End-of-topic test: Energy Flow Through Ecosystems

Unit 8 · Topic 8.2 end-of-topic test

Suggested time: about 48 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

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. Then open the scoring guide and mark your own work against it. Every energy value carries its unit, kilojoules per square meter a year, and every percentage is written with a space: 10 %.

Question 1

A female pond turtle feeds on insects and pondweed all summer. Each day she takes in more energy than she uses, and by autumn she carries a thick layer of fat.

Which of the following happens to the surplus energy she takes in each day?

A. She stores it as fat, and later builds it into her eggs B. It leaves her body as heat, with no use to her C. She uses it to keep her body warmer than the pond water D. It passes out of her body undigested

Question 2

A weasel in January takes in 610 kJ per day from the mice it catches. It uses 690 kJ per day keeping warm and hunting.

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

A. −1 300 kJ per day B. −80 kJ per day C. +80 kJ per day D. +1 300 kJ per day

Question 3

Food eaten per gram of body each day by a hamster and a tree frog of the same mass.

A hamster and a tree frog of the same mass live in the same cool room. The table gives the food each one eats per gram of body each day.

Which of the following explains the difference in the table?

A. The tree frog digests its food less completely than the hamster does B. The hamster is active at night, and moving about uses most of the food it eats C. The tree frog makes its own sugar, so it needs little food D. The hamster is an endotherm, so it respires food to make the heat that keeps its body warm

Question 4

Sockeye salmon feed in the ocean for two or three years and store fat. Then they swim up a river, eating nothing on the way, and breed once at the end of the journey.

Which of the following strategies is the salmon using to match its breeding to the energy on offer?

A. Switching from asexual to sexual reproduction as food falls B. Timing its mating so the young hatch when their food does C. Pausing breeding through a poor season, then breeding again D. Storing energy in one period and breeding on it later

Question 5

Five rows from the marsh survey: the species, the marsh, the date and the adults seen.

Ecologists survey two salt marshes and record the rows in the table below.

Which of the following groups of adults is one population?

A. The saltmarsh sparrows and the seaside sparrows seen in North Marsh on May 12 B. The saltmarsh sparrows and the river otters seen in North Marsh on May 12 C. The saltmarsh sparrows seen in North Marsh on May 12 D. The saltmarsh sparrows seen in North Marsh on May 12 and on August 12

Question 6

What entered and left the sealed sphere over one year.

A sealed glass sphere holds water, algae, small snails and bacteria. Nothing but light and heat can cross the glass. Over one year the ecologists who keep it measured what entered and left the sphere, in the table below, and its living things stayed alive.

Which of the following explains why the sphere needs 1 500 000 kJ of light every year but no new carbon atoms or nitrogen atoms?

A. Heat leaves at every transfer and cannot come back, while the same atoms go round between the water, the algae and the snails B. The algae use up light faster than they use up carbon atoms, so light must be replaced far more often than carbon C. Photosynthesis makes brand-new carbon atoms inside the sphere, but nothing inside the sphere can make new light D. The snails breathe out carbon atoms they make themselves, so the sphere always has a fresh supply of them

Question 7

Where the phosphate tracer was found three weeks after it was added to the pond.

Ecologists add a small amount of traceable phosphate to a fenced-off pond and, three weeks later, measure how much of the tracer sits in each part of the pond. The table gives the results.

Which of the following is the largest abiotic reservoir the tracer reached?

A. The algae B. The mud at the bottom C. The pond water D. The snails

Question 8

At dawn, drops of water form on a spider’s web from the water vapor in the air. By noon the web is dry.

In order, which two processes did the water in a drop pass through?

A. Precipitation, then evaporation B. Evaporation, then condensation C. Condensation, then transpiration D. Condensation, then evaporation

Question 9

A city closes its coal-burning power station and gets the same amount of electricity from a solar farm. Nothing else about the city changes.

Which of the following happens to the carbon dioxide in the air above the city over the following years, and why?

A. It falls, because the solar panels take carbon dioxide out of the air B. It stays the same, because the city uses the same amount of electricity C. It falls, because less fossil-fuel carbon leaves as carbon dioxide by combustion D. It rises, because the coal left in the ground decomposes

Question 10

A nitrogen atom in the protein of a dead bean leaf on the soil ends up, months later, in a molecule of nitrogen gas in the air.

In order, which steps of the nitrogen cycle carried the atom from the leaf to the air?

A. Nitrogen fixation, assimilation, denitrification B. Ammonification, nitrification, denitrification C. Nitrification, ammonification, denitrification D. Ammonification, assimilation, nitrogen fixation

Question 11

A grower raises lettuce with its roots in a tank of water and bubbles air through the tank. The air is about 78 % nitrogen gas. The lettuce turns yellow until the grower adds nitrate to the water.

Which of the following explains why the lettuce stayed short of nitrogen while the air bubbled through the tank?

