slip: Calling 14.0 % a mistake in the survey because it is not 10 %. The 10 % rule is a rounded working figure; measured transfers run from about 5 % to about 20 %.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

Suppose a long drought dries out a bulrush marsh. The bulrushes die and lie on the exposed mud, and soil microbes feed on them. Two years later heavy rains flood the marsh again, and the flooded mud holds little oxygen.

(a) Describe the form in which the bulrushes took up their nitrogen while they were alive. (1 point)

A full-credit answer: The bulrushes took up nitrogen through their roots as ammonium or nitrate dissolved in the marsh’s water, never as nitrogen gas.

Check the box for each point your answer earns

as ammonium or nitrate (dissolved, through the roots). Accept with or without: not as nitrogen gas.

Common slip: Saying the bulrushes took nitrogen gas from the air. No plant enzyme breaks the triple covalent bond in nitrogen gas.

(b) Explain how the nitrogen in the dead bulrushes’ protein becomes nitrate in the dry mud. (1 point)

A full-credit answer: Soil microbes break the dead bulrushes down and turn their protein’s nitrogen into ammonium: ammonification.
Soil bacteria then turn the ammonium into nitrite and then into nitrate: nitrification.

Check the box for each point your answer earns

ammonification (dead matter to ammonium) followed by nitrification (ammonium to nitrate), both done by soil microbes. Accept nitrification written with or without the nitrite step.

Common slip: Naming nitrogen fixation. Fixation turns nitrogen gas into ammonia; the bulrushes’ nitrogen was already in a compound.

(c) Predict how the store of nitrate in the mud changes in the months after the marsh floods again. (1 point)

A full-credit answer: The store of nitrate in the mud falls.

Check the box for each point your answer earns

the nitrate store falls (decreases).

Common slip: Predicting a rise because the water carries nitrate in. The flooded mud holds little oxygen, and that is where bacteria turn nitrate into nitrogen gas.

(d) Justify your prediction using the steps of the nitrogen cycle. (1 point)

A full-credit answer: The flooded mud holds little oxygen, so the bacteria that turn nitrate into nitrogen gas thrive: denitrification speeds up.
Denitrification is an arrow out of the mud’s nitrate store, into the air.
A faster arrow out lowers the reservoir it leaves.
So the mud’s store of nitrate falls.

Check the box for each point your answer earns

denitrification (nitrate to nitrogen gas, which leaves to the air) speeds up in the flooded mud, a faster arrow out of the nitrate store, so the store falls.

Common slip: Saying the nitrate washed away with the flood. The flooded mud holds little oxygen, and the nitrate leaves as nitrogen gas.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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