8.7 Disruptions in Ecosystems

1. AIt is a trait that is passed from parents to offspring and helps in the desert’s dry conditions. Correct
   An adaptation is a heritable variation favored by selection because it gives an advantage in a particular environment. Long tubules are an adaptation only because they are inherited; they reduce water loss and increase reproductive success where water is scarce.
2. BIt is a trait each rodent develops during its own life as it adjusts to the dry conditions.
   A student who thinks adaptations are adjustments made by individuals picks this. An adaptation is a heritable trait; a feature each individual develops during its own life is not passed on through its alleles and is not an adaptation.
3. CIt is a trait that arose in the population because the rodents needed to save more water.
   A student who thinks mutations arise because organisms need them picks this. Mutations are not directed by environmental pressures; the variation in tubule length arose at random, and the dry environment favored the long-tubule variants.
4. DIt is a trait that gives its bearers an advantage in every environment where they might live.
   A student who thinks an adaptation is an advantage everywhere picks this. A trait is an adaptation to a particular environment; where water is plentiful, long tubules may give no advantage.

CED 8.7.A.1 · Read this in Fix

1. APeople exposed to malaria produce new sickle-cell mutations, since they need protection.
   A student who thinks mutations arise in response to need picks this. Mutations are not directed by environmental pressures; the allele is common because carriers survive and reproduce better, not because malaria produces it.
2. BCarriers who survive malaria pass on to their children the resistance that they acquired.
   A student who thinks acquired traits are inherited picks this. Resistance in carriers comes from the allele they inherited; immunity or resistance acquired during life is not passed on through genes.
3. CHeterozygotes are fitter than either homozygote, so selection maintains both alleles. Correct
   Where malaria is common, heterozygotes are more likely than normal homozygotes to survive malaria and more likely than sickle-cell homozygotes to avoid severe sickle-cell disease. Their higher relative fitness maintains both alleles in the population.
4. DThe population keeps the allele so that the group as a whole is protected against malaria.
   A student who thinks alleles are kept for the good of the group picks this. Allele frequencies change through the survival and reproduction of individuals; heterozygous individuals leave more offspring than either homozygote.

CED 8.7.A.2 · Read this in Fix

1. AThe insecticide caused the mutations for resistance that the insects needed to survive its use.
   A student who thinks mutations arise because organisms need them picks this. Mutations are not directed by environmental pressures; the insecticide selected among variants already present.
2. BInsects that survived a dose built up resistance in their bodies and passed it to their offspring.
   A student who thinks acquired traits are inherited picks this. Tolerance built up in an insect's body during its life is not passed on through its genes.
3. CEvery insect in the population gradually became more resistant until the whole population was.
   A student who thinks evolution happens by every individual changing picks this. Individuals do not change genetically; the proportion of resistant individuals rose because they survived and reproduced more.
4. DInsects that already had resistance alleles from a random mutation survived and reproduced more. Correct
   Mutations arise at random with respect to need; some insects carried resistance alleles before or independently of exposure. The insecticide killed susceptible insects, so resistant ones left more offspring and the resistance allele became common over generations.

CED 8.7.A.3 · Read this in Fix

1. ANative mussels are unaffected, as they are better adapted to their own lakes.
   A student who thinks natives always outcompete newcomers picks this. An introduced species free of predators can outcompete native species for resources, as the stem's details suggest.
2. BNative mussels decline, as zebra mussels outcompete them for food and space. Correct
   Free of predators, zebra mussels can reach very high densities, remove much of the plankton that native mussels also filter, and cover their shells. Outcompeted for food and space, native mussel populations decline.
3. CNative mussels increase, as a lake with more species is more stable.
   A student who thinks adding species always benefits an ecosystem picks this. Zebra mussels compete with native mussels and attach to them, so their arrival harms rather than helps native mussels.
4. DNative mussels quickly produce the mutations they need to compete and so persist.
   A student who thinks mutations arise in response to need picks this. Mutations are not directed by environmental pressures; native mussels cannot produce useful mutations on demand.

