Intraspecific and Interspecific Competition
Objectives:
 To assess the effect of competition among plants at different population densities on the
growth of plants.
 To assess the relative competitiveness of two species of plants when they are planted
together.
Competition:
Competition is an interaction between organisms in which both are harmed by the interaction.
Competition can occur among members of the same species and is called intraspecific competition or
it can occur between members of different species, in which case it is called interspecific competition.
Competition between animals can involve fights over food, water, nesting sites, or mates or may
involve less overt forms such as a greater height allowing some animal’s access to food that others
cannot reach.
Among plants, competition usually is difficult to visualize. However, certain resources, such as
water, soil nutrients, and sunlight are in limited supply. Plants that have special abilities to capture
these resources should be more successful in competition.
In this investigation we will examine the impact of seed growth at different densities and in
competition with another type of plant.
Materials:
62 Radish seeds
62 Basil seeds
9 small cups for planting
Sand
Hypothesis: Write a hypothesis for Investigation 1 for both the radish seeds and the wheat seeds.
Investigation #1 Intraspecific Competition
1. Obtain 31 radish seeds.
2. Obtain 6 small cups and poke a small home in the bottom. Place a leaf over the hole and fill
within 2 cm of the top with sand.
3. Carefully place radish seeds on the sand and cover with about 1 cm of additional sand as
follows:
 In pot 1 plant one seed
 In pot 2 plant 10 seeds
 In Pot 3 plant 20 seeds
4. Pour water into the pot until it begins to run out the bottom.
5. Place cups with seeds on the windowsill.
6. Examine daily and water the plants when the surface becomes dry.
7. After two weeks remove the plants from each pot, including the roots.
8. Weigh the total biomass produced from each pot, determine the average biomass per plant
and then record the information in the data table.
9. Repeat the same procedures 1- 8 for the basil seeds.
10. Graph the results of the data table.
11. Answer the questions for both radish and basil seed data.
Hypothesis: Write a hypothesis for Investigation 2.
1. Investigation #2 Interspecific Competition
2. Obtain 31 radish seeds and 31 basil seeds.
3. Obtain 3 small cups and poke a small home in the bottom. Place a leaf over the hole and fill
within 2 cm of the top with sand.
4. Carefully place seeds on the sand and cover with about 1 cm of additional sand as follows:
 In pot 1 plant one radish seed and one basil seed
 In pot 2 plant ten radish seeds and ten basil seeds.
 In pot 3 plant twenty radish seeds and twenty basil seeds.
5. Pour water into the pot until it begins to run out the bottom.
6. Place cups with seeds on the windowsill.
7. Examine daily and water the plants when the surface becomes dry.
8. After two weeks remove the plants from each pot, including the roots.
9. Separately weigh the total biomass of radish and basil plants produced from each pot,
determine the average biomass per plant for both the radish and wheat plants and then record
the information in the data table.
10. Graph the results of the data table. (Use two different colors to show the two different kinds of
plants. Be sure to include a legend for your colors.
11. Answer the questions
Investigation 1 Data table
Seeds per pot
Radish: Intraspecific Competition
Total biomass per pot (grams)
Average biomass = Total biomass
Number of seeds per pot
1
10
20
Investigation 1 Data table
Seeds per pot
Basil: Intraspecific Competition
Total biomass per pot (grams)
Average biomass = Total biomass
Number of seeds per pot
1
10
20
Analysis
1. At what density was there a change in the biomass per plant as the density of the plants in the
pots increased?
2. How did it change?
3. What resources might have been limited?
Investigation 2 Data Table
Seeds per pot
Radish and Basil Interspecific Competition
Total biomass per pot (grams)
Average biomass = Total biomass
Number of seeds per pot
1 radish
1 basil
10 radish
10 basil
20 radish
20 basil
Analysis:
1. What kind of plant was most affected by the competition between the two species? (In order
to answer this question you many want to compare your data with those of Investigation 1. An
easy way to do this would be to include those data on this graph with two other colors. If you
chose to do this, be sure to include a legend.)
2. List three reasons one kind of plant might have been more successful than the other (How
might one of the plants have had a competitive advantage?)