Characterizing Community
Structure: Plants
Investigation
Manual
ENVIRONMENTAL SCIENCE
Table of Contents
2 Overview
2 Outcomes
2 Time Requirements
3 Background
5 Materials
6 Safety
6 Preparation
7 Activity 1
7 Activity 2
8 Activity 3
8 Disposal and Cleanup
9 Observations
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CHARACTERIZING COMMUNITY STRUCTURE: PLANTS
Overview
Ecologists catalog population size and species diversity for
many reasons. For example, they must determine the number
of individuals that can be safely removed from a population,
plan controlled burns for forestry management, and determine
community resilience to disruption. In the following activities,
students will use two different sampling techniques—quadrats
and transects—to estimate population size. Students will estimate
population ground cover, use the Shannon index to estimate
species richness, and use the Simpson index to estimate species
diversity.
Outcomes
• Estimate the percent ground cover of various organisms using a
quadrat.
• Calculate population size, species diversity, and species
richness for a transect.
• Compare species diversity and species richness for two sample
locations.
Time Requirements
Activities 1 and 2 will be performed at a field site of your choosing.
Times shown are for time required at the site to perform the activity.
Preparation …………………………………………………………… 45 minutes
Activity 1: Transect Method …………………………………………..2 hours
Activity 2: Quadrat Method …………………………………………….1 hour
Activity 3: Species Richness and Diversity
Calculations …………………………………………………..1 hour
2 Carolina Distance Learning
Background
Ecology is the study of the interactions
between organisms and their environment.
Groups of organisms and abiotic factors that
interact with one another are collectively called
an ecosystem. Living organisms within an
ecosystem comprise the biotic community.
Nonliving components of the environment, such
as water and inorganic compounds, are abiotic
factors. Each biotic community is made up
of several populations (groups of organisms
of the same species). These populations are
highly dependent on one another, either as
prey, as shelter, or through other more complex
relationships. Communities are frequently
characterized by the most obvious sessile
(nonmoving) organisms (e.g., a pine forest or
coral reef).
Ecologists study the environment for a number
of reasons. In addition to increasing our
understanding of the natural world, most studies
seeking to characterize a community have some
form of resource management as their goal. It
is important to know, for instance, how many
individuals are present in an environment when
trying to determine hunting or catch limits.
Frequently, just as important to understanding
the function of the community and the
impact of management on that community
is the diversity, or variety, of the community.
Communities that are more diverse are often
more resilient to disruption in terms of the
degree of a perturbation’s effects and in speed
of recovery. Diversity is usually expressed as
total species richness—the number of species
found in an area. Evenness, whether the
numbers of individuals in various populations
are similar, also affects functional diversity of
a community. If two communities have similar
species richness, the diversity will be higher in
the community with a more evenly distributed
abundance.
Other management goals, however, require
different measurements. For example, fire
management practices use many of the same
techniques to determine cover density, fuel load
(the amount of flammable materials present in an
environment), and the presence of rare species.
Ecological sampling techniques are based on
the concept of a representative sample. It is
uncommon for researchers to have either the
time or money to exhaustively catalog every
organism in a complex environment. Instead,
they sample a subsection of the environment
and assume it to be representative of the
environment as a whole. Small sample sizes
can easily miss rare organisms, so repeated
sampling is important. Ideally, environments
are sampled randomly to avoid the introduction
of bias. Systematic sampling (sampling done
at a fixed, periodic interval), however, may be
more practical when the focus of the research
is narrow, such as examining the transition
between two environments. In this type of
sampling, the chosen data is evenly distributed;
the first sample is chosen at random, but the
rest are taken based on a specific interval of
choice.
Different organisms must be surveyed in
different ways. There are two major sampling
techniques—quadrats and transects—for
organisms that are sessile and/or in aquatic
environments (e.g., plants, fungi, coral, and
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CHARACTERIZING COMMUNITY STRUCTURE: PLANTS
Background continued
continued on next page
4 Carolina Distance Learning
organism touching the line is then identified
and counted. Transects are frequently used
to characterize change along a gradient. For
example, researchers may look at changes in
community structure in moving from an open
field to a forested area or over distance from
a body of water. This method has obvious
drawbacks in a highly scattered environment,
such as a savannah with clumps of trees or a
stretch of coast dotted by tide pools; it also
increases the probability of rare and/or small
organisms being underrepresented in the data.
One solution to this limitation is to use a belt
transect, wherein two parallel lines are drawn
at a fixed width apart from each other and all
organisms between them are counted. Another
solution is to combine quadrats and transects
by placing a quadrat at either random intervals
or at fixed points along a transect.
The choice of method and exact implementation
of that method depend on a number of different
factors, including time, budget, and scale of the
habitat and study. Large mobile animals rarely
stay in one place long enough to obtain accurate
counts using the methods discussed in these
activities.