CED 8.7.B.1 · Read this in Fix

1. ANutrient runoff → algal bloom → algae use O₂ in photosynthesis → fish die
   A student who thinks photosynthesis uses up oxygen picks this. Photosynthesis releases oxygen; the oxygen is depleted mainly by decomposers respiring as they break down dead algae.
2. BNutrient runoff → fertilizer poisons the fish → fish die → algae feed on remains
   A student who thinks fertilizer kills fish as a poison picks this. In eutrophication, fish die because nutrient-driven algal growth and decomposition deplete oxygen.
3. CNutrient runoff → algal bloom → algae die → decomposers use up O₂ → fish die Correct
   Added nitrate and phosphate stimulate rapid growth of algae. When the algae die, decomposers break them down, and their cellular respiration uses up dissolved oxygen, so fish die from lack of oxygen.
4. DNutrient runoff → algal bloom → more food for fish → fish populations grow
   A student who thinks more nutrients always benefit an ecosystem picks this. The extra algae eventually die and decompose, using up oxygen, which can kill fish.

CED 8.7.C.1.ii · Read this in Fix

1. ATheir habitat has shrunk as the forest moved upslope, so their populations are likely to decline. Correct
   As the climate warmed, the tree line rose from 1,800 m to 2,100 m. The alpine meadow can occupy only the area above the tree line, and the summit is at 2,400 m, so the meadow's area is now much smaller and the plants' populations are likely to decline.
2. BThey are likely to move higher up the mountain, so the area of their habitat will stay about the same.
   A student who thinks species can always relocate without losing habitat picks this. The summit is at 2,400 m, so there is no higher ground; the meadow has been squeezed into a smaller area near the top.
3. CEach plant will adapt to forest conditions during its life, so their numbers will not fall.
   A student who thinks individuals adapt during their lives picks this. Adaptations arise through selection on heritable variation over generations; individual plants cannot change their genetic makeup to suit forest conditions.
4. DThey will produce the mutations for warm tolerance they need as the meadow shrinks.
   A student who thinks mutations arise because organisms need them picks this. Mutations are not directed by environmental pressures; a shrinking habitat does not cause useful mutations to appear.

CED 8.7.D.1 · Read this in Fix

1. APlants at the mine site built up tolerance by growing in zinc soil, then passed it on to their offspring.
   A student who thinks acquired traits are inherited picks this. Field plants placed on zinc soil die rather than building up tolerance; tolerance is a heritable variation that selection favored at the mine.
2. BTolerance is a better trait overall, so in time it will spread through the field populations as well.
   A student who thinks an adaptation is an advantage in every environment picks this. On normal soil tolerant plants grow more slowly, so in the fields selection works against tolerance rather than spreading it.
3. CThe zinc in the soil caused the new mutations for tolerance that the plants at the mine site needed.
   A student who thinks the environment produces the mutations organisms need picks this. Mutations are not directed by environmental pressures; zinc favored tolerant variants that were already present by chance.
4. DTolerance helps only where zinc is present; in clean soil, faster-growing intolerant plants are favored. Correct
   Zinc tolerance is an adaptation to a particular environment. At the mine, tolerant plants survive and reproduce while intolerant ones die; in the fields, tolerant plants grow more slowly, so intolerant plants leave more offspring and tolerance stays rare.

CED 8.7.A.1 · Read this in Fix

1. AWhere the parasite is present, AA individuals survive best, as A is the dominant allele.
   A student who thinks the homozygous dominant genotype is always fittest picks this. In the parasite region, AA survival (0.62) was well below Aa survival (0.88); dominance does not determine fitness.
2. BWhere the parasite is present, heterozygotes survive better than either of the homozygous genotypes. Correct
   In the parasite region, survival was 0.88 for Aa, compared with 0.62 for AA and 0.20 for aa, so heterozygotes had the highest survival there. This is the pattern of heterozygote advantage.
3. CHeterozygotes survive better than individuals of either homozygous genotype in both of the two regions.
   A student who thinks heterozygotes are fittest in every environment picks this. Without the parasite, AA survival (0.90) was at least as high as Aa survival (0.88); the heterozygote's advantage depends on the parasite.
4. DSelection against aa individuals will remove the a allele from the populations in both regions.
   A student who thinks selection always eliminates harmful alleles picks this. In the parasite region, Aa individuals carry the a allele and have the highest survival, so selection there maintains the a allele in the population.

