Every population is at its normal level.
Click a circle to select it. Thicker arrows = more food flowing along that link right now.
100% = the normal, balanced population. The selected species is drawn thicker. Yellow lines mark what you changed.
Wet and dry years make every population wobble around normal. Turn it off to see exact balance points. It is always off during a challenge.
Every living thing needs energy. Plants make their own food from sunlight by photosynthesis, so they are called producers. Animals cannot, so they eat plants or other animals; they are consumers. A food chain shows one path the energy takes: grass → grasshopper → frog → snake → hawk. The arrow means “is eaten by”, and it points the way the energy flows.
Most animals eat more than one kind of food, and most are eaten by more than one predator. A mouse eats grass seeds and grasshoppers, and is hunted by snakes, foxes and hawks. When all the food chains in an ecosystem are joined together, they make a food web. Animals that eat only plants are herbivores (primary consumers); animals that eat other animals are carnivores (secondary or tertiary consumers); animals that eat both are omnivores. Decomposers such as fungi and bacteria break down dead things and return nutrients to the soil, so the plants can grow again.
Because they are linked, the populations depend on each other. Remove the foxes, and fewer rabbits are eaten, so the rabbits increase; then the extra rabbits eat more grass and shrubs, so the plants decrease. A change can travel two or three links away: with no frogs, grasshoppers multiply and eat more grass. This chain reaction is called a trophic cascade. Changes can also start at the bottom (a drought means less plant food for everyone, all the way up to the hawks) or in the soil (no decomposers means fewer nutrients, so less plant growth).
Each feeding step is a trophic level. Only a small part of the energy in one level ends up in the next, typically around 10% (it varies from about 5 to 20%), because most is used for living, lost as heat or never eaten. That is why there are thousands of grasshoppers but only a few hawks, and why food chains rarely have more than 4 or 5 levels. Open the Biomass pyramid tab: the mass of living things shrinks at each level up, because each level is fed by only a small share of the energy of the level below. In a balanced ecosystem, negative feedback keeps populations fairly steady: more rabbits feed more foxes, and more foxes then eat more rabbits.
(1) Select Foxes and press Remove; follow the rabbits, then the grass. (2) Press Reset, remove the Frogs, and see what happens to the grass even though frogs never eat grass. (3) Remove the Decomposers and watch the soil nutrients fall. (4) Double the Rabbits and watch the population return to normal. (5) Try every Prediction challenge and see how many you get right.
| Role | In this grassland | What it does |
|---|---|---|
| Producer | Grass, shrubs & berries | Makes food from sunlight (photosynthesis); the energy source for the whole web |
| Primary consumer | Grasshoppers, rabbits, mice | Herbivores (or omnivores) that eat producers |
| Secondary consumer | Frogs, mice, snakes, foxes, hawks | Eat primary consumers |
| Tertiary consumer | Snakes, foxes, hawks | Eat secondary consumers (snakes eat frogs; foxes, snakes and hawks eat mice, which eat grasshoppers; hawks eat snakes) |
| Top predator | Hawks | Nothing here eats them |
| Decomposers | Fungi and bacteria (helped by detritivores such as worms) | Break down dead matter and return nutrients to the soil |
Many animals feed at more than one level, which is why some appear in two or three rows. A food web shows this; a single food chain cannot.
| Change | Main effects | Why |
|---|---|---|
| Foxes removed | Rabbits ↑ to ~160%, shrubs ↓ ~70%, grass ↓ ~85%, hawks ↑ ~160% | Fewer rabbits eaten; more rabbits eat more plants and feed more hawks |
| Frogs removed | Grasshoppers ↑ to ~190%, grass ↓ ~70%, snakes ↓ ~35% | Trophic cascade: no frogs → more grasshoppers → less grass |
| Grasshoppers removed | Frogs die out, grass ↑ ~125%, mice ↑ ~160% | Frogs had only one food; grass is eaten less; mice get more seeds |
| Hawks removed | Snakes ↑ to ~155%, frogs ↓ ~85% | Snakes lose their only predator and eat more frogs |
| Rabbits removed | Foxes die out, shrubs ↑ ~145%, mice and snakes ↑ | Foxes here depend mostly on rabbits; plants are eaten less |
| Decomposers removed | Grass ↓ to ~50%; every animal falls | Nutrients stay locked in dead matter, so plants grow slowly |
| Drought | Every population ↓; hawks ~75%, snakes ~60% | Bottom-up effect: less plant energy enters the food web |
| Ecosystem | What happened | Lesson |
|---|---|---|
| Rocky shore, USA | When ecologist Robert Paine removed the predatory starfish Pisaster, mussels took over and many other species disappeared (Paine 1966) | A keystone species can hold a whole community together |
| Kelp forests, North Pacific | Where sea otters were hunted out, sea urchins multiplied and ate the kelp forests; where otters returned, the kelp recovered (Estes & Palmisano 1974) | A classic trophic cascade |
| Yellowstone, USA | Wolves were reintroduced in 1995–96; elk numbers fell and willows and aspens recovered in some places. Scientists still debate how much of the recovery is due to the wolves | Top predators can affect plants, but real ecosystems have many causes at once |
| Australia | European rabbits released in the 1800s spread with few predators, stripped vegetation and harmed native animals | Invasive species can upset a food web |
Each population grows when it has food and shrinks when it is eaten, starves or dies of old age, using simple predator–prey equations (Lotka–Volterra type) with numbers chosen so that every species starts in balance and the biomass ratios between levels are set near 10%. Decomposers recycle nutrients that help the plants grow. With Weather varies switched on, plant growth goes up or down by about 15% in wetter and drier years (a slowly wandering random number), so all the populations wobble by roughly 5–15% around normal, as real populations do; switch it off to see the exact balance. In the 3D view a fox, a rabbit or a hawk is not one real animal: the number of animals shown follows the model’s population, and when a population has to fall, a predator is usually shown chasing and catching one. Real ecosystems have many more species, seasons, migration and chance events, and real animals switch foods, so real results are less tidy, but the direction of the effects follows the same logic. The animals shown in 3D are a sample; the counts are in the species card.