A. No plant enzyme breaks the triple bond in nitrogen gas, so roots take up nitrogen only as ammonium or nitrate B. Nitrogen gas dissolves in water so poorly that none of it ever reached the lettuce’s roots in the tank C. The roots fix nitrogen gas into ammonia only when soil bacteria are present in the tank to help them D. Lettuce needs almost no nitrogen, so the yellowing came from too little light reaching the leaves

Question 12

For centuries a river’s floods have spread sediment, rich in phosphate, over the soil of its delta. A dam is built upstream, and the sediment now settles behind the dam instead. The delta’s farmers harvest their crops as before.

Over the following decades, what happens to the delta soil’s store of phosphate?

A. It rises, because the water held behind the dam weathers more rock and releases more phosphate B. It stays the same, because the phosphorus cycle has no reservoir in the air to lose phosphate to C. It falls, because the floods that carried phosphate into the soil have stopped while the harvest still carries it out D. It falls, because the phosphate in the delta’s soil slowly turns into a gas and leaves for the air

Question 13

What each flask received and the sugar the bacteria made from carbon dioxide.

Researchers grow bacteria from water deep underground in four flasks. Every flask holds carbon dioxide and the minerals the bacteria need, but no sugar. The table gives what each flask received and how much sugar the bacteria made from carbon dioxide.

Which of the following are the bacteria?

A. Photosynthetic autotrophs B. Chemosynthetic autotrophs C. Heterotrophs that eat the hydrogen sulfide D. Decomposers of the flasks’ dead matter

Question 14

Body temperatures of animals P and Q after two hours in each chamber.

Researchers keep two animals of the same mass, P and Q, for two hours in chambers at three air temperatures. The chambers stop the animals moving to a warmer or cooler spot. The table gives each animal’s body temperature at the end of the two hours.

Which kind of animal is P, and how would it hold its body temperature in the wild?

A. An endotherm; its own respiration makes the heat that holds its body at one temperature B. An ectotherm; it would move between sun and shade C. An ectotherm; its muscles would make heat when it moves D. An endotherm; it would sweat to keep its body cooler than the air

Question 15

On the deep sea floor, hagfish gather at the body of a whale that died and sank. They eat its flesh over many weeks.

Which kind of eater is a hagfish here?

A. A carnivore B. An omnivore C. A decomposer D. A scavenger

Question 16

A wolf kills and eats a moose. The moose’s meat is made of carbon compounds: protein, fat and a little carbohydrate.

Which of the following describes what happens to the moose’s carbon compounds inside the wolf?

A. The wolf’s cells respire all of them, and their carbon leaves the wolf as carbon dioxide B. The wolf’s cells build all of them into the wolf’s own muscle and fat C. The wolf’s cells respire some of them to make ATP and build the rest into the wolf’s own tissue D. The wolf’s cells break them down and rebuild the carbon compounds from carbon dioxide

Question 17

What each organism of the salt lake eats, from the survey.

A survey of a salt lake lists what each organism eats, in the table below. A student builds the lake’s food web from the table, drawing each arrow from the eaten organism to the organism that eats it.

Which of the following arrows is in the student’s web?

A. From the brine shrimp to the flamingos B. From the flamingos to the brine shrimp C. From the algae to the avocets D. From the brine fly larvae to the flamingos

Question 18

A class feeds 45 g of mulberry leaves, measured as dry mass, to silkworm larvae for a week. At the end of the week the larvae have gained 5 g, and the class has collected 16 g of droppings, both as dry mass.

Which of the following accounts for the other 24 g of leaf?

A. The larvae’s cells destroyed it as they used its energy for moving and growing B. The larvae respired it, so its carbon left as carbon dioxide and water and its energy left as heat C. It is still inside the larvae’s guts, undigested, waiting to pass out as droppings D. It became larva, and the class weighed the larvae wrongly at the end of the week

Question 19

The energy the estuary’s eelgrass stores in a normal year and in the year of extra nutrients.

An estuary’s food chain is eelgrass, then the snails that eat it, then the crabs that eat the snails, then the herons that eat the crabs. One year a flood carries extra nutrients into the estuary, and the eelgrass stores twice as much energy as usual. The table gives the two years.

By the 10 % rule, what happens to the number of herons the estuary can feed in the year of extra nutrients?

A. It stays the same, because the herons eat crabs, not eelgrass B. It falls, because the extra eelgrass shades the estuary’s water C. It rises by the whole extra amount, because every level above stores all the energy added at the base D. It rises, because about 10 % of the extra energy passes up at each level

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 · 4 points

Ecologists survey a tidal mudflat. Algae growing on the mud are eaten by mud snails, the mud snails are eaten by wading birds, and the wading birds are hunted by peregrine falcons. The table gives what the algae turned into sugar in a year and what they respired. The pyramid gives the energy stored in the mud snails’ level and in the wading birds’ level, both in kilojoules per square meter a year.

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

(a) Calculate the net primary productivity of the algae, in kilojoules per square meter a year. (1 point)

kJ per square meter a year

Write down the values in the question:

made = 13 500 kJ per square meter a year

respired = 5 400 kJ per square meter a year

Write down the equation:

net primary productivity = made - respired

Substitute the values into the equation and calculate:

net primary productivity = 13 500 - 5 400 = 8 100 kJ per square meter a year

A full-credit answer: The net primary productivity of the algae is 8 100 kJ per square meter a year.

