Ecology: Predator Prey Interactions
Competition between predators: “Survival of the Sweetest”
Thomas Sobat, PIE Fellow, Biology Department, Ball State University, Muncie Indiana 47306
STANDARDS ADDRESSED:
6.4.8 Explain that in all environments, such as freshwater, marine, forest, desert,
grassland, mountain, and others, organisms with similar needs may compete with one
another for resources, including food, space, water, air, and shelter. Note that in any
environment, the growth and survival of organisms depend on the physical conditions.
6.4.9 Recognize and explain that two types of organisms may interact in a competitive or
cooperative relationship, such as producer/consumer, predator/prey, or parasite/host.
OBJECTIVES
Students will be able to identify the interactions that exist between predators and prey.
Students will be able to identify and define the many ways in which prey avoid capture,
including mimicry, camouflage, speed, coloring, size, etc.
Students will be able to define competition and the competitive exclusion principle.
Students will be able to quantify predation success given predator and prey interactions.
Students will be able to observe and make predictions about predator/prey interactions.
MATERIALS
Life Savers (five or six colors: green, brown, red, orange and white; 2 of each
color/student; candy may be substituted by a similar reusable item)
Test tube holders (1 per student)
Forceps (1 per student)
Plastic Spoons (1 per student)
Plastic drinking straws (1 per student)
Plastic forks (1 per student)
Plastic lunch bag (1 per student)
INTRODUCTION
Populations of animals interact with each other and their environment in a
variety of ways. One of the primary interactions a population has with its
environment and other populations is linked to its feeding behavior.
Populations of animals that feed on other organisms are called predators.
The populations on which predators feed are called prey; often, predator and
prey populations cycle in complex interactions. When prey resources are
abundant, predator numbers increase until the prey resources wane. When prey
numbers drop, predator numbers dwindle as well. If the environment provides
adequate refuge and resources for prey, their numbers may again increase and
the cycle begins again.
The concept of competitive exclusion suggests that two species that
require identical resources cannot coexist in the same location. The reasoning
behind this concept is that one of those two species will be better adapted to that
environment and be more successful to the point of excluding the lesser-adapted
species from the environment. Yet it has been found that many species with
similar requirements do coexist. Because the environment is varied, competing
species can use resources in different ways when competition is intense,
allowing space for one another.
When two interacting species, for example predator and prey, evolve
together, they can influence the evolution of the other. This is referred to as
coevolution. Sometimes coevolution results in two species that influence
(positively or negatively) each other in a relationship referred to as symbiosis.
The various types of symbiosis include:



Parasitism - one species (parasite) benefits more than the other species
(host)
Commensalism - one species benefits while a second species is neither
helped nor injured
Mutualism - both species benefit from the interaction
The predator-prey relationship is important in maintaining balance among
different animal species. Adaptations that are beneficial to prey ensure that the
species will survive. The ways animals avoid falling prey to a predator include
the use of camouflage or protective coloration. One form, cryptic coloration,
allows an animal to blend in with its environment to avoid being detected. A few
animals are poisonous or unpleasant-tasting, and predators soon learn to leave
such animals alone. These poisonous animals are often brightly colored
(aposomatic), as well, which acts as a warning to predators. A few animals rely
on trickery and copy the defenses of other animals to protect themselves; these
are referred to as mimics.
During this lesson, students will participate in an outdoor lab activity that will
simulate competition for resources between predators. This exercise will
demonstrate to students that predator populations are regulated through
community level competition. Students will be divided into four groups, and each
group will be assigned a different utensil (plastic spoon, plastic straw, forceps or
test tube holder). Utensils will represent an unequal ability to gather resources.
Resources in the form of individually wrapped hard candies (Life Savers) of
four colors will be distributed within the boundaries of the outdoor lab in limited
numbers. Students will be instructed to collect as many candies as possible
using only their utensils (no hands) within a given time period. Although it will not
be encouraged, taking food from other students will be acceptable and can be
explained as parasitism. Following the given time period students will record
their catch, and calculate its value (colors will be assigned differing values).
