Net production of consumers (N) = I – (F + R), where I = chemical energy in ingested food, F = chemical energy lost in faeces and urine, R = respiratory losses
Animals cannot use all the energy in their food. Net production is the energy left over after losses in faeces, urine, and respiration — energy available to build new biomass.
Formula
N = I − (F + R)
Real World
A dairy cow ingests about 55 000 kJ day⁻¹ but loses roughly 20 000 kJ in faeces and urine and 30 000 kJ through respiration, leaving only about 5 000 kJ day⁻¹ as net production in milk and body growth.
Exam Focus
Clearly distinguish F (faeces and urine) from R (respiration) — listing them separately earns both marking points.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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