No population grows for ever. Something always pushes back: the weather, the soil, a predator, a disease, or simply too many neighbours wanting the same meal. This page is about those pushes — which ones are living, which are not, and how they combine to set the number of individuals an area can hold.
📚 What you need to know
The distribution and abundance of a population are set by biotic (living) and abiotic (non-living) factors.
Abiotic factors include temperature, light, pH, salinity, dissolved oxygen, soil texture, moisture, nutrients, wind and carbon dioxide. All of them can be measured.
A niche is the full set of conditions a species needs and the role it plays. Two species cannot share an identical niche indefinitely.
Populations interact through herbivory, predation, parasitism, mutualism, disease and competition.
Intraspecific competition is within one species; interspecific competition is between different species.
Predator and prey numbers rise and fall in linked cycles, with the predator peak always coming after the prey peak.
These interactions raise or lower the carrying capacity of the populations involved.
Abiotic factors: the conditions
Abiotic factors decide where a species can live at all. Get below a plant’s minimum temperature or outside a fish’s salinity range and no amount of food will help.
Abiotic factor
What it changes
How it is measured
Temperature
Rate of photosynthesis, growth and reproduction. Most species survive only inside a narrow band.
Thermometer or temperature probe
Light
More light usually means faster photosynthesis and faster plant growth.
Light meter
pH
Controls which nutrients dissolve and are available to roots; too far either way and species die out.
pH meter or probe
Salinity
Decides which aquatic organisms can control their water balance and survive.
Conductivity meter
Dissolved oxygen
Low oxygen kills fish and invertebrates; a classic sign of polluted water.
Oxygen probe
Soil texture and moisture
Controls how much water and air the soil holds and how easily roots push through.
Sieving, drying and weighing samples
If an exam question asks how you would investigate why a plant grows on one side of a field and not the other, name the abiotic factor and the instrument. “Measure light with a light meter at each quadrat” scores; “check the conditions” does not.
Niche: the job, not the address
A habitat is where a species lives. A niche is everything about how it lives there — what it eats, when it is active, where exactly it feeds and shelters, which conditions it tolerates.
Definition
niche = the conditions a species needs + the role it plays in the ecosystem
Here is the important consequence. If two species try to occupy exactly the same niche, they are competing for exactly the same things. One will be slightly better at it, and over time the other is pushed out, forced into a slightly different niche, or lost from the area altogether. That is why, in a healthy ecosystem, species that look like rivals usually turn out to be doing subtly different jobs.
This is niche separation. It looks like sharing, but each species has quietly specialised so that direct competition is avoided.
The six ways populations interact
Interaction
What happens
Who gains
Herbivory
An animal eats a plant. More plants means a higher carrying capacity for the herbivore.
Herbivore gains, plant loses
Predation
One animal hunts and eats another, lowering the carrying capacity of the prey.
Predator gains, prey loses
Parasitism
A parasite lives on or in a host, taking food and shelter and often carrying disease.
Parasite gains, host loses
Mutualism
Both species benefit, so the carrying capacity of both goes up. Bees get nectar, flowers get pollinated.
Both gain
Disease
Pathogens spread through a population and raise the death rate, lowering carrying capacity.
Pathogen gains, host loses
Competition
Two individuals or species need the same limited resource, so both get less of it.
Both lose out
Competition comes in two flavours
Intraspecific — between members of the same species. Two grey squirrels after the same acorn. It gets fiercer as the population grows denser, which is exactly what stops the population growing for ever.
Interspecific — between different species with overlapping niches. A grey squirrel and a red squirrel after the same acorn. This one can end with a species disappearing from the area.
Remember it by the prefix. “Intra” means inside, as in intranet — inside one species. “Inter” means between, as in international — between species.
Predator and prey: linked cycles
In a stable community, predator and prey numbers do not sit still. They cycle, and the two cycles are locked together with a delay. Follow the loop:
🧩 The predator–prey loop
Plenty of prey, so predators find food easily and predator numbers rise.
More predators eating, so prey numbers fall.
Less prey to catch, so predators starve and predator numbers fall.
Fewer predators hunting, so prey numbers recover and the loop starts again.
Notice the predator curve is also flatter. It takes many prey animals to support one predator, so predator numbers are always lower.
The lag is the whole answer to most exam questions on this graph. Predators cannot breed on food they have not eaten yet, so their rise comes after the prey rise, never at the same time and never before it.
Introduced species: competition at its most obvious
When a species arrives somewhere new, the local species have not evolved alongside it. Two things often follow. The newcomer may be better at using a shared resource, out-competing the native species until too few are left to breed. It may also carry a pathogen the native species has no resistance to. Grey squirrels in Britain did both to red squirrels; introduced foxes in Australia did the first to small native mammals.
Worked examples
WORKED EXAMPLE
Two beetle species were grown separately and then together. Explain the results.
Grown alone in identical jars of flour, species A reached 480 individuals and species B reached 450. Grown together in one jar, species A reached 390 and species B fell to zero by week 14.
Step 1: name the interaction
Interspecific competition — two different species needing the same limited resource (flour and space).
Step 2: explain the outcome
Their niches overlap almost completely. Species A uses the resource more efficiently, so it takes a larger share.
Step 3: explain the zero
Species B gets too little food to breed successfully, so its numbers fall until none are left.
Species A out-competes species BNote that species A also ends up lower than 480 — competition costs the winner as well.
WORKED EXAMPLE
On a predator–prey graph the prey peak occurs in year 4 and the predator peak in year 6. Explain the two-year gap.
Step 1: start with the cause
More prey means more food available to predators.
Step 2: give the delay a reason
Predators need time to feed well, breed and raise young before their numbers actually rise, so the increase shows up later.
Step 3: close the loop
By year 6 the extra predators are eating heavily, prey numbers are already falling, and the predator peak then falls too.
The predator peak must follow the prey peakAlways answer in the order cause, delay, consequence. That is how the marks are written.
WORKED EXAMPLE
State whether each example is intraspecific or interspecific competition.
(a) Two male deer fighting over territory. (b) Ivy and a young oak competing for light. (c) Seedlings of the same wildflower growing so densely that most die.
(a) same speciesintraspecific(b) different speciesinterspecific(c) same species, and made worse by densityintraspecificPart (c) is also a density-dependent factor — the next page picks that idea up.
💡 Exam tip
Answer with a chain: factor changes → something happens to the organisms → population size changes. One link is rarely enough for two marks.
For any graph of two species, check which peaks first before writing anything. That single observation drives the whole answer.
Use the phrase carrying capacity when describing what an interaction does. Predation lowers the prey’s carrying capacity; mutualism raises both.
When naming an abiotic factor, pick one you could measure with an instrument. It makes the rest of the answer easier to write.
Learn one named introduced-species example properly. It works for competition, disease and human impact questions alike.
⚠ Common mix-up
Habitat and niche. Habitat is the address; niche is the job description. Three species can share a habitat but not a niche.
Saying predators wipe out their prey. If they did, the predators would starve. In a stable community both cycle instead.
Parasitism confused with predation. A predator kills quickly; a parasite lives with the host and harms it slowly.
Mutualism confused with parasitism. In mutualism both species gain. Check the second species before choosing.
Reading the predator peak as the cause. The prey rise comes first and causes the predator rise.
Calling competition a biotic factor only when animals fight. Plants compete hard for light, water and nutrients without moving at all.
Up next: Patterns of Population Growth — carrying capacity, J-curves, S-curves and the negative feedback that keeps a population near its limit.
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