Ecology-Plant Competition
Introduction
Competition occurs when organisms in the same community seek the
same limiting resource. This resource may be prey, water, light, nutrients,
nest sites, etc. Competition among members of the same species is
intraspecific. Competition among individuals of different species is
interspecific. Individuals of all species experience both types of
competition, but the relative importance of the two types of competition
varies from population to population and species to species. From the
perspective of an individual, competition is best defined as a decrease in the
fitness of an individual as the density of conspecifics/heterospecific
individuals in the immediate environment increases.
Types of Competition
Exploitation competition occurs when individuals use the same
limiting resource or resources, thus depleting the amount available to
others.
Some variants of this are overgrowth competition, which occurs
when plants shade each other and block out each other’s access to
light, and preemptive competition where the plant that grows in a
space first preempts other plants from growing there.
Interference competition occurs when individuals interfere with the
foraging, survival, or reproduction of others, or directly prevent their
physical establishment in a portion of a habitat. When it occurs in
plants, interference competition is usually via allelopathy-the process
by which individuals secrete or release toxic chemicals into the
environment to kill the competition. Sometimes, indirect interactions,
via a mutualistt (almost always an ant species), can have this same
effect.
In lab today, you begin an experimental study to determine the effects
of interspecific, and intraspecific competition, in a species of annual grass.
Wheat, Triticum aestivum, is a cereal grass native to the Old World, probably
a hybrid of wild grasses and other species of wheat that still grow there. It is
widely planted in temperate areas of the Northern Hemisphere. Because it
grows quickly, is an annual species, and sets easy-to-count seeds, it is easy
to measure the reproductive output of an individual. Since each seed set is
essentially an offspring, the number of seed set is proportional to fitness.
Thus, if an individual were to suffer from interspecific, or intraspecific
competition, we could quantify this by noting a decrease in seed set as the
density of other individuals in the immediate environment increases.
Switchgrass, Panicum virgatum, is a perennial warm-season grass native to
North America. Many aspects of its biology are similar to those of wheat,
though it is a perennial grass, and has a different type of photosynthesis (C4
rather than C3, which is present in wheat.)
Pre-Lab Discussion
1) Before coming to class, look up at least one study of competition in
plants, and be prepared to discuss it. Write a five sentence summary
of what you learned and turn it in to your TA at the beginning of
discussion.
QUESTIONS FOR GROUP DISCUSSION
A ) What were the goals of the competition study you researched? What
was its outcome?
Laboratory Exercise
Directions: Your lab instructor has provided you with the following:
1) Plastic 2” by 2” pots.
2) Switchgrass seeds
3) Wheat seeds
4) Potting mix
5) Plant labels.
This experiment will have three treatments, ALONE, INTERSPECIFIC
COMPETITION, INTRASPECIFIC COMPETITION. Each table in your lab
section will set up, and count, one replicate for each of the following.
1) Wheat alone.
2) Wheat at 5 individuals per pot
3) Wheat, alone, but with 4 switchgrass per pot.
1) Obtain three pots, the appropriate number of seeds (4 switchgrass, 7
wheat), a sharpie, and 5 plant tags. Fill your pots with mix. Label your
tags 1 through 3.
2) For all your pots, plant a wheat seed directly in the center, about a half
inch deep.
3) For treatment 1, use the tag you labeled 1 and leave it alone, it is,
essentially, a control for both interspecific and intraspecific competition
treatments.
4) For treatment 2, add 4 more wheat seeds, at the corners of the pot.
5) Likewise, for treatment 3, apply four switchgrass seeds, at four corners of
the pot, respectively.
6) Place your pots in the planting tray when you are done. You will not be
counting the particular pots you set up at the end of the experiment, nor will
you be responsible for watering them.
They will be watered by the UIC greenhouse staff, ad libitum, under ambient
light. They will not be fertilized, though there is a small amount of fertilizer
in the potting mix.
Comparing Germination and Growth
At about week 4 or 5, your lab group will compare growth rates of your
wheat seedlings, as a sort of stand-in for fitness. Growth rates are more or
less proportional to seed set for this species, because wheat is an annual
species that has not evolved to set aside resources for future years.
Seedlings have been selected to grow as quickly as possible, over the years,
via artificial selection, so a decrease in growth rate can reasonably be
expected to reflect the possible effects of competition.
1) You and the other members of your lab group should locate five pots, one
from each treatment, and a ruler. For each pot. Measure the plant in the
center, the FOCAL PLANT (because we will be measuring the effects of
competition on this one individual, and not its effects on the other
individuals), from the base of the soil to the tip of the longest leaf. Report
your results to your laboratory instructor.
You can analyze the class data using an ANALYSIS OF VARIANCE. The
theory behind this test is a little bit beyond the scope of the class, but it is, in
essence, a way of seeing if the treatment in which a seedling found itself
affected its growth.
Was there a significant effect of the treatment on the growth of your focal
seedlings?
Interpret the results.
Comparing Seed Set
The procedure for this part of the experiment is similar to the procedure
described above, but in this case, count seeds from the focal individual.
Was there a significant effect of the treatment on the number of seeds set by
your focal seedlings?
Interpret the results.