Assignments

For your ease in completing the following assignments, the background text relevant to the experiment that you will perform is in orange text, instructions for each assignment are indicated by plain text, and questions or activities that you will be asked to provide answers for are indicated by bold text.

Assignment 1: Getting to Know PopEcoLab:Single Species Population Growth
This assignment is designed to help you become familiar with the operation of PopEcoLab by studying the population growth of brown sparrows. The first screen that appears in PopEcoLab shows you an input parameter page with a table listing the default parameters for the laboratory conditions that you can manipulate when setting up your experiments.

Before you can set up any experiment in PopEcoLab, you must be familiar with the input parameters that you can manipulate. You will study the effects of these different parameters in future assignments. A brief description of each input parameter is provided below. Refer back to this section as needed when you are working on different assignments.

Click on the Change Inputs button to see all the parameters you can manipulate for this lab. A new page will open with buttons for each of the input parameters located at the left side of each page (initial population size will be open as the first input parameter). Click on each input parameter and read the descriptions below. Change each parameter so that you can become familiar with how each input parameter operates.
Initial Population— the initial population for brown sparrows is set at 200 birds while the default value for blue sparrows and hawks is zero. Population size of each species can be manipulated by clicking and dragging the slider bar.
Clutch Size—clutch size is the number of eggs that a female bird lays in her nest. The default value for brown sparrows and blue sparrows is three eggs while the default clutch size for hawks is two eggs. Controlling clutch size is one way to influence the rate of reproduction for all three species of birds.
Life Span—brown and blue sparrows have a shorter life span than hawks. Notice that the default life span for sparrows is one year while the life span for hawks is three years. This feature allows you to manipulate mortality rates (independent of predation) to influence life span.
Flight Speed—the ability of sparrows to avoid predation by hawks depends on their flight speed. The default value for both brown and blue sparrows is 6 meters per second (m/sec).
Competition—although brown sparrows primarily eat seeds and blue sparrows primarily eat insects, the two sparrows are potential competitors because brown sparrows can also eat insects and blue sparrows can also eat seeds. The relative rates of consumption of these "alternative resources" can be varied independently.
Resource Densities—seed density and insect density directly influence the carrying capacity of brown and blue sparrows, respectively. The default values are 100 seeds per meter-squared (m2) and 100 insects/m2.
After you have finished this introduction to the input parameters, click the Reset button at the left of the screen to return all input parameters to their default values.
  1. Population Growth of Brown Sparrows:Now that you are familiar with the basic parameters in PopEcoLab, set up the following experiment to help you understand some of the factors that affect population growth of a single species.
    1. Leaving all input parameters at their default values, run a simulation of population growth by clicking the Run Experiment button. Note: The number of years to run each experiment can be manipulated in 100-year increments from 100 years to 500 years by using the popup menu in the lower left corner of the input parameter page. The default value is 100 years.

      When the experiment has finished running, a separate page will appear that presents the results of your experiment. Note: Any of the following data views can be saved to disk or printed by clicking the Export button. Clicking on this button will open a separate window with your plot or table. From this window you can then save your data to your hard drive or a disk, and you can print your data by using the print feature of your browser software.

      The following data can be examined:

      Population Size—represented as a plot of the number of birds in the population versus years of the experiment.

      Phase Space—a plot of the number of one species (e.g., blue sparrows) against another (e.g., brown sparrows) as population numbers change over time. This type of plot is valuable for examining relationships between species. You can select the species for each axis.

      Textual Data—text columns of raw data for population size of brown sparrows, blue sparrows, and hawks.

      Input Summary—a summary table of the input parameters for the experiment that you carried out.

      Examine the population size plot. What is your estimate of the carrying capacity for the population of brown sparrows?

      Repeat this same experiment at least three or four times to determine if the results from this experiment are consistent.

    2. Effects of Changes in Mean Clutch Size
      Formulate a hypothesis to predict the effects of a decrease in clutch size on the population number of the brown sparrow.

      Test this hypothesis by carrying out three different experiments with mean clutch size set to different values such as 0, 0.5, 1.0, 1.5, and so on, up to the maximum of 10 eggs, keeping all other parameters at their default values. Repeat each experiment several times, study the plots of population size, and then answer the following questions.

