Unit 8 · Topic 8.6 end-of-topic test
Suggested time: about 47 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
Suppose iron-rich water spills from an old coal mine’s flooded workings and spreads over four grassed heaps of mine waste in one week, killing much of the life on them. The table below gives the number of species of plant and insect counted on each heap before the spill and one, two and six years after it.
Which heap is the most resilient?
Suppose the small birds of two old graveyards, Ashby Graveyard and Welby Graveyard, eat three kinds of insect: loopers, lace bugs and leaf miners. A cold spring kills every insect that spent the winter in the open, on twigs and bark. In Ashby Graveyard all three kinds spend the winter in the open. In Welby Graveyard the loopers and the lace bugs do, and the leaf miners spend the winter inside fallen leaves on the ground.
In which graveyard do the small birds keep a food source?
Suppose a summer drought dries the top layer of soil on two village greens, Harby Green and Kirby Green, while the soil deeper down stays moist. Harby Green holds two grasses, both shallow-rooted. Kirby Green holds the same two grasses and three deep-rooted herbs. Kirby Green keeps most of its plant cover through the drought, and Harby Green loses most of its cover.
Which of the following is the reason Kirby Green keeps more of its plant cover?
Suppose a spill of slurry from a burst farm tank kills much of the plant cover on two wet meadows, Selby Meadow and Danby Meadow. Selby Meadow holds 13 species of plant, about 4 800 plants in all. Danby Meadow holds 34 species of plant, about 1 300 plants in all.
Predict which meadow’s plant community returns to its former state first.
Suppose a wood of wild cherry trees holds 34 species. Ecologists remove each animal species from its own plot of the wood and count the species left on that plot two years later. The table below gives how many of each species live in the wood and the species left when it is removed.
Which species is the keystone species of the wood?
Suppose a new leaf disease of rubber trees reaches two plantations in the same month and spreads through every field on both. Welton Plantation grows one variety of rubber tree. Orby Plantation grows four varieties in equal shares. The table below gives each plantation’s harvest of latex the year before the disease arrived and the year after.
Which of the following is the reason Welton Plantation lost its whole harvest while Orby Plantation kept most of its harvest?
Suppose four farms in one valley each grow oil palms.
Which of the following farms practices monocropping?
Suppose a few dozen manatees, large plant-eating sea mammals, graze a fast-growing turtle grass, a kind of sea grass, in the shallows off a sandy coast. About 45 species of smaller sea plant grow between the grazed patches. Now imagine every manatee is caught and removed.
Which of the following happens in the shallows over the following years?
Suppose a farmer plants a shelter belt, a strip of trees along one side of a field, to shelter the field from the wind. The belt is one kind of tree. The farmer wants the belt to recover better from the next pest or disease that reaches it.
Which of the following changes would make the shelter belt more resilient?
Suppose the stonework of a small harbor holds oysters, sea firs, gutweed and chitons among its species, and a few dozen sea bream feed along it. A disease kills every sea bream. The table below gives the share of the stonework each species covered before the sea bream died and two years after.
Which species did the sea bream hold in check?
Suppose a winter flood scours the plants off a river’s sand bar. An ecologist wants to judge how resilient the sand bar’s plant community is.
Which of the following does the ecologist need to measure?
Suppose ecologists count the species living on the floor of a disused chalk pit one spring and again six springs later. Between the two counts, the community lost one of its supports. The table below gives the counts.
Which of the community’s supports was lost?
Suppose ecologists set up plots on the grassy slope of a reservoir dam under four treatments. Every roof is clear plastic and stands open at the sides, so insects reach every plot. After two summers the ecologists count the plant species in each plot. The table below gives the light at the ground, what reached the soil from above, and the count.
Which support did the roof that keeps the rain off take away from the plots beneath it to which the ecologists added nothing?
Suppose the only insects that carry pollen between the flowers of a valley’s lime trees die out over one winter. A lime tree’s flowers make seeds only after an insect carries pollen to them from another lime tree.
Which of the following happens to the lime trees in the first summer after the insects die?
Suppose a disease kills every field maple in a valley within one summer. Leaf miners, tiny insects whose larvae eat only field maple leaves, live in the valley. Long-eared bats eat the leaf miners and several other kinds of insect.
Which of the following is the short-term change in the valley?
Suppose a disease kills every golden eagle that hunts over a mountain corrie, a hollow high on a mountainside, within one summer. The golden eagles ate the corrie’s mountain hares, and the hares graze the corrie’s grasses and low herbs. A student predicts that fifteen years on, the corrie holds fewer species.
