Energy Transfer and Productivity

The Source of Energy

• Almost all energy in ecosystems comes from sunlight
• Producers (autotrophs, mainly plants) convert light energy into chemical energy via photosynthesis
• Only about 1–3% of the light energy reaching a plant is actually converted to chemical energy in organic molecules
• Reasons for low efficiency:
  • Light may be the wrong wavelength (only certain wavelengths are absorbed by chlorophyll)
  • Light may miss the leaves (hits soil, water, or is reflected)
  • Light may hit non-photosynthetic parts of the plant
  • Photosynthetic reactions are not 100% efficient

Energy Transfer Between Trophic Levels

• Energy is transferred from one trophic level to the next when organisms are eaten
• At each level, approximately 10% of energy is transferred to the next (the 10% rule is a rough average)

Where Does the Energy Go?

At each trophic level, energy is lost through:

1. Respiration — the largest energy loss. Organisms respire to release energy for life processes (movement, growth, reproduction, maintaining body temperature). This energy is ultimately lost as heat
2. Excretion — energy in waste products (urine, CO₂)
3. Egestion — energy in faeces (food that was not digested and absorbed)
4. Not all organisms are eaten — dead organisms and waste are broken down by decomposers (bacteria and fungi)

Gross Primary Productivity (GPP)

• The total rate of energy fixed by photosynthesis (total chemical energy stored in organic molecules by producers)
• Measured in $\text{kJ m}^{-2} \text{yr}^{-1}$

Net Primary Productivity (NPP)

• The energy available to primary consumers (herbivores) and decomposers
• It is GPP minus the energy lost through the plant's own respiration ($R$):

$NPP = GPP - R$

• NPP represents the rate of production of new biomass available for the next trophic level

Net Production at Consumer Levels

For consumers: $\text{Net production} = \text{Energy ingested} - \text{Energy lost in faeces} - \text{Energy lost in respiration} - \text{Energy lost in urine}$

Or more simply: $N = I - F - R$

Where:

• $N$ = net production (available for growth/next trophic level)
• $I$ = energy ingested (food eaten)
• $F$ = energy lost as faeces and urine
• $R$ = energy lost in respiration

The efficiency of energy transfer between trophic levels:

$\text{Efficiency} = \frac{\text{Net production of the trophic level}}{\text{Net production of the previous level}} \times 100\%$

Typically 5–20% (average ~10%)

Why Are Food Chains Short?

• So much energy is lost at each level that after 4–5 transfers, there is insufficient energy to support another trophic level
• Top predators need huge territories to obtain enough food energy

Endotherms vs Ectotherms

• Endotherms (mammals, birds) use a lot of energy from respiration to maintain body temperature → less efficient energy transfer (more energy lost as heat)
• Ectotherms (reptiles, fish, invertebrates) rely on external heat sources → more efficient energy transfer (less energy lost in respiration for thermoregulation)
• This is why farming ectotherms (e.g., fish) can be more energy-efficient than farming endotherms (e.g., cattle)

• Pyramids of energy show the rate of energy flow through each trophic level
• Always pyramid-shaped (cannot be inverted — energy is always lost at each level)
• More accurate than pyramids of numbers or biomass
• Units: $\text{kJ m}^{-2} \text{yr}^{-1}$

Question: A grassland ecosystem has a GPP of $20{,}000 \text{ kJ m}^{-2} \text{yr}^{-1}$. Plants use $12{,}000 \text{ kJ m}^{-2} \text{yr}^{-1}$ in respiration. Primary consumers ingest $2{,}000 \text{ kJ m}^{-2} \text{yr}^{-1}$ and their net production is $200 \text{ kJ m}^{-2} \text{yr}^{-1}$. Calculate the NPP and the efficiency of energy transfer from producers to primary consumers. (3 marks)

Solution:

$NPP = GPP - R = 20{,}000 - 12{,}000 = 8{,}000 \text{ kJ m}^{-2} \text{yr}^{-1}$

$\text{Efficiency} = \frac{\text{Net production of primary consumers}}{NPP} \times 100 = \frac{200}{8{,}000} \times 100 = 2.5\%$

• Energy enters ecosystems via photosynthesis (only 1–3% of light energy is captured).
• $NPP = GPP - R$; NPP is the energy available to consumers.
• Only ~10% of energy transfers between trophic levels; the rest is lost to respiration, faeces, urine, and decomposition.
• Endotherms lose more energy in respiration than ectotherms (less efficient farming).
• Farming practices (restricting movement, controlling temperature, selective breeding) aim to maximise energy conversion to useful biomass.