This manual provides instructions for performing
transect and quadrat sampling procedures.
Since access to specific environments varies,
please refer to your instructor’s directions for
choosing a location for your sampling.
Using the methods provided, you will collect
data to calculate several indexes of species
richness and diversity. Since estimates of
species richness (S = total number of species)
oysters). These methods can be employed
separately or in combination.
In practice, quadrats usually measure 0.5 m2
or 1 m2
, although they may be of any size.
Researchers place quadrats throughout the test
site and then inventory what they find inside
of each. Critical to this sampling technique
is the placement of quadrats throughout the
habitat. If an external condition creates a bias
in quadrat placement (e.g., resulting in the
placement of quadrats where there appear to
be higher concentrations of the target species),
the subsequent statistical analyses will be
inaccurate. The area to be studied can be
divided into grids on a map, and quadrats can
either be placed systematically or randomly on
the grid. Figure 1 shows both of these sampling
styles, with black outlined squares representing
grids and orange squares inside of the grids
representing quadrats.
Figure 1.
The fastest and usually the least expensive
method of determining the abundance and
diversity of organisms within an area is the line
transect. In this method, a straight line is drawn
between two points, usually by laying down
a line of twine or string. Alternatively, the line
may also be drawn on a larger scale by moving
in a straight line from point A to point B; any
Systematic Placement Random Placement
are highly dependent on sampling effort,
species diversity is frequently reported relative
to the total number of individuals sampled. The
appropriate technique to use in creating such
indexes has been the subject of much scientific
debate. Two commonly used indexes are the
Shannon index (also called the Shannon-Weiner
or Shannon-Weaver index), which measures
species richness, and the Simpson index,
which is a measure of diversity. In both cases, pi
is the proportion of a species in the community
(i.e., the number of individuals counted of a
given species divided by the total number
of individuals counted of all species in the
community). S is the total number of species in
the community.
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Shannon index: Simpson index:
i = 1
D =
s
pi
2 ∑
i = 1
H´ = –
s
∑ pi ln pi
Needed but not supplied:
• Paper
• Marker
• Scissors
• Calculator capable of taking the natural
logarithm (ln) of a number
• Camera or cell phone capable of taking
photographs
Reorder Information: Replacement supplies
for the Characterizing Community Structure:
Plants investigation (item number 580815)
can be ordered from Carolina Biological
Supply Company.
Call: 800-334-5551 to order.
Materials
Included in the materials kit:
String, 200
feet
Measuring
tape, 150 cm
8 Orange flag
markers
CHARACTERIZING COMMUNITY STRUCTURE: PLANTS
6 Carolina Distance Learning
Read all the
instructions for this
laboratory activity before beginning. Safety
while performing fieldwork is of greatest
importance. Follow the instructions closely and
observe established laboratory safety practices,
including the use of appropriate personal
protective equipment (PPE).
Never conduct fieldwork on your own,
especially in remote areas; always take a
responsible partner with you. Prepare and
leave a field safety plan, complete with planned
survey locations and expected return time,
with a responsible party. Wear appropriate
clothing (long pants and close-toed shoes are
usually best), sunblock, and insect repellant if
necessary. Be familiar with the potential hazards
in your chosen survey areas. Please review
the “Field Work” portion of the Laboratory
Safety Manual or your school’s specific safety
guidelines for more information. Make sure
that you are healthy enough to participate in
fieldwork. If in doubt, consult your medical
practitioner.
Preparation
1. Read through the activities.
2. Obtain all materials.
3. To prepare for the transect sampling method
before going into the field:
a. Collect the string, scissors, and two flags.
b. Measure and cut 3 m (300 cm) of string.
c. Tie one end of the string to one flag.
Carefully wind the remaining string around
the flag so it will not tangle.
4. To prepare for the quadrat sampling method
before going into the field:
a. Collect the string, scissors, a marker, four
flags, and the measuring tape.
b. Measure and cut a little more than 4 m
(400 cm) of string.
c. Tie one end of the string to one flag.
d. Measure 100 cm on the string from the
flag, and mark the string with the marker.
e. Continue to measure at 100-cm increments.
There should be a mark at 100 cm, 200 cm,
and 300 cm. Carefully wind the string
around the flag so it will not tangle.
Safety
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ACTIVITY 1
A Transect Method
1. Take supplies for both the transect method
and quadrat method to your field site, and
set aside the string prepared for the quadrat
method.
2. Using the materials prepared for the transect
method, push the flag with the string tied
around it into the ground at the chosen start
of the transect.