(b) Calculate the transfer of energy between the algae and the mud snails as a percentage of the algae’s stored energy, to one decimal place. (1 point)

The mudflat’s energy pyramid: the algae’s slice unlabeled, the mud snails’ slice 891 kJ per square meter a year, the wading birds’ slice 71 kJ per square meter a year.

Write down the values:

mud snails = 891 kJ per square meter a year

algae = 8 100 kJ per square meter a year

Write down the equation:

transfer = \frac{energy in the level above}{energy in the level below} \times 100

Substitute the values into the equation and calculate:

transfer = \frac{891}{8 100} \times 100 = 11.0 %

A full-credit answer: The transfer between the algae and the mud snails is 11.0 %.

(c) A student says: “Exactly 10 % of the energy passes up at every step of this mudflat.” Use the data to support or refute the student’s claim. (1 point)

A full-credit answer: The data refute the claim.
The transfer between the algae and the mud snails is 11.0 %.
The transfer between the mud snails and the wading birds is 71 ÷ 891 × 100, about 8 %.
Neither step is exactly 10 %, though both sit inside the range ecologists measure, about 5 % to about 20 %.
So 10 % is a working rule, not an exact law of this mudflat.

Check the box for each point your answer earns

the judgement (the claim is not supported) AND the ground (at least one measured step is not 10 %: 11.0 % between the algae and the snails, or about 8 % between the snails and the birds). Accept with or without: both values sit inside the measured range of about 5 % to 20 %.

Common slip: Agreeing because the values are close to 10 %. The claim says exactly, and the data give 11.0 % and about 8 %.

(d) By the 10 % rule, explain why the mudflat is unlikely to sustain a population at a fifth trophic level, above the peregrine falcons. (1 point)

A full-credit answer: By the 10 % rule the peregrine falcons’ level stores about 10 % of 71, about 7 kJ per square meter a year.
A fifth level would store about 10 % of that, under 1 kJ per square meter a year.
A population needs energy every year to stay alive, grow and raise young.
So too little energy would be left to feed a population above the falcons.

Check the box for each point your answer earns

each level stores only about 10 % of the level below, so a fifth level would store under about 1 kJ per square meter a year (about 10 % of the falcons’ 7), too little to feed a population.

Common slip: Explaining the missing level by the lack of a hunter. The chain ends where too little energy is left, not where a hunter is missing.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

The food web below was drawn from a survey of an Arctic sea. Ice algae grow on the underside of the sea ice. In the web, each arrow is drawn from the eaten organism to the organism that eats it.

The Arctic sea’s food web, drawn from the survey.

(a) Identify the trophic level, or levels, at which the seabirds sit in this web. (1 point)

A full-credit answer: The seabirds sit at two levels: secondary consumer, by their copepod meals, and tertiary consumer, by their Arctic cod meals.

Check the box for each point your answer earns

secondary consumer (copepods) AND tertiary consumer (Arctic cod), the seabirds at two levels.

Common slip: Giving one level only. An organism with two foods at different levels sits at both.

(b) Suppose fishing boats remove almost all the Arctic cod. Explain why the ringed seals lose their food while the seabirds still have food. (1 point)

A full-credit answer: One arrow arrives at the ringed seals’ box, from the Arctic cod, so the cod are the seals’ only food in this web.
Two arrows arrive at the seabirds’ box, from the copepods and from the Arctic cod.
When the cod are removed, the seals have no other path, while the seabirds still have the copepods.

Check the box for each point your answer earns

the Arctic cod are the seals’ only food (one arrow arriving), while the seabirds have two foods (two arrows arriving), so the seabirds still have the copepods.

Common slip: Tracing the loss to one consumer only. Every path the removed organism fed must be followed.

(c) The cod recover. A company plans to feed a coastal town from this sea and must choose between harvesting Arctic cod and harvesting ringed seals. Predict which harvest feeds more people from the same sea. (1 point)

A full-credit answer: Harvesting the Arctic cod feeds more people.

Check the box for each point your answer earns

the Arctic cod.

Common slip: Choosing the seals because each seal is a larger animal. The size of one animal does not set how much energy its level stores.

(d) Justify your prediction using the 10 % rule. (1 point)

A full-credit answer: Each trophic level stores only about 10 % of the energy stored in the level below.
The ringed seals are one level above the Arctic cod, so the seals’ level stores about 10 % of the cod’s.
People eating cod are one level above the cod; people eating seals are two levels above the cod.
So people eating seals store about 10 % of what people eating cod would store, and the cod harvest feeds more people.

Check the box for each point your answer earns

the seals are an extra trophic level between the cod and the people, and each level stores only about 10 % of the level below, so eating cod directly leaves about ten times as much energy for the people.

Common slip: Saying seals are richer food. The 10 % rule counts the levels crossed, not the richness of the meal.

Free-response score: 0 of 4

Feedback and scoring guides appear after you submit.

Multiple choice checked: 0 of 19 correct.

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