# Investigating populations

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• Created by: emilyn
• Created on: 28-01-13 21:36

## 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 *****, 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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• 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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