Food Chains and Webs Simulation

About this experiment

A meadow food web links grass, three plant eaters, a frog, a snake and two top predators. Every species starts in balance. Click any species to remove it and watch the effects ripple through the web over the following years: some populations grow, some shrink and some disappear completely. Change the rainfall to see how a drought at the bottom of the web reaches the animals at the top.

Learning goals

Identify producers, primary consumers, secondary consumers and top predators in a food web
Explain that arrows in a food web show the direction of energy flow
Predict how removing one species affects its predators, its prey and its competitors
Describe how a change in the producers spreads up the food web

How to use it

1Name the producer and every consumer, and trace each food chain from the grass to the hawk.
2Predict what will happen to the rabbits, then click the fox to remove it and press Play.
3Reset, remove the mice and explain why the snakes and hawks die out.
4Reset and lower the rainfall to 60%. Which species are hit first, and why?

The model

\frac{d N _{i}}{d t} = N_{i} (r_{i} + j \sum a_{ij} N_{j})

Generalized Lotka-Volterra model. Feeding links add to the predator and subtract from the prey.

r_{grass} > 0, r_{animals} < 0

Grass grows on its own. Every animal declines without food.

a_{ii} < 0

Crowding limits every species, so the community settles to a stable state

Assumptions

This is a qualitative teaching model. Populations are shown as a percentage of their starting values, not as counts.
Each feeding link transfers biomass at a fixed rate, and every species is limited by crowding.
A species whose population falls below 1% of its starting value is treated as locally extinct and does not come back unless you add it again.
Rainfall only changes how fast the grass grows. Seasons, disease and migration are ignored.

Try this in class

Q1Removing the fox makes the rabbits increase. Why does the grass then decrease a little?
Q2Removing the mice wipes out the snakes and hawks. Why does the fox survive?
Q3Removing the grasshoppers causes the frogs to die out, yet the foxes increase. Explain this chain of effects.
Q4Which single species, if removed, causes the largest number of other species to disappear? What does this tell you about it?

Frequently asked questions

What is the difference between a food chain and a food web?

A food chain is one path of feeding from a producer to a top predator. A food web joins all the chains in a habitat, showing that most animals eat, and are eaten by, more than one species.

Why do the arrows point from prey to predator?

The arrows show the flow of energy and matter. When a fox eats a rabbit, energy moves from the rabbit to the fox.

Does the model show real numbers of animals?

No. It shows trends. Real food webs have many more species, and their strength of interaction is measured in the field. The direction of the changes is what students should compare.