Unit 8 test

Unit 8 · Unit 8 end-of-unit test (also the test-out)

Suggested time: about 80 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

Answer every question, then press Submit the test. Feedback and the scoring guides appear after you submit. For the three 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. After you submit, mark your own free-response work against each scoring guide. Every case that opens with ‘Suppose’ is imagined for the question.

Question 1

Suppose a student tips 20 small soil animals of one kind into the middle of a dish and shines a lamp on one end of it.

Which of the following observations is a taxis?

A. The animals move faster in the light and slow down in the dark, in no set direction B. The animals turn more often in the light than in the dark C. The animals curl up where they are when the lamp comes on D. The animals turn away from the lamp and move straight to the dark end

Question 2

The snails counted in each section of the tray after ten minutes.

Suppose a student sets up a tray with four sections of sand: dry, slightly damp, damp and wet. She tips 40 small snails of one kind into the middle. The null hypothesis is that the snails show no preference among the sections. After ten minutes she counts the snails in each section, as the table shows.

Which of the following is the chi-square value for these counts?

A. 1.6 B. 7.4 C. 18.5 D. 74

Question 3

Suppose a kind of babbler, a small bird, lives in groups. Some young adults spend a year helping to feed chicks instead of raising chicks of their own. Biologists propose that helping spreads the helper’s own alleles.

Which of the following observations would best support the biologists’ explanation?

A. Helpers feed their own parents’ chicks far more often than other pairs’ chicks B. Helpers feed the chicks of whichever nest is nearest to them C. Helpers feed the chicks of the pair that holds the best territory in the group D. Helpers feed the chicks as often as the parents do

Question 4

Suppose a grower raises a kind of cosmos, a garden flower, indoors under lamps that stay on for 16 hours a day. The plants grow tall and leafy but never flower. The grower then leaves the lamps on for 10 hours a day, at the same room temperature, and the plants flower within three weeks.

Which of the following explains why the plants flowered?

A. Fewer hours of light let the plants store more sugar for flowers B. The plants grew toward the lamps and reached them C. The plants time their flowering by the length of the night D. The air was cooler with the lamps on for fewer hours, and the cold set off flowering

Question 5

Suppose the males of a kind of ortolan, a small bird, each sing one song. A biologist wants to know whether the song is learned.

Which of the following observations would show that the song is learned?

A. Every adult male in the population sings the same song B. Chicks begin to sing at three months old, whichever nest they hatched in C. Males sing more often in spring than in winter D. Chicks raised where they hear no adult sing grow up singing a different song

Question 6

Suppose a kind of bluethroat, a small bird, and a kind of thornback, a small ray, of the same mass are kept in one room, the thornback in a tank of sea water, each with food to spare. The room’s heating fails, and the room cools from 22 °C to 11 °C and stays there.

Which of the following describes the change in the food each animal eats per day?

A. The bluethroat eats more, and the thornback eats less B. The bluethroat eats less, and the thornback eats more C. Both eat more D. Both eat less

Question 7

Suppose a mountain lake’s tiny floating plants store 46 000 kJ of energy per square meter a year. Tiny floating animals eat them, small fish eat the tiny animals, and fish-eating birds eat the small fish.

By the 10 % rule, how much energy is stored in the fish-eating birds’ level?

A. 4.6 kJ per square meter a year B. 46 kJ per square meter a year C. 460 kJ per square meter a year D. 4 600 kJ per square meter a year

Question 8

Suppose ecologists measure a lowland lake for a year. Its tiny floating plants store 42 500 kJ of energy per square meter a year, and the tiny floating animals that eat them store 5 100 kJ per square meter a year.

What percentage of the energy stored by the tiny floating plants is stored by the tiny floating animals?

A. 1.2 % B. 8.3 % C. 12 % D. 88 %

Question 9

Suppose a farmer’s field stays waterlogged every spring. Bacteria in the wet soil turn its nitrate into nitrogen gas, which escapes to the air. The farmer digs drains, and from the next spring air reaches the soil.

Which of the following is the process the drains slow down?

A. Nitrogen fixation B. Nitrification C. Assimilation D. Denitrification

Question 10

A food web of a sandy sea floor. Each arrow is drawn from the eaten organism to the organism that eats it.

