The second law of thermodynamics limits energy transfer efficiency because energy transformations are inefficient: some energy is dispersed as heat and becomes unavailable for biomass production. Therefore, transfer between trophic levels can never be efficient.
The Mechanism
This is tested in Subtopic 2.2: Energy and biomass in ecosystems. Producers transform light energy into chemical energy stored in organic compounds. Consumers obtain energy by feeding, but only a fraction becomes new biomass.
During cellular respiration, chemical energy supports movement, growth and active transport. Some is inevitably transformed into heat and dispersed. Ecosystems therefore require a continuous energy input, usually solar radiation. At HL, this dispersal is linked to increasing entropy.
| Fate of energy | Effect on transfer to the next trophic level |
|---|---|
| Respiration and heat loss | Dispersed energy cannot become biomass |
| Movement and metabolism | Energy is used without directly producing biomass |
| Uneaten material | It does not enter the next consumer |
| Faeces and excretion | Organic matter is not assimilated |
| Growth and reproduction | Stored biomass remains available to consumers |
For example, if a trophic level receives and stores as biomass available to the next level, its ecological efficiency is:
Ecological efficiency calculations are HL only; SL students explain energy losses qualitatively. A common misconception is that energy is recycled. It is not: energy flows and becomes dispersed as heat, while matter cycles.
Declining energy availability limits food-chain length. Progressively less energy is available from phytoplankton to zooplankton to cod in the North Atlantic. Harvesting or eating at lower trophic levels can be more resource-efficient, although cultural preferences and equitable food access also matter.
Exam Technique
For an explain question, connect energy transformation to heat loss, reduced biomass transfer and shorter food chains. At HL, add entropy and ecological efficiency. Do not call a fixed law or true average; efficiency varies among ecosystems, trophic levels and species.