Unit 3 · 10–15% of the exam Beta

Unit 3 practice test: Populations

12 exam-style questions from the unit, free, with an explanation for every option. Most have four answer choices, as on the real exam, and a few ask you to type a calculation with its units; sets share one data table, graph, model or passage.

Covers: Generalist and Specialist Species; K-Selected r-Selected Species; Survivorship Curves; Carrying Capacity; Population Growth and Resource Availability; Age Structure Diagrams; Total Fertility Rate; Human Population Dynamics; Demographic Transition.

Passage

Malthus and the food supply

In 1798 the English economist Thomas Malthus argued that a population left unchecked grows in a doubling series (1, 2, 4, 8, 16), while the food supply can grow only by steady additions (1, 2, 3, 4, 5). Population, he concluded, will always press against the food supply, and famine, disease and war will keep it in check unless people choose to have fewer children. In 1800 there were about 1 billion people on Earth; by 2022, according to the United Nations, there were 8 billion. Over the same period, food produced per person rose in most of the world, as farmers used new crop varieties, fertilizer and machines. Some scientists answer that this success depends on soil, fresh water and fuel that are being used faster than they are replaced.

Invented passage for practice, summarizing Malthus’s 1798 essay and United Nations population estimates.

1. What did Malthus claim about population and food?

  1. Food grows faster than population, so famine will end
  2. Population and food grow at the same rate over time
  3. Population grows faster than food, so hunger, disease or war will check it
  4. Population will stop growing by itself at 1 billion
Show the answer

The passage states Malthus’s claim in its first two sentences.

  • Food grows faster than population, so famine will end: That is the opposite of his claim.
  • Population and food grow at the same rate over time: He argued they grow in different ways, one doubling, one by additions.
  • Correct: Population grows faster than food, so hunger, disease or war will check it: Right: a doubling series outruns steady additions, in his argument.
  • Population will stop growing by itself at 1 billion: He did not predict a stop at a set number.

2. Which assumption does Malthus’s argument depend on?

  1. People will choose to have more children each year
  2. Disease does not affect human populations
  3. Population growth stops at 8 billion
  4. Food production can grow only by steady additions
Show the answer

An assumption is a premise the argument needs but does not prove.

  • People will choose to have more children each year: He assumed populations grow if unchecked, not that families keep getting larger.
  • Disease does not affect human populations: He listed disease as one of the checks.
  • Population growth stops at 8 billion: The 8 billion figure is from 2022, long after his essay.
  • Correct: Food production can grow only by steady additions: Right: if food could also grow fast, population would not have to outrun it.

3. Which evidence in the passage most weakens Malthus’s prediction?

  1. Food per person rose while population grew eightfold
  2. About 1 billion people were alive in 1800
  3. Malthus wrote his essay in England in 1798
  4. Some scientists worry about soil and water
Show the answer

Evidence weakens a prediction when it shows the opposite of what was predicted.

  • Correct: Food per person rose while population grew eightfold: Right: if food had fallen behind, food per person would have dropped.
  • About 1 billion people were alive in 1800: That is a starting count, not a test of his prediction.
  • Malthus wrote his essay in England in 1798: Where and when he wrote does not test his claim.
  • Some scientists worry about soil and water: That concern supports his question about limits; it does not weaken it.

Passage

The reindeer of St. Matthew Island

In 1944, 29 reindeer were released on St. Matthew Island, a remote island in the Bering Sea with no wolves or other large predators. The island’s lichens, the reindeer’s main winter food, had built up over centuries into thick mats. By 1963 the herd had grown to about 6,000 animals, and biologists found the lichen mats grazed down and trampled over much of the island. During the severe winter of 1963 to 1964, most of the reindeer starved. A survey in 1966 found 42 animals, and the herd later died out. Lichens grow only a few millimeters a year, so the island’s winter food would take decades to recover.

Invented passage for practice, summarizing published accounts of the St. Matthew Island reindeer, studied by the biologist David Klein (US Fish and Wildlife Service and University of Alaska).

4. Which claim does the passage best support?

  1. Wolves caused the reindeer population to crash in 1964
  2. Lichens grow quickly and recovered within a year
  3. The reindeer died out because they were overhunted
  4. The reindeer outgrew their winter food and crashed
Show the answer

The passage traces growth, overshoot, food loss and dieback.

  • Wolves caused the reindeer population to crash in 1964: The passage says the island had no wolves.
  • Lichens grow quickly and recovered within a year: The passage says lichens grow only a few millimeters a year.
  • The reindeer died out because they were overhunted: The passage blames starvation, not hunting.
  • Correct: The reindeer outgrew their winter food and crashed: Right: the herd grew to 6,000, grazed down the lichen and then starved.

5. Which assumption is the passage making when it calls the 6,000 reindeer an overshoot?

  1. The island could not supply that much lichen year after year
  2. The reindeer would have kept growing without the hard winter
  3. The first 29 reindeer were already too many for the island
  4. Reindeer herds tend to crash after about 20 years
Show the answer

Overshoot means resources are being used faster than they are replaced.

  • Correct: The island could not supply that much lichen year after year: Right: overshoot means using food faster than it grows back, which the grazed-down lichen shows.
  • The reindeer would have kept growing without the hard winter: The passage does not assume unlimited growth; it describes damaged food.
  • The first 29 reindeer were already too many for the island: The herd grew for years, so 29 was well below carrying capacity.
  • Reindeer herds tend to crash after about 20 years: The passage is about this island’s food supply, not a fixed time span.

