Intraspecific and Interspecific Competition

Intraspecific and Interspecific Competition

Objectives:

v  To assess the effect of competition among plants at different population densities on the growth of plants.

v  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 grass seeds

9 small cups for planting

Sand

Hypothesis: Write a hypothesis for Investigation 1 for both the radish seeds and the grass seeds.

Investigation #1 Intraspecific Competition

1.  Obtain 31 radish seeds.

2.  Obtain 6 small cups and poke a small hole 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:

v  In cup 1 plant one seed

v  In cup 2 plant 10 seeds

v  In cup 3 plant 20 seeds

4.  Pour water into the cup until it begins to run out the bottom.

5.  Place cups with seeds on the windowsill.

6.  Repeat the same procedures 1- 5 for the grass seeds.

7.  Examine daily and water the plants when the surface becomes dry.

8.  After two weeks remove the plants from each cup, including the roots.

9.  Weigh the total biomass produced from each cup, determine the average biomass per plant and then record the information in the data table.

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 grass seeds.

3.  Obtain 3 small cups and poke a small hole 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:

v  In cup 1 plant one radish seed and one grass seed.

v  In cup 2 plant ten radish seeds and ten grass seeds.

v  In cup 3 plant twenty radish seeds and twenty grass seeds.

5.  Pour water into the cup 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 cup, including the roots.

9.  Separately weigh the total biomass of radish and grass plants produced from each pot, determine the average biomass per plant for both the radish and grass 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 Radish: Intraspecific Competition

Investigation 1 Data table Grass: Intraspecific Competition

Analysis

1.  At what density was there a change in the biomass per plant as the density of the plants in the cups increased?

2.  How did it change?

3.  What resources might have been limited?

Investigation 2 Data Table Radish and Grass Interspecific Competition

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?)