Investigating Abundance & Distribution

• Distribution refers to where the organisms are.
• Sampling

  Ideally, it would be great to know exactly how many organisms there are, and where each of them is, so that we could determine the abundance and distribution exactly. However, in real life, this would be extremely time consuming or even impossible.
• Instead, ecologists use a method called 'sampling', where they only measure a subset of the organisms, and use that subset to make predictions about the whole population. 
• There are two main sampling methods you need to know about

  1. Quadrats - to measure abundance
  2. Transects - to measure how distribution changes
• Quadrats are square frames, that have been subdivided into lots of smaller squares. We can use them to sample a habitat and estimate the size of a population.

  
• Describe how you might use a 1m x 1m quadrat to estimate the number of dandelions in a 100m x 100m field.

  • Place multiple tape measures along the length and width of the field, forming a large grid.
  • Use a random number generator to generate 10 pairs of random coordinates.
  • Place quadrats at those coordinates and count how many dandelions are found in each quadrat.
  • Calculate the mean number of dandelions per m2.
  • Estimate the total population size using our mean number of dandelions per m2 and the total area of the field.
• Transects are used to study distribution

  Transects are used to observe and record changes in species distribution across an area.

  (transects are not used to estimate the abundance)
• Example - using transects to study the distribution of dandelions in an open field stretching from a lake to a woodland


  1. A transect line is laid out in a straight line between the lake and woodland.
  2. Quadrats are placed at regular intervals along the transect line. 
  3. The abundance of dandelions is measured by counting the number of dandelions in each quadrat along the transect line, from the lake to the woodland.
  4. Steps 1-3 are repeated using new transect lines (these must be parallel to the first). By using multiple transect lines, the data gathered is more representative of the area.