Random Sampling (AQA A Level Biology): Revision Note

Random sampling

• Measuring the different levels of biodiversity within an ecosystem is not an easy task

• Finding out which species live in an ecosystem and the size of the populations requires the identification and cataloguing of all organisms present to build a species list

• This is possible for areas that are very small or where the species are large such as trees

• However, for larger and more complex ecosystems like rainforests, it is simply impossible to find, identify and count every organism that exists there

• When this is the case, different samples of the area can be taken and used to estimate the total species numbers in the area

• Sampling is a method of investigating the abundance and distribution of species and populations

• There are two different types of sampling:

  • Random

  • Systematic

• In random sampling, the positions of the sampling points are completely random or due to chance

  • This method is beneficial because it means there will be no sampling bias by the person who is carrying out the sampling that may affect the results

• In systematic sampling, the positions of the sampling points are chosen by the person carrying out the sampling

  • There is a possibility that the person choosing could show bias towards or against certain areas

  • Individuals may deliberately place the quadrats in areas with the fewest species, as these will be easier and quicker to count

  • This is unrepresentative of the whole area

• When a sampling area is reasonably uniform or has no clear pattern to the way the species are distributed, then random sampling is the best choice

Frame quadrats

• Some ecosystems are very complex, with large numbers of different species of different sizes

• For the sake of logistics, sampling is often used to estimate the distribution and abundance of species

  • The distribution of a species describes how it is spread throughout the ecosystem

  • The abundance of a species is the number of individuals of that species

• When carrying out sampling, square frames called quadrats can be used to mark off the area being sampled

• Quadrats of different sizes can be used depending on what is being measured and what is most suitable in the space where the samples are being made.

• Quadrats must be laid randomly in the area to avoid bias

  • This random sampling can be done by converting the sampling area into a grid format and labelling each square on the grid with a number

  • A random number generator is used to pick the sample points

• Once the quadrat has been laid on the chosen sample point, the abundance of all the different species present can be recorded

Apparatus

• Quadrat

• Random number generator

Method

• Mark up a grid on a map or a scale drawing of the area being studied, and label the grid with coordinates

  • The area must be big enough to get a representative estimate for the specific habitat/ecosystem

Calculating population density

1. Use a random number generator to choose a set of coordinates

2. Place the quadrat at each generated coordinate.

3. Count the number of individuals within each quadrat.

4. Population density is expressed as the number of individuals per square metre (m²).

5. Use a running mean to determine how many quadrats are needed for a reliable estimate:

   • Calculate the mean number of individuals after 2 quadrats, then after 3, then after 4, and so on.

   • Stop when the mean stabilises (shows little or no change with additional quadrats).

6. To estimate the total population size:

   • Multiply the mean number of individuals per quadrat by the total area of the habitat.

Calculating the abundance of a species using percentage cover

1. This method is typically used for plants

2. Place a grid quadrat at the selected coordinate

3. Each square in the grid represents 1% cover

4. Count the number of squares where the species covers more than half of the square

5. The percentage cover equals the number of occupied squares

   • E.g., if 30 squares are occupied, percentage cover = 30%

6. This method is subjective, so the same person should assess all quadrats to maintain consistency

Calculating the abundance of a species using frequency

1. Place a frame quadrat at several randomly generated coordinates

2. Record whether the species is present or absent in each quadrat

3. Calculate frequency as:

   • (Number of quadrats where the species is present ÷ Total number of quadrats) × 100

   • E.g., if the species is found in 3 out of 10 quadrats, the frequency is 30%.

Results

• The results from the quadrats can be used to calculate the predicted frequency and density of a species within an area

• Species frequency

  • The probability that the species will be found within any quadrat in the sample area

  • (Number of quadrats where species is present ÷ Total number of quadrats) × 100

  • E.g., if bluebells are found in 18 out of 50 quadrants:

    • Frequency = (18 ÷ 50) × 100 = 36%

• Species density

  • indicates how many individuals of that species there are per unit area

  • Total number of individuals counted ÷ Total area of quadrats

  • E.g., 107 bluebells in 50 quadrats (1 m² each)

    • Density = 107 ÷ 50 = 2.14 individuals per m²

• Percentage Cover

  • This is used when it's difficult to count individual organisms (e.g. grass or moss)

  • The quadrat is divided into 100 smaller squares

  • Count the squares where the species occupies over half

  • Percentage cover = number of occupied squares

• Using percentage cover and frequency together provides a more complete picture of a species' distribution within a habitat

  • A species with a high percentage cover but low frequency is likely found in dense patches in specific areas rather than spread evenly

  • This pattern suggests the species may have a preference for certain environmental conditions, such as:

    • light vs. shade

    • wet vs. dry

    • high vs. low pH

    • crowded vs. open areas

    • exposed vs. sheltered locations

  • This type of data helps identify habitat preferences and ecological patterns.

Limitations

• Quadrats and transects can only be used for sessile and slow-moving species

• Some species can be counted to find their abundance, but others that are very small or in high numbers require abundance to be calculated using percentage cover or frequency techniques

  • The frequency technique shows how common a species is, but it does not give information on the estimated number of individuals or the size of the population