Laboratory Activity
Do plants of the same species compete with one another?
Unit IV : Interdependence of Organisms
Time Allotment: Day 1 20 minutes, 10 minutes once per week, Final day 50 minutes
PASS Objectives:
Content Standard (CS): 4.3 Living organisms have the capacity to produce populations
of infinite size, but the environment and resources limit population size (i.e., carrying
capacity and limiting factors)
EOI Item Specifications*
Emphasis: Identify, analyze, and evaluate factors affecting population size. Predict
possible changes in populations.
Content Limits: Items may include limiting factors such as predation food supply, and
habitat. Assessable concepts include biotic potential, carrying capacity, natality,
mortality, immigration, emigration, colonization, and population density. Interpret the
population dynamics resulting in J- shaped and S-shaped growth curves.
Process Standard 1-6
Textbook Correlation: Biology the Dynamics of life Copyright 2005 published by
Glencoe, McGraw-Hill
[Chapter 4 and Section 1]
Introduction
In animal populations, competition is typically studied by examining changes to birth and
death rates. Ecologists often study plant competition by comparing the biomass of
individual plants in plant populations. In this lab, you will study intraspecific
competition—competition among plants of the same species. As with most ecological
studies, you will need to collect data for several weeks.
Question: Do plant populations of various densities grow differently due to competition?
Learner Objectives:
The student will be able to:
 Write a hypothesis to predict the effect of density on average biomass
 Measure and average plant height in mm

Materials:
Based on class of 30 working in groups of 2)
186 radish seeds ( number will vary per group)
15 9-cm plastic pots (1 per group)
clean potting soil (enough to fill each pot ¾ full)
15 metric rulers
3 shallow tray for pots ( 1 per 5 groups)
15 masking tape (1 per group)
15 permanent markers (1 per group)
3 balances (accurate to 0.1 g) (1 per 5 groups)
graduated cylinders
Teacher Note:
Each group of two students are working on 1/5 of the experiment. Assign each group the
number of radish seeds that they will be planting, observing and measuring during the
experiment. Also assign them to one of the 3 main lab groups: A, B, or C. At the end of
the experiment, join together the 5 A lab groups, 5 B lab groups and 5 C lab groups .
This large group of 10 students will collect the final data from the 5 different densities of
radish seeds. Based on a class of 30 students, this will give you 3 repeats of the
experiment.
Procedure:
1. Obtain a clear plastic pot, fill it ¾ full of moist potting soil.
2. Plant the assigned number of radish seeds in the pot as instructed by the
teacher. That number may be: 2, 4, 8, 16, or 32. Seeds should be covered
by no more than a ½ cm of soil. Water soil with 40 mls of tap water.
3. Using a marker and masking tape, write name, hour and number of seeds
in pot and group letter A, B or C.
4. Place all the A pots in a shallow tray, repeat for B and C pots. Continue to
keep the soil moist—not drenched—throughout the course of the
experiment.
5. After the seeds have sprouted, check to see that the pot has the correct
number of seedlings and weed out any extra plants so that you have the
correct density.
6. Once a week measure the shortest and tallest plant in the pot in mm and
average the two numbers. Construct a data table to record weekly average
height and observations on plant appearance (color, number of leaves)
7. At the end of the experiment, the 5 lab pairs of groups A, B, and C will get
together with the 5 plant pots to measure the biomass of the plants. For
each pot, cut all the plants at soil level, place them on the balance and
record their mass in grams. Record this number as the total biomass for the
pot. Next calculate the average per plant biomass of each pot, by dividing
the pot biomass by the number of plants in that pot. Record this as average
biomass per plant.
Data Analysis, Conclusions, and Questions
1. Graph Data Prepare a graph showing the relationship between the average plant
biomass and the density of plants. Draw a best-fit line for your data points. What
was the effect of plant density on the average biomass of each pot's population?
Does this graph support your hypothesis?
2. Infer Draw a second graph that compares the total biomass for each population
to the number of plants in each population.
Think Critically Based on your results, infer how human population growth is
affected by population density.
3. Error Analysis What sources of error might have affected your results?
References:
Glenco: Lab Manager: Biology: Population Ecology: Do plants of the same species
compete with one another?