Practice questions · Topic 8.6

Unit 8 · Practice for the Topic 8.6 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: Biodiversity, summed up

Video coming soon

Resilience, read from the return toward the former state; more species and more variety, more paths for the energy; the one-crop field and monocropping; the keystone species and what happens when it goes; the producers, light, water and mineral nutrients, and the pollinators as the supports; the short-term and the long-term change, two predictions with their own reasons.

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

Each flat’s plant cover in the five years after the flood.

Suppose a winter flood buries two river flats, Mill Flat and Weir Flat, under gravel and strips their plants. Before the flood, plants covered the same share of each flat. The graph below shows each flat’s plant cover in the five years after the flood.

Which of the following describes Weir Flat in year 5?

A. Weir Flat is back at its cover before the flood B. Weir Flat is below its cover before the flood and still climbing C. Weir Flat is below its cover before the flood and no longer climbing D. Weir Flat is above its cover before the flood

Question 2

Suppose a few dozen whooper swans, large wintering wildfowl, graze a fast-growing stonewort, a plant-like alga, on the bed of a shallow lake each winter, and many kinds of smaller water plant grow between the grazed patches. The swans stop coming for eight winters, and then a flock returns and stays.

Which of the following happens on the lake bed over the winters after the swans return?

A. The stonewort covers less of the lake bed B. The stonewort covers more of the lake bed C. The stonewort covers the same share of the lake bed as before the swans returned D. The stonewort dies out

Question 3

Suppose two flooded brick pits, Kelby Pit and Firsby Pit, hold the same 23 species of plant and animal. Kelby Pit holds about three times as many individual plants and animals as Firsby Pit. A disease that kills one species of water plant reaches both pits.

Predict which pit’s community returns to its former state first.

A. Kelby Pit returns first B. Firsby Pit returns first C. Both pits return at about the same time D. Neither pit returns to its former state

Question 4

The four places: natural or planted, plant species before the dry summer, and plant cover two years after it as a percent of the cover before.

Suppose a dry summer kills much of the plant cover in four places. Two of the places are natural, and two were planted by people. The table below gives each place’s number of plant species before the summer and its plant cover two years later, as a percent of its cover before.

Which of the following conclusions does the table support?

A. The planted places recovered more than the natural places B. The natural places recovered more than the planted places C. The places recovered by the same share whatever their number of species D. The places with fewer species recovered less, whether natural or planted

Question 5

In which of the following cases is the named species a keystone species?

A. A river’s top predator, whose loss changed the count of the one fish it ate and nothing else B. A rare predator whose loss let the shellfish it ate fill a sandy bay and most of the bay’s other species disappear C. The most abundant beetle in a wood, whose loss left the wood’s other species as they were D. A pond’s algae, which capture most of the energy the pond’s animals live on

Question 6

Suppose a few dozen whooper swans, large wintering wildfowl, graze a fast-growing stonewort, a plant-like alga, on the bed of a shallow lake each winter, and many kinds of smaller water plant grow between the grazed patches, with the water insects that live on them. Now imagine the swans stop coming.

Which of the lake bed’s species are affected over the following years?

A. The stonewort, the smaller water plants and the insects that live on them B. The stonewort that the swans grazed only C. The stonewort and the smaller water plants between the patches only D. The insects that live on the smaller water plants only

Question 7

Suppose a disease kills every plant on a cliff ledge in one summer. The ledge’s insects eat its plants, and the ledge’s small birds eat those insects.

Which of the ledge’s animals lose their food?

A. The insects only B. The small birds only C. None of the ledge’s animals D. The insects and the small birds

Question 8

Suppose road builders divert the spring that fed a hillside flush, a patch of ground kept wet by a spring, so the flush dries out. A student is asked to predict the change in the flush’s number of plant species over the following years.

Which of the following is a prediction that names a direction?

A. The flush’s plants are affected by the loss of the spring B. The number of plant species in the flush falls C. The number of plant species in the flush changes D. The flush is a different place after the spring is diverted

Question 9

Which of the following observations would show that a species is the keystone species of its community?

A. Removing the species from a plot leaves far fewer species there than any other removal leaves B. The species has more individuals in the community than any other species has C. The species captures more of the community’s energy than any other species does D. The species is the community’s top predator

Question 10

Which of the following describes a pollinator?

A. A plant that makes pollen B. An animal that carries pollen from flower to flower C. An animal that spreads a plant’s seeds D. An insect that eats pollen

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 a Model or Visual Representation · 5 points

Suppose a small wood of aspens and hornbeams holds 38 species. Psyllids, tiny sap-sucking insects, feed only on the aspens. Inchworms, the larvae of one day-flying moth, eat the leaves of both trees. Swifts catch the adult psyllids and the inchworm moths in the air. The food web below shows these five species. A disease kills every aspen in the wood within two summers.

Five of the wood’s species drawn as a food web, with the aspens and every arrow leaving them faded.

(a) State the number of unfaded arrows that arrive at the psyllids after the aspens die. (1 point)

Hint: Count the arrows into the psyllids box, then check where each one starts.

A full-credit answer: After the aspens die, no unfaded arrow arrives at the psyllids.

Check the box for each point your answer earns

none (the psyllids’ one arrow came from the aspens).

Common slip: Counting the arrow from the aspens as still there. That arrow is faded with the aspens.