3. Walk in as straight a line as possible, letting
the string unwind behind you to 3 meters in
length for the transect. If you must detour
around a tree trunk or an obstruction,
return as soon as possible to the line of the
transect. Pass the string over or through
any shrubbery, keeping the line as close to
straight and the string as close to the ground
as possible.
4. When the string is completely unwound, tie
the free end to a second flag and push this
flag into the ground.
5. Identify and familiarize yourself with
the different species of plants
touching the transect. Take a photo of each
type found for later reference. Your instructor
may want you to identify each species using
a local plant guide. Write a brief description of
each in Data Table 1.
6. Count the number of individuals of each plant
species touching the transect, and record it
in Data Table 1. If there aren’t many plant
species touching the transect, place your
measuring tape on the ground at a right angle
to a random spot on your transect and count
the number of plant species touching the
measuring tape.
7. Pick up the transect, and take the flags
and string to a different location at the
current site.
8. Repeat Steps 2–6 for a second transect.
Collect all data for Transect 2, and record it
in Data Table 2. Leave Transect 2 in place
to use with Activity 2.
ACTIVITY 2
A Quadrat Method
1. Using the materials prepared for the
quadrat method, choose a point at either
end or at a random point along Transect 2.
2. Push the quadrat flag with the string tied
around it into the ground at this point.
3. Run the string along a line perpendicular to
the transect, and place your second flag at
the first mark on your quadrat string.
4. Using a piece of paper or a notebook for
geometric reference, run the string at a
right angle to the first segment of string
(see Figure 2).
Figure 2.
ACTIVITY
ACTIVITY
ACTIVITY 2 continued
5. Place the third flag at the second mark
along the string, and again, using paper or
a notebook for geometric reference, run the
string perpendicular to the second segment
and parallel to the first.
6. Place the fourth flag at the third mark along
the string. Run the string around the fourth
flag and tie it off at the first flag.
7. Familiarize yourself with the different
species of plants within your quadrat.
Take a photo of each type found for later
reference. Your instructor may want you
to identify each species using a local plant
guide. Write a brief description of each in
Data Table 3.
8. Estimate the percentage of quadrat covered
(or percent cover) for each plant type in
your quadrat by looking down on your
quadrat from above. Record each of
these percentages in Data Table 3. When
estimating percent cover, it may be useful
to imagine a grid overlaying your quadrat,
or if you want to be more precise, use the
remaining string on the roll provided to mark
out the grid. Determine how many of those
smaller squares are occupied by each plant
species, then divide that number by the total
number of squares. Remember that you
are measuring how much of the ground is
covered by each plant and not how much
of the ground is covered. Because the
area occupied by each plant species often
overlaps with that of another species, the
sum of these percentages will likely be more
than 100%.
ACTIVITY 3
A Species Richness and Diversity
Calculations
1. Calculate the total number of individuals (N).
This is the sum of all the individuals of each
species (ni) for Transect 1. Record N in Data
Table 1.
2. Calculate pi for each species, and record the
value in Data Table 1.
pi = ni/N
3. Calculate pi
2 for each species, and record the
value in Data Table 1.
pi
2 = pi x pi
4. Calculate the Simpson index. This is the sum
of all pi
2 values for the species in Data Table
1. Record this value in Data Table 1.
5. Calculate piln(pi
) for each species, and record
the value in Data Table 1.
piln(pi) = pi x ln(pi)
6. Calculate the Shannon index. This is the
negative sum of all piln(pi) values for the
species in Data Table 1. Record this value in
Data Table 1.
7. Repeat Steps 1–6 for Transect 2, and record
the values in Data Table 2.
8. Calculate the total percentage of ground cover
for the species found in the quadrat in Data
Table 3. This is the sum of the percentages
in Data Table 3. This value may be more than
100%. Record the value in Data Table 3.
Disposal and Cleanup
Before leaving the field site, pick up the string,
flags, and any other material you brought with
you to return to your home.
8 Carolina Distance Learning
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Data Table 1.
Transect 1
Species Description
Number of
Individuals (ni) pi = ni/N pi
2 = pi x pi
piln(pi) =
pi x ln(pi)
Total number of
individuals (N)
Simpson index
Shannon index
Observations
dead leaves
small, thin twigs
13
5
grass (blades of grass) 30
large tree 1
49
ACTIVITY
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Data Table 2.
Transect 2
Species Description
Number of
Individuals (ni) pi = ni/N pi
2 = pi x pi
piln(pi) =
pi x ln(pi)
Total number of
individuals (N)
Simpson index
Shannon index
Observations continued
grass (blades of grass)
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Data Table 3.
Quadrat
Species Description
Percentage of
Quadrat Covered
(Percent Cover)
Total ground
cover percentage
ENVIRONMENTAL SCIENCE
Characterizing Community Structure:
Plants
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