Energy enters almost every ecosystem as sunlight, is captured by producers, and moves through consumers as they eat. At each step most of it is lost as heat. Nothing sends it back to the start, which is why energy flows rather than cycles.
Food chains
A food chain shows one path energy takes. The arrows always point in the direction the energy travels, which is toward the eater.
A simple food chain
sun -> grass -> grasshopper -> frog -> snake -> hawk grass: producer grasshopper: primary consumer frog: secondary consumer snake: tertiary consumer hawk: top predator
Drawing the arrows backwards is the most common error in this topic. The arrow means is eaten by, so it runs from the food to the feeder.
Food webs
A single chain is a simplification. Real organisms eat several things and are eaten by several things, and a food web shows those overlapping chains together. Webs also show why removing one species can ripple outward: everything that ate it must find other food, and everything it ate is now less controlled.
The energy pyramid
Only about ten percent of the energy at one level becomes available to the next. The rest is used for the organism's own living and movement, or lost as heat, or contained in parts that are not eaten.
| Level | Energy available |
|---|---|
| Producers | 10,000 units |
| Primary consumers | about 1,000 units |
| Secondary consumers | about 100 units |
| Tertiary consumers | about 10 units |
Two consequences follow. First, food chains are rarely longer than four or five links, because there is too little energy left to support another level. Second, top predators are always comparatively few in number, and the biomass at each level shrinks as you climb.
Common mistakes
Pointing the arrows toward the food
Arrows follow the energy, so they point from the eaten to the eater.
Calling energy flow a cycle
Matter cycles. Energy passes through once and leaves as heat.
Check yourself
In the chain grass to rabbit to fox, which way do the arrows point?
From grass to rabbit to fox, following the energy toward whatever does the eating.
Why are there fewer hawks than grasshoppers?
Because only about a tenth of the energy passes to each higher level, so the higher levels can support far fewer individuals.
If producers hold 20,000 units of energy, roughly how much reaches the secondary consumers?
About 200 units. Around a tenth passes to the primary consumers, and a tenth of that again to the secondary consumers.
Why are food chains rarely longer than five links?
Because so much energy is lost at each step that after four or five levels there is not enough left to support another one.
See also
Sources
- Food chains, food webs and the ten percent rule as taught in United States upper elementary life science. Figures are the standard classroom approximation. Examples on this page are our own.