Name: __________________________________________________
Period: _____
Determining Population Density and Dispersion
Background:
The task of taking an inventory of the different kinds of organisms and their population sizes in an environmental site
can be very difficult, especially if the area is teeming with life. Since it would be impractical, if not impossible, to count
each individual organism in a large area, ecologists randomly choose small portions of the whole area and classify and
count the organisms in each small portion. They can then estimate the size of each population in the larger community.
This process is called the quadrat method.
The goal of the quadrat method is to estimate the population density of each species in a given community. Population
density is the number of individuals of each species per unit area. Small square areas, called quadrats, are randomly
selected to avoid choosing unrepresentative samples. Once the population densities for all quadrats are determined, the
population size within the larger area can be estimated using the following formula:
N = (A/a) * n
Where,
N = the estimated total population size
A = the total study area
a = the area of the quadrat
n = the number of organisms per quadrat
Procedure Part 1:
1) Select, at random, an area within the site to be your first 1 m2 quadrat as directed by your instructor.
2) Identify 1 species of plant within your quadrat. Record the population size of this plant species in the data table.
Data Table:
Species
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Using the data above, calculate the estimated total population size (N) of the organism below:
Average (n)
To determine the dispersion of a population, specific locations must be identified within the quadrat. In the three
examples below, various organisms were marked on a sub-quadrat and tallied. The resulting graphs identify patterns
found in the three types of dispersion.
Random dispersion: a standard curve is shown in the percentage of individuals per sub-quadrat
Uniform dispersion: a specific number is found in ~100% of the sub-quadrats
Clumped dispersion: large numbers are found in some of the sub-quadrats but a large % of sub-quadrats will be empty
Using the sub-quadrat provided below; fill in the data table, construct a graph, and identify the population dispersion.
Organisms
in
Sub-Quadrat
0
Total
Found
%
Found
1
2
3
4
5
Population Dispersion: ____________________
Procedure Part 2:
1) Select, at random, an area within the site to be your first 1 m2 quadrat as directed by your instructor.
2) Identify 1 species within your quadrat.
3) Record the population size and sub-quadrat locations of this species.
4) Move the quadrat to another location and repeat step 3.
Species
Quadrat 1
Quadrat 2
Quadrat 3
Quadrat 4
Quadrat 5
Quadrat 6
Using the data above, calculate the estimated total population size (N) of the organism below:
Quadrat 1
Quadrat 2
Quadrat 3
Quadrat 4
Quadrat 5
Quadrat 6
Average (n)
Using the data from the previous 6 quadrats; construct a data table, graph, and identify the population dispersion.
Analysis Questions:
1. Draw a diagram of the site location showing the dimensions used to estimate the total area:
2. Which pattern of distribution most accurately estimates population sizes and why?
3. Why do scientists use estimation techniques to determine population sizes?
4. What are some of the limitations to estimation techniques?
5. How does the dispersion of a population limit the size of the population?
6. Identify a natural phenomenon which would result in random dispersion?