Ecosystem Structure & Energy Flow — Practice · Grade 12 Science

Chapter: Ecology, Populations & Biodiversity

What this practice covers

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

9 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

In an energy pyramid, which trophic level has the LEAST energy available to it?
  1. Primary consumers
  2. Secondary consumers
  3. Top predators
  4. Producers
Answer and explanation

Top predators

Because only ~10% of energy transfers at each step, the top predators at the peak have the least energy available.

Problem 2

A wetland's producers have a GPP of 18,000 kcal/m²/yr and spend 7,000 kcal/m²/yr on respiration. What is the net primary productivity (NPP)?
Answer and explanation

11000 kcal/m²/yr

NPP = GPP - respiration = 18,000 - 7,000 = 11,000 kcal/m²/yr.

Problem 3

Producers store 80,000 kcal/m²/yr as NPP. Using the 10% rule, how much energy is available to the secondary consumers (two levels up)?
Answer and explanation

800 kcal/m²/yr

Primary consumers: 80,000 × 0.10 = 8,000; secondary consumers: 8,000 × 0.10 = 800 kcal/m²/yr.

Problem 4

Net primary productivity (NPP) is best described as
  1. the energy lost as heat by top predators
  2. the energy released by decomposers
  3. the total energy producers capture from the sun before any is used
  4. the energy producers store as biomass after paying their own respiration cost
Answer and explanation

the energy producers store as biomass after paying their own respiration cost

NPP = GPP - respiration, the energy left as biomass and available to consumers.

Problem 5

Explain how limited trophic-transfer efficiency can constrain higher-level production. Does it establish a universal maximum of four or five trophic levels?
Answer and explanation

Production generally decreases along successive transfers because respiration, uneaten material, and other pathways limit transfer into new biomass. Actual efficiencies and food webs vary; the10% estimate does not establish a fixed universal maximum number of trophic levels. Organism size, production, turnover, and feeding relationships also matter.

Problem 6

A producer level has 4000 kJ and efficiency 20%. Find next-level production. Show the reasoning, not just the conclusion.
Hints

Supplied efficiencies are conditional model values.

Answer and explanation

800 kJ.

Problem 7

Why should decomposers appear connected to several trophic levels?
Hints

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

Answer and explanation

They process dead material and wastes from producers and multiple consumer levels.

Problem 8

Energy transfer and matter cycling need separate accountsEnergy transfer and matter cycling need separate accountsSunlight inputBiologicaltransfersHeat tosurroundingsMatter can cycle through reservoirs.Energy flows through; elements are recycled through processes.
Original qualitative energy flow and material-cycle distinction; no numerical fluxes are implied.
Interpret the supplied visual. Can decomposers recycle the heat lost by respiration into the original food energy without a new energy source?
Answer and explanation

No. Nutrients can be reused, but restoring chemical energy requires an energy input and appropriate processes.

Energy enters many ecosystems as sunlight and eventually disperses as heat; it is not recycled through a closed biological energy loop. Matter such as carbon, nitrogen and water moves among reservoirs through physical, chemical and biological processes. Decomposers transform organic material and return accessible nutrients, while their respiration also transfers energy. Draw separate energy arrows and element inventories so that a material cycle is not mistaken for unlimited energy recovery.

No. Nutrients can be reused, but restoring chemical energy requires an energy input and appropriate processes.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 9

A habitat has GPP12000 kJ/m²/yr and producer respiration 5000. Consumers produce 840 kJ/m²/yr. Find NPP and the NPP-to-consumer production efficiency.
Answer and explanation

NPP=7000 kJ/m²/yr. Efficiency=840/7000=12%. The fraction uses NPP, not GPP, and does not imply every intermediate process has 12% efficiency.

NPP=7000 kJ/m²/yr.

Efficiency=840/7000=12%.

The fraction uses NPP, not GPP, and does not imply every intermediate process has 12% efficiency.

Response criteria

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

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