Lab #8
SIMULATION OF PREDATOR-PREY DYNAMICS
IN A COMPLEX ENVIRONMENT
In this laboratory you will use the simulation package BIOTA, part of BIOQUEST,
to evaluate the role of environmental complexity in promoting the persistence of predatorprey systems. The simulation is based on a classic experiment by C.B. Huffaker (1958),
published in Hilgardia (see attached photocopy from Begon et al. (1986) Ecology:
Individuals, Populations and Communities for a summary).
To begin, bring down the BIOQUEST pop-up, and select the BIOTA folder. Click
on the license agreement to continue to the logo. Select NEW SIMULATION. An
Untitled-1Map Window should now appear. With this Window open, then go back to the
Biota folder and open the Biota Tutorial folder. Then double click on Guide to Biota. Skip
to “Starting a New Simulation” (p. vii). With both the New Simulation and Guided Tour
windows open and visible, proceed through steps 1-19, reading the Guided Tour and
executing the instructions on the New Simulation.
When done with the GUIDED TOUR, go to the FILE pop-up, and QUIT. Again,
return to BIOQUEST, open the Biota folder and Biota 1.2. This time, at the logo, select
OPEN DOCUMENT. Choose (in order) the MacIntosh HD/Applications/Bioquest
Alias/Biota/Simulations/Huffaker’s Mites. You are first provided with field notes. Read
through them. Huffaker's primary objectives were to determine what combinations of A)
environmental complexity, and B) food availability would result in the greatest persistence
of the 2-species system.
Close the FIELD NOTES (click on the square at the upper left), and HUFFAKER'S
MITES MAP will appear. A map with 16 regions should appear. Answer the following
questions on the initial conditions of Huffaker’s Experiment (the same questions appear on a
separate page at the end of the lab, to be turned in when done).
Q1: How many units of orange are provided, and to what percent of the regions?
Q2: What growth model simulates the growth of the prey mites, what is the initial
density and what is the carrying capacity?
Q3: What growth model simulates the growth of the predator mites, what is the
initial density and what is the carrying capacity?
Q4: What percent of individuals of the predator and prey populations migrate, and
do they migrate to adjacent regions or show longer distance dispersal?
A. First manipulate the number of regions. Start with just 2 regions (1x2). Place oranges,
prey and predators in both regions. Before running the simulation, first go to the
GRAPHICS pop-up and select SIMULATION PLOTS. Click on SUM under the combined
data to obtain the sum of all regions in the summary window at the top. OK. Now go to the
Species pull-down, then Run Simulation. First change the number of cycles to 25. Now run
the simulation to its conclusion. Notice that the entries in the graphs are hard to quantify,
but that in all probability one or both mite species went extinct rather early during the
experiment. You can enlarge the summary graph by double clicking on it as a rough
approximation. The Y axis can be further changed using the Graphics pull-down to provide
greater visual sensitivity (setting an appropriate maximum Y value). Also change the colors
for oranges, prey and predators so they may be readily distinguished in the graph. Examine
the trajectories of orange units, prey and predator in the entire area in the summary graph at
the upper right. Enter the following summary data in Table 1: a) the numbers of prey and
predators present at time t=10 and b) the times till extinction of predator and prey. When
done, click on RESET, and run the simulation again to get a sense of the variability
introduced by the randomness of introductions of food and the randomness of migration.
Enter summary data as before, and also enter your data on the White Board to later compute
class means. Then click on the square at the upper left to close (DON'T SAVE, now or at
any time). Now change your environment to 16 (4x4) regions. Randomly populate 6 of the
16 regions with oranges, prey and predators (their degree of co-ocurrence will likely have a
big effect on the outcome). Repeat the data collection. Again run the simulation a second
time and write the data on the White Board.
B. As you can see, the Lotka-Volterra model, coupled with a simple two-species system,
make long-term survival a challenge. Now try manipulating the food supply by increasing
the probability of a region receiving food at any time (cycle) from 5% to 20%. This should
provide additional opportunities for prey to show rapid growth. Take data for just the 16region grid, running two trial simulations as before. Again record your results in the Table
and write the data on the White Board.
C. (optional 1 pt Extra Credit) Altering the rates of emigration between regions might be
expected to further alter the long-term persistence of predators and prey. On your own,
systematically manipulate the emigration patterns of one or both mite species, summarizing
the nature of the manipulation and describing its "success" in terms of promoting persistence
of the two mite species Table 1 as the numbers of prey and predators at t=10 and the times
till extinction.
Table 1. Summary Data for Huffaker’s Mite Experiment.
TREATMENT
5% food, 2 regions – Trial 1
5% food, 2 regions – Trial 2
5% food, 2 regions - Class mean
5% food, 16 regions – Trial 1
5% food, 16 regions – Trial 2
5% food, 16 regions (class mean)
20% food, 16 regions – Trial 1
20% food, 2 regions – Trial 2
20% food, 2 regions - Class mean
Prey
(t=10)
No.
Pred.
(t=10)
No.
Ext. time
(Prey)
Ext.
(Pred)
time
Name: ___________
(Download and add to this page by providing answers to the questions below: )
Q1: How many units of orange are provided, and to what percent of the regions?
Q2: What growth model simulates the growth of the prey mites, what is the initial
density and what is the carrying capacity?
Q3: What growth model simulates the growth of the predator mites, what is the
initial density and what is the carrying capacity?
Q4: What percent of individuals of the predator and prey populations migrate, and
do they migrate to adjacent regions or show longer distance dispersal?
Did increasing the complexity of the environment have an effect on the
persistence of the two mite populations? Explain, using summary class data drawn from
Table 1 in support of your answer.
Did increasing the incidence of oranges alter the densities of either mite species?
Did it affect the persistence of the two species? Again, include information from Table 1
in support of your statement.
(Optional Extra Credit) Describe any additional manipulations performed to
maximize the persistence of predator and prey, and summarize the “success” of the
manipulation in promoting long-term persistence of the two-species system.