Energy Flow in Ecosystems — Drills · Grade 10 Science

Chapter: Ecosystems & Conservation Decisions

What these drills covers

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

I can classify organisms as autotrophs or heterotrophs and place them on trophic levels in a food web.
I can use the 10% rule to calculate the energy available at each level of an energy pyramid.

6 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

In a food chain, organisms that capture energy from sunlight occupy the

first trophic level
second trophic level
third trophic level
decomposer level only

Answer and explanation

first trophic level

Producers form the first trophic level; all higher levels depend on the energy they capture.

Problem 2

Using a 10 percent transfer, how much of a producer level's 20,000 kJ reaches the second trophic level?

Answer and explanation

2000 kJ

20,000 × 0.10 = 2,000 kJ, the rest being respired, lost as heat, or not consumed.

Problem 3

Using a 10 percent transfer at each step, how much of a producer level's 20,000 kJ reaches the fourth trophic level?

Answer and explanation

20 kJ

20,000 × 0.1³ = 20 kJ. Three transfers reduce the available energy a thousandfold.

Problem 4

Decomposers are essential to an ecosystem because they

add energy to the system
return nutrients from dead material to inorganic forms producers can use
occupy the first trophic level
prevent energy loss

Answer and explanation

return nutrients from dead material to inorganic forms producers can use

Decomposers close the matter cycle by releasing elements locked in dead tissue. They add no energy; energy still enters only through producers.

Problem 5

Explain why food chains rarely have more than four or five levels.

Answer and explanation

Only about a tenth of the energy at one level becomes available at the next, the rest being used in respiration, lost as heat, or never consumed. After three or four transfers the energy remaining is a thousandth or a ten-thousandth of what the producers captured, which cannot support a viable population of another level. The limit is energetic rather than anatomical.

Response criteria

State a biologically accurate conclusion.
Show the data, mechanism, or calculation that supports it.
Include units, conditions, or limits where relevant.

Problem 6

A pyramid of numbers for one ecosystem is wider at the second level than the first. Explain how this is possible when the energy pyramid cannot be.

Answer and explanation

A pyramid of numbers counts individuals regardless of size, so one very large producer — a single tree — can support many thousands of insects, making the second level wider. Energy is a different quantity: the tree contains far more stored energy than the insects feeding on it, and transfer losses mean the second level always holds less energy than the first. Counting individuals and measuring energy answer different questions, which is why the shapes can differ.

Response criteria

State a biologically accurate conclusion.
Show the data, mechanism, or calculation that supports it.
Include units, conditions, or limits where relevant.

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