Predator-Prey Cycles Part II
Predator-Prey Simulation
Go to http://www.ucopenaccess.org/course/view.php?id=13 . Scroll down to The Biosphere > Simulation
Lab > Run Lab.
OBJECTIVES
 Understand the inter-connected relationship between the growth and decline of predator and prey
populations.
 Understand the effects of growth rate, death rate and capture efficiency on predator-prey population
cycles.
LAB DESCRIPTION
In a closed system, the relationship between predator and prey can be described in a simplistic way by a set of
coupled equations. The equations model the facts is that predators consume prey and thus the population size
of one depends on the other. If prey is available the predator population can grow; however, as the predator
population grows, it consumes more prey, and as a result the prey population decreases, causing a decrease in
the predator population. A cycle develops with the two population sizes increasing and decreasing over time.
The maximum and minimum size of each population, as well as the time scale of the cycle over which the
populations increase and decrease, depend on various parameters such as the prey growth rate, the predator
death rate, and how difficult it is for the predator to capture the prey.
In this lab you will examine the cycle of growth and decline in the population sizes of prey and predator, and
determine the role different parameters play in this cycle.
The main goals are to understand the relationship between predator and prey population sizes and why a
change in a particular parameter can lead to changes in the population growth cycle of both the predator and
prey. Press the start button and watch the increase and decrease of the two populations in the graph. Reset the
simulation and try changing one of the parameters to see what happens to the curves. Try this for a few
different sets of parameters to get a general feel for how the predator and prey growth cycles are affected.
Assumptions:
 An endless food supply is available such that the prey can grow without bound (i.e. there is no carrying
capacity for the prey). The population size of the prey depends only on the predator capture coefficient
and the number of predators that are present.
 The death rate of the predators is instantaneous in the absence of prey. Thus if there were no prey the
predator population size would begin an immediate and steep decrease.
 When the predator or prey population decreases to less than one, the population is assumed extinct
(population=0) for the rest of the simulation run.
Initial Conditions:
 Seals are considered to be the prey and killer whales are the predators.
 The initial size of the prey (seals) population is set to 100 and there are initially 50 predators (killer
whales).
 The prey growth rate is 0.5 and the predator death rate is 0.6.
 The capture efficiency is set to 0.004
Input Variables:
Prey Initial Size: The total number of prey at the beginning of the simulation run.
Prey Growth Rate: The inherent capacity of the prey population size to increase (related to birthrate). The
larger the growth rate the faster the prey population will increase.
Predator Initial Size: The total number of predators at the beginning of the simulation run
Predator Death Rate: Instantaneous death rate of the predators when no prey are present. A larger death rate
means predators will die out quicker in absence of prey.
Capture Efficiency: This number represents the ability of the predator to capture the prey over some time
interval. A larger value for this parameter means that the predators have a better chance of capturing prey. In
contrast, a smaller value means the prey is better at eluding the predator.
Output Variables:
Time: Instantaneous time in arbitrary units of the simulation.
Prey Size: Number of prey at a given time.
Predator Size: Number of predators at a given time.
Units:
Predator and Prey Size: Numbers represent the sum of the individuals of each population at any given time
(i.e., no units).
Prey Growth Rate, Predator Death Rate, Capture Efficiency: 1/time
Graph formats:
Task:
How would increases or decreases in any of the variables (prey initial size, prey growth rate, predator initial
size, predator death rate, and capture efficiency) affect the rate of change of either the predator or prey
populations? How does the shape of the graph change?
Use the Snip-It tool or Screen Shots to illustrate each variable and then write a short summary of how the
variable impacted the prey population, the predator population, and how it changed the graph.