types of sampling

• sampling

  -used to estimate the number of organisms in an area.

  -number of individuals of a species present in an area= abundance of organisms.

  -can be used to measure a particular characteristic of an organism.
• Random sampling

  = selecting individuals by chance, each individual in the population has an equal likelihood of selection.

  • random number computers are used.
  • mark out grid on grass using tape measure.
  • use random numbers to determine co-ordinates.
  • take a sample at each of the co-ordinates pairs generated.
• opportunistic sampling

  -weakest form, may not be representative of population.

  -uses organisms that are conveniently available.
• stratifies sampling

  = some populations can be divided into a number of strata based on a particular characteristic.

  • A random sample is then taken from each of these strata proportional to it's size.
• systematic sampling

  -different areas within an overall habitat are identified. eg: how plant species change as you move inland from the sea.

  • line transect= mark a line along the ground between 2 points, take samples at specific points which touch the line.
  • belt transect= 2 parallel lines marked, samples taken from area between them.