Efficiency measures biomass transfer

1. Only part of the biomass available at one trophic level is converted into new biomass at the next level.
2. Transfer efficiency describes how large that transferred part is compared with the starting amount.
3. For a biomass transfer, use $efficiency=\frac{biomass\; transferred\; to\; the\; next\; level}{biomass\; available\; at\; the\; previous\; level}\times 100\%$.
4. Place the amount transferred in the numerator and the starting amount in the denominator.
5. Use quantities measured on a comparable basis, including the same area and time period where those are specified.
6. The masses must have the same units before division, so convert units first if necessary.
7. Because identical units cancel in the ratio, an efficiency is given as a percentage, not in grams or kilograms.

Use the supplied values

1. The often-used figure of about $10\%$ is an illustration, not a fixed value that applies to every ecosystem or feeding step.
2. Differences in food digestibility, the proportion eaten and the amount used in respiration can produce different transfer efficiencies.
3. An efficiency for one step cannot automatically be used for another step unless the question states that they are equal.
4. Use the relevant data to calculate each transfer rather than replacing the data with an assumed percentage.

Convert units before dividing

1. A mass stated in kilograms can be converted to grams using $1\mathrm{kg}=1000\mathrm{g}$.
2. Writing the conversion before the substitution makes it clear which quantities are being compared.
3. Keep unrounded values during intermediate steps, then round the final result to the precision requested.

Calculate an unknown biomass

1. To calculate the biomass transferred, multiply the starting biomass by the efficiency expressed as a decimal fraction.
2. Use $biomass\; transferred=starting\; biomass\times \frac{percentage\; efficiency}{100}$.
3. To calculate the starting biomass required, rearrange to $starting\; biomass=\frac{biomass\; transferred}{percentage\; efficiency/100}$.
4. A percentage must be divided by $100$ before it is used as a multiplier, so $12\%$ becomes $0.12$.

Repeated transfers reduce available biomass

1. Each additional feeding step retains only a fraction of the biomass available at the previous level.
2. For successive transfers, multiply the fractional efficiencies rather than adding the percentage efficiencies.
3. This means that the overall efficiency across several steps is smaller than the efficiency of an individual step.

Energy and biomass data

1. An energy-transfer efficiency uses the same ratio method, but both quantities must be energy values, such as values measured in $\mathrm{kJ}$.
2. Do not divide a biomass value in $\mathrm{g}$ by an energy value in $\mathrm{kJ}$ and label the result a percentage transfer efficiency.
3. The retained energy-pyramid diagram shows the declining energy supply through feeding levels and supports the explanation for limited food-chain length.
4. A snapshot of biomass and a measured transfer over time describe different things, so interpret the quantities exactly as the data labels specify.
5. When efficiencies are positive but below $100\%$, energy and biomass available through that feeding route become progressively smaller, rather than suddenly disappearing at a fixed level.

Transfer losses limit food-chain length

1. Biomass contains chemical energy that consumers need for maintenance, movement, growth and reproduction.
2. At each feeding step, material fails to become new consumer biomass through the routes explained previously, leaving less biomass and associated energy for another level.
3. Eventually the supply reaching a further level may be too small to maintain a viable population of consumers.
4. This helps explain why food chains usually contain only a few trophic levels, rather than extending indefinitely.
5. There is no single compulsory maximum number of levels: the amount produced at the base and the efficiencies of the transfers both affect what can be supported.
6. Large top predators often occur at low densities and range over large areas to obtain enough prey.
7. The percentages in this illustration show one possible sequence of transfers and should not replace measured values in a calculation.