CED 8.7.A.2 · Read this in Fix

1. AReject it, as cells from colonies never exposed to the antibiotic were already resistant. Correct
   Colonies 3 and 8 grew on the antibiotic replica, and cells taken from the same colonies on the master plate, which had never contacted the antibiotic, also survived the antibiotic. The resistance was present before exposure, which contradicts the hypothesis that the antibiotic caused it.
2. BReject it; this proves that the environment can never affect the rate at which mutations occur.
   A student who thinks one rejected hypothesis proves a sweeping rule picks this. The data show that this antibiotic did not cause these resistance mutations; they say nothing about other factors, and some, such as UV light, do increase mutation rates.
3. CSupport it, as colonies 3 and 8 mutated to resistance once they were put on the antibiotic plate.
   A student who thinks mutations arise in response to need picks this. Cells from colonies 3 and 8 on the master plate were resistant without ever meeting the antibiotic, so the mutations were present before exposure.
4. DSupport it, as the cells in colonies 3 and 8 had adapted to the antibiotic during the replica test.
   A student who thinks individuals adapt during their lives picks this. The master-plate cells, which were never in the replica test, were already resistant, so their resistance did not arise as a response to the antibiotic.

Working Prediction if the hypothesis were true: resistance appears only after contact with the antibiotic, so cells from master-plate colonies (never exposed) should be killed. Observed: cells from master colonies 3 and 8, never exposed, survived the antibiotic; colonies 1 and 5 did not grow on the replica and their cells were killed. Resistance existed before exposure, so the data contradict the hypothesis: reject it.

CED 8.7.A.3 · Read this in Fix

1. ANative shrub cover held steady, as natives are best adapted to the forest.
   A student who thinks native species always outcompete newcomers picks this. Native shrub cover fell from 62% to 15% over the 12 years.
2. BThe data prove that the vine outcompeted the native shrubs for light.
   A student who thinks a correlation proves a particular mechanism picks this. The data show that the vine increased as the shrubs declined, which is consistent with competition, but they do not show which resource, or whether competition caused the decline.
3. CNative shrubs lost ground over the same years in which the vine spread. Correct
   From year 0 to year 12, vine cover rose from 1% to 68% while native shrub cover fell from 62% to 15%; the two changed in opposite directions, most steeply between years 4 and 8.
4. DThe vine's cover kept increasing every two years with no sign of slowing.
   A student who thinks an invasive population grows without limit picks this. The vine's cover rose by 24 percentage points between years 4 and 6 but only by 2 between years 10 and 12, so its growth slowed.

CED 8.7.B.1 · Read this in Fix

1. ADo insects eat less leaf tissue from shrubs in the new region than from shrubs in the native range? Correct
   The explanation depends on a measurable difference in insect feeding between the two ranges. Measuring the leaf tissue eaten by insects in each range tests that difference directly.
2. BWhy does the shrub want to spread into the new region so much more than it does in its native range?
   A student who thinks questions about what an organism wants are scientific picks this. A shrub's 'wants' cannot be measured, so the question cannot be tested.
3. CIs the shrub a harmful species that people should remove from the region where it was introduced?
   A student who thinks experiments can decide value questions picks this. Whether people should remove the shrub is a value judgment; data can inform it, but no experiment can answer it.
4. DIs the shrub more abundant than most of the native plants in the region where it was introduced?
   A student who thinks that confirming a pattern tests its explanation picks this. The shrub's abundance is what needs explaining; the question does not measure insect feeding, so it cannot test the proposed cause.

CED 8.7.B.1 · Read this in Fix

1. AHuman activity played no part, since people did not cut down the elm trees the disease killed.
   A student who thinks humans change ecosystems only by directly killing organisms picks this. People did not need to cut down the elms: transporting the fungus in logs was a human activity that drove the change.
2. BThe loss of the elms affected only the elms; the rest of each forest community was unchanged.
   A student who thinks the loss of one species affects only that species picks this. Elms provided shade, food and habitat for other organisms, so their loss changed conditions for many other species.
3. CThe elms will produce new mutations for resistance now that the fungus has reached them.
   A student who thinks mutations arise because organisms need them picks this. Mutations are not directed by environmental pressures; the fungus cannot make the elms produce resistance mutations.
4. DHuman transport carried the fungus across an ocean, much faster than natural spread by beetles. Correct
   Shipping logs carried the fungus across an ocean far faster than bark beetles could have spread it, introducing it to elm populations with no previous exposure. Human activity thus accelerated a change in North American forests.

