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What is a Plot?

A plot is an area demarcated by a particular shape making up it's boundary, in which the species and individuals belonging to those species are noted when studying biodiversity. A plot which is in the shape of a rectangle or square is called a quadrat. Ofcourse since the underlying principle and use of the shape to measure biodiversity is the same regardless of the shape of the plot, the term quadrat has become popular and can often be used to denote plots that are not rectangular in shape.


What are the assumptions relating to the use of Plots/Quadrats in a study design?

When plots are used to measure biodiversity there are certain assumptions which must be met to ensure that the data being collected in the plots is accurate with respect to the aims of the study. These assumptions are as follows -

When plots are used to measure biodiversity there are certain assumptions which must be met to ensure that the data being collected in the plots is accurate with respect to the aims of the study. These assumptions are as follows -

  1. The organisms being studied or counted in the plots are relatively immobile or if mobile, they will not leave the plot boundary before having a chance to be counted. –

The plots are utilised to provide a representative sample of the area being studied and when data is collected from them it gives us an idea of what species, and how many individuals belonging to the different species there are in the plot. Obviously if the organisms move around then you will not get data that is accurate or representative of the area as organisms that have relatively high mobility in a relatively low plot area may leave or enter before they are counted and the data collected will be inaccurate.

  1. The area of the plot is known.-

Basically this means that the boundaries of the plot have to be demarcated so that there is no confusion and organisms outside these boundaries are not counted.

  1. The number of individuals belonging to the different species in each quadrat is counted as far as their detection permits. -

This is absolutely necessary to gain an understanding of the relative abundance and densities of the different species available in the study area.

  1. The plots are representative of the study area -

This should not be a problem if proper randomization of the laying out of plots is undertaken.


What groups/taxa of biodiversity are plots/quadrats used for studying?

Plots are generally used for studying organisms that can be studied with strategies that adhere to the assumptions given above. Therefore they are generally used for sampling immobile organisms like plants or organisms with low mobility such as molluscs. Ofcourse they can also be used to study organisms with higher mobility by creating an impassable barrier around the boundary of the plot to ensure that organisms inside the plot cannot leave it, as is often done when sampling herpetofauna.


How do you decide the Plot Size and Shape? 

There are multiple criteria that are used to decide the shape and size of a plot. The following are some of these criteria-

  1. Over the course of time scientists who specialise in studying a particular group have used standard plot shapes and sizes for particular groups based on strategies required for studying these groups and their collective experience. You can search the scientific literature for what plot shapes and sizes are used for your group of interest (if plots are used to study that group) and use the same shapes and sizes. Ofcourse this isn't a very good option as conditions may differ widely from one geographic region to another and different strategies may need to be used for studgying different sub groups of organisms depending on their characteristic differences so having one size and shape that is consistently used for a single group/taxa in every circumstance and area is not wise.

  2. Edge effect minimization as a determining factor of shape -

The edge effect is a term used to denote the changes that can be seen in species community structure and composition at the boundary of two habitats. The edge effect exists at all scales of size, even at the scale of patches of plants such as herbs scattered around an area in clumps (in this hypothetical example the edge effect will be seen at the boundary of the clumps of vegetation scattered over an area). Thus the edge effect also affects the results of data collected from plots when plots layed out fall over the edges of micro habitats. Since the permiter to surface area ratio is lower in some shapes of plots than others these shapes should be prefered as they naturally minimise the amount of edge area falling in the plot and decrease the variability of plot results. Thus according to this strategy of picking a shape for a plot, a circle is better than a square which is better than a rectangle.

3. The minimal area method of obtaining the size of the plot based on calculations from the species area curve -


Plots are layed out and species enclosed within them are counted. Each successive plot is larger than the preceding plot and contains the preceding plot within it's boundary (this is called nesting as each plot is "nested" within the next). According to an ecological concept called the species area curve relationship, the number of species in an area grows with an increase in the size of the area (i.e larger areas tend to have more species when all the other circumstances such as geographic area, climate etc are the same.) So each successively larger plot will have more species, when the increase in the size of the plot fails to bring a significant increase or a lack of increase in the number of species contained in it, the area of this last plot is considered to be the minimal area required to characterize a community (the term community is used to denote a number of species belonging to the same group found in an area together like a plant community for example . Click here  for more information on the species area curve relationship and a detailed methodology, with an example, for carrying out this method of estimation of the size of the sampling plot).


4.Determining optimal plot size based on relative cost of sampling and relative variance of plot counts -


A number of different plot sizes are used and their relative cost as well as the relative variance of their counts are estimated and multiplied. The quadrat size which has the lowest figure for the product of it's relative cost and relative variance is chosen as the optimal plot size. Click here for a detailed method with an example for this technique.



How is sampling undertaken using plots? 


Once the size and shape of the plot is decided upon, sampling is undertaken by laying plots  randomly or systematically over the study area. Plots can be laid by using plot or quadrat frames (where a plastic frame of the plot is laid on the study site and the individuals inside the frame counted) or by using point quadrats (where the points of the outer boundaries of the plot or quadrat are marked before counting the individuals within). The species and the number of individuals belonging to each species is then noted by the researcher. This constitutes the data that is collected.



Calculating Density of Individuals of a Species from Quadrats -



Provided that  all the quadrats are laid randomly and there is a sufficient sample size of quadrats laid, the density of a species can be estimated using the following formula -


Density of Species A per unit area  = Total number of individuals of species A found in all quadrats

Total area covered by all the quadrats


quadrat frame


                                                                     A frame used as a quadrat for sampling plants. Source - Wikimedia Commons.


About the Author of this Article


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