IB Biology HLPopulations & CommunitiesPaper 1 & 2~12 min read
Limiting Population Size: Examples
Three named ways populations get held in check: predators eating prey, control arriving from the top or the bottom of a food web, and plants that chemically poison their competitors. The last one explains where penicillin came from.
📘 What you need to know
Predators kill and eat other animals; the animals eaten are prey.
In a stable community, predator and prey numbers rise and fall in a predator–prey cycle that limits both populations.
The Canada lynx and the snowshoe hare are the famous example.
Top–down control: a population limited by predators (or plants limited by herbivory).
Bottom–up control: a population limited by the availability of resources (or plants limited by light intensity).
Top–down control has lethal and non–lethal effects — predators kill prey, and their presence also changes prey behaviour.
Allelopathy: organisms secrete secondary metabolites that harm other organisms, reducing interspecific competition.
Antibiotic secretion by microorganisms is a form of allelopathy.
Predator–prey cycles
The cycle is four statements in a loop. Learn them in order and you can describe any predator–prey graph an examiner puts in front of you.
🧩 The cycle, in order
The number of predators increases when there is more prey available.
The number of prey decreases in response to the increase in predators.
The number of predators decreases in response to the decrease in prey.
The number of prey increases in response to the decrease in predators.
The cycle repeats.
Relationships this simple — one predator, one prey — are unlikely to exist in nature. There will be other predator and prey species, and other factors affecting the sizes of both populations.
Top–down and bottom–up control
Populations in a community can be controlled from either end of the food chain. Ask a single question: is this population limited by what eats it, or by what it eats?
A coastal seagrass ecosystem is mainly bottom–up, controlled by nutrient availability. Overfishing that removes marine predators can temporarily switch dominance to top–down control.
Top–down cascades. In a food web, a change in one predator ripples outwards. A decrease in foxes could lead to an increase in rabbits, which could lead to a decrease in the growth of grass. And because the grass → rabbit → fox chain does not exist in isolation, that effect will influence other parts of the food web too.
Top–down control shapes an ecosystem through lethal and non–lethal effects, and students almost always forget the second one.
Lethal: predators kill prey, influencing their numbers and therefore their effect on the rest of the ecosystem.
Non–lethal: the presence of predators affects prey behaviour — what they choose to eat, and where they choose to spend time. This can also alter the structure of an ecosystem.
The non–lethal effect is worth real marks. A herd that avoids an open meadow because wolves hunt there will graze somewhere else instead, and that meadow’s plants recover. The wolves changed the vegetation without eating a single plant, and without necessarily killing anything.
Allelopathy
Species compete with each other for resources — that is interspecific competition. Some species have strategies that increase their ability to outcompete others, and those strategies work in one of two ways:
By increasing their own survival chances, e.g. camouflage.
By decreasing the survival chances of a competitor, e.g. secreting harmful chemicals into the environment.
Those harmful chemicals are secondary metabolites, as opposed to primary metabolites, which are molecules essential for survival. Allelopathy is the strategy of secreting secondary metabolites that harm other organisms into the surroundings.
How plants release them
Example species
What it does
Through roots into the soil
Garlic mustard
Produces sinigrin, which reduces seed germination and root growth in other plants
As gases via the stomata into the air
Bracken ferns
Thought to release toxins into surrounding soil, and their fronds contain toxic chemicals released as they decay
Stored in leaves, released when leaves fall and break down
Himalayan balsam
Thought to secrete allelochemicals into the soil that limit the growth of other plants
Antibiotic secretion
Allelopathy is not only a plant trick. The secretion of antibiotics is a form of allelopathy found in some microorganisms — the antibiotic penicillin was discovered in the fungus Penicillium, and antibiotics are also secreted by some bacteria.
Antibiotics kill bacteria, for example by preventing cell wall formation or inhibiting protein synthesis. That reduces interspecific competition, and so increases survival and reproduction in the species producing the antibiotic. The organism is not being medicinal — it is winning a competition.
Worked examples
WE 1
Reading a predator–prey graph
A graph shows lynx and hare numbers over 90 years. Explain why the lynx peaks occur after the hare peaks. (3 marks)
Point 1: predators respond to food
A rise in hare numbers means more prey available, so more lynx survive and reproduce and lynx numbers rise afterwards.
Point 2: prey then fall
The increased number of lynx eat more hares, so the hare population decreases.
Point 3: predators then fall
With fewer hares to eat, lynx numbers decrease in turn, allowing hares to recover and the cycle to repeat.
Each population responds to the other, so the curves are out of stepthe lag is the answer — predators can only increase after their food has
WE 2
A top–down cascade
Foxes are removed from an area of grassland. Predict and explain the effects on the rabbit and grass populations. (3 marks)
Step 1: the rabbits
Removing their predator lifts top–down control, so rabbit numbers increase.
Step 2: the grass
More rabbits means more herbivory, so the growth of grass decreases.
Step 3: the honest caveat
This chain does not exist in isolation, so the change will also influence other parts of the food web.
Fewer foxes, more rabbits, less grassthe caveat about the wider food web is often the third mark
WE 3
Explaining allelopathy
Explain how allelopathy gives a plant an advantage in interspecific competition. (3 marks)
Point 1: what is released
The plant secretes secondary metabolites into its surroundings, for example through its roots into the soil.
Point 2: the effect on rivals
These chemicals harm other species, for example by reducing their seed germination and root growth.
Point 3: the advantage
This decreases the survival chances of competing species, so there is less interspecific competition for light, water and minerals, and the allelopathic plant survives and reproduces more successfully.
Damage the competitor, and you win the resources by defaultsay secondary metabolites, not just “chemicals” — the term is the mark
💡 Exam tips
Learn the predator–prey cycle as four linked statements, and always mention the lag.
Name the pair: Canada lynx and snowshoe hare.
Add the caveat that real food webs are more complex than a single predator and single prey.
Top–down = limited by predators; bottom–up = limited by resources.
Remember the non–lethal effects of predators on prey behaviour.
For allelopathy, use the words secondary metabolites and interspecific competition.
⚠ Common mistakes
Saying predators and prey peak at the same time. The predator curve always lags.
Claiming predators cause prey numbers to fall to zero. Predators decline first, letting prey recover.
Mixing up top–down and bottom–up. Ask what limits the population: what eats it, or what it eats.
Thinking only one type of control can operate. Both can act at once, with one dominant.
Describing allelopathy as a plant “defending itself”. It is attacking a competitor’s survival.
Saying fungi make antibiotics to help humans. They make them to outcompete bacteria.
Up next: Population Growth Curves (Skills). Time to put a name to the shape you have already met twice — and to learn the log–scale trick that proves growth really is exponential.
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