Limiting Factors
Objective:
 Analyze how population size is determined by births, deaths, immigration, emigration, and limiting factors (biotic and abiotic) that
determine carrying capacity. AA
Purpose of the Lab/Activity:
 Identify and describe the essential components of habitat.
 Describe the importance of good habitat for animals.
 Define “limiting factors.”
 Recognize that some fluctuations in wildlife populations are natural as ecological systems undergo constant change.
 Graphing the data collected to show how population size differs over time and with changes in the environment.
Prerequisites:
 An understanding of the components of an ecosystem, organism’s role and interactions that occur between them.
 The difference between abiotic and biotic factors.
 The resources necessary for an organism to survive.
Problem Statement: How will resource availability affect the population of a species in an ecosystem?
Background:
A variety of factors affects the ability of wildlife to successfully reproduce and to maintain their populations over time. Disease/parasitism,
competition, predator/prey relationships, varying impacts of weather conditions from season to season (e.g., early freezing, heavy snows,
flooding, and drought), accidents, environmental pollution, and habitat destruction and degradation are among these factors.
Some limiting factors depend on how many organisms live nearby (Density-dependent). For example, disease/parasites spreads faster in
heavily populated or through migration areas.
It is also easier for predators to spot animals in large numbers. Note that herding also decreases prey chances of being the one preyed upon.
Typically, if you are too large to hide well and/or too slow to outrun a predator, there are advantages to living in a group. It offers protection by
numbers through greater strength in defense and lower probability of being the one caught. Predation is certainly density dependent. The
number of prey available limits predation and the number of predators limit prey (see predator-prey relationships below and the Hare Lynx
Population Ecology Lab).
Competition also depends on numbers. Survival within an ecosystem depends on access to nutrients and resources. A nutrient is a chemical
substance that an organism consumes and needs to survive. A resource is something that facilitates survival, but may or may not be required.
The most fundamental of life’s nutrients are food and water. Without these essential components, animals cannot survive. As far as resources,
shelter/protection and mate-availability are perhaps the most important for long-term survival and reproduction. Recall from the concept of
niche that each organism has a unique role and place within an ecosystem. Since two organisms cannot fulfill take the exact same place,
competition also exists in that sense. Considering that long-term survival of any species involves reproduction, an important point is that
organisms also compete for mating opportunities (sometimes to lethal extents). Also, note that competition exists between and within species.
Competition becomes worse if more organisms are present because there comes a point when there are not enough
nutrients/resources/mates/etc to sustain/satisfy all.
Note that nutrient and resources such as mates, shelter, habitat, food, and water are often also referred to as “limiting factors”. But if there is
plenty of food for everyone, it is not necessarily a limiting factor (though animals will sometimes still compete for the best there is). Be careful
of how you think of it. For example, food is not what limits population growth. It is the lack of food or competition for it. Therefore,
resources/nutrients are not actually limiting factors. The competition for these things is what limits population growth.
Pollution, habitat destruction, natural disasters also kill animals. However, this does not directly depend on numbers. Such factors are densityindependent. A volcanic eruption will destroy most life in an area. It does not matter how many were living in it. Naturally, such factors often
reduce resources, which end up increasing competition between the organisms that remain. They also affect the numbers upon which disease
spreading and predation and depend on. Therefore, in a way, density-independent factors often indirectly cause worsening of density-dependent
factors.
Wildlife populations are not static. They continuously fluctuate in response to a variety of stimulating and limiting factors. Natural limiting
factors tend to maintain populations of species at levels within predictable ranges. This kind of “balance in nature” is not static, but is more like
a teeter-totter than a balance. For example, if there is plenty of prey, the number of predators goes up because they have plenty to eat. However,
this leads to too many predators hunting prey. This makes the number of prey to go down. Competition for prey increases and excess predators
starve and die off. This means prey are hunted less often and their numbers grow. The cycle restarts (predator-prey relationship).
Without limiting factors populations would grow exponentially. (2-4-8-16-32-48). However, naturally-caused and culturally-induced limiting
factors prevent wildlife populations from reproducing in numbers greater than their habitat can support (maximum number = carrying
capacity). This is called logistic growth. An excess of such limiting factors, however, leads to threatening, endangering, and eliminating whole
species of animals.
