Investigating populations


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Systematic sampling along transects

  • Important when there’s transition in community 
  • A line transect uses a tape/string stretched across the ground in a straight line. Any organism the crosses the line is recorded 
  • A belt transect is a strip, usually a metre wide, marked by putting a second line parallel to the first. The species occurring within the belt between the lines are recorded
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Measuring abundance

  • Abundance is the number of individuals of a species within a given space 
  • Frequency – the likelihood of a species occurring in a quadrat. Useful when species is hard to count; gives quick idea of general distribution within area. Doesn’t give info about density and detailed distrubution. 
  • Percentage cover – an estimate of the area within a quadrat that a plant species covers. Useful when species is difficult to count, data can be collected quickly and individual plants don’t need to be counted. Less useful in overlapping layers
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Random sampling

  • Used to avoid bias
  • Lay out 2 tape measures at right angles along 2 sides of the area 
  • Get coordinates using random number generator eg calculator 
  • Place quadrat in the coordinates
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Quadrats

  • Size of quadrat – depends on size of plant of animal being counted and how they are distributed. Large species, large quadrats. Groups of species, lots of small quadrats
  • Number of sample quadrats to record – more quadrats means more reliable results. More species in the area means more quadrats needs to be used 
  • Position of each quadrat – random sampling must be used.
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Mark-release-recapture

  • A known number of animals are caught, marked in some way and then released
  • Later a given number of individuals is collected randomly
  • Marked number of individuals is recorded 

Techniques assumes:

  • Proportion of marked to unmarked individuals in 2nd sample is the same as the proportion of marked to unmarked individuals in the population as a whole
  • Marked individuals distribute themselves evenly within the rest of the population
  • No immigration into or emigration out of the population
  • Few deaths and births within the population
  • Method of marking is not toxic and it doesn’t make the individual more obvious to predators
  • Mark not lost during investigation

 


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Analysing data

  • qualitative data can be presented in a graph/ table to make comparisons easier and more precise
  • however when comparing 2 sets of data different factors may affect it
  • significance tests can be used to show if the results are due to a particular facter or just chance
  • statistical tests used to calculate the strength and direction of any correlation between 2 variables eg spearmans rank
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