Plot sampling (frequently referred to as quadrat sampling) is used to determine the population size of one or more species in a community. The technique works best with sessile (stationary) or slow-moving organisms such as plants. In addition to population size, the data collected can provide information about the distribution and relative importance of each species in the community.
Sampling is done by locating plots in different locations in the community and counting the number of organisms in each plot. The size of the plot depends on the size of the species being sampled. One square meter or less is usually sufficient for sampling small, herbaceous plants but trees may require plots of 100 m2. A sufficient number of plots should be quantified so that adding additional plots to the sample does not reveal additional new species.
Plots can be located randomly in the community or at regular intervals. In either case, grid coordinates can be used to determine location. If random sampling is desired, random numbers can be used to determine the coordinates.
The calculations below will be performed for each species. Density is a measure of the number of individuals per unit of space. Dominance is the area of ground covered by a species. A species with many large individuals will have a high dominance measurement. Frequency is a measure of how the individuals are distributed. Individuals that are found only in one corner will have a low frequency value while those that are scattered throughout the sampling area will have a high value. An indication of how important a species is in a community can be obtained by calculating density, dominance, and frequency and then adding the three values together.
Number of Individuals
Sum of all plot areas
Area covered by a species
Sum of all plot areas
Number of plots which have at least 1 individual of a species
Total number of plots sampled
Each of the above calculations should be converted to a relative value by dividing it by the total for all species as follows:
Relative Density = density/total density of all species Equation #4 Relative Dominance = dominance/total dominance for all species Equation #5 Relative Frequency = frequency/total frequency of all species Equation #6
The maximum value that any of these relative calculations can be is now 1. An index of relative importance can be calculated as the sum of the relative density, dominance and frequency:
Importance Value = relative density + relative dominance + relative frequency
12 inch ruler or tape measure
One 0.1 m2 sampling ring - This is a wire ring that encloses an area that is 0.1 m2. The ring shown in the photograph was constructed using 8 gauge aluminum TV antenna ground wire purchased at Radio Shack (item number 15-035). Any stiff wire can be used for this purpose.
Cut a piece of wire 50 inches (1.27 meters) long and put a mark 3 inches (7.6 cm) from one end. Measure 44.1 inches (1.12 m) from this mark and put a second mark. The second mark will be approximately 3 inches from the other end. Bend the wire to form a ring. The ring will be the correct size when the two marks touch each other. While the two marks are lined up near each other, wrap the 3-inch ends to hold the ring together as shown below.
This sampling technique will be used to sample populations of herbaceous (nonwoody) plants in a forest or in an abandoned field.
Procedure for Online Students:
Find a woodlot or field that is 20 paces (steps) or larger on each side. A smaller area may be used if necessary. If the area is larger than 20 paces X 20 paces, limit your sampling to a corner that is 20 paces X 20 paces. This will cut down the amount of time needed to sample the woodlot but your results will apply only to the 20 pace X 20 pace area. If you were interested in obtaining data for the entire woodlot, it would be necessary to sample the entire area.
A 0.1 m2 wire sampling ring will be used to sample the plants. The location of each sample will be determined using random numbers. Obtain two 2-digit random number between 0 and 20. This can be done by using the table of random numbers provided with this laboratory exercise. If you do not know how to use the table, read the instructions for using the table of random numbers.
These random numbers correspond to the location of the first sample point. For example, if your numbers were 12 and 07, you would walk along the baseline 12 paces and then walk into the woodlot 7 paces as shown in the figure below. The circle in the diagram represents the location of the first sample.
Procedure for Campus Students:
Unroll a tape measure on one side of a field to be sampled and a second tape measure along an adjacent side as shown in the diagram below.
Lay a tape measure along one side
of a field as shown by line A below.
Lay a second tape measure along an
adjacent side of the field as shown by line B.
In order to sample this field, you will need to know where to place the sampling ring. This can be done using random numbers to determine sample location.
Note the length and width of the field in meters. Use random numbers to find a location on the A tape. Use random numbers to find a location on the B tape. If the length of the tape is less than ten meters, you will need a 1- digit random number. If the length is between 10 and 99 meters, you will need a 2- digit random number and if the length is between 100 and 999 meters, you will need a 3- digit random number. The random numbers corresponds to the meter marks on the tape and will serve as coordinates.
Random numbers can be obtained by using a table of random numbers. If you do not know how to use the table, read the instructions for using the table of random numbers.
For example, suppose 66 meters by 41 meters as shown on the diagram below.. You will need to select two 2-digit random numbers. Suppose that your first random number is 55 and your second number is 30. Your sample will be located as shown by the broken line below.
The intersection of the two broken lines in the diagram above is where the sample is taken. The sampling ring is placed at that location and the number of plants of each species is counted. A more unbiased sample can be taken by standing at this location and then dropping the sampling ring behind you so that you cannot see where it lands.
In order to minimize human bias, you should turn around and drop the sampling ring behind you so that you cannot see where it will land.
Use a ruler to estimate the approximate diameter of each plant in the ring and record the data in the raw data table. An example of diameter is shown by the red arrow in the photograph below.
For grasses and clumps of small plants, the diameter of the base of the clump near ground level can be used as shown below.
You may record your measurements in inches if you do not have a meter stick or a metric ruler but these measurements will need to be converted to centimeters before you perform any calculations. Inches can be converted to centimeters by multiplying your measurements by 2.54 (discussed later). Measurements should be rounded to the nearest 0.01 cm. For example: 7.24 cm.
After this sample is recorded, the ring is moved to a new random location.
Most students do not know the species names, so refer to them "species 1", "species 2", etc. For example, if the first sample had 3 individuals of species 1, and the diameters of these individuals were 20 mm, 15 mm, and 20 mm, the first line of the data table would look like this:
Species 1 20, 15, 20 cm
It might be helpful to briefly describe the species so that if you see it again in later samples, you will know what number you gave it.
Repeat this procedure until 5 samples have been taken. A sample size of 5 plots is adequate for our purposes but if you were interested in an estimate which is more accurate, you would need to collect more samples. Your samples must contain at least 5 different species. If you do not have at least 5 species, continue taking samples until you have 5 species. When you enter the data into the spreadsheet, be sure to enter the total number of samples taken.
Online Students: Return to the Angel website and go to the document "Plot Sampling Questions" to submit your data and answer questions about this exercise.
Campus Students: Submit a copy of your spreadsheet and the answers to the plot sampling questions.