A food web is a diagram of all the feeding relationships in an ecosystem: who eats whom. It is made of many food chains joined together, because most animals eat more than one kind of food and are eaten by more than one predator. Arrows point from the food to the animal that eats it, showing the direction in which energy flows.
Key takeaway: a food web = all the connected food chains in an ecosystem.A food chain shows a single path of energy, for example grass → rabbit → fox. A food web shows many chains connected together, so it is closer to real life. In a web you can see that one animal can have several foods and several predators, and that a change to one population can affect many others.
Key takeaway: a chain is one path; a web is many paths linked together.From the food to the eater: grass → rabbit means the rabbit eats the grass. The arrow shows the direction that energy (and matter) moves. A common mistake is to draw the arrow from the eater to the food.
Key takeaway: arrows follow the energy: food → eater.The species that ate it lose a food source and may decline or switch to other foods. The species it ate lose a predator or consumer and often increase. These changes then spread to other species linked to them. For example, removing foxes lets rabbits increase, which then reduces the grass and shrubs. Try it in the simulation.
Key takeaway: removing one species affects its food, its predators, and others further along the web.Producers (plants, algae) make their own food by photosynthesis. Consumers eat other organisms: herbivores eat plants, carnivores eat animals and omnivores eat both. Decomposers (fungi, bacteria) break down dead organisms and wastes, returning nutrients to the soil for producers to use again.
Key takeaway: producers make food, consumers eat it, decomposers recycle it.Only about 10% of the energy at one trophic level is passed on to the next. The rest is used for movement, growth and keeping warm, is lost as heat, or is in parts that are not eaten. So each level can support far less living material than the one below: many grasshoppers, fewer frogs and snakes, and only a few hawks.
Key takeaway: about 90% of the energy is lost at each step, so top levels are small.A trophic cascade is when a change in a predator affects not just its prey but the level below that too. For example, when sea otters were hunted out, sea urchins (their prey) multiplied and ate the kelp forests. In this simulation, removing frogs lets grasshoppers multiply, which reduces the grass.
Key takeaway: effects that pass down through two or more feeding levels.A keystone species has a much larger effect on its ecosystem than its numbers suggest, like the keystone that holds up an arch. Ecologist Robert Paine showed that removing the starfish Pisaster from rocky shores let mussels crowd out many other species. Top predators and ecosystem engineers such as beavers are often keystone species.
Key takeaway: remove a keystone species and the whole community changes.Without decomposers, dead plants, dead animals and droppings would pile up, and the nutrients locked inside them (such as nitrogen and phosphorus) would not return to the soil. Plants would run short of nutrients and grow less, and every animal that depends on plants would decline. Matter cycles in an ecosystem; decomposers keep the cycle going.
Key takeaway: decomposers recycle nutrients so plants can keep growing.Real populations are never perfectly steady. Rainfall, temperature and disease change from year to year, so plants grow more in some years and less in others, and that change is passed up the food web to the herbivores and then the predators. Ecologists say a balanced ecosystem is in dynamic equilibrium: the numbers wobble around a normal level instead of staying exactly on it. Switch Weather varies on and off in the simulation to compare.
Key takeaway: a balanced ecosystem wobbles around normal levels (dynamic equilibrium).An invasive species is one brought to a new place where it spreads and causes harm. It may have no natural predators there and compete with native species for food, or prey on animals that have no defence against it. European rabbits in Australia stripped vegetation and harmed native wildlife.
Key takeaway: invasive species can upset the balance of a food web.