School › Food chains › Ages 15–16
Energy flow, pyramids and biomagnification
Calculate productivity and trophic transfer efficiency, compare pyramids of numbers, biomass and energy, and model how persistent pollutants become concentrated at the top of a food chain.
Read
Producers capture energy by photosynthesis (gross primary productivity, GPP). Part is used in their own respiration and lost as heat; what remains is net primary productivity (NPP), available to consumers. At Silver Springs, GPP was 20,810 kcal/m²/yr, and 13,187 kcal/m²/yr of it (about 63%) was lost to respiration and heat.[4]
Trophic level transfer efficiency: TLTE = (production at one level ÷ production at the level below) × 100%. At Silver Springs it was about 14.5% from producers to primary consumers; across ecosystems the average is often quoted as about 10%.[4][3]
Energy leaves the food chain because not all sunlight reaches producers, much is released as heat in respiration, some food is not digested and is excreted, and organisms die without being eaten; decomposers then lose most of that energy as heat.[3]
Net production efficiency differs between animals: ectotherms (such as a caterpillar, about 18%) turn about 10 times more of their assimilated energy into new biomass than endotherms (such as a squirrel, as low as 1.6%).[4]
Pyramids: a pyramid of numbers can be inverted (about 200 trees but 150,000 herbivorous insects per 0.1 ha), and so can a pyramid of biomass (English Channel: phytoplankton 4 g/m², zooplankton 21 g/m², because phytoplankton turn over quickly). A pyramid of energy is always upright. After four to six transfers too little energy remains for another level.[4]
Biomagnification: persistent chemicals become more concentrated at higher trophic levels. DDT made the eggshells of fish-eating birds fragile and was banned in the US in the 1970s; in Saginaw Bay (Lake Huron), walleye contained more than four times the PCBs of the phytoplankton.[4]
Try it: Modelling energy loss and biomagnification
You need
- About 100 dried beans or paper counters per group, 10 of them marked with a pen (the 'pollutant')
- Three cups or trays labelled phytoplankton, small fish, big fish
- Graph paper or a spreadsheet
- Calculator
Safety
Do not eat the beans. Wash hands after handling counters. Clear spilled beans from the floor.
What to do
- The numbers in this model are made up to show the idea; they are not real data.
- Energy: the producers in a model pond have NPP = 50,000 kJ/m²/yr. Using a transfer efficiency of 10% at each step, calculate the energy at levels 2, 3 and 4.
- Draw the pyramid of energy to scale (or on a log scale) and explain its shape.
- Biomagnification: put 100 counters (10 marked) in the phytoplankton tray. Each counter is a unit of phytoplankton biomass.
- The small fish 'eat' all the phytoplankton but, with 10% transfer, keep only 10 units of biomass: keep 10 counters and ALL 10 marked counters, because the pollutant is not broken down or excreted.
- The big fish eats the small fish and keeps 1 unit of biomass plus all the pollutant. Calculate the fraction of marked counters at each level.
What happens, and why?
Energy: 50,000 → 5,000 → 500 → 50 kJ/m²/yr, so the energy pyramid is always upright and narrows fast. Pollutant: the fraction of marked counters rises from 10/100 (0.1) in the phytoplankton to 10/20 (0.5) in the small fish and 10/11 (about 0.9) in the big fish. Because the biomass shrinks while a persistent chemical stays, its concentration rises up the chain, as measured for PCBs in Saginaw Bay.[4][3]
Play: True or false?
Decide whether each statement about energy flow and food chains is true or false.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11