Suppose ecologists survey a sandy sea floor and record the food web below. Each arrow is drawn from the eaten organism to the organism that eats it.

Which organism in the web feeds at two trophic levels?

A. The anglerfish B. The scad C. The dab D. The small crustaceans

Question 11

Suppose farmers plow up an old grassland to grow crops. Decomposers in the turned soil break down its dead roots and old plant matter far faster than before.

Which of the following describes the movement of carbon that the plowing causes?

A. Carbon moves from the air into the soil as carbon dioxide B. Carbon moves from the soil into the crop as carbon dioxide C. Carbon moves from the soil into the air as carbon dioxide D. Carbon stays in the soil as carbon dioxide

Question 12

Suppose the phosphate dissolved in a lake’s water falls by half over ten years, while the lake’s algae, water plants and fish all increase.

Which of the following explains the fall in the water’s phosphate?

A. The phosphorus atoms were destroyed in the algae’s respiration B. The phosphorus left the water as a gas and escaped into the air C. The phosphorus was turned into nitrate D. The phosphorus moved into the living things and the mud

Question 13

Suppose a survey of a salt pan’s edge finds that its producers make 23 400 kJ of sugar per square meter a year and respire 10 700 kJ per square meter a year.

What is the net primary productivity of the producers?

A. 10 700 kJ per square meter a year B. 12 700 kJ per square meter a year C. 23 400 kJ per square meter a year D. 34 100 kJ per square meter a year

Question 14

Suppose a population of nutcrackers, birds of mountain pinewoods, holds 850 birds. In one year 205 chicks hatch and 128 birds die.

If the population keeps rising by the same number each year, how many birds will it hold three years from now?

A. 927 birds B. 1 004 birds C. 1 081 birds D. 1 465 birds

Question 15

Suppose a tank holds 520 water bugs of one kind, with food to spare and nothing that eats them. Their maximum per capita growth rate, rmax, is 0.15 per week.

What is the population’s rate of change, dN/dt?

A. 7.8 water bugs per week B. 78 water bugs per week C. 598 water bugs per week D. 3 467 water bugs per week

Question 16

The stint counts on the shore over four years, on a log-scale y-axis.

Suppose a kind of stint, a small bird that feeds along the tide line, is counted on one shore every year. The graph below plots the counts on a log-scale y-axis, and the points lie on a straight line sloping down.

Which of the following describes the change in the population?

A. The count falls by the same fraction each year B. The count falls by the same number of birds each year C. The count falls faster each year than the year before D. The count has settled at the shore’s carrying capacity

Question 17

Suppose the food on a sandy sea floor can support no more than 2 300 brill, a flatfish: K is 2 300 brill. The area holds 575 brill, and their maximum per capita growth rate, rmax, is 0.16 per year.

What is the population’s rate of change, dN/dt?

A. 23 brill per year B. 69 brill per year C. 92 brill per year D. 276 brill per year

Question 18

Suppose about 3 800 megrim, a flatfish, live on one stretch of muddy sea floor off a sandy point, near the most its food can support. A storm covers about 30 % of the stretch with coarse sand, and the megrim’s food dies out there.

Which of the following describes the megrim population over the following years?

A. The population stays near 3 800 B. The population falls to zero C. The population falls at first and then climbs back to 3 800 D. The population falls and settles near a new, lower carrying capacity

Question 19

The density of each pen and the share of its fish that died in the month the parasite arrived.

Suppose young saithe, a sea fish, are raised in four net pens of the same size, each at a different density, and a gill parasite reaches all four pens in the same month. The table gives each pen’s density and the share of its fish that died that month.

Which of the following conclusions do the data support?

A. The parasite kills the same share of fish at every density B. The parasite’s effect on the fish shrinks as the pen gets more crowded C. The parasite’s effect on the fish grows with the density of the pen D. The fish in the densest pen resist the parasite best

Question 20

The flowering plants counted on the riverbank.

Suppose a botanist counts the flowering plants of one riverbank and finds three species, as the table shows. The formula sheet gives Simpson’s Diversity Index as $1 - \sum {(\frac{n}{N})}^{2}$ .

What is Simpson’s Diversity Index for the riverbank, to two decimal places?

A. 0.40 B. 0.44 C. 0.46 D. 0.54

Question 21

What happened to the remaining species in the plots where the other was removed.

