BL / ENVS 403
Fall 2008
Population Ecology Laboratory
Introduction: This is an on-line laboratory that will model different aspects of populations and
then predict outcomes of the population size based on the input parameters. You will be looking
at three species in this lab: brown sparrows, blue sparrows, and hawks. You will then
investigate what various changes in the life history, competition, predation, and habitat mean for
the population size. You should keep notes on this paper, or a separate paper, but then type up
your answers to each question and turn them in next week in laboratory. You may work with a
partner, and you may turn in one set of answers for the two of you, but do be sure you understand
all the concepts for the laboratory final.
To begin: When online, go to www.biologylabsonline
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).
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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.
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.
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?
The carrying capacity will be approximately 1000 birds
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Repeat this same experiment at least three or four times to determine if the results from this
experiment are consistent.
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.
Prove or disprove 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? .
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?
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?
Effect of Life Span on Minimum Viable Clutch Size: 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.
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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.
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?.
Both carrying capacities will equal approximately 1000 birds.
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.
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.
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.
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.
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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?
As sparrow flight speed increases, eventually hawks become extinct because they are not
successful enough at capturing prey.
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?
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?
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.
Increasing the seed density (carrying capacity) of the brown sparrow causes predator-prey
oscillations of increasingly larger amplitude which eventually results in extinction of the predator
and often the prey.
Assignment 4: 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, this 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.
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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: Extinction Risk
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 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.
Write down your hypothesis (i.e. clutch size and life span in relation to extinction risk) and
briefly describe the data that supported your hypothesis.
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