(b) Predict the short-term change in the number of psyllids in the wood. (1 point)

Hint: The psyllids fed on the aspens alone; think about an eater whose one path is gone.

A full-credit answer: In the first year, the number of psyllids falls.

Check the box for each point your answer earns

the psyllids decrease (fall). A prediction that names no direction earns nothing.

Common slip: Writing that the psyllids are affected. The point wants the direction: they fall.

(c) Predict the long-term change in the number of species in the wood. (1 point)

Hint: Count what the wood has lost: a producer, and any eater that depended on it alone.

A full-credit answer: Over the following years, the number of species in the wood falls.

Check the box for each point your answer earns

the number of species decreases (falls). A prediction that names no direction earns nothing.

Common slip: Predicting that the number of species stays at 38 because only the aspens died. The eaters that fed on the aspens alone go too.

(d) Justify your prediction in part (c). (1 point)

Hint: Name the part lost, the eater that lost its only path, and what the swifts have left to eat.

A full-credit answer: The wood lost the aspens, one of its two producers, so the psyllids, which fed only on the aspens, lost their only path and died out.
The swifts lost one of their two paths and kept the other, so they stay but with less food.
So the wood holds fewer species than before.

Check the box for each point your answer earns

the evidence (the aspens, a producer, are gone) and the reasoning (the psyllids’ only path is gone, so they are lost; the swifts keep one path) that lead to fewer species.

Accept reasoning consistent with a wrong prediction in part (c): award the point only for a chain, each step with its direction, that leads to the direction the student predicted.

Common slip: Writing ‘the wood lost a species, so it has fewer species.’ The reasoning names the chain: the aspens, the psyllids’ only path, the swifts’ paths.

(e) Explain how the loss of the aspens changes how well the wood recovers from the next change. (1 point)

Hint: Think about what the swifts eat now, and what they would eat if the inchworms failed too.

A full-credit answer: After the aspens died, the wood’s energy has fewer other paths when the next species fails.
The swifts now eat inchworm moths alone, so if a cold spring kills the inchworms the swifts have nothing left.
So the wood returns toward its former state more slowly, or not at all.

Check the box for each point your answer earns

fewer species give the energy fewer other paths when the next species fails (accept: the swifts have one path left, so one more loss leaves them nothing), so the wood recovers more slowly.

Common slip: Writing that the wood is less resilient because it is smaller. The point is the paths: the swifts are down to one.

Free-response score: 0 of 5

Free response 2 · Analyze a Model or Visual Representation · 4 points

Suppose two dune slacks, damp hollows among sand dunes, lie a kilometer apart. The food webs below show the species of each slack and what eats what: harvester ants gather seeds, pollen beetles feed on the flowers, and the moths’ larvae eat silverweed leaves. A disease kills every silverweed plant in both slacks in one summer.

The two slacks’ food webs, with the silverweed and every arrow leaving it faded in each.

(a) Describe the difference between the two slacks’ webs in the number of paths energy could take to the harvester ants before the disease. (1 point)

A full-credit answer: In Ranby Slack three arrows arrive at the harvester ants, one from each producer.
In Saxby Slack one arrow arrives at the harvester ants, from the silverweed.

Check the box for each point your answer earns

three paths (arrows) to Ranby Slack’s harvester ants against one to Saxby Slack’s (accept: two paths against none after the silverweed died).

Common slip: Counting the arrows into the moths. The task asks about the harvester ants.

(b) Predict what happens to the number of harvester ants in Saxby Slack over the following years. (1 point)

A full-credit answer: The number of harvester ants in Saxby Slack falls.

Check the box for each point your answer earns

the harvester ants decrease (fall). A prediction that names no direction earns nothing.

Common slip: Predicting that the harvester ants are affected. The point wants the direction: they fall.

(c) Explain how the loss of the silverweed affects the community of each slack. (1 point)

A full-credit answer: In both slacks the moths’ larvae lose their only food, and the moths go.
In Ranby Slack the harvester ants still eat centaury and dune-grass seeds, so the harvester ants stay and the slack loses one animal species.
In Saxby Slack the harvester ants ate silverweed seeds alone, so the harvester ants go too, and the slack loses every animal species it had.

Check the box for each point your answer earns

Ranby Slack’s harvester ants (and pollen beetles) have other producers to eat, so only the silverweed and the moths are lost, while Saxby Slack’s harvester ants had the silverweed alone, so its animals are all lost (the other paths named in the case’s words).

Common slip: Saying Saxby Slack is hit harder because it had fewer plants. The point is the paths: Saxby Slack’s harvester ants had one path, Ranby Slack’s had three.

(d) A student says: “A slack keeps its harvester ants after the disease only if its silverweed carried many alleles.” Evaluate the student’s claim. (1 point)

A full-credit answer: The claim is wrong.
Every silverweed plant in both slacks died, whatever alleles it carried.
Ranby Slack keeps its harvester ants because its centaury and dune grass still feed them: a difference in species diversity, not in genetic diversity.

Check the box for each point your answer earns

the judgement (the claim is wrong) AND the ground (the slacks differ in their number of producer species and paths — species diversity; the silverweed’s alleles decided nothing once every plant died). Accept the ground as: Ranby Slack’s harvester ants have two other seeds to eat.

Common slip: Judging the claim right because more alleles mean more resilience. Every silverweed plant died in both slacks; what saved Ranby Slack’s harvester ants was its other producer species.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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