Every species could, in theory, fill the planet. A single pair of rabbits breeding freely would bury a country in a few years. They do not, and the reason is that habitats run out of things. This page is about what runs out, and what happens when it does.
📘 What you need to know
Carrying capacity (K) is the maximum number of individuals of a species that an ecosystem can support.
Abiotic limiting factors are non-living: light, temperature, water, soil minerals.
Biotic limiting factors are living: competition, predation, disease.
Density-dependent factors hit harder when the population is crowded. Density-independent factors hit equally hard whatever the density.
Density-dependent factors create negative feedback, so populations fluctuate around K rather than sitting exactly on it.
Population density = the number of individuals per unit area of habitat.
Carrying capacity
Give a population plenty of space and food and it grows quickly. Keep going and the growth slows, then stops. The level it stops at is the carrying capacity.
Definition to learn
Carrying capacity (K) = the maximum number of individuals of a species that an ecosystem can support
Read the shape, not just the end point. The steep middle section is the population growing almost unchecked; the flat section is the habitat saying no.
What actually runs out?
Limiting factors come in two families. Abiotic factors are the non-living conditions. Biotic factors are the other living things.
Type
Factor
Why it lowers carrying capacity
Abiotic
Light
Less light means less photosynthesis, so plants make less food and fewer can survive
Abiotic
Temperature
Very high or very low temperatures slow the enzyme-controlled reactions of metabolism
Abiotic
Soil minerals
Without nitrate and magnesium, plants cannot build proteins and chlorophyll
Biotic
Competition
Energy spent competing for food, light or mates is energy not spent on growth and reproduction
Biotic
Predation
Prey are killed, and the survivors spend energy hiding and fleeing instead of breeding
Biotic
Disease
Energy goes into fighting the pathogen, so less is left for growth and reproduction
Notice the pattern running down the biotic rows: energy spent on one thing cannot be spent on another. If you can say that sentence, you can explain any biotic limiting factor you are given, even one you have never met.
Density-dependent or density-independent?
This is the split examiners test most. The question is simple: does crowding make this factor worse?
Density-dependent
Density-independent
The effect gets stronger as the population gets denser
The effect is the same at any density
Disease spreads faster when individuals are packed together
A flood drowns a sparse population just as readily as a crowded one
Competition for food bites harder when more mouths share the same supply
A hard frost kills a set proportion regardless of numbers
Predators are drawn to dense patches of prey
A volcanic eruption or a wildfire hits everything in its path
Mostly biotic factors
Mostly abiotic factors
Quick test. Ask yourself: “if I doubled the number of individuals in the same space, would this factor kill a bigger fraction of them?” Yes means density-dependent. No means density-independent.
WORKED EXAMPLE
Classifying limiting factors
A population of freshwater fish lives in a small lake. For each factor, state whether it is density-dependent or density-independent, and give a reason: (a) a fungal infection of the gills, (b) a severe winter that freezes the lake, (c) competition for insect larvae.
(a) fungal infectiondensity-dependentfish are closer together at high density, so the fungus passes between them more easily(b) severe winterdensity-independentthe freeze affects the same proportion of fish whether there are 50 or 5000(c) competition for larvaedensity-dependentmore fish share the same food supply, so each one gets less
Why populations wobble around K
Real populations do not settle exactly on the carrying capacity and stay there. They overshoot, drop back, overshoot again. That pattern is not random — it is negative feedback doing its job.
Follow the chain. Numbers rise above K → competition and disease increase → more deaths and fewer offspring → numbers fall. Numbers drop below K → plenty of food, less disease → more survive and breed → numbers rise. Each time the population moves away from K, the same factors push it back.
The wobble is the evidence. A population held by negative feedback never sits still, it just never wanders far.
🧩 Writing a negative feedback answer
Start with the change. “The population rises above the carrying capacity.”
Name the density-dependent factor. Competition for food, spread of disease, or predation.
Say what it does to individuals. Death rate rises, birth rate falls.
Finish with the result. “So the population size decreases back towards K.”
Then do the mirror image for a population that drops below K.
What about positive feedback?
Positive feedback pushes a change further in the same direction. More individuals means more breeding pairs, which means even more individuals. That is what drives the steep part of a growth curve. It cannot last: sooner or later a density-dependent factor kicks in and negative feedback takes over.
WORKED EXAMPLE
Explaining the wobble
The graph of a rabbit population on an island shows numbers rising and falling repeatedly around a fixed level. Explain, using named factors, why the population does not simply settle at the carrying capacity. [4]
Point 1: overshootrabbits keep breeding, so numbers rise past KPoint 2: density-dependent factors increasecompetition for grass increases and disease spreads faster in the crowded populationPoint 3: numbers fall backdeath rate rises above birth rate, so the population drops below KPoint 4: the cycle repeatsless competition below K means more survive and breed, so numbers rise againthe word examiners want somewhere in here is negative feedback
💡 Exam tip
Learn the definition of carrying capacity as a sentence, including the word maximum.
Never write “there is not enough food” on its own. Say competition for food increases, so fewer individuals survive and reproduce.
Abiotic and biotic is a different split from density-dependent and independent. A question may ask for either, so read carefully.
On a graph, the carrying capacity is the level the curve levels off at, not the highest single point.
Use the energy argument for biotic factors: energy used competing, hiding or fighting infection is not available for growth and reproduction.
If a question says “explain”, you need a chain with the word so or therefore in it at least twice.
⚠ Common mix-up
Carrying capacity is not the starting population. It is the ceiling the population grows up to.
Predation is not always density-dependent in exam answers by default — you must say why: predators are attracted to dense prey populations.
Natural disasters are density-independent, even though they kill huge numbers. Size of effect is not the test; dependence on crowding is.
Negative feedback does not mean the population shrinks. It means it is pulled back towards K from either direction.
Do not say the population “stops growing because K is reached”. That is circular. Name the factor that stops it.
Density is per unit area, so a bigger habitat with the same number of individuals has a lower density.
Up next: Limiting Population Size: Examples — time to put real species into these ideas, starting with the most famous predator-prey pair in biology.
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