Name: ______________________________
BIOTIC and ABIOTIC FACTORS IN A COMMUNITY
INTRODUCTION
The biotic factors of an environment are all organisms
found with the environment. Organisms are divided into five
kingdoms: Monera (bacteria, blue-green algae), Protista (singlecelled organisms such as amoebas), Fungi (including mushrooms
and molds), Plantae (plants), and Animalia (animals). A group of
organisms of the same species found within the same environment
is a called a population. Populations of different species sharing
the same area and interacting with each other make up a
community. One way of studying the interactions of organisms in a
community is by taking an inventory of all the species in the area
and comparing the sizes of their populations.
The organisms of a community are constantly interacting.
Changes in one population affect other populations in the
community. Abiotic factors such as soil type, moisture, slope of the
land, wind, light, and temperature also affect the composition of
the community. Scientists who study communities of organisms
benefit from observing changes in the abiotic factors of an
ecosystem. Like the biotic factors, abiotic factors change from
season to season and from year to year. Organisms in the ecosystem either adapt to changing conditions
or they die, thereby changing the composition of the community. In autumn, for example, as temperatures
decrease, leaves fall from trees and some animals prepare for winter hibernation. Days become shorter
and sunlight becomes less intense. Although many plants dies during a winter freeze, they may have
dropped seeds that sprout the following spring.
In this laboratory, you will observe the abiotic factors of soil moisture, light intensity, and
temperature in two study sites. You will then classify the species of plants and animals in each site by
measuring randomly chosen quadrats within the each site and counting the individuals of each population
within each quadrat. With this data you will estimate the sizes of plant and animal populations within the
community and infer the relationships between biotic and abiotic factors.
WARM UP
Before scientists can design an experiment, they must first make observations on which to base
their hypotheses. Scientists have many different methods of collecting data. To measure the abiotic
factors in this laboratory you will use a soil moisture sensor, a light intensity meter, and a temperature
sensor. The soil moisture sensor uses capacitance to measure the water content of soil and reports the
volume of water in the soil as a percentage. The light sensor measures lux, or lumens/m2, which is a
quantity of visible light emitted/absorbed per unit area. Finally, the temperature sensor measures in
degrees Celsius (ºC). Light, temperature, and moisture are common abiotic factors that determine which
plants and animals exist in any given community.
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 large community. This process is called the quadrat
method.
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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.
For example, if a 10 m x 10 m (100 m2) site is being surveyed, three quadrats of a smaller size,
perhaps 1m x 1m (1 m2), might be selected at random. If the population densities of a particular species at
the three quadrats are 10, 12, and 14 individuals per m2, an average is taken [(10 + 12 + 14)/3 = 12
individuals per m2]. That number is multiplied by the ratio of the larger area to the area of each quadrat
(100 m2/1 m2) to calculate the estimated population size within the site (12 x 100 = 1200 individuals).
This process is repeated for all species in the community.
MATERIALS
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Metric ruler or tape measure
8 stakes
Rubber mallet
Right triangle tool
Marking tape
Scissors
Light sensor
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Soil moisture sensor
Temperature sensor
Lap top computer
LabPro or Labmate interface
PVC quadrats
Field guides to plants and animal species
Note pad
PROCEDURE
1. With your instructor, select two areas that are nearby but possibly different in their abiotic
factors (i.e. different light intensities, temperatures, soil moisture). In each area use the tape
measure to mark off a square 10m on each side and drive a stake into the ground at each
corner. Use the right triangle tool to make each square as precise as possible. Half of the
class will investigate each area.
2. Loop the marking tape around the four stakes to mark the boundaries of the study site, then
cut the tape and tie the ends. Be sure that the tape is taut.
3. Choose 12 areas within the study site. Select areas six where you and your classmates believe
the following three abiotic factors will be highest and six where they will be lowest. Your
group will be assigned to measure one of the abiotic factors listed below in one of the study
areas. Follow the assigned procedure explained by your instructor.
a. Soil Moisture: Measure the soil moisture using the soil moisture sensor and record
your data in Table 1. Mark the area of highest soil moisture and lowest soil moisture
in each site.
b. Light Intensity: Measure the light intensity with the light sensor and record your
data in Table 1. Mark the area of highest and lowest light intensity in each site.
c. Temperature: Measure the temperature with the temperature sensor and record your
data in Table 1. Mark the area of highest and lowest temperature in each site.
