Practice questions · Topic 8.7

Unit 8 · Practice for the Topic 8.7 end-of-topic test

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 case 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. Every case that opens with ‘Suppose’ is imagined for the question.

Question 1

Suppose a kind of figwort, a wayside herb, carries an allele N. Plants with one copy of N survive a smut, a disease that destroys the flowers, and set the most seed. Plants with two copies of N grow poorly and set few seeds. The frequency of N has stayed level for many generations. Then the smut disappears from the region.

Which of the following describes the frequency of N over the generations that follow?

A. The frequency falls by the same amount every generation B. The frequency falls fast, then more slowly C. The frequency falls to zero in one generation D. The frequency stays level

Question 2

Suppose a kind of St John’s wort, a yellow-flowered plant from another continent, spreads across a dry hillside where nothing eats it. At home, the caterpillars of one kind of moth eat its leaves, and its patches stay small.

Which of the following changes would slow the spread of the St John’s wort most?

A. Clearing the native grasses from around each patch of St John’s wort B. Watering the hillside through the dry months C. Fencing the hillside so that nothing tramples it D. Releasing that moth onto the hillside

Question 3

Suppose a metal that living things cannot break down or excrete washes into a lake. Tiny floating animals eat the lake’s tiny floating plants, small fish eat the tiny animals, and a kind of fish-eating bird eats the small fish. At each step up the chain the metal’s concentration is ten times higher, and each level’s living tissue, all together, is about 10 % of the level below it.

Which level contains the most of the metal in total?

A. The four levels contain about the same total each B. The fish-eating birds, at the top of the chain C. The tiny floating plants, at the bottom of the chain D. The small fish, in the middle of the chain

Question 4

Suppose a town’s drains carried its waste straight into the river downstream of the town. Every August that stretch of the river turned green, and dead fish floated at its edge. The town builds a treatment works that removes the nutrients from the waste before the water reaches the river.

Predict what happens to the dissolved oxygen in that stretch of river in the Augusts that follow.

A. The dissolved oxygen falls B. The dissolved oxygen stays the same C. The dissolved oxygen rises D. The dissolved oxygen swings up and down from one August to the next

Question 5

Suppose a kind of white-eye, a small bird that eats ripe fruit in the treetops, is brought to an island in cages, escapes, and spreads through the island’s woods.

Over the following years, which of the island’s native birds falls in number?

A. A kind of fantail that eats insects in the treetops B. A kind of pitta that eats worms on the ground C. A kind of parrot that eats ripe fruit in the treetops D. A kind of tropicbird that eats fish at sea

Question 6

Suppose a kind of alpine poppy grows only on the top 200 meters of a mountain, where the summer air is cool enough for its seeds to ripen. Lower down the slope the summer air is too warm for them. Over the next century the region’s climate warms by 3 °C.

Which of the following happens to the poppy’s range on the mountain?

A. The range moves down the slope B. The range stays where it is C. The range moves 200 meters up the slope D. The range shrinks toward nothing

Question 7

The yearly percent change in the dipper count on the sixteen stretches, drawn as one box-and-whisker plot.

Suppose dippers, small birds that feed in fast streams, are counted each spring on sixteen stretches of one river, and each stretch’s yearly percent change is worked out. The plot below shows the sixteen changes.

What is the smallest number of the sixteen stretches that can have lost dippers over the year?

A. 4 B. 8 C. 12 D. 16

Question 8

Which of the following is a dead zone?

A. Water colored green by algae that have multiplied B. Water whose oxygen concentration has fallen too low for its animals to live C. Water poisoned by a metal that living things cannot excrete D. Water where a newcomer has eaten every native fish

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

Suppose a fish farm moors its cages in a firth, a wide arm of the sea. The waste feed and the fish’s droppings sink into the water. Each summer, divers count the scallops, shellfish that live on the sea floor, on ten patches near the cages and on ten patches at the firth’s mouth, and work out each patch’s yearly percent change.

(a) Identify the nutrients that the waste feed and the droppings release into the firth’s water. (1 point)

Hint: Fertilizer and a town’s waste set off the same chain; think about what the feed and the droppings carry into the water.

A full-credit answer: The fish farm adds nitrate and phosphate to the firth’s water.
The waste feed and the droppings release those nutrients as they rot.

Check the box for each point your answer earns

nitrate and phosphate (accept: nitrogen and phosphorus compounds; accept one of the two named). Do not award for: ‘waste’, ‘feed’, ‘droppings’ or ‘nutrients’ alone.

Common slip: Writing ‘the waste’ or ‘the feed’. The feed and the droppings rot, and what they release into the water is the nitrate and phosphate.

(b) Predict, with a direction word, what happens to the scallop count near the cages, and give the reason as a chain. (1 point)

Hint: Follow the chain from the added nutrients to the oxygen the scallops need.

A full-credit answer: Near the cages the scallop count falls.
The added nutrients feed the firth’s algae, so the algae multiply near the cages.
The algae die and sink, and decomposers respire them.
So the dissolved oxygen near the sea floor falls, and the scallops suffocate.

