End-of-topic test: Community Ecology

Unit 8 · Topic 8.5 end-of-topic test

Suggested time: about 56 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. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. 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. The formula sheet gives Simpson’s Diversity Index, Diversity Index = 1 − Σ(n/N)², where n is the total number of organisms of a particular species and N is the total number of organisms of all species.

Question 1

A biologist studies the living things on the wooden pilings of one wharf: wracks, which are seaweeds, dog whelks, which are sea snails, and the sea bass that hunt among them.

Which of the following groups is the pilings’ community?

A. All the dog whelks living on the pilings B. Every species living on the pilings, together with the pilings’ wood, the seawater and the sunlight C. The pilings’ wood, the seawater and the sunlight around them D. Every species living on the pilings: the wracks, the dog whelks and the sea bass

Question 2

The butterflies counted on the downland.

Suppose a student walks one stretch of downland, dry chalk grassland, at noon and counts every butterfly seen. The table below gives the count of each species.

Which of the following is the species composition of the downland’s butterflies?

A. 28 fritillaries, 12 hairstreaks and 5 skippers B. 45 butterflies counted in all C. Three species of butterfly D. Fritillaries, the most numerous species

Question 3

The count of each species of flowering plant in the two water meadows.

Suppose a botanist counts the flowering plants of the same five species in two water meadows, Meadow Q and Meadow U. The table below gives the count of each species in each meadow.

Which meadow has the greater species diversity, and why?

A. Meadow Q, because it holds more species than Meadow U B. Meadow Q, because its plants are shared more equally among the same five species C. Meadow U, because its most numerous species holds more plants than any species in Meadow Q D. Neither: the two meadows hold the same five species, so their species diversity is the same

Question 4

Four pie charts, Q, U, Y and Z, each claiming to show the levee’s counts.

Suppose a survey of the grass bank of a levee, a raised bank built to hold back a river’s floods, counts 60 plants of four species: 27 knapweed, 18 wild carrot, 9 chicory and 6 teasel. The four pie charts below, Q, U, Y and Z, each claim to show these counts.

Which pie chart shows the levee’s counts correctly?

A. Pie Q B. Pie U C. Pie Y D. Pie Z

Question 5

Suppose one species’ share of the birds counted on an airfield rises from 0.13 one year to 0.39 ten years later.

Which of the following gives that species’ squared share, ${(\frac{n}{N})}^{2}$ , at the start and ten years later?

A. 0.0169, then 0.0507 B. 0.0169, then 0.1521 C. 0.13, then 0.39 D. 0.26, then 0.78

Question 6

Suppose a survey of a saltpan’s edge counts 60 plants of three species, 24 of them samphire. A student works out Simpson’s Diversity Index for the plants from the formula sheet’s line.

Which of the following is N in that line, for this survey?

A. 0.4 B. 3 C. 24 D. 60

Question 7

Suppose two oxbow lakes, old river bends cut off from their river, hold the same five species of fish. The first lake’s Simpson’s Diversity Index is 0.78 and the second lake’s is 0.47.

Which of the following does the difference between the two values say about the first lake?

A. The first lake’s fish are shared more equally among the five species B. More species of fish live in the first lake than in the second C. One species holds most of the first lake’s fish D. The first lake is 78 % diverse, so it is the better place for fish

Question 8

A sundew is a bog plant with sticky hairs on its leaves. Small flies land on the leaves and stick fast, and the sundew digests them.

Which kind of interaction is this?

A. commensalism B. mutualism C. parasitism D. predation

Question 9

A bullfinch is a plump songbird with a stout, rounded bill and a pink breast. In one valley a student counts 40 bullfinches. Each spring they eat the buds of the valley’s fruit trees.

Which of the following is part of the bullfinch’s niche?

A. Its stout, rounded bill B. The buds of the fruit trees it eats C. The count of 40 bullfinches in the valley D. Its pink breast feathers

Question 10

Suppose two species of parakeet nest in one wood, and each species nests only in the hollows of the wood’s old trees. The wood holds few hollows. Whenever both species want one hollow, the larger species drives the smaller species out, so the larger species’ pairs take nearly all the hollows each spring.

Predict what happens to the count of the smaller species over the following years.