CED 8.7.C.1 · Read this in Fix

1. ALarger animals take up more pesticide directly from water across their bigger bodies.
   A student who thinks larger bodies absorb more directly picks this. Concentration is per unit mass, so a bigger body alone would not raise it; the pesticide accumulates mainly through food, which is why the concentration rises at each feeding level.
2. BEach consumer makes more of the pesticide in its body as it digests contaminated prey.
   A student who thinks digestion produces more of the pollutant picks this. Organisms do not make the pesticide; all of it comes from the prey they eat, and it builds up because it is retained.
3. CEach consumer eats many prey over its life and stores the pesticide from them in its fat. Correct
   The pesticide is fat-soluble and not easily broken down or excreted, so a consumer retains most of the pesticide in all the prey it eats over its life. Because it eats many times its own mass in prey, its concentration rises above theirs, and the effect compounds at each level (biomagnification).
4. DThe pesticide goes up the food chain with energy, which builds up toward the top level.
   A student who thinks energy accumulates toward the top of a food chain picks this. Energy decreases at each higher level; the pesticide becomes more concentrated precisely because it is kept while most of the energy is lost as heat.

CED 8.7.C.1.i · Read this in Fix

1. A0.4 mg/kg
   A student who thinks a pollutant keeps the same concentration from prey to predator picks this, giving the fish's concentration. The heron retains the pesticide from 25 kg of fish in a 2.0 kg body, so its concentration is much higher.
2. B4.5 mg/kg Correct
   Pesticide eaten = 25 kg × 0.40 mg/kg = 10 mg. Pesticide retained = 0.90 × 10 mg = 9.0 mg. Concentration in heron = 9.0 mg ÷ 2.0 kg = 4.5 mg/kg, about 11 times the concentration in its prey.
3. C0.5 mg/kg
   A student who applies the 10% rule for energy to the pesticide picks this: 25 × 0.40 × 0.10 ÷ 2.0 = 0.5 mg/kg. The table shows that 90% of the pesticide eaten is retained, not 10%.
4. D9.0 mg/kg
   A student who treats the total amount as the concentration picks this: 9.0 mg is the amount retained, which must be divided by the heron's 2.0 kg body mass to give mg/kg.

Working Pesticide eaten = 25 kg × 0.40 mg/kg = 10 mg. Retained = 0.90 × 10 mg = 9.0 mg. Concentration = 9.0 mg / 2.0 kg = 4.5 mg/kg.

CED 8.7.C.1.i · Read this in Fix

1. ARemove tank 1, as a tank with no added phosphate tells us nothing.
   A student who thinks an untreated group gives no information picks this. Tank 1, with no added phosphate, is the baseline needed to show what phosphate does.
2. BAdd a tank with more phosphate, more light, and a higher temperature.
   A student who thinks changing several variables at once is acceptable picks this. Another tank in which all three variables differ still would not show which variable affects algal growth.
3. CKeep the conditions, but measure the algae in every tank once each day.
   A student who thinks more measurements make any comparison valid picks this. Daily measurements give more data, but light and temperature would still vary with phosphate, so their effects could not be separated.
4. DGive all three tanks the same light period and the same temperature. Correct
   In the current design, light and temperature rise along with phosphate, so any difference in algal growth could be caused by any of the three. Holding light and temperature constant leaves phosphate as the only independent variable.

CED 8.7.C.1.ii · Read this in Fix

1. APlants would decline, but animals would be unaffected, as animals do not need sunlight to live.
   A student who thinks only plants depend on sunlight picks this. Animals obtain their energy from producers, directly or indirectly, so the loss of sunlight would reduce the energy available to them as well.
2. BPhotosynthesis would fall, so food webs would collapse from the base, causing extinctions. Correct
   Less sunlight would reduce photosynthesis and primary productivity. With less energy entering food webs, herbivores and then the predators that depend on them would decline, so many species, especially those needing large amounts of food, would be likely to go extinct.
3. CLarge predators would be the least affected, as they hold the most energy in their food webs.
   A student who thinks top predators hold the most energy picks this. Top predators have the least energy available to them and depend on all the levels below, so they are often the most vulnerable.
4. DMost animals would adapt to the darkness during their own lives, so few species would be lost.
   A student who thinks individuals adapt during their lives picks this. Individuals can adjust only slightly, and cannot change their genes; a sudden, large loss of energy at the base of food webs would cause many populations to die out.

CED 8.7.D.1 · Read this in Fix