Suppose a kind of bittercress, a small alpine plant, grows almost only within the cushions of a kind of pearlwort, a plant that grows in dense cushions close to the ground, which keep the soil beneath them damp. Ecologists remove one species from each of several plots and record what happens to the other, as the table shows.

Which pair of signs describes the interaction, the bittercress first and then the pearlwort?

A. + / 0 B. + / + C. + / − D. 0 / 0

Question 22

The counts of the two species, alone and together, at weeks 0, 4, 8 and 12.

Suppose two species of small crustacean, species J and species K, are raised in tanks of the same size. Neither species eats other animals. Ecologists raise each species alone in its own tank and the two together in a third tank. The table gives the counts at weeks 0, 4, 8 and 12.

Which of the following conclusions do the data support?

A. Species J eats species K B. Species K eats species J C. The two species use the same food, and species J excludes species K D. The two species use different foods, and both persist together

Question 23

Suppose a survey of one pond’s water plants finds four species, with counts of 60, 20, 10 and 10 plants.

Which of the following changes would raise the pond’s species diversity?

A. The commonest species doubles to 120 plants B. One of the two rarest species doubles to 20 plants C. The rarest species dies out D. Every species doubles

Question 24

Each island’s insect species after the spray, as a percentage of the count before it.

Suppose an insecticide sprayed on the fields beside a river drifts over two of its islands, Nether Ait and Over Ait, and the number of insect species on each island falls to 30 % of the number before the spray. The graph below shows each island’s insect species over the next four years, as a percentage of the number before the spray.

Which of the following statements do the graphs support?

A. Over Ait’s community resisted the spray better B. Over Ait’s community recovered faster in the first year C. The two communities lost the same share and recovered at the same rate D. Nether Ait’s community is the more resilient

Question 25

Four of the grassland’s animals: each one’s share of the animals counted, and the species lost from plots after it was removed.

Suppose ecologists study a dry grassland of about 90 species. For four of its animals, the table gives that animal’s share of all the animals counted and the number of species that disappeared from fenced plots two years after that animal alone was removed.

Which of the four animals is the grassland’s keystone species?

A. The mole-rat B. The seed-eating mouse C. The groundhopper D. The pratincole

Question 26

Suppose desmans, small river mammals of the Pyrenees, live in two mountain rivers and eat river invertebrates. In the Garonne the desmans eat five kinds of invertebrate; in the Ariège they eat two kinds. A chemical spill kills the same one kind of invertebrate in both rivers. The Garonne loses few desmans that year, and the Ariège loses most of its desmans.

Which of the following explains why the Garonne’s desmans came through the year?

A. The Garonne held more invertebrates in total before the spill B. The spill killed fewer invertebrates in the Garonne C. The Garonne’s desmans kept feeding on the kinds of invertebrate the spill spared D. The Garonne’s desmans are a hardier kind that survives a spill

Question 27

The mean burdock cover after two years against the number of native species sown with it.

Suppose a kind of burdock from another continent spreads along a road cutting. Ecologists clear twelve plots on the cutting, sow each with burdock seed, and sow 0, 2, 4 or 8 species of native plant on top, three plots per number. The table gives the mean burdock cover of the three plots after two years.

Which of the following explains the fall in the burdock’s cover?

A. The native plants attract the insects that eat the burdock in its home range B. The native plants take up the light, water and minerals the burdock would have used C. The native plants add nitrate and phosphate to the soil, which harm the burdock D. The burdock cannot grow in soil where other plants have grown before

Question 28

Suppose ten dry years shrink a shallow lake to nothing. The lake’s water plants, snails and fish disappear, and grasses spread over the dry bed.

Which of the following is the cause of the change in where this ecosystem is found?

A. A geological event B. An invasive species C. A human activity D. A meteorological event

Question 29

The yearly percent change in the houting count in the estuaries of each coast.

Suppose houting, fish that spawn upriver, are counted each spring in twelve estuaries on each of four coasts, and each estuary’s yearly percent change is worked out. The plots below show the four coasts.

On which coast did every estuary lose houting?

A. The north coast B. The east coast C. The south coast D. The west coast

Question 30

The southern and northern edges of the argus’s range, in kilometers north of the south coast, in three years.

