You already know energy transfer is inefficient. This page puts a number on it. One equation, a couple of unit traps, and one honest caveat: the famous 10% figure is a rough guide, not a law.
📘 What you need to know
Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next.
Only around 10% of energy is typically available to store in the consumer’s tissues; roughly 90% is lost.
Energy is lost through uneaten parts, undigested material egested as faeces, heat from respiration, and excretion of metabolic waste.
These losses limit food chain length to about five trophic levels.
Efficiency = (energy used for new biomass ÷ energy supplied) × 100.
The 10% value is not fixed and varies between ecosystems, trophic levels and species.
The equation
Ecological efficiency
efficiency (%) = (energy used for new biomass ÷ energy supplied) × 100
The bar is drawn to scale for a real example: a toad turning 700 of 11 000 kJ into new tissue. That green sliver is what a predator eating the toad has to live on.
Why so much is lost
When a consumer ingests another organism, only a small share of the chemical energy in its food ends up as consumer biomass. The reasons are the ones you met earlier, now grouped as the four standard losses:
Not every part is eaten. Roots and woody parts of plants, or the bones of animals, are left behind, and the energy stored in those tissues is lost to the environment.
Not everything eaten is digested. Cellulose in plants and fur on animals pass through, so some material is egested as faeces and its chemical energy leaves the food chain.
Heat from respiration. Energy is released to the environment as heat whenever consumers respire.
Excretion of metabolic waste. Compounds such as urea in urine carry energy out of the body.
Whatever survives all four is available to fuel life functions and be stored as biomass during growth — and only that stored portion can be passed upwards.
Why chains stay short. Eventually the energy remaining is insufficient to support another trophic level. That is why most terrestrial ecosystems cannot support more than about five levels.
The 10% figure, honestly
Ecological efficiency is defined as the percentage of energy received by one trophic level that is passed on to the next. The number varies a great deal between ecosystems, trophic levels and species — the commonly used 10% is not a fixed amount and not a true average. It is a convenient teaching figure.
Real measured values regularly come out anywhere from around 1% to over 20%. Cold-blooded animals tend to be more efficient than warm-blooded ones, because they do not spend energy maintaining a constant body temperature.
If a question gives you real data, use the data. Quoting “10%” when the numbers in front of you say 6.4% is one of the easiest marks to throw away in this whole topic.
WORKED EXAMPLE
A caterpillar consumes blackberries containing 42 J of energy on its first day. Of this, 5.6 J is used to form new caterpillar biomass. Calculate the ecological efficiency of this step.
Step 1: Check the units matchBoth figures are in joules, so no conversion is neededStep 2: Substitute into the equationefficiency = (5.6 ÷ 42) × 100Step 3: Calculate= 0.1333 × 10013.3%
WORKED EXAMPLE
Toads introduced to control insect pests ingest 11 000 kJ m−2 yr−1. They lose 7 500 as heat from respiration and 2 800 in faeces and urine. Calculate the ecological efficiency of the transfer from insects to toads.
Step 1: Add the losses7 500 + 2 800 = 10 300 kJ m−² yr−¹Step 2: Find the energy used for growth11 000 − 10 300 = 700 kJ m−² yr−¹Step 3: Substitute into the equationefficiency = (700 ÷ 11 000) × 100 = 6.366.4%Well below 10% – always use the data you are given
WORKED EXAMPLE
Producers in a grassland have an NPP of 9 000 kJ m−2 yr−1. The primary consumers store 810 kJ m−2 yr−1. If efficiency stays the same, estimate the energy stored by secondary consumers.
Step 1: Find the efficiency of the first transfer(810 ÷ 9 000) × 100 = 9%Step 2: Apply the same efficiency again810 × 0.09 = 72.9About 72.9 kJ m−² yr−¹Say “estimate” – efficiency rarely stays identical between levels
💡 Exam tip
Check the units match before substituting. Converting is often a separate mark.
Work out the energy used for growth first if the question gives you losses instead.
Give the answer as a percentage, usually to one or two decimal places.
Use the given data rather than assuming 10%.
If asked why efficiency is low, name all four losses, starting with heat from respiration.
⚠ Common mix-up
Forgetting to multiply by 100. A ratio is not a percentage.
Using energy ingested as the numerator. The top of the fraction is the energy used for new biomass.
Treating 10% as a rule. It is a rough guide with a very wide real range.
Mixing joules with kilojoules. Convert first.
Confusing ecological efficiency with net productivity. One is a percentage, the other is a rate with units.
Up next: Nutrient Cycles in Ecosystems — moving from energy to matter, and the stores, sinks and sources that carry it round.
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