Unbeknownst to the students, survival of individuals will depend upon resources
collected, and populations will be limited by the reduction of students due to
insufficient resources. Following the population reduction a second run can be
attempted if time permits.
PROCEDURE
1.) Distribute two lifesaver of each color per student, two cryptic (green), four
neutral (brown and white), and two aposomatic (red) in the outdoor lab
area.
2.) Students should be equally divided and assigned to one of four groups
delineated by the utensils supplied, and be given a plastic lunch bag (give
students an opportunity to name their group). Instruction should be
limited to the idea that they are predators, and information regarding
location of the prey items (outdoor lab). Students can only use their
utensil to capture prey (no hands). Once prey has been captured
students must transfer prey to their lunch bags using their utensil. Allow
prey collection for a period of four minutes, and then have students return
to a central location. Have each group tally their prey by color and record
the catch on a data sheet (one per group).
3.) Inform students that prey items are not worth the same number of points,
and have them calculate the value of their catch using the prey value key
located on the data sheet. Scores will be used to establish population
sizes for round two of predation. Each group will need 5 points per
individual, and groups with excess points will incorporate those students
excluded from other groups. At this point it would be appropriate to give
students time to discuss the outcome of their efforts, and establish
strategies for the next run of the activity. If students have been excluded
from a group, and there is no space for them in another group, start a new
group (use plastic forks).
4.) Have a discussion with the students that leads to a general understanding
of what has taken place; be sure to include the idea of competitive
exclusion, cryptic and aposomatic species. If students are stealing prey
from one another you will need to discuss parasitism (cleptoparasitism in
this case). Students should be encouraged to discuss instances of
predator prey interactions they may have observed, personally, in text or
on television. Have the students predict the outcome of another bout of
predation and develop a testable hypothesis (one per group).
5.) Redistribute prey items in the outdoor lab, and allow students to collect for
an additional four minutes applying their new knowledge to a strategy that
might best suit their population. Follow instructions outlined in steps 1
and 2.
6.) Have students discuss the outcome of their efforts, be sure to tie up loose
ends (discuss parasitism if it was observed in run 2, but not in 1). Have
them predict what might happen if you continued with another run, or if
you added another predator group.
ASSESMENT
Students can be assessed using traditional means (quiz) or students can be held
responsible for a written project outlining predictions and results.
EXTENSIONS
Have students enter their data into a spreadsheet and graph the results of their
predation events. Add additional colors to the different categories of prey
(cryptic, neutral, and aposomatic). Have one of the groups receive positive
points for collecting aposomatic prey.
Glossary
Aposomatic - Conspicuous coloring of poisonous or distasteful organisms that enables
potential predators to easily see and recognize it.
Coevolution - Evolution involving a series of reciprocal changes in two or more
noninterbreeding populations that have a close ecological relationship and act as agents
of natural selection for each other, as the adaptations of a predator for pursuing and of its
prey for fleeing.
Community - An assemblage of interacting plant and animal populations occupying a
given area.
Commensalism - (of an animal, plant, fungus, etc.) living with, on, or in another,
without injury to either.
Competition - The struggle among organisms, both of the same and of different species,
for food, space, and other vital requirements.
Cryptic - Fitted for concealing; serving to camouflage
Mimics - The close resemblance of an organism to a different organism, such that it
benefits from the mistaken identity, as in seeming to be unpalatable.
Mutualism - A relationship between two species of organisms in which both benefit from
the association.
Parasite - An organism that lives on or within a plant or animal of another species, from
which it obtains nutrients.
Population - All the individuals of one species in a given area
Predator - An animal that hunts and seizes other animals for food.
Prey - An animal hunted or seized for food
Symbiosis - The living together of two dissimilar organisms, as in mutualism,
commensalism, or parasitism.
Predator Prey Data Sheet
Group
name:
color
Student
Date:
color
color
score
score
color
score
color
score
Prey Item Values Aposomatic colors -2; Cryptic colors 2; Neutral colors 1.
score