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 ECOLOGY : FIELD TRIP TO CORREZE

ESTIMATING THE SIZE OF A POPULATION OF A PLANT SPECIES

Plants are sessile, they do not move around. They can be sampled using a quadrat.

Our aim is to estimate the size of the population of a plant species.

You will need to:

  • Clearly identify the species of plant you are investigating.
  • Determine the area in which the population is to be sampled using tape measures.
  • Use a suitable sized quadrat to sample the plants

DISTRIBUTING QUADRATS RANDOMLY

For samples to be representative of an area all the parts of the area must have an equal chance of being sampled. This can be achieved by using a regular grid, sampling at precise points over the whole area, or by using a random sampling technique. This exercise will use the random sampling technique.

MATERIALS

Quadrat, pencil, random number tables, notebook, TI 83 plus calculator.

METHOD

  1. Determine the limits of the area to be sampled. Spin a pencil and heading in the direction indicated by the pencil, pace out a random number of steps using the numbers given in the random number tables. If you encounter the boundary of your sampling area continue pacing, as though you have bounced of the boundary inside your sampling area. See diagram below:
     

     

  2. After the correct number of paces, place the quadrat on the ground and count the number of plants of the species that you have chosen.
The edge effect: With quadrats it is always a problem to know what to do with plants which touch the edge. Are they counted "in" or "out"? The rule is if they touch the right side or the top, count them "in". If they touch the bottom or the left side, count them "out". When you are working as a group you must decide between you, which is the top of the quadrat, before you start counting.
 

© Paul Billiet 2004