Who eats whom, and why the top of the chain is so lonely

How to use this simulation

1. Look at the web on the left. Each circle is a species and its number is its trophic level. Arrows point from the eaten to the eater, and the small dots running along them show energy moving.
2. Look at the pyramid on the right. The producers sit at the foot with all the energy; each level above holds a tenth of the one below. The bar lengths use a log scale, because a linear scale would make the top invisible.
3. Drag the energy slider. Every level scales together, and the longest-chain reading tells you how many levels still have at least 1 kJ.
4. Drag the passing-on slider from 10 % towards 20 %, then down to 5 %. Watch the top level gain or all but vanish.
5. Use the Remove one species control. Rings and triangles mark each species: declining, rising, pulled both ways, or starved out. The panel under the pyramid lists them in words.
6. Press reset to return to the default setting, then compare your readings with the solved problems below.

Why you never see a lion eat a lion

Walk through a rice field at dusk. Grasshoppers jump away from your feet, a frog snaps one up, and a snake slides towards the frog. High above, an eagle is watching the snake. Every one of these animals is alive because of a single thing: energy that the rice and the grass captured from sunlight.

That energy does not circle round and round. It enters through the green plants, passes upward each time something is eaten, and leaks away as heat at every step. This one-way journey is called energy flow, and it is the reason there are thousands of grasshoppers but only a few eagles.

This page lets you test it. You will build the pyramid yourself with the sliders, change how efficient each step is, and then delete species from the web to find out which ones the rest depend on.

Food chain or food web?

A food chain is a single line: grass, grasshopper, frog, snake, eagle. It is easy to draw and easy to learn, but real animals rarely eat only one thing. The mouse in the simulation eats both paddy and grass, and is eaten by both the snake and the eagle.

Put every chain of a habitat together and you get a food web: a tangle of crossing arrows that shows who eats whom. Webs are more realistic, and they are also more stable, because an animal that loses one food source can switch to another.

Trophic levels in the simulation

A trophic level is a feeding position. The levels are counted from the producers upward, and the simulation gives every species its level.

Words you need first

Ecology is full of terms that sound alike. Learn them in pairs.

The 10 % rule

In 1942 Raymond Lindeman measured the energy in a lake and found that each level held only a small part of the one below. The rule of thumb that came out of it is the 10 % rule (Lindeman's law): about 10 % of the energy at one trophic level is passed to the next, and the rest is lost.

Where does the other 90 % go? Most of it is burnt in respiration to keep the animal warm and moving. Some leaves as faeces and urine, some is in bones, fur and shells that the next animal cannot digest, and some is in the body of an animal that dies uneaten and goes to the decomposers.

The arithmetic is just repeated multiplication. Starting with 10,000 kJ in the producers, level 2 has 10,000 × 10 % = 1,000 kJ, level 3 has 100 kJ, level 4 has 10 kJ and level 5 has 1 kJ. The simulation uses exactly this calculation, with the passing-on rate you set.

Three kinds of pyramid

Ecologists draw pyramids for three different quantities. Only one of them is always upright.

What happens when you remove a species

The simulation applies two plain rules to the web. First, if every food a species eats is gone, it starves out and the loss passes upward. Second, if a species loses only some of its foods it declines, and if a species loses a predator it rises. A species can be pulled both ways.

Try it in the simulation

Spend ten minutes on these experiments. Each one is a question an examiner could ask.

• Leave the energy at the default and read the energy at every level. Is each one exactly a tenth of the one below?
• Raise the passing-on rate to 20 %. How much energy does the eagle get now, and how many levels can the producers support?
• Lower it to 5 %. What happens to the top level, and what does the longest-chain reading say?
• Set the energy to 100,000 kJ at 10 %. How many levels can the food web now have?
• Remove each of the seven species in turn and write down how many are affected. Which one disturbs the web most?
• Remove a producer. Why do the herbivores decline but not starve out?
• Hide the labels and name the five trophic levels from memory, then show them to check.

Solved problems

Each solution states the formula first, then substitutes the numbers. Every figure is computed by the same code that drives the simulation.

Common mistakes

These are the slips that cost marks every year.

• Drawing the arrow from the eater to the eaten. Arrows show the flow of energy, so they point to the eater.
• Saying energy is recycled. Nutrients are recycled; energy flows one way and leaves as heat.
• Forgetting the decomposers. They are in every real food web, and they recycle nutrients from dead matter.
• Saying 10 % of the organisms are passed on. The rule is about energy, not numbers or even mass.
• Thinking the top predator has the most energy. It has the least.
• Counting the sun as a trophic level. The first level is the producer.
• Treating the 10 % as exact. It is an average that ranges from about 5 % to 20 % in real systems.

Where you see it in real life

Fish farmers in Bangladesh know the rule. Fish that eat plankton (a low trophic level) are cheap to raise because the energy loss is small, while a carnivorous fish needs several kilograms of smaller fish to gain one.

Removing a top predator is a real danger. When tigers are lost from the Sundarbans, deer and boar numbers can rise and damage the young mangroves. When vultures fell sharply across South Asia in the 1990s, carcasses piled up and wild dog numbers climbed.

Poisons add another twist: DDT and mercury do not break down, so they concentrate at each level and are highest in the top consumer. This is called biomagnification and it is why eagles and large fish are the first to be harmed.

Exam corner

Typical questions, and what a full-mark answer contains.

Revision summary

Producers capture solar energy, consumers pass it up through trophic levels, and at each step about 10 % moves on while the rest is lost as heat. With 10,000 kJ in the producers, the five levels hold 10,000, 1,000, 100, 10 and 1 kJ, and 99.99 % never reaches the top. That is why chains are short, top predators are rare, and eating lower on the chain feeds more people. Food webs are more stable than chains, but removing a widely eaten or widely eating species can spread through many others.

Frequently asked questions