Productivity and energy transferEdexcel International A Level Biology: Revision notes

Productivity is the rate at which energy (or biomass) is made in an ecosystem, in units such as kJ m⁻² year⁻¹ or g m⁻² year⁻¹.

• Gross primary productivity (GPP) is the total energy fixed by producers in photosynthesis per unit area per unit time.
• Respiration (R) is the energy producers lose as heat in respiration.
• Net primary productivity (NPP) is the energy left stored in new plant biomass.

NPP = GPP − R

Worked example: GPP is 14 500 and R is 6 000 kJ m⁻² year⁻¹, so NPP = 14 500 − 6 000 = 8 500 kJ m⁻² year⁻¹. This is the energy available to consumers and decomposers.

Energy enters a food chain as light energy captured by producers, and passes to primary consumers, secondary consumers and so on. At each level only a fraction of the energy is transferred.

Efficiency of transfer (%) = (energy or biomass in the trophic level ÷ energy or biomass in the previous level) × 100

Worked example: producers have 12 000 kJ m⁻² year⁻¹ and primary consumers 1 080 kJ m⁻² year⁻¹, so efficiency = 1 080 ÷ 12 000 × 100 = 9.0%.

The efficiency is typically about 10% (5 to 20%). Only about 1 to 3% of the light energy reaching a crop ends up as NPP.

Between trophic levels, energy is not passed on because:

• Not all biomass is eaten: roots, bones, fur, or organisms that die uneaten.
• Not all biomass eaten is digested and absorbed: undigested material (for example cellulose and lignin) is lost in faeces.
• Excretion: energy is lost in urea and other excretory products.
• Respiration: a large part of the energy assimilated is used for movement, active transport and maintaining body temperature and lost as heat.

Only the energy stored in new biomass passes on. Plant material is harder to digest than animal tissue, so transfer from plants to herbivores is often less efficient than from herbivores to carnivores.

Only a small percentage of the light reaching a plant is used in photosynthesis because:

• some light is reflected, transmitted through the leaf, or falls on non-photosynthetic tissue or bare ground;
• some wavelengths are not absorbed by the pigments;
• photosynthesis may be limited by carbon dioxide, temperature or water;
• some of the energy fixed is lost as heat in respiration (so NPP is less than GPP).

This low efficiency, and the losses at each trophic level, means long food chains support few organisms at the top. Farming crops for direct human consumption therefore feeds more people per hectare than rearing animals, although other factors also matter.