Suppose a kind of argus, a small butterfly, is recorded across a country every twenty years. The table gives the southern and the northern edge of its range, as the distance north of the country’s south coast. Over those forty years the country’s summers grew warmer.

Which of the following explains the pattern in the table?

A. A disease killed the butterfly in the south, and the survivors flew north together B. Collectors caught most of the argus in the south of the country C. Warmer summers let the butterfly breed farther north, and the far south grew too warm for it D. Cooler summers pushed the butterfly’s range toward the north

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 · Interpreting and Evaluating Experimental Results with Experimental Design · 9 points

Suppose ecologists mark twelve plots of the same size on one hillside grassland, all on the same soil, and sow nothing. Each spring for five years they spread nitrogen fertilizer on the plots at one of four rates: 0, 50, 100 or 200 kg of nitrogen per hectare, three plots at each rate. Every plot is mown on the same day each July, and the cut plants are dried and weighed. In the fifth summer the ecologists count the plant species in each plot before mowing. The table gives the mean of the three plots at each rate, with the ±2SE range.

Plant species per plot and dry mass of plants cut per plot, in the fifth summer, at each fertilizer rate (mean of three plots ± 2SE).

(a)(i) The fertilizer adds nitrate to the soil. Describe how a grass plant in the plot uses that nitrate to build its proteins. (1 point)

A full-credit answer: The grass takes up the nitrate from the soil water through its roots.
It builds the nitrogen from the nitrate into amino acids.
It joins the amino acids into its proteins.

Check the box for each point your answer earns

the plant takes up the nitrate through its roots and builds its nitrogen into amino acids or proteins (assimilation).

Accept with or without the word assimilation.

Common slip: Saying the plant takes nitrogen from the air. A plant’s roots take up nitrate; the nitrogen gas of the air is of no use to it.

(a)(ii) Explain why the grass depends on bacteria for any nitrogen it gets from the air around it. (1 point)

A full-credit answer: Nitrogen gas is two nitrogen atoms held by a very strong bond.
A plant has no way to break that bond.
Only certain bacteria can, turning the gas into ammonia.
So the grass can use nitrogen only after bacteria have fixed it, or as nitrate.

Check the box for each point your answer earns

the plant cannot break the bond in nitrogen gas, so the nitrogen must first be fixed into ammonia or ammonium by bacteria.

Accept: nitrogen gas is unreactive, so the plant can take up only ammonium or nitrate.

Common slip: Saying there is too little nitrogen gas in the air. Nitrogen gas is most of the air; the plant cannot break its bond.

(b)(i) Identify a dependent variable in the ecologists’ experiment. (1 point)

A full-credit answer: The number of plant species per plot.

Check the box for each point your answer earns

one of the following: the number of plant species per plot; the dry mass of plants cut per plot.

Common slip: Naming the fertilizer rate. The rate is what the ecologists changed: the independent variable.

(b)(ii) Justify the ecologists’ marking all twelve plots on one grassland with the same soil. (1 point)

A full-credit answer: Plots on one grassland with the same soil start the same.
So any difference among the plots after five years comes from the fertilizer rate.
A difference in soil or in the plants already there would give a difference of its own.

Check the box for each point your answer earns

it makes the plots the same at the start, so a difference between them is due to the fertilizer rate and not to the soil or the plants already there.

Common slip: Saying it is easier to reach. Convenience is no justification; the point is that the plots differ in the fertilizer alone.

(b)(iii) Justify the ecologists’ mowing every plot on the same day each July. (1 point)

A full-credit answer: Mowing on the same day keeps the mowing the same for every plot.
Then the fertilizer rate is the only thing that differs among the plots.
If plots were mown on different days, a difference in mass or species could come from the mowing instead.

Check the box for each point your answer earns

it keeps every other condition the same, so the fertilizer rate is the only variable that differs among the plots.

Accept: a plot mown earlier or later would have grown for a different time, which would change its mass on its own.

Common slip: Saying mowing feeds the plants. The mowing is a condition held the same; it is the fertilizer that differs.

(c)(i) Determine, using the ±2SE ranges, whether the mean dry mass cut at 100 kg of nitrogen per hectare differs from the mean at 200 kg. Give the evidence your decision rests on. (1 point)

A full-credit answer: The data cannot show that the two means differ.
At 100 kg the range is 4.6 to 5.2 kg; at 200 kg it is 4.8 to 5.4 kg.
The two ranges overlap.
So the data cannot tell the two means apart.

