No population grows forever. Something always stops it — running out of food, being eaten, catching a disease, or simply the weather. This page sorts those brakes into biotic and abiotic, then into density-dependent and density-independent, which is the split examiners really care about.
📚 What you need to know
Biotic factors come from other living things: predation, herbivory, parasitism, mutualism, disease and competition.
Abiotic factors are physical and non-living: temperature, light, water, pH, salinity, oxygen, soil and wind.
Density-dependent factors hit harder as a population gets more crowded.
Density-independent factors hit just as hard whatever the density — a flood drowns a crowded field and an empty one alike.
Density-dependent factors drive negative feedback, pulling a population back towards its carrying capacity.
Intraspecific competition is within one species; interspecific competition is between different species.
Two kinds of factor
Everything that limits a population is either alive or not alive. That is the whole distinction, and it is worth one easy mark whenever it appears.
Abiotic factors are quantifiable, which is exactly why fieldwork so often pairs a species count with a set of physical measurements.
How populations interact
Interaction
What happens
Effect on numbers
Herbivory
An animal eats a plant.
More plants means the herbivore population can grow; heavy grazing knocks the plants back and then the herbivores fall too.
Predation
One animal hunts and eats another.
Predators hold prey numbers down; prey numbers then limit the predators.
Parasitism
One species lives on or in a host and takes resources from it.
The parasite gains, the host is weakened, so the host population can support fewer individuals.
Mutualism
Both species gain, such as bees and flowering plants.
Both populations can support more individuals than they could alone.
Disease
A pathogen spreads through the population.
Death rate rises, so numbers fall until survivors with resistance recover.
Competition
Two individuals need the same limited resource.
The weaker competitor gets less, breeds less and may be pushed out entirely.
Two flavours of competition
Intraspecific competition happens within a single species: two grey squirrels going for the same acorn. Because both animals need exactly the same things, this is the fiercest competition there is, and it gets worse as the population grows.
Interspecific competition happens between species with overlapping needs: a grey squirrel and a red squirrel after the same acorn. Here one species is usually better at getting the resource, so the other slowly declines. In Britain grey squirrels outcompeted red squirrels for food and nest sites, and also carried a virus the reds had no resistance to.
Prefix trick: intra means inside (think intranet, inside a company) and inter means between (think international, between nations). Same species inside, different species between.
Predator and prey rise and fall together
Predator and prey numbers do not stay flat. They chase each other in cycles, and the predator peak always comes after the prey peak. That delay is the whole point of the graph.
Read the cycle in four steps: prey rise, predators rise, prey fall, predators fall — then it repeats.
Density-dependent and density-independent
This is the split that separates a grade 5 answer from a grade 7 one. Ask yourself: does crowding make this factor worse?
Type
Examples
Why it depends on crowding
Density-dependent
Competition for food, predation, disease and parasites
The more individuals packed into an area, the less food each gets, the easier prey are to find, and the faster infection spreads.
Density-independent
Drought, flood, frost, fire, volcanic ash
A late frost kills the same proportion of a crowded population as a sparse one. Numbers make no difference.
Negative feedback keeps numbers steady
Density-dependent factors are what create negative feedback. A rise in numbers triggers changes that push numbers back down; a fall triggers changes that let numbers climb again. The population therefore wobbles around a set point rather than shooting off in one direction.
Negative feedback is the reason a stable ecosystem does not simply fill up with one species.
Worked examples
WORKED EXAMPLE
Reading a competition data set
Two beetle species were kept in the same store of flour. The table shows the mean number of adults counted each month.
Month
Species A
Species B
0
120
115
3
168
84
6
214
41
9
247
12
Describe the trend and explain what is most likely causing it. [4]
Step 1: describe both trends with dataA rises from 120 to 247 while B falls from 115 to 12 over nine monthsStep 2: name the interactioninterspecific competition — different species, same foodStep 3: explain the mechanismA is better adapted to use the flour, so it gets more food, breeds more and outcompetes BB declines towards local extinction as A takes the shared resourcealways quote two numbers when a question says “describe the trend”
WORKED EXAMPLE
Sorting factors correctly
A colony of seabirds nests on a cliff. In one year an unusually severe storm destroys many nests, and in another year an outbreak of avian flu spreads through the crowded colony. Classify each event and justify your choice. [4]
The stormabiotic (physical, non-living) and density-independentit destroys the same proportion of nests however many birds there areThe flu outbreakbiotic (caused by a living pathogen) and density-dependentcrowding means birds are closer together, so infection spreads fasterstorm: abiotic, density-independent. Flu: biotic, density-dependenttwo classifications are needed for each event — do not stop at “abiotic”
💡 Exam tip
Use the crowding test to decide density-dependence: if packing them in tighter makes the factor worse, it is density-dependent.
Predation, disease and competition are the three safe density-dependent examples. Weather events are the safe density-independent ones.
On predator-prey graphs, always say the predator peak lags behind the prey peak.
Data questions want numbers. Quote a starting value and a finishing value with units.
Negative feedback answers should include the word “returns” or “restores” — the system is being pulled back to a set point.
If a question says “explain”, give the chain: more crowding → less food each → fewer survive → population falls.
⚠ Common mix-up
Thinking negative feedback is harmful. Negative means stabilising, not bad.
Swapping intra- and interspecific. Intra is inside one species, inter is between two.
Saying predators wipe out their prey. If they did, the predators would starve; that is why the cycle exists.
Calling drought density-dependent. A drought does not care how many individuals are present.
Listing a factor without a mechanism. “Competition” alone rarely scores; say what is being competed for.
Forgetting mutualism. Not every biotic interaction is harmful — some raise both populations.
Up next: Patterns of Population Growth — you know the brakes, now look at the shapes those brakes produce on a graph.
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