Ecosystem Structure & Energy Flow — Drills · Grade 12 Science

Chapter: Ecology, Populations & Biodiversity

What these drills covers

By the end of Ecosystem Structure & Energy Flow you should be able to:

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

Vocabulary used: autotroph, heterotroph, trophic level, gross primary productivity (GPP), net primary productivity (NPP), ecological efficiency (10% rule).

Problem 1

Which organism is an autotroph (producer)?
  1. An oak tree
  2. A gray wolf
  3. A mushroom
  4. A red-tailed hawk
Answer and explanation

An oak tree

An oak tree makes its own food by photosynthesis, so it is an autotroph; the others must eat other organisms.

Problem 2

A forest has a GPP of 12,000 kcal/m²/yr, and its producers respire 4,500 kcal/m²/yr. Find the net primary productivity (NPP).
Answer and explanation

7500 kcal/m²/yr

NPP = GPP - respiration = 12,000 - 4,500 = 7,500 kcal/m²/yr.

Problem 3

Producers store 30,000 kcal/m²/yr as NPP. Using the 10% rule, how much energy reaches the primary consumers?
Answer and explanation

3000 kcal/m²/yr

30,000 × 0.10 = 3,000 kcal/m²/yr.

Problem 4

About 90% of the energy at a trophic level never passes to the next level. Most of that energy is
  1. used to create additional sunlight
  2. used for life processes and lost as heat through respiration
  3. stored as new biomass for the next level to eat
  4. turned into new nutrients by producers
Answer and explanation

used for life processes and lost as heat through respiration

Most energy is spent on movement, growth, and staying alive, and ultimately radiates away as heat.

Problem 5

Explain why an ecosystem can support many more producers than top predators.
Answer and explanation

Because only about 10% of energy transfers at each step, far less energy is available at the top of the pyramid than at the base. The little energy that reaches top predators can support only a small population, while the energy-rich base can support enormous numbers of producers.

Problem 6

Producers capture 200,000 kcal/m²/yr. Using the 10% rule, how much energy is available to the tertiary consumers (three levels above the producers)?
Answer and explanation

200 kcal/m²/yr

200,000 × 0.10 × 0.10 × 0.10 = 200 kcal/m²/yr.

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