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What are point transects/counts? -

This is a method where data is collected at points dispersed either randomly or systematically across an area or a habitat where sampling needs to be done. Data is not collected at areas which lie between these points even when they are passed through to reach the actual points.

Often a researcher that enters a habitat will cause the certain organisms such as birds to go quiet or hide as they are startled by what they perceive to be a predator. Due to this they may not be detected as easily by the researcher. Point transects or point counts are utilized where the biodiversity being studied (such as birds) need to be given time to settle down and acclimatize to the presence of the intruding researcher.

 

What are the assumptions relating to the use of point transects/counts in a study design?

 

The sampling method has certain assumptions which need to hold true for accurate estimates and results to be derived from this method. The greater the violation of the assumptions the greater the reduction in accuracy of the estimates. While it may not always be possible to undertake any sampling strategies such as this one without the violation of one or more assumptions at times, the degree of violation has to be reduced as much as possible. The assumptions are given below -

 

  1. Individuals being sampled are detected at their initial location prior to any movement in response to the observer.

  2. Individuals are detected independently. This means that the detection of one individual should not be related or influenced by the detection of another. This assumption is often violated to an extent, especially if the population under study consists of individuals living and moving in groups.

  3. Point transects are laid either  randomly or systematically.

 

How is sampling undertaken using point counts?

Points are laid out in the habitat or area in which sampling needs to be done in a randomly or systematically manner. The “point” actually consists of a circle of a radius decided upon by the researcher prior to sampling based on the biodiversity group/taxa being sampled. Researchers reach the site and wait at the centre of this circle silently for a period of time over which they collect data on all the individuals of the different biodiversity groups/taxa they are studying that lie within the circle. The period of time for which a researcher remains at a point collecting data, is decided upon by the researcher based on factors such as the taxon being studied, the available cover in the location and time, money and effort constraints. All the points in the study are standardized with respect to this amount of time.

 

Calculating Density of Individuals of a Species from Point Counts-

 

Density of individuals of species A =  Total number of individuals of species A             

           Number of circular plots X πX (radius of circle)2

Where πris basically just the area of the circle

 

Point count method

 

 

 

 

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References

Southwood T. , and P. Lenderson. Ecological Methods - Published by Blackwell Science Ltd.