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

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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

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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

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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

1 m

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 pil n(pi) for each species, and record the value in Data Table 1. pil n( pi) = pi x ln( pi)

6. Calculate the Shannon index. This is the negative sum of all pil n(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.

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Data Table 1.

Transect 1 Species Description

Number of Individuals (ni) pi = ni/N pi 2 = pi x pi

pi l n(pi) = pi x ln(pi)

Total number of individuals (N)

Simpson index

Shannon index

Observations

ACTIVITY

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Data Table 2.

Transect 2 Species Description

Number of Individuals (ni) pi = ni/N pi 2 = pi x pi

pi l n(pi) = pi x ln(pi)

Total number of individuals (N)

Simpson index

Shannon index

Observations continued

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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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