Introduction

Literature on competition theory abounds. The basic tenet is that similar, but not identical, species compete or have competed in the past. That is, species with similar requirements, such as habitat and food requirements, are in competition with each other for those requirements. Competition coefficients quantify the magnitude of the competitive effect of one species on another. If two species are perfect competitors, if they have the same competition coefficients, one of the two species will go extinct. Because species coexist even in the face of competition, they must not be perfect competitors. Competition theory states that perfect competitors cannot coexist and that in order to coexist, species must utilize resources differently and have different competitive abilities. Species may utilize different food sources, habitats, or times of activity to avoid competition. Species may become specialized to be the best at utilizing a particular resource, or they may be generalists and be only marginally good at exploiting a wide variety of resources to avoid competition. Competitive displacement occurs when competition leads to the evolution of different resource utilizations or competitive abilities of species exploiting similar resources.

According to competition theory, many of the niches that species currently occupy are the result of competition between species that occurred in the past. Current niche diversification that is driven by competition may be difficult to observe. Invasive species may offer an opportunity to observe the effects of competition as they occur. Invasive species enter a habitat where they have not previously existed and may be better competitors than the native species that already live in that habitat. Invasive species often drive native species to extinction through their superior competitive ability. This is an example in nature where one may witness competition leading to either extinction or competitive displacement. It may also be useful to use computer simulations to model the effects of competition.

In this exercise, you will examine competition between two species. One species is a superior grower and one is a superior colonizer. You will examine the effects of habitat heterogeneity , cost of living, and food value on competitive outcomes of the speciesí interactions.

Lab Instructions

Outline of the Lab Construction

The purpose of this model is to examine the concept of competition and the idea of competitive exclusion. The lab is divided into two scenarios, both of which look at the competition occurring between two species, one a grower and the other a colonizer.

The grid of the simulation contains two habitats upon which the colonizer and grower interact. There is a "good habitat" where the cost of living is low for both the colonizer and grower, and a "bad habitat" where an increased cost of living is imposed upon the organisms. Food is introduced each time step only to the good habitat.

There are three parameters that will be modified in the lab: 1) cost of living in the good habitat, 2) cost of living in the bad habitat, and 3) food values. Cost of living is the amount of energy used to maintain the organism in the habitat that it is in for one time step. The food value is the amount of energy that is received when the organism eats a piece of food. The differences between the grower and colonizer are a result of changing the three parameters just mentioned. All other parameters are the same. The grower has a higher food value but must pay a higher cost of living when moving through the bad habitat. The colonizer has a lower food value, but has a lower cost of living in the bad habitat. As a result, the grower is able to derive more energy from a unit of food than the colonizer, but the colonizer is able to move through the empty habitat at a lower cost.

The lab will be divided into two scenarios, one with a uniform good habitat and the other with a patchy good habitat. The uniform good habitat has the entire good habitat in a single large square with bad habitat around it. The patchy good habitat divides the good habitat into smaller squares with bad habitat in between. The total area of good habitat in both scenarios is the same.

Procedure

Basic Start-up and Commands

  1. Run EcoBeaker by double clicking on the icon.
  2. Open Situation File: "Patchhabitat.sit"
  3. Maximize EcoBeaker to full screen and move the windows so that nothing is overlapping and everything can be seen.
  4. The two scenarios are saved as markers. To open a scenario look at the top bar of EcoBeaker. Click on "Marker" and a menu will pop down with "Uniform Good Habitat" and "Patchy Good Habitat" at the bottom. To select a scenario, click on the name and it will open.
  5. General instruction for running a scenario: Scenarios are controlled using the CONTROL PANEL window. "GO" starts the scenario. "STOP" pauses the scenario and it can be unpaused by pressing "GO" again. "RESET" causes the scenario to stop and return to the starting conditions at time step zero.
  6. Changing parameters in the CHANGE PARAMETER window: To change a parameter, place the cursor in the box with the value you want to change. Enter the new value. Press "CHANGE" at the bottom of the CHANGE PARAMETER window. You can change parameters while a scenario is running. To change the cost of living, you must first click on the button "HAB ARRAY" to bring up the value boxes for that species in each habitat. Remember to press change after entering any new value.

 

Uniform Good Habitat Scenario

  1. Open the "Uniform Good Habitat" scenario under the "Marker" heading.
  2. Which organism do you expect to dominate on a uniform good habitat, the grower or colonizer?
  3. In the CONTROL PANEL window press "GO" to start the scenario and watch the line graph of the populations of growers and colonizers.
  4. How long did they coexist?
  5. What would happen if the food value of the grower was changed from 15 to 5?
  6. What does this demonstrate about the Competitive Exclusion Principle?

Patchy Good Habitat

  1. Open the "Patchy Good Habitat" scenario under the "Marker" heading.
  2. After watching the Uniform scenario, which organism do you expect to be dominant and how long will the two organisms coexist?
  3. Run the scenario.
  4. How long did they coexist? What does this demonstrate about mediated competition?
  5. Change the cost of living of the grower in the bad habitat to 8 energy units per time step. What happens?
  6. Change the cost of living of the grower in the bad habitat to 12 energy units per time step. What happens?
  7. Try varying the food values and other values. Reopen the marker to get the original values of the parameters.

Questions:

  1. In what other ways might you vary the habitat structure? How would these variations affect competitive interactions between the grower and the colonizer?
  2. Besides varying spatial structure, how might niches vary such that they do not overlap?
  3. What other competitive trade-offs might species evolve in response to inter- or intraspecific competition?
  4. How would adding more competing species to a system affect species interactions and the population dynamics of the species? How would adding predators/herbivores affect competition?

 

References

Connell, J. H. 1961. The influence of interspecific competition and other factors on the distribution of

the barnacle Chthamalus stellatus. Ecology 42: 710-723.

Elton, C. S. 1958. Ecology of Invasions. John Wiley & Sons, Inc., New York.

Gause, G. F. 1934. The Struggle for Existense. Williams & Wilkins, Co., Baltimore.

Hardin, G. 1959. Nature and Manís Fate. Rinehart & Co., Inc., New York.

Lotka, A. J. 1932. The growth of mixed populations: Two species competing for a common food supply.

J. Wash. Acad. Sci. 22: 461-469.

Volterra, V. 1926. Variations and fluctuations of the number of individuals in animal species living

together. Trans. in Chapman. 1931. Animal Ecology 409-448.