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Quiz: Energy Flow in Ecosystems

Test your understanding of how energy enters, moves through, and exits ecosystems with these review questions.

1. What percentage of incoming solar energy do plants and other photosynthetic organisms typically capture?

  1. About 10-15%
  2. About 1-2%
  3. About 25-30%
  4. About 47%
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The correct answer is B. Of all the solar energy that reaches Earth's surface, plants and other photosynthetic organisms capture only about 1-2% of it through photosynthesis. Despite this small percentage, that captured energy powers virtually every food chain and every living organism on the planet. Much of the remaining energy is reflected, absorbed as heat, or hits areas with sparse vegetation.

Concept Tested: Solar Energy Input

2. What is the relationship between gross primary productivity and net primary productivity?

  1. NPP equals GPP plus autotroph respiration
  2. GPP equals NPP minus heterotroph respiration
  3. NPP equals GPP minus autotroph respiration
  4. GPP equals NPP plus heterotroph respiration
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The correct answer is C. Net primary productivity (NPP) equals gross primary productivity (GPP) minus autotroph respiration (Ra). GPP is the total energy fixed by producers through photosynthesis, while NPP is what remains after producers use energy for their own metabolic needs. NPP represents the energy available to all other organisms in the ecosystem, typically 30-60% of GPP.

Concept Tested: Net Primary Productivity

3. Why do endotherms typically have lower trophic efficiency than ectotherms?

  1. Endotherms eat less food per unit body weight than ectotherms
  2. Endotherms use a large portion of food energy to maintain constant body temperature
  3. Endotherms have slower metabolic rates that limit energy absorption
  4. Endotherms consume only producers while ectotherms consume multiple trophic levels
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The correct answer is B. Endotherms (warm-blooded animals like mammals and birds) burn enormous amounts of energy to maintain a constant body temperature. A mouse might use 98% of its food energy on metabolism, converting only 2% into new body mass. Ectotherms do not spend energy on temperature regulation, so they convert a much higher percentage of consumed energy into growth and biomass.

Concept Tested: Trophic Efficiency

4. A forest has a GPP of 10,000 kcal/m2/yr, autotroph respiration of 6,000 kcal/m2/yr, and heterotroph respiration of 3,500 kcal/m2/yr. What is the net ecosystem production?

  1. 4,000 kcal/m2/yr
  2. 6,500 kcal/m2/yr
  3. 500 kcal/m2/yr
  4. 3,500 kcal/m2/yr
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The correct answer is C. First calculate NPP: GPP minus autotroph respiration equals 10,000 minus 6,000, which gives 4,000 kcal/m2/yr. Then calculate NEP: NPP minus heterotroph respiration equals 4,000 minus 3,500, which gives 500 kcal/m2/yr. Since NEP is greater than zero, this forest is acting as a carbon sink, storing more energy than it releases.

Concept Tested: Net Ecosystem Production

5. How does chemosynthesis differ from photosynthesis?

  1. Chemosynthesis produces oxygen while photosynthesis produces carbon dioxide
  2. Chemosynthesis occurs only in freshwater while photosynthesis occurs only on land
  3. Chemosynthesis uses chemical energy from inorganic molecules instead of sunlight
  4. Chemosynthesis is performed by plants while photosynthesis is performed by bacteria
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The correct answer is C. Chemosynthesis uses chemical energy from inorganic compounds such as hydrogen sulfide, methane, or ammonia to produce organic molecules, while photosynthesis uses light energy. Chemosynthetic bacteria at hydrothermal vents form the base of food webs in complete darkness, demonstrating that life does not require sunlight. Both processes are autotrophic.

Concept Tested: Chemosynthesis

6. Which law of thermodynamics explains why energy pyramids always narrow at the top?

  1. The first law, because energy is created at lower trophic levels
  2. The zeroth law, because thermal equilibrium must be maintained
  3. The third law, because entropy approaches zero at absolute zero
  4. The second law, because every energy conversion loses some energy as heat
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The correct answer is D. The second law of thermodynamics states that every energy conversion increases entropy and results in some energy being lost as unusable heat. This means energy is degraded at every trophic transfer, making less energy available at each successive level. This unavoidable energy loss explains why energy pyramids are always wider at the base than at the top.

Concept Tested: Thermodynamics in Ecology

7. What does it mean when an ecosystem has a negative net ecosystem production?

  1. The ecosystem is a carbon sink that is growing rapidly
  2. The ecosystem has no primary producers and depends on external energy
  3. The ecosystem is a carbon source releasing more carbon than it stores
  4. The ecosystem has reached maximum biodiversity and is in equilibrium
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The correct answer is C. When NEP is negative, the ecosystem's total respiration exceeds its gross primary productivity, meaning it releases more carbon dioxide than it absorbs. This makes the ecosystem a carbon source. This can happen after a wildfire, during prolonged drought, or due to deforestation. If many ecosystems shift from sinks to sources, it could accelerate climate change through a positive feedback loop.

Concept Tested: Net Ecosystem Production

8. Why can ocean ecosystems sometimes have an inverted biomass pyramid?

  1. Ocean consumers are always smaller than ocean producers
  2. Phytoplankton reproduce so rapidly that consumers can outweigh standing producer biomass
  3. Ocean water pressure compresses organisms at lower trophic levels
  4. Marine decomposers add biomass to consumer trophic levels
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The correct answer is B. In many ocean ecosystems, phytoplankton have extremely high productivity but low standing biomass because they reproduce and die rapidly. The biomass of consumers like zooplankton can actually exceed the biomass of producers at any given moment. This illustrates the important difference between productivity (a rate) and biomass (a snapshot quantity).

Concept Tested: Biomass

9. What is the fundamental difference between how energy and matter move through ecosystems?

  1. Energy cycles continuously while matter flows in one direction
  2. Energy flows in one direction while matter cycles continuously
  3. Both energy and matter flow in one direction from producers to consumers
  4. Both energy and matter cycle continuously between organisms and the environment
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The correct answer is B. Energy flows in one direction through ecosystems -- entering as sunlight, passing through trophic levels, and exiting as heat that cannot be recaptured. Matter, including carbon, nitrogen, and water, cycles continuously between organisms and the environment through biogeochemical cycles. This fundamental distinction is a consequence of the laws of thermodynamics.

Concept Tested: Entropy in Ecosystems

10. Which type of organism is classified as an autotroph?

  1. A wolf that hunts deer in a temperate forest
  2. An earthworm that feeds on dead organic matter in soil
  3. A cyanobacterium that converts sunlight into chemical energy
  4. A mushroom that decomposes fallen logs on the forest floor
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The correct answer is C. Autotrophs are organisms that produce their own food from inorganic raw materials. Cyanobacteria are photoautotrophs that use sunlight to convert carbon dioxide and water into glucose through photosynthesis. They were the original oxygen-producing photosynthesizers, dating back 2.7 billion years. Wolves, earthworms, and mushrooms are all heterotrophs that must consume organic matter produced by other organisms.

Concept Tested: Autotrophs