IB Biology HLPopulations & CommunitiesPaper 1 & 2~11 min read
What Limits Population Size
On paper, every population should grow forever — each individual can reproduce, and each offspring can reproduce too. In practice populations level off. This page is about the ceiling they hit, what builds it, and why populations wobble around it instead of sitting still on it.
📘 What you need to know
Carrying capacity (K) is the maximum number of individuals of a species that an ecosystem can support.
Every individual has the theoretical potential to reproduce, but many factors stop every individual surviving and reproducing.
Abiotic (non–living) factors: light availability, temperature, soil mineral availability.
Biotic (living) factors: competition for resources, predation, disease.
Population density is the number of individuals present per unit area of habitat.
Density–dependent factors have a different effect at different population densities.
Density–independent factors have the same effect at any density, e.g. a flood.
Density–dependent factors keep a population at or below K by negative feedback, so it fluctuates around its carrying capacity.
Carrying capacity
The carrying capacity of an ecosystem for a species is the largest number of that species it can support. It is written as K. On a growth graph it is the level at which the curve flattens out: at that point the environmental factors stopping individuals surviving and reproducing mean the population can no longer increase.
Nothing has changed about the lions themselves. What has changed is that there are now enough of them for food, space and disease to start pushing back.
What sets the carrying capacity
Split the factors into abiotic (non–living) and biotic (living). The biotic ones share a single, very quotable explanation, so learn that sentence once and reuse it three times.
Type
Factor
How it lowers carrying capacity
Abiotic
Light availability
A lack of light limits photosynthesis, so fewer plants can be supported
Abiotic
Temperature
Low or high temperatures affect the rate of metabolic reactions
Abiotic
Soil mineral availability
Low minerals limit the building of molecules such as proteins and chlorophyll
Biotic
Competition for resources
A lack of resources limits K, and energy spent competing is not available for growth and reproduction
Biotic
Predation
Energy spent avoiding predators is not available for growth and reproduction
Biotic
Disease
Energy spent fighting off disease is not available for growth and reproduction
🧠
One sentence covers all three biotic factors
Energy an individual spends on X is energy it cannot spend on growth and reproduction, so carrying capacity falls. Swap X for competing, avoiding predators, or fighting disease. Three marks from one memorised line.
Density–dependent and density–independent factors
Population density is the number of individuals present per unit area of habitat. Some limiting factors care about density and some do not, and that difference has a name.
The three classic density–dependent factors are disease, competition and predation. They all become more severe the more crowded the population gets.
Negative feedback around K
Density–dependent factors act to keep a population at or below its carrying capacity. That is a negative feedback effect: a system that keeps conditions within narrow limits by pushing back whenever they stray from an ideal value.
🧩 The loop, in both directions
Population rises above K.
Density–dependent factors intensify: disease spreads, competition for food increases.
Survival and reproduction fall, so the population size decreases.
Population drops below K.
The same factors ease off: less disease, less competition.
Survival and reproduction increase, so the population size increases again.
The result is that a population controlled by negative feedback does not sit exactly on K — it fluctuates around its carrying capacity, overshooting and undershooting in turn.
Compare this with negative feedback in homeostasis. It is the same idea — a value straying from an ideal triggers a response that brings it back.
And positive feedback? A population under positive feedback responds to a change by continuing to change in the same direction. More individuals means more reproduction, which means more individuals again. This is what drives the early, explosive part of a growth curve — and it continues until a density–dependent factor such as competition starts to limit growth.
Worked examples
WE 1
Define and explain carrying capacity
Define carrying capacity and explain why a population stops increasing when it reaches this value. (3 marks)
Point 1: the definition
The maximum number of individuals of a species that an ecosystem can support.
Point 2: what happens there
At this point, environmental factors prevent all individuals from surviving and reproducing.
Point 3: the balance
Deaths balance births, so the population can no longer increase and the growth curve flattens out.
The ecosystem cannot support any more individualsuse “maximum number… an ecosystem can support” — that exact phrasing is the mark
WE 2
Sorting the factors
A population of rabbits is affected by (a) an outbreak of myxomatosis and (b) a severe winter frost. Classify each factor and justify your choice. (4 marks)
(a) Myxomatosis: density–dependent
Disease is a biotic factor that spreads faster through a dense population, so it has a greater effect at high densities.
(a) Why it matters
This means it can regulate the population, pushing numbers back towards carrying capacity.
(b) Frost: density–independent
Temperature is an abiotic factor. A frost is equally likely to kill rabbits whether they are crowded or sparse.
(b) Why it matters
Its effect does not depend on numbers, so it cannot hold the population at a particular level.
Disease responds to crowding; frost does notthe justification is where the marks are — do not just label them
WE 3
Explaining a fluctuating graph
A graph shows a deer population rising above and falling below a dashed line labelled K. Explain this pattern. (3 marks)
Point 1: above K
When numbers rise above carrying capacity, density–dependent factors such as competition for food and spread of disease reduce survival and reproduction, so the population decreases.
Point 2: below K
When numbers drop below carrying capacity those same factors ease, so survival and reproduction increase and the population rises again.
Point 3: name it
This is negative feedback, and it makes the population fluctuate around its carrying capacity rather than settling exactly on it.
Density–dependent factors reverse the change each timename negative feedback explicitly — it is usually a whole mark on its own
💡 Exam tips
Carrying capacity is the maximum number an ecosystem can support, and it is written K.
Sort factors into abiotic and biotic before you write, so your answer has a structure.
Use the energy line for biotic factors: energy spent on X is not available for growth and reproduction.
For density–dependent, always say greater effect at higher density.
Name negative feedback when explaining fluctuation around K.
A population fluctuates around K — it does not sit exactly on it.
⚠ Common mistakes
Saying carrying capacity is the number of organisms living there. It is the maximum the ecosystem can support.
Treating K as fixed forever. It changes if the environment changes.
Calling a flood density–dependent because it kills more in a crowd. The proportion killed is what matters, and that does not change.
Mixing up density–dependent with biotic. They overlap heavily, but they are different classifications.
Confusing negative feedback with “a bad effect”. Negative means opposing the change, not harmful.
Saying predation reduces K only by killing. The energy cost of avoiding predators matters too.
Up next: Limiting Population Size: Examples. Now for the named case studies — predator–prey cycles, top–down and bottom–up control, and plants that poison their neighbours.
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