Biomass, Productivity and Transfer Through Trophic Levels

Overview

Measuring biomass as dry mass, gross and net primary production with calculations, energy losses and efficiency between trophic levels, and drawing and interpreting pyramids of biomass.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is biomass?

   Model answer

   Biomass is the total mass of living material, usually expressed as dry mass per unit area.

2. 2

   Why is dry mass rather than fresh mass used when biomass is measured?

   Model answer

   The water content of an organism varies greatly and contains no energy, so removing it makes the comparison between organisms valid.

3. 3

   Give one disadvantage of measuring biomass as dry mass.

   Model answer

   The organisms must be killed and dried to constant mass, so the same individuals cannot be sampled again.

4. 4

   What is gross primary production?

   Model answer

   It is the total quantity of chemical energy fixed by the producers in a given area in a given time.

5. 5

   What is net primary production, and how is it calculated?

   Model answer

   It is the chemical energy stored in plant biomass after respiratory losses, calculated as gross primary production minus the energy lost in respiration.

6. 6

   The gross primary production of a grassland is 30 000 kJ m⁻² y⁻¹ and the plants lose 8000 kJ m⁻² y⁻¹ in respiration. Calculate the net primary production.

   Model answer

   30 000 − 8000 gives a net primary production of 22 000 kJ m⁻² y⁻¹.

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