Sampling Methods

Objectives

• Identify methods of sampling organisms.
• Calculate population size, species frequency, and species density.

Observation

• Observe species in their natural habitat.
• Record common species of plants and animals.

Quadrat Method

• Place the quadrat randomly in the study area.
• Count the number of slow-moving and stationary organisms within the quadrat.
• This method is best used in flat areas.

Line Transect Method

• Place a piece of string in a straight line with marks at regular intervals.
• Count the slow-moving or stationary organisms that touch the string at the intervals.
• This method is suitable for slopes and areas where one habitat changes to another.

Belt Transect Method

• Place a piece of string of a certain length.
• Randomly throw the quadrat along the length of the string.
• Count the organisms within the quadrat.

Capture-Recapture Method

• Catch a set number of organisms.
• Tag and release them.
• Catch more organisms and record the number that was recaptured.

Calculating Population Size

The formula for calculating population size using the capture-recapture method is:
Total Population = \frac{(Number \, captured \, 1st \times Number \, captured \, 2nd)}{Number \, of \, tagged \, organisms \, that \, are \, recaptured}}

Calculating Species Frequency

The formula for calculating species frequency is:
Species \, Frequency = \frac{(Number \, of \, throws \, where \, the \, species \, is \, seen)}{Number \, of \, throws} \times 100

Calculating Species Density

The formula for calculating species density is:
Species \, Density = \frac{Total \, number \, of \, organisms \, of \, a \, species}{Number \, of \, quadrat \, throws}

• Note: This is applicable if the quadrat is 1m^2.

Population Size Example 1

• A student collects 24 giant African snails, marks them, and releases them.
• 5 days later, she collects 20 snails, 8 of which are marked.
• Calculate the size of the snail population.
  • Number captured 1st = 24
  • Number captured 2nd = 20
  • Number of tagged organisms recaptured = 8
    Total Population = \frac{(24 \times 20)}{8} = 60
• The estimated population size of giant African snails in her garden is 60.

Population Size Example 2

• Elvin collected 22 woodlice, marked them, and released them.
• 10 days later, he collected another 18 woodlice, 3 of which were marked.
• Determine the size of the woodlouse population.
  • Number captured 1st = 22
  • Number captured 2nd = 18
  • Number of tagged organisms recaptured = 3
    Total Population = \frac{(22 \times 18)}{3} = 132
• The estimated population size of woodlice is 132.

Species Frequency and Density Example

• Calculations:
  • Species S:
  • Total number of individuals = 5 + 7 + 5 + 2 + 4 + 7 + 5 + 8 + 0 + 3 = 46
  • Number of throws where species S is seen = 9
  • Species Frequency = \frac{9}{10} \times 100 = 90
  • Species Density = \frac{46}{10} = 4.6
  • Species T:
  • Total number of individuals = 13 + 10 + 11 + 0 + 0 + 15 + 0 + 0 + 9 + 0 = 58
  • Number of throws where species T is seen = 5
  • Species Frequency = \frac{5}{10} \times 100 = 50
  • Species Density = \frac{58}{10} = 5.8
  • Species U:
  • Total number of individuals = 3 + 0 + 4 + 5 + 2 + 1 + 0 + 4 + 2 + 3