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Unlock This CourseEnergy is the fuel that powers all life on Earth. But here's something amazing - when energy moves through a food chain, most of it disappears! This isn't magic, it's science. Understanding how energy flows and why so much gets 'lost' helps us understand why ecosystems work the way they do.
Think about it like this: a cow eats loads of grass all day, but when you eat a burger, you're only getting a tiny fraction of the energy that was originally in all that grass. Where does the rest go?
Key Definitions:
Only about 10% of energy transfers from one trophic level to the next. This means 90% of energy is 'lost' at each step! This fundamental rule explains why food chains are usually short and why there are fewer predators than prey animals.
When we say energy is 'lost', it doesn't just vanish into thin air. Energy can't be created or destroyed, but it can change form. Here's what happens to that missing 90%:
Every living thing uses energy for survival and much of this energy ends up as heat that escapes into the environment. Let's break down exactly where energy goes:
About 60-70% of energy is used for cellular respiration - keeping cells alive, moving, growing and reproducing. Most of this becomes heat energy.
Around 10-20% leaves the body as waste (urine, faeces) that still contains chemical energy but can't be used by that organism.
About 10-20% remains in parts that predators don't eat - bones, shells, roots, bark and other tough structures.
In the African savanna, grass captures about 10,000 units of energy from sunlight. Zebras eating this grass only get about 1,000 units (10%). Lions eating zebras get just 100 units (1% of the original). A parasitic tick on the lion might get only 10 units - that's just 0.1% of what the grass originally captured! This explains why there are millions of grass plants, thousands of zebras, hundreds of lions, but only a few ticks per lion.
Scientists measure energy transfer efficiency using this simple formula:
Energy Transfer Efficiency = (Energy in higher trophic level ÷ Energy in lower trophic level) × 100
For example: If plants have 1000 kJ of energy and herbivores have 100 kJ, the efficiency is (100 ÷ 1000) × 100 = 10%
The 10% rule is an average - actual efficiency can range from 5% to 20% depending on several factors:
Energy transfer creates distinctive pyramid shapes when we diagram ecosystems. These pyramids help us visualise how energy flows and why ecosystems have their characteristic structure.
Energy pyramids are always pyramid-shaped because energy decreases at each level. The base (producers) always has the most energy and each level up has roughly 10% of the level below it. This creates a predictable, tapering pyramid shape.
Phytoplankton (microscopic marine plants) capture 15,000 units of solar energy. Zooplankton eating them get 1,500 units. Small fish like herring get 150 units. Larger fish like cod get 15 units. Seals eating cod get just 1.5 units. This dramatic decrease explains why the ocean has billions of phytoplankton but only thousands of seals.
Oak trees capture 20,000 units of energy. Caterpillars get 2,000 units. Blue tits eating caterpillars get 200 units. Sparrowhawks eating blue tits get 20 units. This explains why one sparrowhawk territory needs hundreds of blue tits and thousands of caterpillars to survive.
Most food chains have only 4-5 trophic levels. This isn't a coincidence - it's a direct result of energy transfer efficiency.
By the 4th or 5th level, there's simply not enough energy left to support another level of predators. The numbers become too small to maintain a viable population.
Higher trophic levels have smaller populations. Eventually, populations become so small they can't find mates or maintain genetic diversity.
Top predators need huge territories to find enough food. There's simply not enough space on Earth for many levels of large predators.
Humans have dramatically altered natural energy flows in ecosystems. Understanding energy transfer helps us make better decisions about food production and conservation.
Modern farming tries to maximise energy transfer efficiency by:
Overfishing of large predator fish like tuna and sharks removes the top of marine food pyramids. This can cause 'trophic cascades' where the removal of predators leads to population explosions of their prey, which then overconsume their food sources. Understanding energy pyramids helps fisheries managers set sustainable catch limits that maintain ecosystem balance.
Energy transfer efficiency is fundamental to understanding how ecosystems work. The 10% rule explains why:
This knowledge helps us make informed decisions about conservation, sustainable agriculture and managing our impact on natural ecosystems. Every time you see a food web or ecosystem, remember - it's all about that 10% rule!