Biology 3A: Ecology: Point-Quarter Sampling
Introduction
There are several methods for sampling organisms in an area. Plot, or quadrat
sampling, is a typical example. The results of quadrat sampling are often related to the size
and shape of the plots used. The method is also very time consuming. Methods that do not
use plots are often useful and much faster for plant (and sessile animal) sampling. Point-
quarter method is a favorite method that is "plotless" and useful in areas of different sizes
and shapes.
The point-quarter method relies upon a random distribution of the organisms in the
area to be sampled. Uniform or clumped distributions will yield erroneous results.
However, this method is one of the most favored for sampling in several habitats including
intertidal and coastal scrub.
Method
A number of randomly determined points are selected in the area to be sampled.
These may be taken using a random number chart and distance along a transect tape, as
random numbers corresponding to grid positions in the area, or as random numbers used to
select previously arbitrarily determined
points in the area. Each point will
represent the center of the measurement
area. From the center, a compass will be
used to define four quadrants. In each of
these quadrants you will determine, and
measure the distance to, the closest plant
from the center point. The center of the
stem, or clump of stem, should be used.
Be sure to measure all relative close plants
to make sure you have the single closest
individual. Next, you should measure the
diameter covered by the closest plant.
This will be used to estimate the area
covered by the plant. All of the raw data
should be recorded in Table 1. All
measurements should be made in meters.
Figure 1. Point to plant distances and plant diameters in
each of four quadrants
Calculations
First, calculate the areas covered as shown in column 5, Table 1 (remember that area
= (diameter/2)2 x π). Compute the total of all point to plant distances as directed at the
bottom of column 6, Table 1. Next fill in the data for number of individuals by species on
Table 2 and coverage area by species on Table 3. Now you are ready to complete the
calculations on Table 4.
In the legend for Table 4 insert the number of points sampled. Next, multiply this
by 4 to get the total number of quadrants (4k). Transfer the total of the point to plant
Point-Quarter Transect Lab Page 1 of 6
distances from Table 1. Finally, in the legend, compute the Total Density (TD) of plants per
square meter as:
TD = 1 / (total point to plant distances / 4k)2
List the species in column 1. In column 2 put the total number of individuals, ni , (all
quadrants) for each species (from column 13, Table 2).
Relative and Absolute Density
The relative density, RDi, is the number of individuals (column 2) divided by the
total number of all plants counted (the sum of column 2, which is also 4k).
RDi = ni / 4k
Absolute density, Di, is Relative Density x Total Density (as you calculated above).
Di = RDi x TD
Frequency
In column 5 enter the data from column 12, Table 2. Calculate Frequency, Fi , as
number of points having the species divided by total number of points (k from the legend).
Fi = ji / k
Next, find the sum of the frequencies and put that number at the bottom of the
column in Totals. The relative frequency, RFi , is the frequency divided by the sum of the
frequencies.
RFi = Fi / ΣF
Coverage
In Table 3, column 12, you calculated the total area covered for all quadrants for
each species. Those numbers should be transferred to column 8. Coverage, Ci , is computed
as the total area covered by a species (Ai ) times the density (column 4) divided by the total
number of individuals in that species (column 2).
Ci = Ai Di / ni
Relative coverage, RCi , is the coverage value you just calculated divided by the total
coverage for all species (last row, column 8)
RCi = Ci / ΣC
Importance Value
The importance value, IVi , of each species can be calculated as the sum of the
Relative Density, Relative Frequency and Relative Coverage.
IVi = RDi + RFi + RCi
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Table 1. Point-Quarter Transect Raw Data
Point No. Quad No. Species Diameter Area Covered Point to Plant
1 1
2
3
4
2 1
2
3
4
3 1
2
3
4
4 1
2
3
4
5 1
2
3
4
6 1
2
3
4
7 1
2
3
4
8 1
2
3
4
9 1
2
3
4
10 1
2
3
4
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Table 2. Summary of number of individuals from Point-Quarter Sampling
Species 1 2 3 4 5 6 7 8 9 10 No. of Total for
points with species
species
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Table 3. Summary of species areas from Point-
Quarter Sampling
Species 1 2 3 4 5 6 7 8 9 10 Total area
for
species
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Table 4. Calculations for Point-Quarter Sampling
Date:
Habitat:
Total Points (k): 4k: Total point to plant distances: Total Density:
Species, i ni Relative Absolute No. of Frequency Relative Total area Coverage Realtive Importance
Density Density points Fi Frequency covered Ci Coverage Value
RDi Di with RFi Ai RCi IVi
species
ji
Totals 4k 1.00 //////////////// //////////////// ΣF= 1.00 //////////////// ΣC= 1.00 ////////////////
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