Check the box for each point your answer earns

no difference can be claimed, because the two ±2SE ranges overlap (4.6–5.2 and 4.8–5.4).

A bare ‘no difference’ with no reference to the overlap does not earn the point.

Common slip: Comparing the two means alone, 4.9 and 5.1. The error bars overlap, so the difference between the means could be chance.

(c)(ii) Calculate the percent change in the plant species counted per plot from the 0 kg plots to the 200 kg plots, giving a fall as a negative value. (1 point)

Write down the values in the question:

species at 0 kg = 24 per plot

species at 200 kg = 6 per plot

Write down the equation:

percent change = \frac{final - start}{start} \times 100

Substitute the values into the equation:

percent change = \frac{6 - 24}{24} \times 100 = −75 %

A full-credit answer: Percent change = (final − start) ÷ start × 100.
(6 − 24) ÷ 24 × 100 = −75 %.

Accept ‘a fall of 75 %’.

(d)(i) A pond lies at the foot of the slope below the 200 kg plots, and rain washes some of their nitrate into it. Predict how the dissolved oxygen in the pond changes in late summer over the following years. (1 point)

A full-credit answer: The dissolved oxygen in the pond falls in late summer.

Check the box for each point your answer earns

the dissolved oxygen falls (decreases).

A prediction that names no direction earns nothing.

Common slip: Predicting more oxygen because more algae photosynthesize. The algae bloom, then die, and their decomposers use up the oxygen.

(d)(ii) Provide reasoning to justify your prediction. (1 point)

A full-credit answer: The nitrate is a nutrient for the pond’s algae, so the algae multiply.
The algae then die, and decomposers feed on them.
The decomposers respire, and respiration uses up the dissolved oxygen.
So the oxygen falls, most of all in late summer when the dead algae are most.

Check the box for each point your answer earns

the added nitrate makes the algae multiply; when they die, decomposers respire them and use up the dissolved oxygen.

The chain must reach the decomposers’ respiration; ‘the algae use the oxygen’ alone does not earn the point.

Common slip: Stopping at ‘the algae bloom’. The bloom comes before the fall; the oxygen falls when decomposers respire the dead algae.

Free-response score: 0 of 9

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

Suppose two mountain lakes, Aldwark Water and Thorley Water, hold the food webs below; ferox trout are large fish that eat other fish. The two lakes hold about the same mass of living things. Each arrow is drawn from the eaten organism to the organism that eats it.

The food webs of the two lakes. Each arrow is drawn from the eaten organism to the organism that eats it.

(a) Describe one feature of a community that lets an introduced species become invasive there, although the same species stays harmless in another community. (1 point)

A full-credit answer: The community holds none of the species that eat the newcomer at home.
So the newcomer is eaten by nothing, and its numbers climb.

Check the box for each point your answer earns

one of the following: the community lacks the newcomer’s predators; it lacks its competitors; it lacks the diseases or parasites that infect it at home.

Common slip: Saying the newcomer breeds fast. Its breeding is the same in both communities; what differs is what holds it in check.

(b) A kind of snakehead, a fish from another continent, is released into Aldwark Water. It eats crucian and schelly. Determine the trophic level at which the snakehead feeds in Aldwark Water, and state what your decision rests on. (1 point)

A full-credit answer: The snakehead feeds as a tertiary consumer.
Crucian and schelly eat snails, small crustaceans and insect larvae, which eat the algae and the bistort.
So crucian and schelly are secondary consumers, and the fish that eats them is a tertiary consumer.

Check the box for each point your answer earns

tertiary consumer (the fourth trophic level), because its prey, crucian and schelly, are secondary consumers that eat the primary consumers.

Accept ‘the same level as the ferox trout’ with that reasoning.

Common slip: Placing the snakehead one level above the algae because it is a fish. The level follows from what it eats, not from what kind of animal it is.

(c) A disease kills every snail in both lakes. Explain why Aldwark Water’s community is likely to recover better than Thorley Water’s. (1 point)

A full-credit answer: In Aldwark Water the crucian also eat small crustaceans and insect larvae.
So energy still reaches the crucian, and the ferox trout above them, along other paths.
In Thorley Water the crucian eat snails only, so they starve, and the ferox trout lose their food.
A community with more species has more paths for energy, so it recovers better.

