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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: 10% Energy Transfer Rule
Every living organism needs energy to survive. This energy originally comes from the sun and flows through ecosystems in one direction. Understanding how this energy moves from one organism to another is key to understanding how ecosystems function and why food chains have limited length.
Key Definitions:
Energy enters most ecosystems from the sun. Plants and other producers capture about 1% of the sunlight that falls on them and convert it to chemical energy through photosynthesis. This energy is then passed along the food chain when organisms eat each other.
Organisms in a food chain are grouped into trophic levels:
• Level 1: Producers (plants, algae)
• Level 2: Primary consumers (herbivores)
• Level 3: Secondary consumers (carnivores that eat herbivores)
• Level 4: Tertiary consumers (carnivores that eat other carnivores)
• Level 5: Apex predators (top carnivores)
When energy passes from one trophic level to the next, only about 10% of the energy is transferred to the higher level. This is known as the 10% rule or 10% law of energy transfer.
If producers (plants) in an area contain 10,000 kJ of energy, then the primary consumers (herbivores) that eat these plants will only obtain and incorporate about 1,000 kJ (10%) of this energy. The secondary consumers will then only receive about 100 kJ (10% of 1,000 kJ) and so on up the food chain.
Energy is lost between trophic levels for several important reasons:
A significant amount of energy is lost as heat during respiration. Animals use energy for movement, maintaining body temperature and other life processes.
Not all the food an animal eats is digested and absorbed. Some passes through the digestive system and is excreted as waste, taking energy with it.
Predators often don't eat all parts of their prey (like bones, fur, or shells). These unused parts contain energy that isn't transferred.
We can use the 10% rule to calculate how much energy is available at different trophic levels:
Example: If a field of wheat contains 5,000,000 kJ of energy:
As you can see, by the time we reach the third consumer level, only 0.1% of the original energy remains available!
The 10% rule explains why we represent energy in ecosystems as a pyramid. Each trophic level has approximately 90% less energy than the level below it.
An energy pyramid shows the amount of energy at each trophic level. The width of each bar represents the amount of energy. The pyramid shape occurs because of the 10% rule - each level has only about 10% of the energy of the level below it.
Energy in food chains is typically measured in joules (J) or kilojoules (kJ). When drawing energy pyramids, we usually show energy per unit area per unit time, such as kJ/m²/year.
Because so much energy is lost at each trophic level, food chains rarely have more than 4 or 5 links. By the fifth trophic level, very little of the original energy remains - not enough to support another level of consumers.
The 10% rule has important implications for human food production and consumption:
Eating plants directly (being primary consumers) is more energy-efficient than eating animals that ate plants. When we eat meat, we're getting only about 10% of the energy that would be available if we ate plants directly.
Producing 1 kg of beef requires approximately 10 kg of plant material. This is why meat production requires more land, water and other resources than plant production for the same amount of food energy.
The 10% rule helps explain why fishing at lower trophic levels (eating smaller fish like sardines or anchovies) is more sustainable than fishing at higher trophic levels (eating large predatory fish like tuna or sharks). For every 1,000 kg of phytoplankton (producers), we might get 100 kg of small fish, but only 10 kg of predatory fish. Overfishing of top predators is particularly damaging to marine ecosystems because these species already exist in smaller numbers due to the 10% rule.
We can calculate the efficiency of energy transfer between trophic levels using this formula:
Efficiency (%) = (Energy transferred to next level ÷ Energy available at current level) × 100
Example: If 5,000 kJ of energy is available in plants and herbivores obtain 400 kJ:
Efficiency = (400 ÷ 5,000) × 100 = 8%
This is slightly less than the typical 10%, which is just an approximate rule.
Let's clear up some common misunderstandings about the 10% rule:
The 10% rule is a fundamental concept in ecology that helps us understand:
Understanding this concept helps us make better decisions about food production, conservation efforts and sustainable use of natural resources.
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