A. It falls to zero B. It levels off at half the wood’s carrying capacity C. It levels off at the same count as the larger species’ count D. It rises to the wood’s carrying capacity

Question 11

Suppose two species of gurnard, bottom-living fish, hunt the same small crustaceans in one bay. One species hunts over the sand at about 10 m deep, the other over the sand at about 40 m deep, and both species persist year after year.

Which of the following names the way the two species of gurnard share the bay?

A. competitive exclusion B. mutualism C. niche partitioning D. predation

Question 12

The ragworms per square of mud and the whimbrels counted on the shore, years 1 to 10.

Suppose ragworms, which live in the mud of a shore, and the whimbrels, wading birds that eat them, are counted on one stretch of shore each autumn for ten years. The graph below plots the ragworms per square of mud against the left y-axis and the whimbrels against the right y-axis.

About how many whimbrels were counted in year 7?

A. 25 whimbrels B. 30 whimbrels C. 500 whimbrels D. 620 whimbrels

Question 13

Suppose ling, large fish of deep water, hunt Norway pout, small fish, and both are counted off one stretch of coast every year for twenty years. In one year the Norway pout count reaches its highest in ten years.

Over the next year, which of the following does the ling count do?

A. The ling count falls B. The ling count falls, then climbs C. The ling count stays level D. The ling count climbs

Question 14

Suppose bleak, small river fish, and the burbot that hunt them are counted in one river every year for twelve years. The bleak counts lie between 320 and 2 800 bleak. The burbot counts lie between 9 and 58 burbot. A student plots both series against the years, the bleak against a left y-axis and the burbot against a right y-axis.

Which of the following pairs of scales fits the two y-axes?

A. Left 0 to 30 000, a gridline every 5 000; right 0 to 60, a gridline every 10 B. Left 0 to 3 000, a gridline every 500; right 0 to 60, a gridline every 10 C. Left 0 to 3 000, a gridline every 500; right 0 to 600, a gridline every 100 D. Left 0 to 3 000, a gridline every 500; right 0 to 3 000, a gridline every 500

Question 15

Suppose a disease killed off the serotines, large bats, of one valley, and now serotines return to the valley. Serotines eat chafers, large beetles, and chafer grubs eat the roots of the valley’s grass.

Which of the following happens to the valley’s grass over the following years?

A. More grass grows B. Less grass grows C. The grass is unchanged D. More grass grows at first, then the grass returns to the amount it had before

Question 16

Four students’ drawings, Q, U, Y and Z, of the horseflies and the gemsbok.

Horseflies bite gemsbok, large antelope, and drink their blood; the gemsbok lose blood and live on. Four students each drew the two populations as circles joined by two arrows, one each way, in drawings Q, U, Y and Z below. The sign at an arrow’s head is the effect on the population the arrow ends on.

Which drawing represents the interaction correctly?

A. Drawing Q B. Drawing U C. Drawing Y D. Drawing Z

Question 17

The mean number of plant species per square on the four stretches of seawall, with ±2SE error bars.

Suppose four stretches of one seawall were built 5, 20, 50 and 100 years ago. On each stretch a student counts the plant species in ten squares of 1 m². The graph below shows each stretch’s mean number of plant species per square, with ±2SE error bars; the two ends of each error bar are printed beside it.

Which of the following pairs of stretches do the data show to differ in their mean number of plant species?

A. The 5-year-old and 50-year-old stretches only B. The 20-, 50- and 100-year-old stretches, each with each of the other two C. The 5-year-old stretch and each of the other three stretches D. Every pair of stretches

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

Suppose a student studies two species of grain beetle, species Q and species U, whose larvae eat stored grain. In the first treatment the student puts 10 adults of species Q into one jar of cracked wheat and 10 adults of species U into a second jar of the same size with the same amount of cracked wheat. In the second treatment the student puts 5 adults of each species into a third jar of the same size with the same amount of cracked wheat. The jars are kept in the same room, and the adults of each species are counted every four weeks. The table below gives the counts.

The adults of each species counted every four weeks: each species alone, and the two species together.

(a) Identify the species that is excluded when the student keeps the two species together in one jar. (1 point)

A full-credit answer: Species U.

Check the box for each point your answer earns

species U (accept: the species whose count falls to 0 in the shared jar).

