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 ECOLOGY : FIELD TRIP TO CORREZE

DETERMINING THE MINIMUM NUMBER OF QUADRATS

How many quadrats do you need to place? Enough for your data to be representative of the whole population. This can be done by eye but it requires some calculation.

Recording and processing data on the TI 83 plus calculator

  1. Press then select 1: Edit… to open a spread sheet.
  2. Just like Excel or other spread sheets you enter the data in columns or lists L1, L2 etc. Type in your quadrat numbers in L1 and the numbers of plants of the species you are studying in L2.

Calculating the mean density of plants per quadrat
  1. Sample at least 10 times then carry out the following operation.

Use the cursor to get to the top of L3. Press then and and select OPS (use the cursor to move one to the right). This opens up a menu of functions rather like in Excel.

Select 6:cumSum( This is the cumulative sum function

Press and the function will appear at the bottom of the screen

L3 = cumSum(

Press and then ). Press and you will see the spread sheet calculate the cumulative sum of all your samples.

  1. Do this periodically to L3 as you add in more data.

    Use the cursor to go to the top of L4. Press

    Press and ¸ and then

    This calculates the cumulative mean density of the plants in your samples.

  2. Do this periodically to L4 as you add in more data.

Graphing your data in the calculator

To monitor the progression of the cumulative mean density with each new sample you should periodically plot a graph using STAT PLOT

Press and STAT PLOT. Open Plot 1 by pressing

Using the cursors and

Select ON

Select the line graph plot

In Xlist: Press and
In Ylist: Press and

Then to see the graph Press and select 9:ZoomStat

You should be able to see a scaled graph showing the variations in your mean.

As you progress these variations should stabilise out.

The Theoretical Minimum Quadrat Number (MQNt) is at the point where they start to become stable.

You can find this value by using the cursor to move the pointer around on the graph. Double the MQNt to find the Safe Minimum Quadrat Number (MQNs).
MOST IMPORTANT Storing your data in the calculator’s archive

If you do not do this your data will be lost. The next time you use a spread sheet the data in L1, L2 etc. will get compressed.

Archiving frees up RAM memory so that you can continue recording more data. Archiving data will remove it from the RAM memory but it will not stop it from being over-written when new data is recorded. It needs to be renamed.

To rename a list of data (e.g. L1)

  1. Select the list to be stored by pressing then then
  2. Open the LIST menu by pressing then LIST and select OPS using the cursor
     
  3. Descend the menu using the cursor to B: L press and type in a code name for the list. It must start with a letter so press the green key and chose a letter. Your list name can consist of one letter and four figures (try today’s date)

To archive or un-archive a list variable (L1) using a Memory Management editor:
  1. Press then MEM to display the MEMORY menu.
  2. Select 2:Mem Mgmt/Del... to display the MEMORY MANAGEMENT/DELETE menu.
  3. Select 4:List... to display the LIST menu.
  4. Press to archive L1. An asterisk (*) will appear to the left of L1 to indicate it is an archived variable.
    To unarchive a variable in this screen, put the cursor next to the archived variable and press     . The asterisk will disappear.
  5. Press then QUIT to leave the LIST menu.

 
  • Estimate the size of this population and its density.
  • Compare with one other group working in a different area.
  • Discuss and evaluate this investigation.

 

© Paul Billiet 2004