18.2.4 Testing for Distribution & Abundance

• 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
• The distribution and abundance of a species in an area can be assessed using different practical methods:
  • Frame Quadrats
  • Line and Belt Transects
  • Mark-release-capture

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
• 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 the samples are being made in
• Quadrats must be laid randomly in the area to avoid sampling 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
  • Then 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

Results from quadrats

• The results from the quadrats can be used to calculate the predicted frequency and density of a species within an area
• Species frequency is the probability that the species will be found within any quadrat in the sample area
  • The number of quadrats that the species was present in is divided by the total number of quadrats and then multiplied by 100
  • For example, if bluebells were found in 18 out of 50 quadrats the species frequency would be (18/50) x 100 = 36%

• Species density indicates how many individuals of that species there are per unit area
  • The number of individuals counted across all quadrats is divided by the total area of all the quadrats
  • For example, if 107 bluebells were found across 50 quadrats that are 1m² each the species density would be 107/50 = 2.14 individuals per m²

• It can sometimes be difficult to count individual plants or organisms. When this is the case percentage cover of the species within the quadrat can be estimated instead
  • The quadrat is divided into 100 smaller squares. The number of squares the species is found in is equivalent to its percentage cover in that quadrat
  • For example, if grass is found in 89 out of 100 squares in the quadrat then it has a percentage cover of 89%

Percentage cover of grass in a quadrat - There may be some squares lacking any species and other squares with multiple species overlying one another. Therefore the total percentage cover of a single quadrat can be over or under 100% sometimes

Line & belt transects

• Throughout some areas, there can be changes in the physical conditions
  • For example, there may be changes in altitude, soil pH or light intensity

• When investigating the species distribution in these kinds of areas systematic sampling is more appropriate
• Methods using transects can help show how species distribution changes with the different physical conditions in the area
  • A transect is a line represented by a measuring tape, along which sample are taken

• For a line transect:
  • Lay out a measuring tape in a straight line across the sample area
  • At equal distances along the tape record the identity of the organisms that touch the line. For example, every 2m
  • This produces qualitative data

• For a belt transect:
  • Place quadrats at regular intervals along the tape and record the abundance of each species within each quadrat
  • This produces quantitative data

A line transect and belt transect is carried out in the field

Mark-release-capture

• The methods above are only useful for stationary organisms
• Different methods are required for estimating the number of individuals in a population of mobile animals
• The mark-release-capture method is used in conjunction with the Lincoln Index
• For a single species in the area:
  • The first large sample is taken. As many individuals as possible are caught, counted and marked in a way that won’t affect their survival
  • They are returned to their habitat and allowed to randomly mix with the rest of the population
  • When a sufficient amount of time has passed another large sample is captured
  • The number of marked and unmarked individuals within the sample are counted
  • The proportion of marked to unmarked individuals is used to calculate an estimate of the population size
  • The formula for the calculation is:

N = n₁ x n₂ / m₂

• • Where:
    • N = population estimate
    • n₁ = number of marked individuals released
    • n₂ = number of individuals in the second sample (marked and unmarked)
    • m₂ = number of marked individuals in the second sample

Worked example: Lincoln index with mark-release-recapture

Scientists wanted to investigate the abundance of leafhoppers in a small grassy meadow. They used sweep nets to catch a large sample of leafhoppers from the meadow. Each insect was marked on their underside with non-toxic waterproof paint and then released back into the meadow. The following day another large sample was caught using sweep nets.

• No. caught and marked in first sample  (n₁) = 236
• No. caught in second sample (n₂) = 244
• No. of marked individuals in the second sample (m₂)  = 71

Using the equation:                 N = n₁ x n₂ / m₂  = 236 x 244 / 71 = 811

N (estimated population size) = 811