Unit 1 · 6–8% of the exam Beta

Unit 1 practice test: The Living World: Ecosystems

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 Ecosystems; Terrestrial Biomes; Aquatic Biomes; The Carbon Cycle; The Nitrogen Cycle; The Phosphorus Cycle; The Hydrologic (Water) Cycle; Primary Productivity; Trophic Levels; Energy Flow and the 10% Rule; Food Chains and Food Webs.

Passage

Two warblers in one spruce forest

A field biologist spent three summers watching two warbler species that eat the same kind of caterpillar in the same spruce forest. She expected one species to push the other out. Instead, both stayed common. Species A fed almost entirely in the top third of the trees, while species B fed on the lower branches near the trunk. In a stand where storms had snapped off the treetops, species A was rare and species B was common. She concluded that the two species can share the forest because each uses a different part of it.

Invented passage for practice.

1. What is the biologist’s main claim?

  1. Species A is better at catching caterpillars than species B.
  2. Storms are the main cause of warbler deaths in spruce forests.
  3. Both species persist because they feed in different parts of the trees.
  4. The two species will keep competing until one of them disappears from the forest.
Show the answer

She concludes the two species share the forest because each uses a different part of it.

  • Species A is better at catching caterpillars than species B.: The passage never compares how well each catches caterpillars.
  • Storms are the main cause of warbler deaths in spruce forests.: Storms appear only as a natural test, not as a claim about deaths.
  • Correct: Both species persist because they feed in different parts of the trees.: Right: that is her conclusion in the last sentence.
  • The two species will keep competing until one of them disappears from the forest.: That was her expectation, which the observations did not support.

2. How does the stand with snapped-off treetops support the claim?

  1. It shows that species B feeds higher when species A is gone.
  2. Without treetops, species A lost its feeding zone and became rare.
  3. It shows that the caterpillars live mostly on the lower branches near the trunk.
  4. Species B became rare there, showing it depends on species A.
Show the answer

Where its part of the tree was missing, species A was rare, which links each species to its own feeding zone.

  • It shows that species B feeds higher when species A is gone.: The passage does not say species B moved up.
  • Correct: Without treetops, species A lost its feeding zone and became rare.: Right: removing the treetops removed species A’s part of the forest.
  • It shows that the caterpillars live mostly on the lower branches near the trunk.: The passage says both eat the same caterpillar; it does not limit where caterpillars live.
  • Species B became rare there, showing it depends on species A.: The passage says species B was common there.

3. Which term names what the two warblers are doing?

  1. Mutualism between the two species
  2. Commensalism
  3. Resource partitioning
  4. Parasitism
Show the answer

Dividing a shared resource by place (or time, or food size) is resource partitioning.

  • Mutualism between the two species: Neither species benefits the other.
  • Commensalism: Neither species benefits from the other while the other is unaffected.
  • Correct: Resource partitioning: Right: they divide the trees by height.
  • Parasitism: Neither species lives on or in the other.

Model or diagram

Phosphate in three streams

00.050.10.150.2Land around the streamPhosphate (mg/L)ForestFarmlandTown

4. About how much phosphate does the farmland stream carry?

  1. About 0.18 mg/L
  2. About 0.02 mg/L
  3. About 0.11 mg/L
  4. About 1.8 mg/L
Show the answer

The farmland bar reaches about 0.18 mg/L.

  • Correct: About 0.18 mg/L: Right: just under 0.2 mg/L.
  • About 0.02 mg/L: 0.02 mg/L is the forest stream.
  • About 0.11 mg/L: 0.11 mg/L is the town stream.
  • About 1.8 mg/L: The axis tops out at 0.2 mg/L.

5. What best explains why the forest stream has the least phosphate?

  1. Roots hold the soil in place.
  2. Forest leaves remove phosphate gas from the air.
  3. Trees release their phosphate into the air as a gas.
  4. Forest soil lacks phosphate, so little can wash in.
Show the answer

Phosphate travels mostly on soil that washes away; forest cover keeps soil in place, and no fertilizer is added.

  • Correct: Roots hold the soil in place.: Right: little erosion and no fertilizer.
  • Forest leaves remove phosphate gas from the air.: There is no phosphate in the air to remove.
  • Trees release their phosphate into the air as a gas.: Phosphorus has no gas form.
  • Forest soil lacks phosphate, so little can wash in.: Forest soils contain phosphate; it just stays put.

6. Which action would most directly lower phosphate in the farmland stream?

  1. Winter cover crops on the fields
  2. Adding more phosphate fertilizer just before heavy rain
  3. Removing the trees that grow along the stream banks
  4. Plowing the fields right before the winter storms
Show the answer

Plant cover holds soil, and phosphate moves on soil. Less soil lost means less phosphate in the stream.

  • Correct: Winter cover crops on the fields: Right: keeping soil in place keeps phosphate out of the water.
  • Adding more phosphate fertilizer just before heavy rain: More fertilizer before rain adds phosphate.
  • Removing the trees that grow along the stream banks: Trees along the bank catch soil; removing them adds phosphate.
  • Plowing the fields right before the winter storms: Bare plowed soil washes away in storms.

