C4.1.5—Carrying capacity
Carrying capacity explains how ecological evidence links organisms, resources and interactions to population size, distribution or community structure in a habitat.
- Syllabus
- First assessment 2025
- Objective
- C4.1.5
- Level
- SL
Exam analysis
Chance of appearing2%of analysed past papers
Latest appearanceNovember 2018
Most common paperPaper1
Typical marks1–7
Common command terms
- Identify
- Describe
- Explain
- Outline
- State
Scoring notes
- Common mistake
- Confusing a population with a community or with all organisms in an ecosystem.
Recent exam appearances
November 2018Paper1 ["SL"] · TZ019[ 1 ]C4.1.5—Carrying capacity
May 2012Paper2 ["SL"] · TZ27(c)[ 7 ]C4.1.5—Carrying capacity
May 2010Paper1 ["SL"] · TZ121[ 1 ]C4.1.5—Carrying capacity
Practice this objective
Coverage 2010–2018 · Updated 16 Jul 2026
Carrying capacity is a resource-limited population size
Carrying capacity is a resource-limited population size.
Carrying capacity is the population size an environment can support over time under specified conditions. Resource limits and feedback make growth slow as the population approaches it.
identify limiting resource; link density to birth/death rate; state that capacity can change.
A drought lowers available food, so the carrying capacity for grazing animals falls even if the species is unchanged.
Carrying capacity is not a fixed species constant; it depends on environment and timescale.
Carrying capacity
Assessment in practice
1–6 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Describe / Explain.
Command terms
Identify / Describe / Explain / Outline / State
What earns marks
Build the answer around this relationship: Carrying capacity must be linked to the correct ecological unit, method or species interaction.
Watch for
Confusing a population with a community or with all organisms in an ecosystem.
Representative question
Question 1
Explain why populations that have grown exponentially reach a maximum size, rather than continue to grow.
Populations and Communities
- A population is one species in an area; a community is all interacting populations there.
- Estimate abundance with unbiased sampling: quadrats for sessile organisms and capture–mark–release–recapture for mobile animals, checking each method’s assumptions.
- Density-dependent competition, predation, disease and waste create negative feedback around carrying capacity; exponential growth slows into a sigmoid curve as limits strengthen.
- Classify interspecific relationships by costs and benefits: predation, herbivory, competition, mutualism, parasitism and pathogenicity.
- Invasive species may escape controls and displace endemic species. Removal experiments can reveal competition and fundamental versus realized niches.
- Chi-squared tests assess species association from observed and expected quadrat counts.
- Predator peaks usually lag prey peaks; top-down control begins with consumers, while bottom-up control begins with resources or producers.
Concept essentials
- Carrying capacity must be linked to the correct ecological unit, method or species interaction.
- The evidence for carrying capacity depends on measurable abundance, distribution, survival or resource patterns.
- Carrying capacity is clearest when cause, ecological process and population outcome are kept distinct.
- Field or graph evidence for carrying capacity needs biological interpretation, not values alone.