Explain how biomass is lost at each trophic level in a food chain, and state one reason why this limits the number of trophic levels that can be supported in any ecosystem.

At each trophic level:
1. Only part of the food is assimilated; the rest is excreted as faeces or egesta.
2. The assimilated biomass is partly used for cellular respiration, releasing CO₂ and heat.
3. The remaining biomass is allocated to growth, reproduction and excretion, so it is not passed on to the next level.
4. Consequently, by the time the energy reaches the top predators, only a very small amount of biomass remains, making it impossible to support another trophic level.

One reason this limits trophic levels is that the energy/biomass available at the highest levels is too low to sustain a further population, so ecosystems can only support a limited number of trophic levels.