Now for the real species. Predators chasing prey, plants poisoning their neighbours, and fungi releasing antibiotics — these are the named examples the IB expects you to be able to talk about, and the ones that turn up in data questions.
📘 What you need to know
In a predator–prey cycle, both populations rise and fall, with the predator peak always lagging behind the prey peak.
The classic example is the Canada lynx and the snowshoe hare.
Top-down control means a population is limited by its predators. Bottom-up control means it is limited by its resources.
A change at the top of a food web can cause a cascade of effects further down.
Allelopathy is the release of chemicals (secondary metabolites) that harm competing species.
Antibiotic secretion by fungi and bacteria is a form of allelopathy.
Predator and prey: a cycle, not a race
Predators eat prey, so you would expect lots of predators to mean few prey. True — but it works the other way round too, and that is what creates the cycle. Follow it round:
Lots of prey means plenty of food, so predator numbers rise.
More predators eat more prey, so prey numbers fall.
With less food, predators starve or fail to breed, so predator numbers fall.
With fewer predators hunting them, prey numbers rise again.
Each step is caused by the one before it, which is why the two curves never peak at the same moment. The predators are always responding to what the prey did a little while ago.
Notice the predator curve is also flatter. There are far fewer lynx than hares, because energy is lost at every step of a food chain.
Be honest about the model. One predator eating one prey species and nothing else does not really exist. Hares are eaten by other animals, and lynx will take other prey when hares are scarce. The graph is a clean teaching model of a messy reality — and saying so in an evaluation question earns marks.
WORKED EXAMPLE
Reading a predator–prey graph
A graph shows a prey population peaking in year 3 and the predator population peaking in year 4. Explain the one-year gap between the two peaks. [3]
Point 1: what the prey peak providesa large prey population means plenty of food for the predatorsPoint 2: why the response is not instantpredators need time to feed, survive better and raise young before numbers risePoint 3: the resultthe predator population only peaks once those extra offspring have grown, one year laterthe word to use is lag — the predator response lags behind the prey change
Top-down and bottom-up control
Ask a simple question about any population: is it held down from above, or held back from below?
Control type
What limits the population
Examples
Top-down
Being eaten — predators or herbivores above it in the food chain
Snowshoe hares limited by lynx; a plant population grazed hard by deer
Bottom-up
Running out of resources from below
Lynx limited by how many hares there are; algae limited by nitrate in the water
Both can act on the same ecosystem at once, but usually one is dominant at any given moment — and which one can switch. A seagrass bed is normally controlled bottom-up by the nutrients available. Take the large fish out by overfishing and the balance can flip, so top-down effects start to dominate instead.
Top-down control is not only about killing. The presence of a predator changes prey behaviour — where they feed, when they move, how much time they spend hiding. Those non-lethal effects reshape an ecosystem just as powerfully as the kills do.
Cascades through a food web
Food chains do not sit in isolation. Change one population and the effect ripples outwards along every chain it belongs to.
Trace one cascade: fewer foxes → more rabbits and mice → less grass. But mice also feed hawks, so hawk numbers shift too.
WORKED EXAMPLE
Top-down or bottom-up?
In a lake, algal growth is restricted by the low concentration of nitrate in the water. The small fish that eat the algae are kept at low numbers by a large pike population. Identify the type of control acting on each population and justify your answer. [3]
Algaebottom-up controllimited by a resource below them, the nitrate they need to make proteinsSmall fishtop-down controllimited by predation from the pike above them in the food chainExtra creditboth act in the same lake, but one is usually dominant at any given time
Chemical warfare: allelopathy
Competition is not always about who grows fastest. Some species fight dirty. Allelopathy is when an organism releases chemicals into its surroundings that damage other species growing nearby.
The chemicals involved are called secondary metabolites. That name matters: primary metabolites are the molecules an organism needs to stay alive, such as amino acids and glucose. Secondary metabolites are not needed for survival — they are the extras, and many of them are weapons.
The logic of allelopathy
You can win a competition two ways: improve your own survival, or damage your competitor’s
Plants deliver these chemicals in several ways: released from the roots into the soil, given off as gases through the stomata, or stored in the leaves and released when the leaves fall and rot.
Species
What it does
Garlic mustard
Releases sinigrin, which cuts down seed germination and root growth in other plants
Bracken fern
Releases toxins into the soil, and its dead fronds release more as they break down
Himalayan balsam
Thought to release chemicals into the soil that hold back neighbouring plants; a successful invader in the UK
Penicillium fungus
Secretes penicillin, an antibiotic that kills nearby bacteria
Antibiotic secretion is allelopathy too
It is easy to think of antibiotics as human medicines. They are not. They are chemicals microorganisms make to kill their neighbours — by stopping bacteria building cell walls, for example, or by blocking protein synthesis.
The payoff for the fungus or bacterium doing the secreting is direct: fewer competitors means less interspecific competition for the nutrients around it, so more energy and raw material are left for its own growth and reproduction.
Allelopathy is a great example to reach for whenever a question asks how one species can limit another’s population size without eating it. It is competition, not predation.
💡 Exam tip
On a predator–prey graph, find the peaks first. Whichever curve peaks first is the prey.
Always explain the lag using time to breed, not just “predators eat prey”.
Learn one named pair (lynx and snowshoe hare) and one named allelopathy example (garlic mustard or Penicillium).
Top-down means controlled by what eats it; bottom-up means controlled by what it eats. Say which and then justify.
For cascade questions, follow the arrows step by step and write each step as a separate sentence.
If asked to evaluate a predator–prey model, mention that other predator and prey species also affect both populations.
⚠ Common mix-up
The two curves do not peak together. If your answer implies they do, you have missed the whole point.
Food web arrows point towards the eater, not towards the food. Get this backwards and every cascade answer collapses.
Allelopathy is not predation. Nothing is eaten; a chemical is released.
Secondary metabolites are not waste products. They have a job, it is just not a survival-critical one.
Top-down control includes herbivores, not only meat-eaters. A grazed plant population is under top-down control.
Antibiotics do not target the fungus that makes them — that is the point. They target its bacterial competitors.
Up next: Population Growth Curves — the S-shaped curve, its three phases, and the log-scale trick for spotting exponential growth.
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