Common slip: Naming species Q because its count is lower together than alone. Species Q still climbs in the shared jar; species U falls to 0.

(b) The student claims that in the third jar each species takes cracked wheat the other species needs. Support the claim with evidence from the table. (1 point)

A full-credit answer: Species Q reaches a lower count with species U than alone: 420 adults by week 24 in the shared jar against 500 alone.

Check the box for each point your answer earns

any ONE of the following: species Q grows more slowly together than alone (80 against 190 adults at week 8); species Q reaches a lower count together than alone (420 against 500 adults by week 24); species U grows more slowly together than alone (45 against 120 adults at week 8); species U reaches a lower count together than alone, or dies out together (0 against 300 adults by week 24).

The evidence must compare the same species alone and together.

Common slip: Quoting the counts of one jar alone. Evidence for the claim compares each species alone with the same species together.

(c) Identify the sign, a plus (+), a minus (−) or a zero (0), that each species’ population gets from the other in the third jar. (1 point)

A full-credit answer: Species Q a minus (−) and species U a minus (−): each species loses to the other.

Check the box for each point your answer earns

a minus for both species (competition harms both populations).

Common slip: Giving species Q a plus because it wins. Species Q reaches 420 with species U present against 500 alone: it loses too.

(d) Explain why, by week 24, the third jar holds adults of one species only, although each species thrives alone. (1 point)

A full-credit answer: Both species’ larvae eat the same cracked wheat in one jar, so the two species have the same niche and compete for every grain.
The better competitor eats more of the wheat.
Each larva of the other species gets less food than it needs, so fewer of them survive to breed and more die.
So the other species’ count falls until none is left: competitive exclusion.

Check the box for each point your answer earns

the two species have the same niche (the same food in the same jar), so the better competitor takes more of the food and the other species gets too little (fewer breed or more die) and its count falls to zero (accept the outcome described without the name competitive exclusion).

Common slip: Saying one species eats the other. Neither species eats the other; both eat the wheat, and one takes more of it.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

Suppose bush dogs, small wild dogs of the South American jungle, hunt pacas, large rodents, in one jungle reserve. Pacas eat the seeds that fall from the reserve’s trees, so where pacas are many, few seeds are left to sprout into seedlings. Both animals are counted every year for twenty years. The graph below plots the pacas against the left y-axis and the bush dogs against the right y-axis.

The pacas and the bush dogs counted in the reserve, years 0 to 20.

(a) Describe the pattern in the timing of the bush dog peaks relative to the paca peaks. (1 point)

A full-credit answer: Each bush dog peak comes about two years after a paca peak: the pacas peak in years 5 and 14, and the bush dogs in years 7 and 16.

Check the box for each point your answer earns

the bush dog peaks come after the paca peaks (accept: about two years after; the bush dog count lags the paca count).

Common slip: Saying the two counts peak in the same years. The dashed bush dog line peaks two gridlines to the right of each solid paca peak.

(b) Explain how the paca count and the bush dog count affect each other over one rise and fall. (1 point)

A full-credit answer: More pacas mean more food for the bush dogs.
So more bush dogs are fed and survive to breed, and the bush dog count climbs.
Many bush dogs kill many pacas.
So the paca count falls.

Check the box for each point your answer earns

more pacas feed more bush dogs, so the bush dog count climbs, AND many bush dogs then kill many pacas, so the paca count falls (both links, each with its direction).

Common slip: Stopping at ‘the bush dogs have more food’. The point wants the bush dog count’s direction and then the paca count’s.

(c) Suppose trappers remove every bush dog from the reserve. The drawing below shows the three populations as a chain, with the arrows drawn but no signs. The sign at an arrow’s head is the effect on the population the arrow ends on. Predict what happens to the number of tree seedlings in the reserve over the following years. (1 point)

The three populations drawn as a chain, with the arrows but no signs.

A full-credit answer: Fewer tree seedlings sprout.

Check the box for each point your answer earns

fewer seedlings (the number of seedlings falls; the direction must be stated). Do not award: ‘the seedlings will be affected’ with no direction.

Common slip: Predicting more seedlings because the bush dogs are gone. The bush dogs never ate seedlings; the change reaches the seedlings through the pacas.

