Calculating biomass transfer efficiency between trophic levels
Ecology • Trophic levels in an ecosystem (biology only)
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Definition of transfer efficiency
Transfer efficiency quantifies the proportion of biomass (or energy) passed from a lower trophic level to the next higher level. The measure uses dry biomass or energy units so that water content does not distort values; scientists often dry samples before measuring biomass to ensure consistency . The formula for transfer efficiency equals (biomass at higher trophic level ÷ biomass at lower trophic level) × 100. The result expresses the fraction of biomass incorporated into new tissue at the higher level as a percentage, allowing direct comparison between stages of a food chain.
Cause of low transfer efficiency
Most consumed biomass does not become consumer tissue because some material is not absorbed and is lost as faeces, and the largest loss occurs via respiration as carbon dioxide and water. High respiration rates cause a large portion of consumed chemical energy to be used for movement, growth maintenance and other life processes rather than growth into biomass . As a result of these losses, energy and biomass decrease at each trophic level. Food chains rarely exceed about six trophic levels because successive low transfer efficiencies leave insufficient biomass to support higher predators .
Practical measurement and units
Biomass measurement uses dry mass (g or kg) or energy units (kJ). Dry mass is standard because water content varies between organisms and over time; drying tissue in an oven removes water to give comparable results . Calculations use totals for all organisms at each trophic level. Field data commonly provide numbers and dry mass per organism; multiplying these gives total biomass per species, then summing species within a trophic level gives the level total for the transfer efficiency formula .
Limiting factors affecting transfer efficiency
Low photosynthetic capture of solar energy limits the biomass generated by producers; plants and algae often use only about 1% of incoming solar energy to form biomass, so the energy base for the whole food chain is small . Other limiting factors include food quality and digestibility (which affect how much is absorbed), metabolic rate of consumers (which affects respiration losses), and environmental conditions that change productivity. Changes in these factors alter the proportion of biomass available to higher trophic levels and therefore change transfer efficiency.
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