Check the box for each point your answer earns

the count falls, with the chain — the nutrients feed the algae, the algae multiply and die, decomposers respire them and use up the dissolved oxygen, the scallops suffocate (accept a chain that omits the algae dying but names the decomposers using the oxygen; accept: decomposers respire the waste feed and droppings themselves and use up the dissolved oxygen, with no algae step). A prediction that names no direction earns nothing.

Common slip: Stopping at ‘the waste poisons the scallops’. The chain passes through the algae and the decomposers that use up the oxygen.

(c) State the null hypothesis for the divers’ survey. (1 point)

Hint: The null hypothesis names the thing changed and the thing measured. What differs between the two groups of patches, and what did the divers measure?

A full-credit answer: Whether a patch lies near the cages has no effect on the yearly percent change in the scallop count.

Check the box for each point your answer earns

a null hypothesis that names the thing changed (whether the patch lies near the cages) and the thing measured (the yearly percent change in the scallop count) and claims no effect. Do not award for: a sentence that predicts a direction; a sentence that promises equal counts.

Common slip: Writing ‘the cages lower the scallop count’. That sentence claims an effect with a direction: the alternative hypothesis, not the null.

(d) The plots below show the two groups of patches after eight years. Give the verdict on the null hypothesis, using the plots. (1 point)

Hint: Read the line across each box, then ask whether the two sit together or apart.

The yearly percent change in the scallop count on the patches near the cages and on the patches at the firth’s mouth.

A full-credit answer: The two medians sit far apart: −6 % near the cages and +1 % at the mouth.
Medians that far apart do not fit a claim of no effect.
So the verdict is reject the null hypothesis.

Check the box for each point your answer earns

reject the null hypothesis, because the two medians sit apart (−6 % near the cages against +1 % at the mouth).

Common slip: Writing ‘the null hypothesis is proved wrong’. Data never prove; the verdicts are reject and fail to reject.

(e) The farm claims that its cages have left the firth’s scallops unharmed. Evaluate the farm’s claim, using the plots. (1 point)

Hint: A verdict on a claim is said with the median that shows it.

A full-credit answer: The claim is not supported.
Near the cages the median yearly change is −6 %, so at least half of those patches lost 6 % or more of their scallops a year.
At the mouth the median is +1 %, so at least half of those patches gained scallops.

Check the box for each point your answer earns

the judgement (not supported) AND the median that shows it (−6 % near the cages; accept a comparison with the +1 % at the mouth). Do not award for: a judgement with no median; ‘proved’.

Common slip: Judging the claim supported because one patch near the cages gained scallops. The median, not the best patch, carries the verdict.

Free-response score: 0 of 5

Free response 2 · Scientific Investigation · 4 points

Suppose a kind of water lettuce, a floating plant from another continent, spreads across a country’s ponds and covers their surfaces. Scientists want to know whether native water plants can slow its spread. In tanks kept at 18 °C, they grow the water lettuce with each of four kinds of native water plant. In a second set of tanks kept at 24 °C, they grow the same combinations. After 10 weeks they measure the mass of water lettuce in each tank.

(a) Describe the effect the spread of the water lettuce has on the species diversity of the ponds it covers. (1 point)

A full-credit answer: The species diversity of the ponds falls.
The water lettuce covers the surface, so the natives beneath it get no light and die out, and each species lost is one species fewer.

Check the box for each point your answer earns

species diversity falls (fewer species), because the natives beneath the cover lose light and die out (accept: the natives lose resources to the newcomer). A direction must be stated.

Common slip: Writing that the ponds are ‘affected’. The point wants the direction: species diversity falls.

(b) Identify a control group the scientists should include in their experiment. (1 point)

A full-credit answer: Tanks holding water lettuce alone, with no native plant, at each temperature.

Check the box for each point your answer earns

water lettuce grown alone (with no native plant); accept at one temperature or at both.

Common slip: Naming the cool tanks as the control. Both sets of tanks hold native plants; the control grows the water lettuce alone.

(c) State the null hypothesis of the experiment in which the water lettuce is grown with native water plants. (1 point)

A full-credit answer: Growing the water lettuce with native water plants has no effect on the mass of water lettuce after ten weeks.

Check the box for each point your answer earns

a null hypothesis naming the thing changed (whether native plants grow with it) and the thing measured (the mass of water lettuce) and claiming no effect. Do not award for: a sentence that predicts a direction.

Common slip: Writing ‘the native plants slow the water lettuce’. That is an alternative hypothesis, with a direction.

(d) At 24 °C the water lettuce grows twice as fast as at 18 °C, while the native plants grow at the same rate at both temperatures. The scientists claim that warmer summers will increase the spread of the water lettuce across the ponds. Justify the scientists’ claim. (1 point)

A full-credit answer: At 24 °C the water lettuce grows twice as fast, and the natives grow no faster.
So in a warmer summer the water lettuce takes light and space from the natives faster than they can hold it.
So it covers more of each pond and spreads to more ponds.

Check the box for each point your answer earns

warmer water speeds the water lettuce’s growth but not the natives’, so the newcomer outgrows the natives and takes their light and space faster (the reasoning names the competition, never the warmth alone).

Common slip: Writing ‘warm water helps plants grow’. The point is the difference: the water lettuce speeds up and the natives do not.

Free-response score: 0 of 4

Multiple choice checked: 0 of 8 correct.

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