Vocabulary: reproduction, predator, prey, degradation, limiting factor, habitat, species, population, resource, carrying capacity, nutrient,
disease, parasitism, competition, natural disaster, niche, exponential growth, logistic growth
Materials (per group): Open space / Note cards of 3 different colors
Pre-Lab:
1. Identify some of the basic needs of animals.
2. What is the difference between a nutrient and a resource? Give an example of each
3. Contrast density-dependent factors from density-independent factors
4. Define vocabulary items using your own words
5. Read over all the lab’s procedures
6. What do you think will be the difference between games 1 & 2 in Part A? Why?
7. What do you think will happen to deer population over time in Part B? Why?
8. In which game do you think predators will fare better in part C? Why?
9. In which game do you think disease will spread faster in Part D? Why?
Procedures:
Part Z (Yes, who says A always has to come first?): A Model of Exponential Growth vs. Logistic Growth (and some competition)
Note: This method is setup to work with classrooms larger than 20. But can be modified to account for smaller classrooms.
Basic Rules: Place several colorful note cards around a large outdoor area. There should twice many cards as students and the same number of
cards of each color. Tell students to hunt for notecards (carefully) as soon as you say go. All games start with 1 student. Student has up to 10
seconds to collect enough points to survive. If student survives, he gets another student (reproduction) and together they repeat the experiment.
Any student without enough points does not survive and cannot multiply.
Game 1: Every card is worth a million points! There is plenty of resources for everyone! Students should write down number of living students
at the end of each trial until they run out of students to continue the experiment.
Game 2 = Limited resources. Certain colors are worth more than others, but teacher will not let student know until after each trial is over. The
values of each color change with each round. Points should be setup so that:
High Card = 10 points
Medium Card = 5 points
Low Card = 3 points
Students need to gather 15 points to survive. Students should write down number alive after each trial. Allow the experiment to continue a
few trials past the point when the carrying capacity is reached. Population seems to stop growing. If it never happens, restart experiment
with less cards.
Part A: Predator-Prey Relationship Graph & Model of Competition and Resource Availability
1. Obtain a number (1 through 4) from your teachers (Deer = 1 & Resources = 2, 3, 4)
2. Go outside. Deer will all stand on one side and all the resources will stand on the opposite side. Everything in between is considered
“habitat”
3. Students 2 – 4 will decide what resource they will be and all the deer will decide what resource they are looking for. Facing away from
each other, deer and resource groups will pick a one color card to signify the resource they will be or hunt for. Each student makes their
own choice. After prey/resource makes a choice, it should spread out throughout the habitat. Students must keep card visible during the
hunt part of the experiment and cannot change colors during it.
Blue / White = Water
Green / Red = Food
Yellow / Other = Shelter
4. When teacher says “GO,” the deer turn around and face the habitat. Resources should hold their card up so deer can see what they are.
When deer sees a student in the habitat holding what they need (same color as they are holding), they should quickly, but calmly, to get
that student and take them back to the deer side. This represents the deer successfully meeting its needs and reproducing. Those deer who
do not meet their needs remain in the environment to provide habitat for the other deer in the next round.
Note: Students can only hold one card at a time and should not arrange which color to pick amongst each other. Any student caught
cheating will be left out of the activity and will receive punishment. In science, it is important that not to tamper with the data.
5. Record the number of deer at the end of each round for graphing later.
6. Predict what will happen in the next round.
7. Repeat steps 3 – 8, fourteen more times for 15 trials total
8. Before starting, students should guess what will happen to deer population after the first round and mark initial population on year 0.
Part B: Human Impact: Habitat Destruction Model
Teacher will reset count to original numbers (3 resources for every deer). Then he/she will wait as usual for all pray to spread out around
the habitat before spreading human-made disasters! Then the deer is released to hunt as usual.
Possibilities (to be done in random order so students never know which one comes next):
 Humans construct large damn or divert water flow to agriculture. Their cities also affect local atmospheric patterns leading to
increasing drought. Whatever water is left is used to sustain their numbers. Ecosystem water levels go down. All water cards are
out of the hunt!

Humans introduce pesticide or facilitate a plague to move from one place to another (global warming causes plague to move from
usual areas or pesticide use makes them avoid natural areas) resulting in a catastrophe that kill most of vegetation. All food cards
are gone!
 Humans need logs for construction or heating purposes, new land for agriculture or cities, or want to mine in the area. Between
pollution and necessary deforestation, all animal shelters are gone. All shelter colored cards are gone from the hunt!
 Humans hunt animals, destroy habitat, and consume water in a vast area. All notecards within a designated area are out!
After each trial students should write down what happened and the number of surviving deer and starting number of deer should be reset.
