Investigating Populations

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Why do we investigate populations of organisms?

To find either ABUNDANCE or DISTRIBUTION of species. 

  • Abundance
    The no. of individuals of one species in a particular area.
    You can simply count plants or mobile organisms. You could also use frequency (how many samples the organism was found in) or percentage cover (how much of the area is covered by the species).
     
  • Distribution 
    Where the species is found in the particular area.  
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Random Sampling

  • To avoid bias you pick the location of your samples randomly e.g. if you were investigating in a field, you could divide it into a grid and use a random number generator to select coordinates. 
     
  • Repeat the process to give a more reliable estimate of the whole area.
     
  • To estimate the no. of individuals in the whole area you take an average of the data collected in each sample and multiplying it by the entire area. Or, if you're calculating percentage cover, you simply take an average. 
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Methods of Investigating Populations

INSECTS ON THE GROUND

  • Pitfalls: steep containers which insects fall into. A disadvantage is that predators may enter the container and eat the insects which could affect the results.
  • Pooters: jars which you can **** insects into. A disadvantage is that it would take a long time or a lot of people to collect enough insects to have a large sample. 

INSECTS IN VEGETATION

  • Beating Trays: trays/sheets which you hold underneath when you shake the vegetation. You can take large samples but a disadvantage is it may not be random as some insects will fall more easily than others. 
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Investigating Plant Populations

MEASURING ABUNDANCE OF PLANT SPECIES

Quadrats

  • Several quadrats are placed on the ground at different locations in the area you are investigating.
  • Can either measure the species frequency or the no. of species in each quadrat. However, it is quicker to measure the percentage cover as you do not have to distinguish individual plants. 
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Investigating Plant Populations

MEASURING DISTRIBUTION OF PLANT SPECIES

Transects

  • Line transect - record any species which touch the tape measure.
  • Belt transect - place quadrats all along the transect to calcuate species frequency and percentage cover along the location. 
  • Interrupted transects - take measurements at intervals. 
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Ethics of Fieldwork

  • Organisms such be studied 'in situ', they should not be removed but if they are numbers should be kept to a minimum and they should be returned quickly  even if they are dead
  • Sufficient time should elapse before further study takes place. 
  • Disturbance or damage to the habitat should be avoided.
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