Check the box for each point your answer earns

Aldwark Water’s consumers have other food sources, so energy still passes up the web when the snails are gone; Thorley Water’s crucian and ferox trout lose their only path.

The label ‘more resilient’ alone does not earn the point; the reasoning must name the other paths.

Common slip: Saying Aldwark Water has more organisms. The two lakes hold the same mass of living things; the difference is in how many kinds, and so how many paths.

(d) A persistent chemical, one that stays in an animal’s body for life, washes into both lakes. Explain why its concentration is highest in the ferox trout. (1 point)

A full-credit answer: Each animal keeps the chemical in its tissue for life.
Each crucian eats many snails and crustaceans and keeps all their chemical.
Each ferox trout eats many crucian and schelly and keeps all of theirs.
Only about 10 % of each level’s mass becomes the next level’s tissue, so the chemical from many prey sits in far less tissue.
So the concentration rises at each step, and the ferox trout, at the top, hold the highest.

Check the box for each point your answer earns

the chemical is stored and passed up the web; each consumer eats many prey over its life while only about 10 % of the biomass is built into the next level, so the concentration rises at each trophic level (biomagnification).

Accept with or without the word biomagnification.

Common slip: Saying the ferox trout swim in the most water. The chemical reaches the ferox trout through their food, concentrated at every step of the web.

Free-response score: 0 of 4

Free response 3 · Analyze Data · 4 points

Suppose a kind of small bird perches on waterbuck, large antelope, in two districts. The birds eat the ticks they pick from the waterbuck’s skin, and they also drink blood from the animals’ wounds. In one district the waterbuck carry few ticks; in the other they carry many. In each district, ecologists compare young waterbuck that the birds visit with young waterbuck kept in bird-proof enclosures, and record each animal’s mass gain over the wet season. The table gives the means.

Mean mass gain of the young waterbuck over the wet season, in kilograms, in the two districts, with and without the birds.

(a) Describe the interaction between the birds and the waterbuck in the district with few ticks. (1 point)

A full-credit answer: In the district with few ticks, the birds gain and the waterbuck lose.
The birds drink the waterbuck’s blood, and the waterbuck with birds gain 4.5 kg less than those without.
So the interaction is parasitism: + for the birds, − for the waterbuck.

Check the box for each point your answer earns

parasitism — the birds benefit (+) and the waterbuck are harmed (−), shown by the lower mass gain with birds where ticks are few.

Common slip: Calling it predation. The birds feed on the living waterbuck without killing them; that is a parasite’s way.

(b)(i) In the district with many ticks, the ecologists add birds to a group of waterbuck they had kept in bird-proof enclosures. Predict how those waterbuck’s mass gain over the next wet season compares with the mass gain of enclosed waterbuck. (1 point)

A full-credit answer: Their mass gain is higher than the enclosed waterbuck’s gain.

Check the box for each point your answer earns

higher (greater) than the enclosed waterbuck’s mass gain.

A prediction that names no direction earns nothing.

Common slip: Predicting a lower gain because the birds drink blood. Where ticks are many, the birds remove far more than they take.

(b)(ii) In the same district, the ecologists keep birds off a group of waterbuck the birds had visited before. Predict how those waterbuck’s mass gain compares with the mass gain of waterbuck the birds still visit. (1 point)

A full-credit answer: Their mass gain is lower than the visited waterbuck’s gain.

Check the box for each point your answer earns

lower (smaller) than the visited waterbuck’s mass gain.

A prediction that names no direction earns nothing.

Common slip: Predicting a larger mass gain because the birds are gone. Without the birds the ticks stay on, and the waterbuck lose blood to them all season.

(c) Identify the interaction between the birds and the waterbuck in the district with many ticks. (1 point)

A full-credit answer: Mutualism: the birds gain food, and the waterbuck gain by losing their ticks, so both benefit (+ / +).

Check the box for each point your answer earns

mutualism (both benefit; + / +).

Common slip: Calling it commensalism. The waterbuck gain too: with the birds they put on 8.5 kg more over the season.

Free-response score: 0 of 4

Feedback and scoring guides appear after you submit.

Multiple choice checked: 0 of 30 correct.