Database results:
    examBoard: Pearson Edexcel
    examType: IGCSE
    lessonTitle: Population Size Investigation
Biology - Ecology and Environment - Organisms and Environment - Population Size Investigation - BrainyLemons
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Organisms and Environment » Population Size Investigation

What you'll learn this session

Study time: 30 minutes

  • How to design and conduct a population size investigation
  • Sampling techniques for estimating population sizes
  • How to use quadrats and transects effectively
  • Methods for marking and recapturing mobile organisms
  • How to analyse and present population data
  • Factors affecting the reliability of population estimates

Introduction to Population Size Investigation

Ecologists need to know how many organisms live in a habitat to understand ecosystem health, monitor endangered species and track changes over time. But counting every single organism in an area is usually impossible! Instead, we use clever sampling techniques to estimate population sizes.

Key Definitions:

  • Population: All the organisms of one species living in a particular area at a particular time.
  • Sample: A smaller, representative portion of a population that is studied to make inferences about the whole population.
  • Abundance: The total number of organisms in a population.
  • Distribution: The pattern of spread of organisms within their habitat.

🌍 Why Study Population Size?

Understanding population size helps us:

  • Monitor endangered species
  • Track invasive species spread
  • Assess the impact of pollution
  • Understand predator-prey relationships
  • Make conservation decisions

🔬 Investigation Planning

A good population investigation needs:

  • A clear research question
  • Appropriate sampling technique
  • Sufficient sample size
  • Control of variables where possible
  • Consideration of ethical issues

Sampling Techniques for Immobile Organisms

Plants and slow-moving organisms can be counted using area-based sampling methods.

Using Quadrats

A quadrat is a square frame (usually 0.5m × 0.5m or 1m × 1m) placed randomly within a habitat to count organisms within it.

📌 Random Sampling

Place quadrats using random coordinates to avoid bias. Count all organisms of your target species within each quadrat.

📏 Systematic Sampling

Place quadrats at regular intervals (e.g., every 5 metres) along a line or grid to detect patterns across the habitat.

📊 Stratified Sampling

Divide the habitat into distinct zones and sample randomly within each zone to account for habitat variation.

Calculating Population Size with Quadrats

To estimate the total population size:

  1. Calculate the mean number of organisms per quadrat
  2. Calculate the total area of the habitat (in m²)
  3. Calculate the area of one quadrat (in m²)
  4. Use this formula: Population estimate = Mean number per quadrat × (Total area ÷ Quadrat area)

Worked Example

You count daisies in 10 random quadrats (each 0.5m × 0.5m) on a school field measuring 50m × 30m.

Quadrat counts: 5, 3, 7, 0, 2, 4, 6, 3, 5, 5

Mean count = 4 daisies per quadrat

Total area = 50m × 30m = 1500m²

Quadrat area = 0.5m × 0.5m = 0.25m²

Population estimate = 4 × (1500 ÷ 0.25) = 4 × 6000 = 24,000 daisies

Transect Sampling

Transects are used to study how populations change across environmental gradients (e.g., from sea to shore, sunny to shaded areas).

📐 Line Transect

A string or tape measure is laid across the habitat. Organisms touching the line are counted, or quadrats are placed at intervals along the line.

🗺 Belt Transect

A wider strip (typically 0.5-1m wide) along which all organisms are counted. Good for larger or less abundant species.

Sampling Mobile Organisms: Mark-Release-Recapture

For animals that move around, we use the mark-release-recapture technique to estimate population size.

The Lincoln Index Method

This method involves three steps:

  1. Mark: Capture a sample of animals and mark them harmlessly (e.g., small dot of non-toxic paint)
  2. Release: Return the marked animals to the habitat and allow time for them to mix with the unmarked population
  3. Recapture: Capture a second sample and count how many are marked

The formula for estimating the total population is:

Lincoln Index Formula

Population estimate (N) = (M × C) ÷ R

Where:

  • M = Number of individuals marked in first catch
  • C = Total number of individuals in second catch
  • R = Number of marked individuals in second catch

Worked Example

You catch 50 woodlice, mark them with a small dot and release them. The next day, you catch 40 woodlice and 10 of them have marks.

M = 50 (marked in first catch)

C = 40 (total in second catch)

R = 10 (marked ones in second catch)

Population estimate = (50 × 40) ÷ 10 = 200 woodlice

Assumptions and Limitations

For mark-release-recapture to be valid:

  • Marked individuals must mix randomly with the unmarked population
  • Marks must not affect survival or behaviour
  • Marks must not wear off between sampling periods
  • The population must be closed (no immigration, emigration, births or deaths)
  • All individuals must have an equal chance of being caught

Analysing and Presenting Your Data

Once you've collected your data, you need to analyse and present it clearly.

📈 Data Analysis

  • Calculate mean, median, mode and range
  • Consider standard deviation to show variation
  • Use statistical tests to compare populations (e.g., t-test)
  • Consider sources of error and uncertainty

📊 Data Presentation

  • Bar charts for comparing different areas
  • Line graphs for showing change over time
  • Scatter graphs for showing correlations
  • Distribution maps showing where organisms were found

Improving Reliability

To make your population estimates more reliable:

  • Increase sample size: More quadrats or larger capture samples give better estimates
  • Repeat the investigation: Take measurements on different days or seasons
  • Use appropriate equipment: Ensure quadrats and measuring tools are accurate
  • Control variables: Try to keep conditions consistent between samples
  • Consider timing: Some organisms are more active at certain times of day

Case Study: Monitoring Butterfly Populations

The UK Butterfly Monitoring Scheme has been running since 1976. Volunteers walk the same route weekly during summer, counting all butterflies seen within 5m. This long-term data has revealed alarming declines in many species, helping to target conservation efforts. For example, the High Brown Fritillary has declined by over 96% since the 1970s, leading to targeted habitat management that has helped stabilise some populations.

Ethical Considerations

When investigating populations, always consider:

  • Minimising disturbance to habitats and organisms
  • Using non-harmful marking methods
  • Returning organisms to their exact capture location
  • Getting permission to access study sites
  • Following safety procedures when working outdoors
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