Unit 2 · 6–8% of the exam Beta

Unit 2 practice test: The Living World: Biodiversity

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: Introduction to Biodiversity; Ecosystem Services; Island Biogeography; Ecological Tolerance; Natural Disruptions to Ecosystems; Adaptations; Ecological Succession.

Data

Moths on dark and light tree bark

Percent of dark-colored moths caught in a forest near a city, by year, as soot from coal smoke first darkened the bark and then was cleaned up (invented data for practice)
YearTree barkDark moths (% of catch)
1850light, covered in pale lichen2
1900darkened by soot70
1950darkened by soot90
1970clean-air laws begin; soot falls85
2000light, lichen returning20

1. Which statement best describes the trend in dark moths from 1850 to 1950?

  1. They fell from 90% to 2% as the bark darkened with soot
  2. They stayed at about 2% through each of those years
  3. They rose, then fell sharply before 1950
  4. They rose from 2% to 90% as the bark darkened
Show the answer

Read the years in order and give the start and end values.

  • They fell from 90% to 2% as the bark darkened with soot: The percent rose over those years.
  • They stayed at about 2% through each of those years: They reached 70% by 1900 and 90% by 1950.
  • They rose, then fell sharply before 1950: The decline came after clean-air laws, after 1950.
  • Correct: They rose from 2% to 90% as the bark darkened: Right: as soot darkened the trees, dark moths went from rare to most of the catch.

2. Birds hunt moths resting on tree bark by sight. Which explanation best links bark color to moth color in these data?

  1. Moths matching the bark were eaten less and bred more
  2. Soot dyed the moths dark as they rested on the trees
  3. Dark moths flew in from the city to escape the clean air
  4. Light moths stopped reproducing when the lichen died
Show the answer

Predators acted as the selecting force: whichever color stood out was eaten more.

  • Correct: Moths matching the bark were eaten less and bred more: Right: camouflage cut predation, so the matching color became more common.
  • Soot dyed the moths dark as they rested on the trees: Wing color here is inherited, not dyed; the share of dark moths changed over generations.
  • Dark moths flew in from the city to escape the clean air: The data show a change in the forest population, not migration.
  • Light moths stopped reproducing when the lichen died: Light moths were eaten more on dark bark; lichen loss did not stop their breeding.

3. Which conclusion is best supported by the change from 1970 to 2000?

  1. Clean-air laws made each moth’s wings lighter
  2. Once bark was light again, selection favored light moths
  3. Dark moths die out quickly in clean air
  4. The population stopped changing after 1970
Show the answer

The same process ran in reverse once the environment changed back.

  • Clean-air laws made each moth’s wings lighter: Laws changed the bark; moth color changed in the population through survival and breeding.
  • Correct: Once bark was light again, selection favored light moths: Right: as soot fell and lichen returned, dark moths fell from 85% to 20%.
  • Dark moths die out quickly in clean air: Dark moths were still 20% of the catch in 2000.
  • The population stopped changing after 1970: The share of dark moths fell from 85% to 20% after 1970.

Data

Snail survival at different salinities

Percent of 200 marsh snails alive after 10 days at each salinity, in parts per thousand (ppt) (invented data for practice)
Salinity (ppt)Snails alive after 10 days (%)Snails that laid eggs (%)
000
5405
108530
159570
209672
259035
30455
3500

4. Based on the table, what is the optimal range of salinity for these snails?

  1. About 15 to 20 ppt
  2. About 0 to 35 ppt
  3. About 5 to 30 ppt
  4. About 25 to 35 ppt
Show the answer

The optimal range is where individuals both survive and reproduce best.

  • Correct: About 15 to 20 ppt: Right: survival is highest and egg laying peaks at 70% and 72% there.
  • About 0 to 35 ppt: At 0 and 35 ppt no snails survive; those are the limits.
  • About 5 to 30 ppt: That is the range where some survive, including the stress zones where few lay eggs.
  • About 25 to 35 ppt: Survival and egg laying fall at 25 ppt and above.

5. At 10 ppt, 85% of snails survive but only 30% lay eggs. What does this show?

  1. 10 ppt is the best salinity for these snails
  2. In a stress zone, snails live but breed poorly
  3. Egg laying does not depend on salinity
  4. Most of the snails tested die at 10 ppt
Show the answer

Reproduction drops off sooner than survival as conditions leave the optimal range.

  • 10 ppt is the best salinity for these snails: Egg laying is much higher at 15 to 20 ppt.
  • Correct: In a stress zone, snails live but breed poorly: Right: survival is fairly high but egg laying is far below its peak, a sign of stress.
  • Egg laying does not depend on salinity: It changes from 0% to 72% across the table.
  • Most of the snails tested die at 10 ppt: 85% survive at 10 ppt.

6. A storm pushes seawater into a marsh and raises its salinity from 18 ppt to 30 ppt for a whole season. Which prediction is best supported by the data?

  1. The population grows, because 30 ppt is the optimal salinity
  2. No snails survive, because 30 ppt is beyond their tolerance
  3. Many snails die and few lay eggs, so the population shrinks
  4. Nothing changes, because snails can close their shells
Show the answer

Moving from the optimal range into a stress zone shrinks a population through deaths and through fewer births.