4. Complete Table 2. Use data from your investigation as well as those of your classmates.
5. Your instructor will designate one of the abiotic factors from step 3 above to serve as your
first quadrat. Make the marked measurement site the center of your quadrat. Lay the PVC
quadrat over the marked site. Make the sides of your quadrat parallel to the sides of the larger
square study site. Use the right triangle or a notebook to help you.
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6. Take an inventory of the different kinds of plants and animals found in this quadrat. Use field
guides or other references to identify the species you observe. When surveying animals such
as insects or worms, look under rocks, on branches, and in the soil, trying your best not to
disturb them. Make a list of the species in your notebook.
7. Record in Table 3, the number of organisms of each plant or animal species within the
quadrat. To count grass or very small insects, select three smaller (10 cm x 10 cm) squares at
random, count the number of individual plants or insects in each smaller square, average the
numbers and multiply by 100 to get an estimate for the full quadrat.
8. Select two additional areas in your study site. Select these areas at random, by closing your
eyes and tossing a small object (rock, coin, etc.) into the square. Make the tossed object the
center of your quadrat and lay your PVC quadrat over the marked site. Make the sides of
your quadrat parallel to the sides of the larger square study site as you did in step 4.
9. Repeat steps 5 and 6 for this additional quadrat.
10. Complete Table 3. For each species, add the number of organisms found in all three quadrats
that you measured and divide by three to calculate the average population density per square
meter. Record the average population densities in Table 3.
11. For each species, multiply the population density by 100 to estimate the total number of
organisms in the larger site. Record the estimated population sizes in Table 3.
12. With your classmates, select between three and five species found in all areas of your study
sites. Create a graph that plots the estimated population sizes of each of these species (which
you calculated in step 10 above) in each of the superlative areas (i.e. highest soil moisture,
lowest soil moisture, most intense light, least intense light, highest temperature, coldest
temperature). Attach your graph to this laboratory.
13. Complete Table 3. You will need to obtain data from the students investigating the other
study site to complete this table.
14. Answer the analysis and conclusion questions on a separate sheet of paper.
OBSERVATIONS
Study Site: _____________________________________________________
Table 1. Soil Moisture, Light Intensity and Temperature of Different Areas of Study Site
Area
1
2
3
4
5
6
7
8
9
10
11
12
Soil Moisture
Light Intensity
Temperature
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Table 2. Comparison of biotic and abiotic factors at two nearby study sites.
Factor
Study Site A: __________
Study Site B: ____________
Highest soil moisture
Lowest Soil Moisture
Most intense light
Least intense light
Highest temperature
Lowest temperature
Table 3. Inventory of Species at three different locations within a 10 m2 quadrat
Species
# in
Quad. 1
Add additional sheets as necessary.
# in
Quad. 2
# in
Quad. 3
Average
population
density
Estimated
population
size in site
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DATA ANALYSIS & CONCLUSION
Answer the following in complete sentences on a separate sheet of paper.
1. Were there any quadrats where two or more superlatives (i.e most intense light, highest
temperature, highest soil moisture etc) were found together? If so, which areas were found
together?
2. Examine Figure 1. How did the estimated population sizes of organisms change around the
areas of highest soil moisture compared to lowest soil moisture?
3. Examine Figure 1. How did the estimated population sizes of organisms change around the
areas of most light intensity compared to least light intensity?
4. Examine Figure 1. How did the estimated population sizes of organisms change around the
areas of highest temperature compared to lowest temperature?
5. Examine Table 3. Can you attribute any differences between each study site with respect to
biotic community composition to abiotic factor differences in each site? Explain your
reasoning.
6. How do your average population densities compare to the population densities of the
individual quadrats? Were the populations spread out evenly over the site?
7. Did you observe any unusual features in one quadrat that were not found elsewhere in the
site? How might these features skew your data?
8. Using the quadrat method, how could you better estimate the actual population sizes of
species within the site?
9. How is the sampling of animal populations more difficult than that of plant populations?
10. How would the removal of the dominant plant species from either site affect the population
sizes of other organisms? Design an experiment to test your hypothesis.
11. How do you expect the abiotic factors measured in this laboratory to change over the course
of a year? How will your study site differ in winter, spring, and summer?
REFERENCES
Bernstein, Leonard. "Investigations 2 and 3." Addison-Wesley Environmental Science: Ecology
and Human Impact : Laboratory Manual. Menlo Park, CA: Addison-Wesley, 1996. 1420. Print.
Guze, Carol. "The Five Kingdoms." Carol's Classroom. Web. 30 Oct. 2011.
<http://www.carolguze.com/images/evolution/5_kingdoms.jpg>.