Methods of estimating population sizes.

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NEWTON COLLEGE

Estimating Population Sizes

ALVINO, Marielle

 Form V-D

        

September, 2009

ESTIMATING POPULATION SIZES

To estimate population sizes when the individuals are hidden or too numerous to count in a given area, it is ideal to estimate these numbers with a random selection of samples taken in the given area with quadrats. Using a given and later chosen area, we will then toss the quadrats and record the number of individuals in that area to estimate the population density.

METHOD:

Experiment A- Using a model in the lab

The equipment that was used for measuring the population and the dimensions of the model were:

  • A quadrat of 0.1m x 0.1m or 100 cm2
  • A marked area on bulky paper where drawn crosses indicate organisms

•     A 100cm ruler

Using a quadrat to estimate the populations of small stationary organisms

First, we measured of the dimensions and area of the paper using the ruler, which are best represented in the diagram below, whose uncertainty is ±0.005 cms:

To find the area of the marked area, I first have to calculate areas A, B and C with some basic formulas:

Area of Rectangle= width x length

Area of Triangle= (width x length) /2

Area A= 98 x 79  7742 cm2

Area B= (130-80) x 70= 50 x 70  3500 cm2

Area C= (98-70)x (130-80)=  28 x 50  1400 cm2

Area A   + Area B +  Area C= Total Area

7742 cm2 +3500 cm2 +1400 cm2 = 12642 cm2 or 1.26 m2

 

In order to randomly throw our quadrat in area we decided to turn around at an initial point. The subject turns away from the area so that she could not see where she was aiming and then throws the quadrat behind her into the area. Then from the position where it landed, the same process is repeated. We chose to choose this method instead of throwing it from the same initial point to avoid biased results.  If any quadrat landed out of the selected area, but significantly close to the perimeter it was moved in the same perpendicular line until it was completely in the area. This process can be shown in the following diagram:

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If it was far off the chosen area, as we encountered in two occasions during experiment B, we decided to invalidate the throws.

The quadrat for experiment A consisted of four wooden sticks whose dimensions were 10 cmx10 cm or 0.1mx0.1m, covering an area of 100 cm2 or 0.01 m2 each time it landed.

The total number of crosses found was 80 which will be divided between:

i) the average number of crosses per throw

 = 80 crosses / 1.2642 m2

=63.282 crosses per m2

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