  • The population grows, because 30 ppt is the optimal salinity: Survival and egg laying are far lower at 30 ppt than at 18.
  • No snails survive, because 30 ppt is beyond their tolerance: Survival is 45% at 30 ppt; the limit is 35 ppt.
  • Correct: Many snails die and few lay eggs, so the population shrinks: Right: at 30 ppt survival falls to 45% and egg laying to 5%.
  • Nothing changes, because snails can close their shells: The data show survival and reproduction both fall at 30 ppt.

7. A nature reserve contains a forest, a meadow, a stream and a marsh. A second reserve of the same size is all forest. Which statement best explains why the first reserve is likely to hold more species?

  1. It has greater habitat diversity, so it offers more different niches
  2. It has greater genetic diversity, because meadows hold more genes
  3. Forests hold fewer species than meadows, streams or marshes of the same size
  4. It has greater species evenness, because marshes are even
Show the answer

Habitat diversity raises species richness, because different habitats offer different niches.

  • Correct: It has greater habitat diversity, so it offers more different niches: Right: each habitat supports species the others cannot, so more habitats mean more species.
  • It has greater genetic diversity, because meadows hold more genes: Genetic diversity is variety within a population; the reserves differ in habitats.
  • Forests hold fewer species than meadows, streams or marshes of the same size: Many forests are rich in species; the point is that four habitats beat one.
  • It has greater species evenness, because marshes are even: Evenness is about population sizes, and a marsh is not "even" in itself.

8. A mangrove forest along a coast traps sediment and slows waves during storms, protecting the town behind it. Which kind of service is this?

  1. A provisioning service
  2. A cultural service
  3. A supporting service
  4. A regulating service
Show the answer

Flood control and protection from waves are regulating services.

  • A provisioning service: The mangroves are not supplying a good here; they are protecting the town.
  • A cultural service: Protection from waves is a physical benefit, not a nonmaterial one.
  • A supporting service: Supporting services feed the others; storm protection is a direct regulating benefit.
  • Correct: A regulating service: Right: reducing flood and wave damage is a natural process that protects people.

9. Which of these is the best example of a habitat island?

  1. A large ocean, for the whales that swim across it
  2. A lake surrounded by land, for its fish
  3. A continent, for the birds that migrate over it
  4. A river delta, for the sediment carried into it
Show the answer

A habitat island is a patch of suitable habitat surrounded by habitat the species cannot use or cross.

  • A large ocean, for the whales that swim across it: Whales move freely through the ocean; it is not an isolated patch for them.
  • Correct: A lake surrounded by land, for its fish: Right: fish cannot cross land, so the lake works like an island for them.
  • A continent, for the birds that migrate over it: A continent is not a small isolated patch for birds that fly across it.
  • A river delta, for the sediment carried into it: Sediment is not a living population, so the idea does not apply.

10. Wildebeest on the Serengeti move hundreds of kilometers each year, following the rains to fresh grass. Which statement best explains this behavior?

  1. They are fleeing volcanic eruptions on the plains
  2. They move to find colder places to live
  3. They migrate to stay near water and fresh grass
  4. They move because the grassland burns each month
Show the answer

Migration is a common response to periodic changes in conditions.

  • They are fleeing volcanic eruptions on the plains: Eruptions are not yearly, and they follow rain, not eruptions.
  • They move to find colder places to live: They follow rain and grass, not temperature.
  • Correct: They migrate to stay near water and fresh grass: Right: moving with the rains keeps them inside their range of conditions.
  • They move because the grassland burns each month: Fire does not happen each month; they follow seasonal rains.

11. Lichens are often the first organisms on bare rock. How do they help other species follow?

  1. They shade the rock so no other species can grow
  2. They eat the seeds of other plants that arrive
  3. They raise the rock’s temperature so trees can grow at once
  4. They weather rock and add dead matter, forming soil
Show the answer

Pioneer species change the site so that later species can live there.

  • They shade the rock so no other species can grow: Lichens are tiny and do not block other species; they make room for them.
  • They eat the seeds of other plants that arrive: Lichens do not eat seeds; they make their food by photosynthesis.
  • They raise the rock’s temperature so trees can grow at once: Trees need soil, which takes a long time to form.
  • Correct: They weather rock and add dead matter, forming soil: Right: lichens release acids that weather rock, and their remains add organic matter, the start of soil.

12. A farmer plants one wheat variety, grown from genetically identical seeds, across all of her fields. A neighbor plants six different varieties. A new fungus that attacks wheat arrives. Which farmer is more likely to lose most of the crop?

  1. The neighbor, because planting six different varieties attracts more fungi to his fields
  2. Both lose the same share, because the fungus attacks wheat of any variety
  3. The first farmer, because her identical plants share any weakness to the fungus
  4. Neither loses much, because fungi feed mostly on dead plant material
Show the answer

Low genetic diversity means one threat can affect nearly every individual the same way.

  • The neighbor, because planting six different varieties attracts more fungi to his fields: More varieties do not attract more fungi; they spread the risk.
  • Both lose the same share, because the fungus attacks wheat of any variety: The varieties differ in traits, which can include resistance, so losses differ.
  • Correct: The first farmer, because her identical plants share any weakness to the fungus: Right: if one plant is vulnerable, every identical plant is too; with six varieties, some are likely to resist.
  • Neither loses much, because fungi feed mostly on dead plant material: Many fungi attack living crops; the question says this one attacks wheat.

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