      What did you discover? Did the results agree with your hypothesis? Why or why not? Is there a threshold clutch size needed to keep the population from going extinct? What happens to the variability in population numbers as clutch size gets smaller? If you were a conservation biologist, what would you say about your ability to predict population numbers when clutch size decreases?

      Formulate a hypothesis to predict the effects of an increase in clutch size on population number, then design and carry out experiments to test your hypothesis.

      What happens to population size as clutch size gets larger? What happens to populations with very large clutch sizes? What do you think is causing the pattern that you see?

    3. Mortality Rates: Investigate the effects of increasing mortality rates by decreasing life span and keeping all other parameters at their default values. Examine the plots of population size.

      Is there a threshold life span needed to keep the population from going extinct? Explain this result.

      Investigate the effects of decreasing mortality rates by increasing life span.

      What happens to the variability in population numbers over time as life spans get longer? If you were a conservation biologist, what would you say about your ability to predict population numbers as life span changes?

    4. Influence of Resource Density:Investigate the effects of decreasing and increasing seed density by running experiments with a decreased resource density for seeds and separate experiments with an increased resource density for seeds. For both experiments, keep all other parameters at their default values. Examine the plots of population size.

      How does changing the amount of available resources affect population size? Why is this so?

  2. Effect of Life Span on Minimum Viable Clutch Size (for use by advanced students): Formulate a hypothesis to explain how changes in life span might affect the threshold clutch size needed to keep a population of brown sparrows from going extinct, then design experiments to test your hypothesis.

    What did you discover? Did the results of your experiment support or refute your hypothesis? Explain your results.

Assignment 2: Two Competing Species
As you learned in the background text for PopEcoLab, interactions between populations of two more different species (interspecific interactions) can affect population size and population density in a number of ways that can benefit or harm one or several species. These assignments are designed to help you learn about interactions that result in competition by following the population growth of both the brown and blue sparrow.

  1. Set the initial number of hawks to zero. Set the initial population numbers for both the brown and blue sparrow equal to 200 birds. Set the relative insect consumption by the brown sparrow equal to zero, and set the relative seed consumption by the blue sparrow equal to zero. Keep the remaining parameters at their default values. Run several simulations and get an estimate of the carrying capacity of each species. Note: You can run ten independent simulations by going to the Initial Population view and clicking the Multiple Run Mode button. With this mode, set the number of runs to 10, then run the experiment.

    What did you observe for the carrying capacity of each species?.

    1. Go to the Competition view and use the slider to increase the relative insect consumption by the brown sparrow, then run this experiment.

      What happened to the density of the two sparrows? Is there a point at which one of the two species goes extinct?

      Now reverse the situation and keep the relative insect consumption by the brown sparrow equal to zero while increasing the relative seed consumption by blue sparrows. Before you run this experiment, formulate a hypothesis to predict the results of this experiment.

      What happened to the density of the two sparrows? Is that what you expected? Explain your results.

    2. Keep the two consumption rates of resources equal, but gradually increase both. Make sure you run several simulations for each set of consumption rates.

      What happens as you increase the amount of competition between the two species of sparrows? Explain these results.

    3. Set both of the relative consumption rates back to their default value of 0.50. Design experiments to investigate the effects of resource densities on species competition, then run these experiments multiple times. Summarize your results.

  2. Advanced assignment: An alternative way to view the type of ecological data that we have studied for these competition experiments is to plot the number of one species against the other species as their numbers change through time. This is called a "phase space" plot and the line that connects the data points is called a "trajectory" in phase space. If one or the other species goes extinct, the trajectory moves toward the x axis or y axis. If both species coexist, the trajectory moves toward a point on the interior of the phase plot.
    Try different combinations of competition between the two sparrow species. Set the number of generations to 500. On the Initial Population screen, click the Multiple Run Mode button and set the number of runs to 10. In the text fields, enter different combinations of initial numbers so that both species have low numbers, both have high numbers, the brown sparrow has low numbers while the blue sparrow has high numbers, and vice-versa. View results in the Phase Space view. On the phase space plot, set the Run Number to "All" to view all the trajectories together. Be sure to try examples where relative consumption rates are low, one is low and the other is high, and both are high.

    Are there any situations where the initial sparrow numbers affect the outcome of competition? Explain why this may or may not be true.

