Why is only about 10% of the biomass transferred from one trophic level to the next?

Sourav PanNovember 12, 2024

Answered

The concept that only about 10% of biomass is transferred from one trophic level to the next is known as the 10% rule in ecology. This rule highlights the inefficiencies in energy transfer within ecosystems and explains why biomass decreases as one moves up the trophic levels. Here’s a detailed explanation of why this occurs:

Reasons for Limited Energy Transfer

1. Metabolic Processes:
   • A significant portion of energy consumed at each trophic level is used for metabolic processes, such as respiration, movement, growth, and reproduction. For instance, animals convert energy from food into heat during metabolism, which is lost to the environment and not available for the next trophic level . This metabolic heat loss is a primary reason why only a fraction of energy is passed on.
2. Waste Production:
   • Organisms do not digest all parts of their food efficiently. Undigested materials (like bones or plant fibers) are excreted as waste, representing a loss of energy that cannot be utilized by the next trophic level . For example, herbivores may consume plant material but only digest a portion of it; the rest is excreted and unavailable to predators .
3. Non-consumed Biomass:
   • Not all organisms at one trophic level are consumed by those at the next level. Some may die without being eaten or may be lost to disease or predation before they can be consumed . This unutilized biomass further reduces the amount of energy available for transfer.
4. Inefficiencies in Energy Conversion:
   • The efficiency with which energy is converted from one form to another also plays a role. For example, plants (producers) convert only about 1% of solar energy into biomass through photosynthesis. This low efficiency compounds the losses as energy moves through the food chain .

Consequences of the 10% Rule

• Decreased Biomass at Higher Trophic Levels: As energy is lost at each step, there is less biomass available to support higher trophic levels. For instance, if a primary producer has 10,000 kilocalories of energy, only about 1,000 kilocalories will be available to primary consumers (herbivores), and even less will be available to secondary consumers (carnivores) .
• Limited Number of Trophic Levels: Due to these energy losses, most ecosystems can support only a limited number of trophic levels—typically three to five—before there isn’t enough energy left to sustain additional levels . This limitation affects food web dynamics and population sizes across different species.

Visual Representation

• The 10% rule can be visually represented in pyramids of biomass or energy, where each successive level has less biomass or energy than the one below it. These pyramids illustrate how energy diminishes as it flows through an ecosystem.