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

A full-credit answer: With the bush dogs gone, fewer pacas are killed, so the paca count rises.
More pacas eat more of the fallen seeds.
So fewer seeds are left to sprout, and fewer seedlings grow.

Check the box for each point your answer earns

fewer pacas are killed so the paca count rises, AND more pacas eat more of the fallen seeds so fewer seedlings sprout (both links, each with its direction).

Accept the two links written as the signs at the arrow heads, a minus at the pacas and a minus at the seedlings, with the direction of each change stated.

Accept reasoning consistent with a wrong prediction in (c): award the point only for two links, each with its direction, that lead to the direction the student predicted.

Common slip: Justifying with the bush dogs alone. The point wants the paca link and the seed link, each with its direction.

Free-response score: 0 of 4

Free response 3 · Analyze Data · 4 points

Suppose a student surveys the fish in two sluice pools, pools held back by the gates of one drain: Pool Q and Pool U. Both pools hold the same four species of fish. The table below gives Pool Q’s counts. Pool U’s Simpson’s Diversity Index is 0.37.

Pool Q’s fish counts, one row per species.

(a) Calculate the squared share, ${(\frac{n}{N})}^{2}$ , of Pool Q’s most numerous species, to four decimal places. (1 point)

Write down the values in the question:

n = 116

N = 200

Write down the equation:

share = \frac{n}{N}

Substitute the values into the equation, and calculate:

share = \frac{116}{200}

share = 0.58

Square the share, and calculate:

{(\frac{n}{N})}^{2} = 0.58 \times 0.58

{(\frac{n}{N})}^{2} = 0.3364 (no unit)

A full-credit answer: The squared share is 0.3364.

(b) Calculate Simpson’s Diversity Index for Pool Q, to two decimal places. (1 point)

Write down the values in the question:

n = 116, 48, 22, 14

N = 200

Write down the equation:

Diversity Index = 1 - \sum {(\frac{n}{N})}^{2}

Square each species’ share, one line per species:

{(\frac{116}{200})}^{2} = 0.58 \times 0.58 = 0.3364

{(\frac{48}{200})}^{2} = 0.24 \times 0.24 = 0.0576

{(\frac{22}{200})}^{2} = 0.11 \times 0.11 = 0.0121

{(\frac{14}{200})}^{2} = 0.07 \times 0.07 = 0.0049

Add the squared shares up:

0.3364 + 0.0576 + 0.0121 + 0.0049 = 0.411

Substitute the values into the equation, and calculate:

Diversity Index = 1 - 0.411

Diversity Index = 0.59 (no unit)

A full-credit answer: Simpson’s Diversity Index for Pool Q is 0.59.

(c) Describe what the difference between the two pools’ indices says about how Pool U’s fish are shared among its four species. (1 point)

A full-credit answer: Pool U’s index is lower than Pool Q’s, and both pools hold the same four species.
So Pool U’s fish are shared less equally among the four species: one species holds most of Pool U’s fish.

Check the box for each point your answer earns

Pool U’s fish are shared less equally (accept: one species dominates Pool U; Pool U has the smaller species evenness). Accept a description consistent with the index calculated in part (b), provided it is compared with 0.37.

Common slip: Saying Pool U holds fewer species. Both pools hold the same four species; with the same richness a lower index comes from less equal shares.

(d) A student says: “Pool U’s index of 0.37 means that 37 % of Pool U’s fish belong to one species.” Evaluate the student’s claim. (1 point)

A full-credit answer: The claim is wrong.
The index is 1 minus the sum of every species’ squared share.
So 0.37 is a plain number with no unit, worked from every species’ share; it is not the share of any one species and not a percentage.

Check the box for each point your answer earns

the judgement (the claim is wrong) AND the ground (the index is 1 minus the sum of the squared shares of every species — a plain number, not one species’ share or a percentage).

Accept as the ground: the index combines every species’ share, so one species’ share is not read from it.

Common slip: Judging the claim right because 0.37 looks like 37 %. The index is a plain number with no unit; no single share can be read from it.

Free-response score: 0 of 4

Feedback and scoring guides appear after you submit.

Multiple choice checked: 0 of 17 correct.

Next: Bouncing back