Energy Flow in Ecosystems — Practice · Grade 10 Science

Chapter: Ecosystems & Conservation Decisions

What this practice covers

By the end of Energy Flow in Ecosystems you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: autotroph, heterotroph, trophic level, biomass, energy pyramid, ten percent rule, standing biomass, transfer efficiency.

Problem 1

An organism that makes its own food from sunlight, carbon dioxide, and water is best described as a(n)
  1. primary consumer
  2. decomposer
  3. autotroph
  4. heterotroph
Answer and explanation

autotroph

Autotrophs (producers) build their own food, usually through photosynthesis; heterotrophs must consume other organisms.

Problem 2

Producers in a pond store 60,000 kcal of energy. Using the 10% rule, how many kcal are available to the primary consumers?
Answer and explanation

6000 kcal

60,000 × 0.10 = 6,000 kcal.

Problem 3

If secondary consumers have 600 kcal of energy available, about how many kcal reach the tertiary consumers above them?
Answer and explanation

60 kcal

600 × 0.10 = 60 kcal.

Problem 4

Which trophic level in an energy pyramid has the LEAST usable energy available?
  1. Producers
  2. Primary consumers
  3. Secondary consumers
  4. Top predators
Answer and explanation

Top predators

Energy is lost at every step, so the top predators at the peak have the least energy available.

Problem 5

Use the 10% rule to explain why a given field of farmland can feed more people if it grows grain for humans than if it grows grain to raise cattle for beef.
Answer and explanation

When cattle eat grain, about 90% of the grain's energy is used for the cattle's own life processes or lost as heat, so only about 10% is stored as beef for people to eat. Eating the grain directly skips that lossy step, so far more of the field's captured energy actually reaches people — meaning the same land can feed many more of them.

Problem 6

A level contains 8000 kJ of production; supplied transfer efficiency to the next is 15%. Find the next level's production. Explain the reasoning in your own words.
Answer and explanation

1200 kJ under the supplied efficiency; remaining energy is not all converted into one identical output.

Convert 15% to 0.15.

8000×0.15=1200 kJ.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

A level produces 2000 kJ and transfer efficiency is 8%. Find next-level production. Show the reasoning, not just the conclusion.
Hints

Production transferred is only part of the total consumed-energy account.

Answer and explanation

2000×0.08=160 kJ.

Problem 8

Why do food-web arrows typically point from food to consumer?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

They represent transfer of matter and energy to the consumer; check the diagram's stated convention.

Problem 9

A small standing biomass can support a larger stock through rapid turnoverA small standing biomass can support a larger stock through rapid turnoverSupplied standing mass at one instantProducer standingbiomass20Consumer standingbiomass100Rapid producer turnover can replenish a small standing stock.Energy-flow rate and standing biomass are different quantities.
Original snapshot biomass example, not a measured aquatic ecosystem or a matched energy-flow pyramid.
Interpret the supplied visual. Does greater consumer standing mass imply energy was created between trophic levels?
Answer and explanation

No. Standing stock differs from production/transfer rate; rapid producer turnover and the stated time boundary matter.

Standing biomass is material present at one time; production is new material per time. A producer community can turn over rapidly, sustaining consumers while its instantaneous mass remains relatively small, as some aquatic comparisons show. Pyramids of numbers can also invert when many consumers depend on a few large producers. A conventional energy-transfer pyramid uses matched rates and boundaries, not the same snapshot quantity. Distinguish stocks, rates and organism counts before declaring a diagram impossible or generalizing to every system.

No. Standing stock differs from production/transfer rate; rapid producer turnover and the stated time boundary matter.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

A supplied chain transfers 15% of 8,000 kJ to its primary consumer level, then 8% of that to a secondary level. Calculate both outputs and explain why 10% should not replace the supplied values.
Answer and explanation

The outputs are 1,200 kJ and 96 kJ. The stated transfer fractions define this comparison; ten percent is an approximation, not a compulsory constant.

The outputs are 1,200 kJ and 96 kJ. The stated transfer fractions define this comparison; ten percent is an approximation, not a compulsory constant.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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