Energy Pyramids & The 10% Rule: Fifth Grade Science
Energy flow through an ecosystem is fundamentally governed by the laws of thermodynamics, illustrated by the 10 Percent Rule of trophic efficiency.
Modeling Energy Loss: The Energy Pyramid
While nutrients and chemical elements cycle continuously through an ecosystem, energy does NOT cycle! Energy enters ecosystems as solar sunlight, flows through food chains, and continuously dissipates into space as metabolic heat. To model the loss of energy at each successive feeding step, ecologists construct an energy pyramid—a graphical diagram shaped like a broad-based pyramid showing how much usable energy is transferred to each higher trophic level.
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| THE ENERGY PYRAMID AND THE 10% RULE |
| |
| /\ |
| / \ |
| / \ |
| / 1 J \ Tertiary Consumer (Hawk) |
| +--------+ |
| / 10 J \ Secondary Consumer (Snake) |
| +------------+ |
| / 100 J \ Primary Consumer (Grasshopper) |
| +----------------+ |
| / 1,000 J \ Primary Producer (Prairie Grass) |
| +--------------------+ |
| |
| Only ~10% of energy transfers up; 90% is lost at each step! |
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The 10 Percent Rule of Ecological Efficiency
The fundamental governing principle of energy pyramids is the 10 Percent Rule: on average, only about 10 percent of the energy stored as biomass in one trophic level is successfully converted into biomass in the next higher trophic level. The remaining 90 percent of the energy is lost between each step!
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| CALCULATING ENERGY ACROSS TROPHIC TIERS |
| |
| PRODUCERS (Grasses): 10,000 Joules of energy |
| | (10% transferred to herbivores) |
| v |
| PRIMARY CONSUMERS (Zebras): 1,000 Joules of energy |
| | (10% transferred to carnivores) |
| v |
| SECONDARY CONSUMERS (Lions): 100 Joules of energy |
| | (10% transferred to apex tier) |
| v |
| TERTIARY CONSUMERS (Scavengers): 10 Joules of energy |
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Where Does the "Lost" 90 Percent Go?
Why is energy transfer so inefficient? Why doesn't 100 percent of the energy in a grass blade transfer into the rabbit that eats it? The 90 percent of energy that fails to transfer is accounted for by three biological realities:
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| WHERE DOES THE 90% OF ENERGY GO? |
| |
| 1. CELLULAR RESPIRATION AND METABOLIC HEAT |
| - The organism burns most of its ingested calories to stay alive: |
| powering heartbeats, brain signaling, running, keeping warm. |
| - This energy turns into thermal heat that radiates into space! |
| |
| 2. UNEATEN BIOMASS |
| - Predators rarely consume the entire body of prey: bones, fur, |
| teeth, and tree roots are left behind uningested. |
| |
| 3. UNDIGESTED WASTE (EXCRETION) |
| - A large portion of consumed food passes out undigested as feces; |
| this energy flows to decomposers, not to the next predator! |
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Only the small fraction of energy that the animal incorporates into its own newly grown living tissue (meat, fat, organs) is available to be passed along to the predator that hunts it.
Why Food Chains Rarely Exceed Four or Five Trophic Levels
Have you ever wondered why there are no super-predators that specialize in hunting lions, or why food chains are never ten or twenty steps long? The answer is pure energetics! Because 90 percent of energy is lost at each tier, energy runs out extraordinarily fast.
- A million Joules of sunlight captured by grass becomes 10,000 Joules in zebras.
- 10,000 Joules in zebras becomes 1,000 Joules in lions.
- If an animal hunted lions, it would receive a tiny fraction of that energy (100 Joules), requiring it to hunt thousands of square miles just to find enough rare lions to avoid starvation.
There simply is not enough energy left in the system to support a viable breeding population of higher-level consumers.
Biomass and Numbers Pyramids
Closely related to energy pyramids are two other ecological pyramids:
- Biomass Pyramid: Measures the total dry weight of living organic tissue at each trophic level (in grams or kilograms per square meter). In terrestrial ecosystems, producer biomass dwarfs consumer biomass: millions of tons of prairie grass support thousands of tons of bison, which support only a few hundred kilograms of wolves.
- Numbers Pyramid: Shows the actual count of individual organisms living at each level. Usually, thousands of producers support hundreds of herbivores, which support a handful of carnivores.
Exercises and Solutions
Exercise 1: Calculating Energy Transfer Through an Aquatic Pyramid. An ocean phytoplankton bloom captures solar energy, storing 50,000 Kilojoules (kJ) of energy in its collective biomass. Microscopic zooplankton eat the phytoplankton; small herring fish eat the zooplankton; and a harbor seal eats the herring. Calculate the exact amount of energy available to the zooplankton, the herring fish, and the harbor seal, applying the 10 Percent Rule.
Solution 1: Applying the 10 Percent Rule (multiply by 0.10 at each successive trophic level): Level 1 (Phytoplankton Producer) stores 50,000 kJ of energy. Level 2 (Zooplankton Primary Consumer) receives 10% of 50,000 kJ = 5,000 kJ of energy. Level 3 (Herring Secondary Consumer) receives 10% of 5,000 kJ = 500 kJ of energy. Level 4 (Harbor Seal Tertiary Consumer) receives 10% of 500 kJ = 50 kJ of energy. Out of the original 50,000 kJ produced, only 50 kJ reaches the seal.
Exercise 2: Evaluating Vegetarian Diets from an Ecological Energetics Standpoint. Ecologists calculate that feeding grain directly to humans can sustain roughly ten times more people than feeding that same grain to cattle and then having humans eat the beef. Explain this calculation using the 10 Percent Rule and energy pyramids.
Solution 2: This calculation is explained directly by the 10 Percent Rule. When humans eat corn or grain directly, humans function as primary consumers occupying the 2nd trophic level, receiving roughly 10 percent of the original solar energy captured by the crops. When the grain is instead fed to livestock cattle, the cattle act as primary consumers (receiving 10% of the crop energy), and 90% of that energy is dissipated as cattle body heat, movement, and manure. When humans subsequently consume the beef, they act as secondary consumers (3rd trophic level), receiving only 10% of the cattle's energy—which is merely 1 percent (0.10 x 0.10 = 0.01) of the original crop energy. Bypassing the intermediate livestock tier eliminates a 90% energetic loss, allowing the same agricultural acreage to feed ten times more human individuals.