WLF 448: Fish & Wildlife Population Ecology
Lab Notes 11, Fall 2004

Lab #11 Problem Set: Competition

Scenario #1

You have both green sunfish (Lepomis cyanellus) and bluegill (L. macrochirus) available for stocking in a 10 ha impoundment. These two species compete to some degree. The pertinent population data are:

Green Bluegill
K1 = 700 K2 = 900
r1 = 0.12 r2 = 0.30
a12 = 1.5 a21 = 1.0

Use POPULUS to answer questions 3 - 6. Assume this is an experimental setup (i.e., the environment is constant) with no exploitation. Turn in your printouts with your written answers.

  1. Is there some level of competition between these 2 species? How do you know?

  2. Would you classify these species as weak, moderate, or strong competitors?  How does density influence if a species is considered a weak, moderate, or strong competitor?

  3. Describe the population growth of 100 green sunfish stocked with no bluegills present. Describe the population growth of 100 bluegill stocked with no green sunfish present. Is this what you would expect for logistic growth? Why or why not? Print out 2 graphs showing the plot of the population growth of each species when alone. Make sure all lines and axes are labeled properly, and provide a descriptive title for your figure.

  4. Describe the population growth of each species if you initially stocked 100 sunfish and 100 bluegill. Note: restrict your analysis to the first 40 years of the simulation. Plot the population growth of each species when in the presence of the other. Make sure all lines and axes are labeled properly, and provide a descriptive title for your figure. Did competition affect the growth of these populations? Explain!

  5. Using data from question 4, create a phase-plane diagram for these two populations (i.e., N1 vs N2). Make sure the axes, lines, and intercept points are properly labeled. Did the populations reach an equilibrium point? Is this a stable or unstable equilibrium? If it is an unstable equilibrium, what factor or factors could cause the balance to shift in favor of one species over the other? Note: In POPULUS a21 = α and a12 = β

  6. Manipulate the input parameters to get a stable equilibrium. Provide a phase-plane diagram and printout of your input data from program POPULUS to verify your success.

Scenario #2: Competition between 2 species of yeast (Gause 1934)

Use the data in the MSExcel file 'yeastdata_problemset.xls' (K:/WLF/448/2004/2004lab/Competition/) and POPULUS to address the following questions.  

1.  During the Inclass exercise you calculated K1 and K2 for the 2 yeast species, Saccharomyces cerevisiae and             Schizosaccharomyces kephir.  Use these values to calculate a12 and a21.

2.  Using the values from 'yeastdata_problemset.xls and the values you calculated, plot the population growth of each species when mixed with the other species using POPULUS.  Also plot the phase-plane diagram.  Is this a stable or unstable equilibrium?  Why?

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Revised: 23 October 2004