6. Which detail best explains why the island could not support a large herd again soon after the crash?

  1. The island lies far out in the cold Bering Sea
  2. Lichens grow just a few millimeters a year
  3. The first reindeer were released in 1944
  4. The survey in 1966 counted 42 animals
Show the answer

Carrying capacity depends on how fast resources are replaced.

  • The island lies far out in the cold Bering Sea: Location alone does not explain slow recovery.
  • Correct: Lichens grow just a few millimeters a year: Right: slow regrowth of the main winter food keeps carrying capacity low for decades.
  • The first reindeer were released in 1944: The release date does not explain why food is slow to return.
  • The survey in 1966 counted 42 animals: The count shows the crash, not why food stays scarce.

7. Monarch butterfly caterpillars feed on milkweed plants, and few other plants. Farm weed control has removed much of the milkweed from field edges. Which outcome is most likely?

  1. Monarchs switch to eating crops in the fields
  2. Monarch numbers rise because weed control kills their predators
  3. There is no change, because adult butterflies drink nectar
  4. Fewer caterpillars survive, so monarch numbers fall
Show the answer

A specialist at any life stage is vulnerable when its key resource is removed.

  • Monarchs switch to eating crops in the fields: Specialist caterpillars do not switch to a different food plant.
  • Monarch numbers rise because weed control kills their predators: Removing milkweed removes their food; that outweighs any change in predators.
  • There is no change, because adult butterflies drink nectar: Adults can feed on many flowers, but the caterpillars need milkweed to grow.
  • Correct: Fewer caterpillars survive, so monarch numbers fall: Right: as specialists, the caterpillars lose their food plant and cannot switch.

8. Why are K-selected species, such as large whales and great apes, more likely than r-selected species to become threatened when people hunt them?

  1. They replace losses slowly, so hunting outpaces births
  2. They are smaller and easier to catch than most r-selected species
  3. They live in more habitats than r-selected species
  4. They produce more offspring than hunters can find
Show the answer

Slow reproduction means slow recovery from any extra deaths.

  • Correct: They replace losses slowly, so hunting outpaces births: Right: few offspring, late breeding and long gaps between births mean slow recovery.
  • They are smaller and easier to catch than most r-selected species: Whales and great apes are large; size is not the reason.
  • They live in more habitats than r-selected species: Habitat range is not what makes K-selected species vulnerable.
  • They produce more offspring than hunters can find: They produce few offspring, which is the problem.

9. A student writes: "Oysters have a Type III curve, so Type III means r-selected." What is the best correction?

  1. The student is right: Type III and r-selected mean the same
  2. Oysters are K-selected, so the student has the species wrong
  3. Type III curves belong to K-selected species instead
  4. They are separate ideas: deaths by age versus reproduction
Show the answer

Keep the two ideas apart, while knowing they tend to go together.

  • The student is right: Type III and r-selected mean the same: They describe different things: when deaths happen versus how a species reproduces.
  • Oysters are K-selected, so the student has the species wrong: Oysters release millions of eggs with no care, which is r-selected.
  • Type III curves belong to K-selected species instead: Type III curves are typical of r-selected species, not K-selected.
  • Correct: They are separate ideas: deaths by age versus reproduction: Right: r-selected species often have Type III curves, but the curve and the strategy are different ideas.

10. The population of 5,000 keeps growing at 4% a year. Using its doubling time of 17.5 years, how large will it be after 35 years?

Type a number. Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

35 years is two doubling times: 5,000×2×2=20,0005{,}000 \times 2 \times 2 = 20{,}000.

  • Answer: 20,000

11. Which shape of age structure diagram is typical of a country whose population is shrinking?

  1. A wide base that narrows steadily with age
  2. A column with equal bars up to old age
  3. A base narrower than the middle
  4. A wide top and a wide base
Show the answer

Narrow base: decline.

  • A wide base that narrows steadily with age: That is the shape of rapid growth.
  • A column with equal bars up to old age: That is the shape of slow or no growth.
  • Correct: A base narrower than the middle: Right: fewer young than middle-aged people means fewer future parents.
  • A wide top and a wide base: That is not one of the usual shapes.

12. A country’s total fertility rate falls from 2.4 to 2.0 over ten years, but its population is still growing. Which explanation is best?

  1. A TFR of 2.0 means fast growth
  2. Large young cohorts now have children
  3. The TFR was measured wrongly
  4. Fewer babies are born each year than before
Show the answer

TFR describes family size; momentum describes the number of parents.

  • A TFR of 2.0 means fast growth: A TFR of 2.0 is below replacement; growth here comes from momentum.
  • Correct: Large young cohorts now have children: Right: population momentum keeps growth going after TFR falls below replacement.
  • The TFR was measured wrongly: There is no need to assume an error; momentum explains it.
  • Fewer babies are born each year than before: Fewer births would slow growth; the explanation is the size of the parent cohorts.

Keep going

Practice the whole unitTake a timed unit testPrint the unit sheet

Practice has every question in the unit, with feedback after each one; the notes for every topic are free. Other units: Unit 1 · Unit 2 · Unit 3 · Unit 4 · Unit 5 · Unit 6 · Unit 7 · Unit 8 · Unit 9.