How biomass is lost between trophic levels
Ecology • Trophic levels in an ecosystem (biology only)
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Definition of biomass
Biomass is the mass of biological material in organisms when water is removed; measurements commonly use dry mass to avoid variation from water content. Dry mass provides a consistent measure of living tissue available for transfer between trophic levels . Measurements of biomass often rely on estimates or sampled individuals because direct drying of large numbers would be destructive.
Trophic transfer efficiency (the 10% rule)
Approximately 10% of the biomass or energy in one trophic level becomes biomass in the next level; about 90% is lost at each transfer. This low efficiency limits the length of food chains and reduces the number of organisms at higher trophic levels . Producers capture only a small fraction of incoming solar energy (about 1%), and consumers convert only a fraction of consumed biomass into new tissue, so only a small percentage passes up the chain .
Loss by not being eaten (uneaten biomass)
Cause: Parts of organisms (roots, bones, shells, uneaten fragments) remain uneaten by the consumer. Effect: That biomass never enters the consumer’s digestive system and therefore does not contribute to consumer biomass. Uneaten biomass becomes detritus for decomposers or remains as untransferred material in the ecosystem.
Loss by egestion (faeces and waste)
Cause: Some consumed biomass passes through the gut unabsorbed and leaves as faeces. Effect: Energy and mass in faeces do not become consumer tissue and are instead available to decomposers. Significant proportions of consumed energy can be lost this way, reducing net transfer to the next trophic level .
Loss by respiration and metabolic processes
Cause: Organisms use organic molecules for respiration to release energy for movement, growth, reproduction and other life processes. Effect: Respiration converts organic carbon into carbon dioxide and water; these products leave the organism and reduce biomass. Respiration represents the largest single loss of biomass and energy between trophic levels . Example energy accounting: of 100 kJ consumed, only a small fraction becomes new tissue; much is used in respiration and released as CO2, water and heat .
Other specific losses (movement, temperature, excretion)
Cause: Energy used for movement, maintaining body temperature, and excreting nitrogenous wastes consumes organic material. Effect: These processes convert biomass into heat, CO2, water and excreted compounds, further reducing the fraction available as new tissue. Animals with high metabolic rates or active lifestyles transfer proportionally less biomass to predators.
Role of decomposers and recycling
Cause: Dead organisms, uneaten parts and faeces are broken down by decomposers (bacteria and fungi). Effect: Decomposers return mineral ions to soil and release CO2 by their respiration; much original biomass is recycled rather than transferred up the food chain, so it does not increase consumer biomass directly .
Limiting factors that affect biomass transfer
Cause: Environmental and biological factors such as light availability, nutrient supply, temperature, digestive efficiency and organism activity level. Effect: Reduced primary production (e.g., low light) lowers producer biomass and therefore limits biomass available to higher trophic levels. High respiration rates or poor digestion reduce transfer efficiency further . Conclusion: The combination of low capture of solar energy, losses through egestion and respiration, and environmental limits explains why pyramids of biomass are always smaller at higher trophic levels .
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