TL;DR:
Producers capture sunlight, consumers eat other organisms, and decomposers recycle dead material.
Arrows in a food web point from the organism that is eaten to the one that eats it, the direction energy moves.
Only a small part of the energy at one level reaches the next, so top levels support fewer organisms.
A hawk circles over a field. On the ground, a snake eats a frog that ate a grasshopper that ate grass. Draw that as a line and you have a food chain. Add the rabbit that eats the grass, the hawk that eats the rabbit as well, and you have a web. A food web shows how energy moves through a community, and where it goes missing.
The idea in plain words
Every food web starts with producers, usually plants or algae, that capture light energy in photosynthesis and store it in sugar. Consumers get energy by eating. A herbivore that eats producers is a primary consumer. An animal that eats primary consumers is a secondary consumer, and so on up the levels.
Decomposers, such as fungi and many bacteria, break down dead plants and animals and waste. They release nutrients back into the soil, where producers can use them again. Without them, nutrients would stay locked in dead material.
Why arrows point the way they do
An arrow means 'energy goes to'. So the arrow runs from grass to grasshopper, not the other way. A common slip is to draw it as 'eats', which reverses every arrow. Check one pair before you draw the rest.
Worked example: a grassland food web
Take this grassland. Grass is eaten by grasshoppers and rabbits. Frogs eat grasshoppers. Snakes eat frogs. Hawks eat rabbits and snakes. When anything dies, fungi and bacteria break it down.
Organism | Role | Trophic level |
|---|---|---|
Grass | Producer | 1 |
Grasshopper, rabbit | Primary consumer | 2 |
Frog | Secondary consumer | 3 |
Snake | Tertiary consumer | 4 |
Hawk | Secondary (eats rabbit) and quaternary (eats snake) | 3 or 5 |
Fungi, bacteria | Decomposer | Feed on all levels |
Use the web to answer questions. If the grasshoppers disappear, frogs lose food, so frogs decline, and snakes that depend on frogs may also fall. Rabbits and grass are not directly affected, though rabbits may be eaten more as the hawks switch to them. Webs make these knock-on effects easy to trace.
Where the energy goes
Only a small part of the energy in one level ends up in the next one. A grasshopper does not eat all the energy in the grass: some of the plant is not eaten. Of what is eaten, some is not digested, and much of the rest is used for the animal's own life processes. Respiration releases it as heat, which cannot be reused by other organisms.
Just a fraction goes into growth, and only that stored energy is available to whoever eats the animal. The result is that each step up the chain has less energy to share.
For practice, make up a simple number. Say the grass in an area captures 10,000 units of energy, and only one tenth of the energy passes up at each step. Then grasshoppers and rabbits receive 10,000 × 1/10 = 1,000 units. Frogs get 1,000 × 1/10 = 100, snakes get 100 × 1/10 = 10, and anything that eats snakes would get 1 unit. The fraction in real ecosystems varies, but the pattern is real: the amount falls quickly.
That is why food chains rarely have more than four or five levels, and why top predators are fewer and need large areas.
Energy pyramids in one picture
If you stack the levels as bars, with producers at the bottom, the bars shrink as you climb. That is an energy pyramid. The bottom bar is widest because it holds the most energy, and each bar above it holds only a fraction. Counts of organisms and total mass often shrink in the same way, since a few large predators live on many small prey.
The pyramid also explains a practical idea: eating lower on the food chain, plants rather than animals, passes on more of the original energy, because fewer steps are lost.
A second question: what if hawks vanish?
Try one more prediction with the same web. If hawks disappear, rabbits and snakes lose their main predator, so both can increase. More snakes means fewer frogs, which means more grasshoppers, which means more grass eaten. Notice that a change at the top can ripple all the way down.
In an exam answer, state the first effect, then the next, and use the words 'increase' and 'decrease' clearly. Say what is likely, not what is certain, because real populations also depend on disease, weather and other food.
How to check your answer
Every arrow should point from food to eater.
Every chain should begin with a producer, never an animal.
Decomposers should connect to every level, since everything dies.
If you used numbers, they should shrink at each level, never grow.
Common mistakes
Drawing arrows from the eater to the food.
Saying energy is recycled. Matter cycles, but energy flows through and leaves as heat.
Saying energy is destroyed. It is converted to forms organisms cannot reuse, mainly heat.
Leaving out the decomposers, or placing them only at the top.
Where you will use it next
Food webs lead to population dynamics, human impact, conservation and the recycling of matter. See the matter side in the carbon cycle, where carbon moves through the same organisms and is returned to the air.
Frequently asked questions
Is a food web the same as a food chain?
A chain is one path. A web joins many chains and is closer to what really happens.
What if an organism eats both plants and animals?
It is an omnivore, and it appears at more than one level in the web.
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