Most tested B10.3

Biodiversity and Human Interactions

This topic covers how we measure the variety and number of organisms in an ecosystem using sampling techniques, and how to analyse the effects of human activities like farming and pollution on this biodiversity.

Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • Random sampling with quadrats is used to estimate the abundance (population size) of organisms, assuming an even distribution.
  • Systematic sampling with a belt transect is used to investigate how the distribution of organisms changes along an environmental gradient (e.g., soil pH, light intensity).
  • Eutrophication is a sequence: excess nutrients (e.g., from fertiliser runoff) cause an algal bloom, which dies and is decomposed by bacteria that consume oxygen, leading to the death of fish and other aquatic life.
  • Acid rain, primarily from sulfur dioxide emissions, damages ecosystems by making soil and water more acidic, which can leach essential minerals needed by trees and release toxic aluminium.
  • Intensive fish farming can harm local biodiversity when waste food and faeces accumulate, causing deoxygenation of the water below the cages and killing wild organisms.
  • Biodiversity is a measure of both the number of different species (species richness) and the population size of each species (abundance).

Formulae

Estimated Population = (Total individuals in samples / Total area sampled) × Total habitat area

To estimate the total number of organisms in a habitat based on data from random quadrat sampling. Remember that 'Total area sampled' = (Number of quadrats) × (Area of one quadrat).

Definitions

Biodiversity
A measure of the variety of life in an area, incorporating both the number of different species present (richness) and the relative numbers of individuals of each species (abundance).
Quadrat
A square frame of a known area used for sampling the abundance of plants and slow-moving animals.
Belt Transect
A sampling method where quadrats are placed at regular intervals along a straight line to investigate how species distribution changes across a habitat.

Worked example

A student estimates the population of dandelions in a rectangular school field measuring 20m by 100m. They use a 0.5m x 0.5m quadrat. They take 10 random samples and count a total of 60 dandelions. What is the estimated total population of dandelions in the field?

  1. 1

    1.

    Calculate the area of a single quadrat:

    Area = 0.5m × 0.5m = 0.25 m2
  2. 2

    2.

    Calculate the total area sampled:

    Total Sample Area = 10 quadrats × 0.25 m2/quadrat = 2.5 m2.

  3. 3

    3.

    Calculate the total area of the field:

    Total Field Area = 20m × 100m = 2000 m2.

  4. 4

    4.

    Calculate the density of dandelions per square metre:

    Density = Total dandelions counted / Total sample area = 60 / 2.5 m2
  5. 5

    5.

    To calculate 60 / 2.5 without a calculator, you can calculate 60 / (5/2) = 60 × (2/5) = 120 / 5 = 24.

    So, the density is 24 dandelions/m2.

  6. 6

    6.

    Estimate the total population by scaling up:

    Total Population = Density × Total Field Area = 24 dandelions/m2 × 2000 m2 = 48000.

Answer: 48000

Common mistakes

  • ×Errors in scaling up calculations. A frequent mistake is to multiply the mean per quadrat by the total area, forgetting to first divide by the quadrat's area to find the density per m2.
  • ×Mixing up units. Ensure the quadrat area and the total habitat area are in the same units (e.g., both in m2) before you perform the final calculation.
  • ×Confusing the methods for abundance and distribution. Remember: random quadrats for 'how many' (abundance), line/belt transects for 'where' (distribution).
  • ×Forgetting the assumptions of sampling. Your estimate assumes the samples are representative of the whole area, which is why random placement is crucial for abundance studies.

No-calculator tips

  • ✓To divide by decimal quadrat areas, use fractions or multiplication. Dividing by 0.25 is the same as multiplying by 4. Dividing by 0.5 is the same as multiplying by 2.
  • ✓Calculate the density (organisms per m2) as an intermediate step. This often gives you a simpler whole number to work with for the final scaling-up multiplication.
  • ✓Break down large multiplications. For 24 × 2000, calculate 24 × 2 = 48, then add the three zeros to get 48,000.

Read this topic in the official UAT-UK ESAT guide →

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