Investigating populations


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1.2 Investigating Populations


Factors to consider:

- Size of the quadrat to use, Larger species require larger quadrats. A species which occur in groups rather thanbeing evenly distributed throughout the area, a large number of small quadrats will give more represntative results than a small number of large ones.

- The number of sample quadrats to record within the study area, the larger the number of sample quadrats, the more reliable the results will be. The greater the number of different species present in the area being studied, the greater the number of quadrats required to produce valid results.

- the position to each quadrat within the study area, - to produce statistically significant results a technique known as random sampling must be used. 

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Random Sampling

- 'random number generator'

- 1. Lay out two long tape measures at right angles, along two sides of the study area.

2. Obtain a series of coordinates by using random numbers taken from a table or generated by a computer.

3. Place a quadrat at the intersection of each pair of coordinates and record the species within it.  

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Systematic sampling along transects

- It is sometimes more important to measure the abundance and distribution of a specis in a systematic rather than a random manner. 
- Belt transect. 

Measuring abundance

Abundance is the number of individuals of a species within a given space. IT can be measured in several ways, depending on the size of the species being counted and the habitat. 

- Frequency, which is the likelyhood of a particular species occuring in a quadrat. If, for example a species occurs in 15 out of 30 quadrats, the frequency of its occurence is 50%. This method if usefull where a species, such as grass is hard to count. It gives a quick idea of the species present and their general distribution within an area. However, it does not provide informatino on the density and detailed distribution of a species. 

- Percentage cover, which is an estimate of the area within a quadrat that a particular plant species covers. It is useful where a species is particularly abundant or is difficualt to count.  Data can be collected rapidly.

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Mark-release-recapture techniques

Animals are captured, marked in some way, and then released back into the community. Some time later, a given number of individuals is collected randomly and the number of marked individuals is recorded. The size of the population is then calculated as follows:

Estimated population =   total number of individual in the first sample x
size                                 number of individuals in the second sample
bumber of marked individuals recaptured.

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