Transfers of energy and matter practice questions.

In a food chain, organisms that convert light energy into chemical energy are called

• consumers
• producers
• decomposers
• detritivores

Answer: producers

Producers (autotrophs) synthesise organic molecules from inorganic sources, typically by photosynthesis.

Organisms that break down dead organic matter and release nutrients are called

• producers
• decomposers
• primary consumers
• apex predators

Answer: decomposers

Decomposers (e.g. bacteria and fungi) recycle nutrients back into the ecosystem.

A pyramid of energy shows

• the number of organisms at each level
• the energy at each trophic level
• the mass of producers only
• the age of an ecosystem

Answer: the energy at each trophic level

Pyramids of energy are always narrower towards the top because energy is lost between trophic levels.

A food chain shows gross primary production of 20 000 kJ m⁻² yr⁻¹ by producers. Herbivores assimilate 1800 kJ m⁻² yr⁻¹ from the producers. Calculate the approximate efficiency of energy transfer from producers to herbivores.

• 0.9 %
• 9 %
• 18 %
• 90 %

Answer: 9 %

Efficiency = (energy at next level ÷ energy at previous level) × 100 = (1800 ÷ 20 000) × 100 = 9 %, close to the typical ~10 % rule.

State the term for the total quantity of energy or organic matter fixed by producers per unit area per unit time, before any is lost in respiration.

• Net primary production (NPP)
• Gross primary production
• Secondary production
• Standing crop

Answer: Gross primary production

Gross primary production (GPP) is the total organic matter fixed by producers; net primary production (NPP) = GPP − respiration by producers.

In an ecosystem, gross primary production is 12 000 kJ m⁻² yr⁻¹ and producer respiration is 4500 kJ m⁻² yr⁻¹. Calculate the net primary production.

• 3000 kJ m⁻² yr⁻¹
• 7500 kJ m⁻² yr⁻¹
• 16 500 kJ m⁻² yr⁻¹
• 8500 kJ m⁻² yr⁻¹

Answer: 7500 kJ m⁻² yr⁻¹

NPP = GPP − respiration = 12 000 − 4500 = 7500 kJ m⁻² yr⁻¹.