Assignment 3: Predator and a Prey
Predation is a fact of life. Predators develop abilities to locate and capture prey, and prey can evolve to develop mechanisms designed to minimize predation. However, if predators are more effective at capturing prey than prey species are at avoiding predation, then the population of prey species will be affected. Conversely, for some species, too little predation can also affect population numbers in a negative way if the prey species population exceeds its carrying capacity.

  1. Evading Predators - Set the initial number of blue sparrows to zero. We will focus on the population growth of the brown sparrow and the hawk. Set the initial population number for the brown sparrow equal to 200 birds and for the hawk equal to 20 birds. Design experiments to test the effects of increasing flight speed of the brown sparrow. Try several different flight speeds.

    What happens? How does increasing flight speed affect the population numbers of sparrows and hawks?

    Design experiments to test the effects of decreasing flight speed of the brown sparrow.

    What happens to the population numbers of the sparrow and hawk?

  2. The Effect of Clutch Size on Predator-Prey Dynamics - Design an experiment to test the effects of changing sparrow clutch size on the predator-prey dynamics.

    What happens to the population numbers of the sparrow and hawk as sparrow clutch size increases? What happens when sparrow clutch size decreases?

    Design experiments to test the effects of changing hawk clutch size on the predator-prey dynamics.

    What happens to the population numbers of the sparrow and hawk as hawk clutch size increases? What happens when hawk clutch size decreases?

  3. Paradox of Enrichment - Rosenweig (1972) identified a phenomenon he called the "paradox of enrichment." He discovered that increasing the carrying capacity of a species‹for example, by adding nutrients to a pond‹can destabilize the system when the species is at risk of predation. Design experiments to test this idea.

    What did you discover? Explain your answers.

Assignment 4: Advanced Assignments: Two Competing Prey and a Predator
Ecologists have identified a phenomenon called "predator-mediated coexistence" (Caswell 1978), where a predator feeds on a dominant competitor, which allows the weaker competing species to coexist.

Design experiments where the brown sparrow always outcompetes the blue sparrow in the absence of any hawks, but by adding hawks that feed more heavily on the brown sparrow, all three species coexist.

Were you able to achieve these conditions? Explain your results.

Some predators are "specialists," which survive by focussing on one or a few abundant prey species. Others are "generalists," which need to eat a variety of less common prey to survive.

Design an experiment where a predator is unable to survive with only one prey source, but can persist when two prey species are available.

Were you able to achieve these conditions? Explain your results.

Ecological systems are complex. Sometimes one species can indirectly affect another through its effect on a third species.

Design an experiment where hawks eat only brown sparrows (i.e., blue sparrows have the maximum flight speed needed to avoid hawks) and hawks coexist with brown sparrows when blue sparrows are absent. However, when blue sparrows are present, their competition with brown sparrows is enough to drive hawks to extinction.

What did you discover? What parameters enabled you to create this situation?

Assignment 5: Group Assignment
The experiments that you have conducted so far were designed to help you learn about the many factors and interactions that can influence the size and growth of a population. Since ecosystems are affected by random events such as changes in weather, applied ecologists must often make an assessment of "extinction risk." They do this by constructing simulation models similar to PopEcoLab and running multiple simulations. This group assignment is designed to have you use PopEcoLab to evaluate extinction risk by running multiple simulations (e.g., 100 simulations) and noting the frequency with which a population becomes extinct.

Design experiments to study how this risk of extinction changes for a combination of parameters such as clutch size and life span.

Examine how the extinction risk varies for different time windows, for example, 100 years, 200 years, and so on. Be systematic in your experimental design. Devise a plan to divide your investigation among different members of your group and run multiple simulations for each experiment. Tip: If you go to the Initial Population screen and click on the Multiple Run Mode button, you can set the number of runs to 10. This will give you ten independent simulations that you can examine by using the spinners on the output screen.

Did the results of your experiments support your hypothesis? If not, reformulate a new hypothesis and carry out additional experiments. Explain how the experimental conditions you set up were responsible for your results. Once you think you have an explanation for your findings, design and carry out additional experiments to support or refute your thesis.

Discuss the results of your experiment with your instructor to clarify questions that you may have about interpreting the results of your experiment and understanding the effects of the experimental conditions you created.