7. Why do the soils of tropical rainforests tend to be poor in nutrients even though the forest is lush?

  1. Heavy rain washes nutrients away; plants absorb the rest fast.
  2. The cold slows the breakdown of dead leaves.
  3. Little rain falls, so dead matter stays dry and does not break down.
  4. Fires burn the dead leaves each dry season.
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Warmth and moisture make dead matter decompose fast; heavy rain then washes nutrients out, and the dense plants take up the rest, so little stays in the soil.

  • Correct: Heavy rain washes nutrients away; plants absorb the rest fast.: Right: rain washes nutrients out and plants absorb the rest quickly.
  • The cold slows the breakdown of dead leaves.: Tropical rainforests are warm all year, not cold.
  • Little rain falls, so dead matter stays dry and does not break down.: Tropical rainforests are the wettest biome.
  • Fires burn the dead leaves each dry season.: Yearly fires are a feature of savannas and grasslands.

8. Why is an estuary such a rich place for young fish and shellfish?

  1. Rivers bring nutrients, and the shallow, sheltered water gets plenty of sunlight.
  2. Its water is colder and saltier than the open ocean.
  3. It has no predators, so young animals are safe.
  4. It is deep, so the water is dark and calm.
Show the answer

Nutrients from rivers plus sunlit, shallow water let plants and algae grow fast, and marsh plants or mangroves shelter the young.

  • Correct: Rivers bring nutrients, and the shallow, sheltered water gets plenty of sunlight.: Right: nutrients plus light mean plenty of food, and the young find shelter.
  • Its water is colder and saltier than the open ocean.: Estuaries mix fresh and salt water, so they are less salty than the ocean.
  • It has no predators, so young animals are safe.: Predators live in estuaries too; the young find shelter, not safety from every predator.
  • It is deep, so the water is dark and calm.: Estuaries are shallow, and light reaches the bottom.

9. Why does burning fossil fuels increase the CO2 in the atmosphere so much faster than natural processes remove it?

  1. It releases in decades carbon buried over millions of years.
  2. It turns the oxygen in the air directly into carbon dioxide.
  3. It stops photosynthesis in the forests near the fuel.
  4. It releases methane, which then hardens into rock.
Show the answer

Fossil fuels are ancient carbon that the slow part of the cycle buried. Combustion releases it far faster than photosynthesis, the ocean and sedimentation can take it back up.

  • Correct: It releases in decades carbon buried over millions of years.: Right: slow-cycle carbon is released at a fast-cycle rate.
  • It turns the oxygen in the air directly into carbon dioxide.: Combustion joins carbon from the fuel with oxygen; the carbon comes from the fuel.
  • It stops photosynthesis in the forests near the fuel.: Photosynthesis continues; it is just slower than the release.
  • It releases methane, which then hardens into rock.: Methane does not turn into rock.

10. Why can most plants not use the N2 gas that surrounds them?

  1. The bond holding the two atoms is too strong for plants.
  2. N2 gas is poisonous to the cells of most plants.
  3. N2 gas is found in soil air, not in the air around leaves.
  4. Plants take up nitrogen as a gas through their leaves.
Show the answer

The triple bond in N2 is very hard to break. Only some bacteria (and lightning, or industry) can convert N2 into ammonia or ammonium that plants can use.

  • Correct: The bond holding the two atoms is too strong for plants.: Right: the bond is too strong for plants to break.
  • N2 gas is poisonous to the cells of most plants.: N2 is not toxic; it is simply unusable to most plants.
  • N2 gas is found in soil air, not in the air around leaves.: N2 makes up most of the air around leaves.
  • Plants take up nitrogen as a gas through their leaves.: Plants take up nitrogen through their roots as nitrate and ammonium.

11. Water vapor rises, cools and forms the droplets of a cloud. What is this step?

  1. Condensation
  2. Evaporation
  3. Transpiration
  4. Infiltration
Show the answer

Cooling vapor turns back into tiny liquid droplets: condensation.

  • Correct: Condensation: Right: vapor to liquid is condensation.
  • Evaporation: Evaporation is liquid to vapor.
  • Transpiration: Transpiration is vapor from plants.
  • Infiltration: Infiltration is water entering the soil.

12. Why is net primary productivity (NPP) always smaller than GPP?

  1. Producers use some of it in respiration.
  2. Plant-eating animals eat part of the GPP before it is measured.
  3. Sunlight is lost before it reaches the leaves of the plants.
  4. Decomposers break down part of the GPP each night.
Show the answer

NPP=GPP−R\text{NPP} = \text{GPP} - \text{R}: the plants’ own respiration uses part of what they capture, leaving NPP as new plant matter.

  • Correct: Producers use some of it in respiration.: Right: plant respiration is subtracted.
  • Plant-eating animals eat part of the GPP before it is measured.: Grazing removes NPP later; it is not why NPP is smaller than GPP.
  • Sunlight is lost before it reaches the leaves of the plants.: Light that misses leaves is never counted in GPP.
  • Decomposers break down part of the GPP each night.: Decomposers act on dead matter, not on GPP itself.

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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.