Energy flows through ecosystems — Biology, 14–17 years
An ecosystem links organisms with the physical conditions around them. Energy enters mainly through sunlight, moves through feeding relationships and is gradually lost as heat.
Energy moves, matter cycles
Plants and algae capture some sunlight and store it as chemical energy. Animals gain energy by eating plants or other animals, and decomposers break down remains. At every step, organisms use energy for movement, growth and keeping warm, releasing much of it as heat. Atoms such as carbon and nitrogen can be reused, but energy is not recycled in the same way.
Why are food chains limited?
An ecosystem must account for a basic problem: organisms need energy, but no transfer is perfectly efficient. A plant uses sunlight to build food, yet its own life processes use much of that energy. A herbivore receives only part of what the plant stored, and a predator receives less again. This limits the number of feeding levels and usually leaves fewer top predators.
Example: a simple energy pyramid
Suppose grass stores 10,000 units of chemical energy. Grasshoppers eat it, but only about 1,000 units become grasshopper biomass; the rest is used or lost as heat. Frogs receive roughly 100 units, and snakes about 10 units. The exact values vary, but the pattern shows why a field can support many plants, fewer herbivores and still fewer predators.
Trap: energy is recycled
This mistake is reasonable because diagrams show arrows going from one organism to another, and matter really does cycle through ecosystems. But the arrow for energy has a direction: sunlight enters, organisms transfer part of it, and heat leaves. Decomposers recycle atoms from dead material; they do not return the lost energy to the food chain.
Where this appears
This helps people plan food production and protect habitats. Eating lower on a food chain generally uses less land and energy, because fewer transfers have occurred. Conservationists also protect plants and small organisms, not only impressive predators: removing a producer or decomposer can affect many links at once. Real ecosystems have branching webs, so one change may have several effects.
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