Which of the following lines justifies the student’s prediction?
Suppose a landowner lets a river flood one dry, closely grazed field each winter, so the field becomes a wet meadow. Each flood soaks the field’s soil, and the landowner keeps the cattle off the field. The dry field held six kinds of plant. The wet meadows along the same river hold about thirty kinds of plant.
Predict what happens to the number of plant species in the field over the following twelve years.
Which of the following is a species’ abundance?
(a) Describe the change in Ash Fen’s number of species over the fifteen years, compared with Elm Fen’s. (1 point)
A full-credit answer: Ash Fen’s count fell from 58 species to 23, most of the fall after the first year.
Elm Fen’s count stayed at about 60 species.
Check the box for each point your answer earns
Common slip: Describing Ash Fen alone. The task asks for the change compared with Elm Fen.
(b) Predict how the plant cover of Ash Fen changes in the first year after the pumping begins. (1 point)
A full-credit answer: Ash Fen’s plant cover falls.
Check the box for each point your answer earns
Common slip: Predicting a change with no direction. The point wants the direction: the cover falls.
(c) Explain how pumping the water from under Ash Fen led to the loss of animal species there. (1 point)
A full-credit answer: With less water in the soil, the fen’s plants made less sugar, and many of them died.
Less energy entered the fen’s web at the bottom.
The animals that ate the plants had less food, and the animals that ate them had less too.
So species at every level thinned out, and some disappeared.
Check the box for each point your answer earns
Also award: the animals that lived in the wet soil or its pools (snails, larvae, amphibians) lost the wet ground they lived in, so those species were lost — a chain through the habitat, not through thirst.
Do not award for: the animals died of thirst (the river still flows); the fen lost a keystone species.
Common slip: Jumping from the water to the animals. The chain passes through the plants: they make less, so less energy enters the web.
(d) A hard winter later kills the same insect species on both fens. Determine which fen returns to its former state first, and give the evidence from the table that your decision rests on. (1 point)
A full-credit answer: Elm Fen returns first.
Fifteen years on, Elm Fen holds 61 species and Ash Fen holds 23.
Elm Fen’s many species give its energy other paths when the frost kills one insect species, so its animals keep their food.
Check the box for each point your answer earns
Common slip: Choosing Ash Fen because it has fewer species that can fail. Fewer species give the energy fewer other paths, so Ash Fen recovers more slowly.
(a) Predict how the loss of the elephants affects the number of species on the wooded grassland over the following fifteen years. (1 point)
A full-credit answer: The number of species falls.
The spreading thorn trees shade out the grasses, so the grazing animals and the spurfowl lose their food and their nesting ground, and species are lost across the whole community.
Check the box for each point your answer earns
Common slip: Stopping at the thorn trees. The change reaches the grasses, the grazers and the birds: the whole community, not one food chain.
(b) Explain how the loss of the elephants changes the number of thorn trees on the grassland. (1 point)
A full-credit answer: The elephants browsed the young thorn trees and knocked down the old ones, so the trees stayed in scattered clumps.
With every elephant gone, nothing browses the young trees or knocks down the old ones.
So the young trees survive and grow, and the trees spread across the grassland.
Check the box for each point your answer earns
Common slip: Saying the trees spread because they had more room. The room was there before; what changed is that nothing browses the young trees now.
(c) Explain why the elephants, about 80 animals among thousands of thorn trees, are the wooded grassland’s keystone species. (1 point)
A full-credit answer: The elephants were far fewer than the thorn trees or the grasses.
Yet killing them let the thorn trees spread and cost the grassland many of its species.
So about 80 elephants held about 60 species in place.
Check the box for each point your answer earns
Common slip: Writing the definition of a keystone species alone. The point wants the case: 80 elephants, and what their loss did to the trees and the species count.
(d) Ecologists claim that the wooded grassland will now recover more slowly from a drought than it did with its elephants. Provide reasoning to support the claim. (1 point)
A full-credit answer: With its elephants, the grassland held about 60 species, so its energy had many paths from the plants to the animals.
Now it holds fewer species, so its energy has fewer other paths when a drought kills some of them.
So the animals that lose one food have fewer other foods, and the community returns more slowly.
Check the box for each point your answer earns
Common slip: Writing that the grassland is less resilient because it lost its keystone species. The reasoning names the chain: fewer species, fewer paths, a slower return.