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Learn5090Paper 1 · Multiple ChoiceRelationships of organisms with one another and with the environmentPopulations and sampling
Relationships of organisms with one another and with the environment · Subtopic 5 of 5

Populations and Sampling

A population is all the organisms of one species in an area; a community is all the populations of different species; an ecosystem is a community and its physical environment. Population size depends on biotic factors (food, predators, disease, competition) and abiotic factors (temperature, light, water). Populations often follow a sigmoid curve: lag, exponential (log) growth, stationary (births = deaths) and sometimes death phase. Population size is estimated with quadrats: count organisms in random quadrats, find the mean, and scale up to the total area.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define population, community and ecosystem.
  • Describe factors affecting population size.
  • Interpret the sigmoid growth curve.
  • Use quadrat data to estimate population size.
Pause and think

How could you estimate how many daisies are on a school field without counting them all?

Section 2 of 14

Core idea

Start hereCounting the uncountable

Ecologists can't count every snail in a field. Instead they sample small areas and scale up, using statistics to estimate whole populations.

The big idea

Population size depends on biotic and abiotic factors. Populations often grow in an S-shaped curve. Random sampling with quadrats estimates population size.

1. Growth curves

Phases of population growth

From few to carrying capacity.

2. Quadrat sampling

Worked exampleEstimating buttercups

Eight 0.5 m² quadrats contain a total of 24 buttercups. The meadow is 1200 m².

Goal
Estimate the population.
Strategy
Mean → density → total.
  1. 24÷8=3 per quadrat24\div8=3\ \text{per quadrat}
    Mean.
  2. 3÷0.5=6 per m23\div0.5=6\ \text{per m}^2
    Density.
  3. 6×1200=72006\times1200=7200
    Scale up.
AnswerAbout 7200 buttercups.
✓ CheckMore quadrats would make it more reliable ✓.
Exam tipRandom placement Use a random number generator for coordinates on a grid of tape measures to avoid bias.
Pause and think

Why would counting snails only near a hedge overestimate the field's population?

Summary

  • Biotic and abiotic factors.
  • Sigmoid curve.
  • Mean ÷ quadrat area × total area.
Key words
Community
All the populations in an area.
Quadrat
Square frame used for sampling.
Carrying capacity
Maximum population an environment can support.
HistoryMalthus. In 1798 Thomas Malthus argued that populations grow faster than food supply, so growth must eventually be limited by famine or disease. His essay later inspired Darwin.
Fun factLemming myth. Lemmings don't jump off cliffs; their numbers boom and crash in roughly four-year cycles driven by food and predators.
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