How transfer efficiency shapes trophic populations

Ecology • Trophic levels in an ecosystem (biology only)

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Why are food chains usually short?

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Food chains are short because insufficient biomass remains after several transfers to sustain further trophic levels.

Key concepts

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Definition: Biomass and trophic levels

Biomass is the dry mass of living or recently dead organisms, measured to avoid variation from water content. A trophic level represents organisms that share the same position in a food chain, such as producers, primary consumers and secondary consumers. Measuring biomass normally uses dried tissue or estimates from sampled data.

Transfer efficiency: amount passed on

Transfer efficiency describes the proportion of biomass or energy passed from one trophic level to the next. Typical transfer efficiency between levels averages about 10%, so roughly 90% of biomass or usable energy is lost at each step. This low efficiency arises because organisms use most energy for life processes and release it as heat or excreted material. The limited percentage passed on explains why energy and biomass decline sharply up a food chain.

Causes of biomass loss

A large portion of biomass is lost through respiration as carbon dioxide and water when organisms release energy for movement, growth and other processes. Additional losses occur because not all material in eaten food is absorbed; some is lost as faeces or remains uneaten (bones, roots, indigestible parts). Decomposers return remaining material to the system but do not transfer it directly to the next consumer level. These processes together reduce the amount of biomass available to higher trophic levels.

Population effects: fewer organisms at higher levels

Cause: Low transfer efficiency leaves only a small fraction of biomass available for the next trophic level. Effect: Fewer individuals can be supported at higher trophic levels because available food mass is smaller. Predator populations therefore remain smaller and more spread out than prey populations. Food chains rarely extend beyond a few levels because insufficient energy remains to sustain additional levels. The pyramid shape of biomass visually reflects this reduction in organism numbers and total mass.

Limiting factors and practical implications

Limiting factors for transfer efficiency include the proportion of plant material consumed, the digestive efficiency of consumers, metabolic rates (respiration losses), and environmental effects on primary productivity. Lower primary productivity or higher metabolic losses reduce biomass at all higher levels and cause further declines in consumer numbers. In managed systems, increasing primary productivity or shortening food chains increases the mass of edible biomass per unit area.

Key notes

Important points to keep in mind

Biomass measurements use dried tissue to avoid water variation.

Producers capture only a small fraction of solar energy (about 1%).

Only about 10% of biomass is transferred between trophic levels on average.

Major losses occur through respiration, faeces and uneaten material.

Lower transfer efficiency causes smaller biomasses and fewer organisms at higher levels.

Short food chains result from compounded energy losses at each transfer.

Higher metabolic rates reduce transfer efficiency by increasing respiration losses.

Improving primary productivity increases biomass available to all higher trophic levels.