Part C: A Predator vs. Alien Model of the Effects of Population Density on Predation
1. Teacher will divide class so there are 10 times more predators than aliens. Predators can run. Aliens can only walk. Predators start by
looking away from aliens. All captured aliens must sit on the floor until the game is over. When teacher says go, predators must turn and
hunt aliens according to the rules of each game:
Game 1: All aliens must gather together in a herd by holding hands. Each holds hands with as many as 5 different people (putting hands
over each other). This connection is their protection. Predators must touch deer to hunt them, but a “capture” touch is only possible when
all connections between aliens are broken. Aliens cannot resist when predators attempt to break a connection (This is not about fighting or
a battle of strength. Any predator/alien resisting or pushing will be quickly and summarily deported from the game). Aliens can rebuild
connections if the predators are not holding their hands. Each predator’s hand can only hold one alien hand at time. Therefore, to capture
aliens, predators must work together to remove connections between aliens and the rest of the herd. Each game will last 1-2 minutes and
will be repeated three times to calculate average number of predations per game.
Game 2: Aliens avoid each other as much as possible so that they do not draw predators’ attention all to one spot. Otherwise, same rules
apply. All the predators have to do to capture is chase and touch (Remember that prey can only walk and that aliens can run). Therefore,
before killing aliens, predators must work together to remove connections between aliens. Predators will hunt for the same amount of time
as the first game and will have the same number of trials to calculate average number of predations per game.
Part D: A Model of Effects of Population Density on Epidemic Rate
1.
2.
3.
4.
5.
6.
All students join the habitat as a population.
One student is “infected” with a disease/parasite. Students are confined to small area (a subsection of habitat). Students tag other students
who then become infected and can tag others, and so forth. This should be allowed to continue for a 10 seconds.
Count number of “infected” and “healthy” students at the end (Students must be honest and admit if they were tagged). All infected
students died. However, healthy students can reproduce and double their numbers (if possible). Therefore, after each trial, many will die,
but then some will come back as the offspring of the surviving ones.
Continue doing this for at least 5 generations
Repeat the same experiment. However, in Game 2, have fewer students participate and/or spread them out as much as possible in the trials.
Write all data on results table
Alternative: A cool variation of this is to do it in the classroom with a glow-in-the dark powder. Place powder in one student’s hand (no one
knows who). Have students each stand in a circle that they cannot leave (can be done with hula-loops). Place circles as close to each other as
possible. Have students shake hands with each other randomly for 30 seconds without ever leaving their circle. Then shut down the light and
turn on a black light to show a clear “infection” spread. Repeat with less students participating, wider circles (several hula-loops), for same
amount of time to show density effects.
Observation/Data:
Trial
Start
1
2
3
4
5
6
7
A Model of Exponential Growth vs. Logistic Growth (and some competition)
# Living After Trial Completed
Game 1
Game 2
1
1
8
9
10
11
12
Year (round)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Predator-Prey Relationship Graph & Model of Competition and Resource Availability
Deer Population (#)
Prediction for Next Round
Human Impact: Habitat Destruction Model
Starting
# of
Deer
Event Description
Deer After Human Impact
A Predator vs. Alien Model of the Effects of Population Density on Predation
# of Captured Aliens
Game
Trial 1
Trial 2
Trial 3
Average
1
2
A Model of Effects of Population Density on Epidemic Rate
# Alive to Reproduce
Generations
Start
1
2
3
4
5
High Density Trial
Low Density Trial
Data Analysis:
Graph the results from your data table for trials of part B. Show the rise and fall of the population. Create a line graph using the data from the
table above; make sure to label your axis(s) and include the units of measurements, as well as a title.
Graph the results from your data table for trials of part A. Show the rise and fall of the deer population and resource availability (one line for
each). Create a line graph using the data from the table above; make sure to label your axis(s) and include the units of measurements, as well as
a title.
Create a bar graph with 5 bars showing what happened to the deer population before / after human impact events of Part B (1 bar for before
count, which is the same for all and 1 for each of the after counts)
Create a bar graph to compare averages of herding vs. not herding in part C
Graph the results from your data table for trials of part D. Show what happened to both conditions (1 line for each). Create a line graph using
the data from the table above; make sure to label your axis(s) and include the units of measurements, as well as a title.
Discussion Questions:
1. List the basic needs of animals.
2. Describe the relationship between resource availability and population growth or decline.
3. Define “limiting factors” and provide three examples of density-dependent and 1 example of density-independent factors.
4. Explain which type of “limiting factor” applied to each part of the lab
5. Can limiting factors change over time? Explain.
6. Explain the difference in results between game 1 & 2 of Part A
7. In part B, once the deer population goes significantly above carrying capacity, describe what happens to the deer population in the years
following.
8. How did you determine the carrying capacity in Part B?
9. Explain differences in game 1 & 2 in Part C
10. How would part C be different if aliens could use their fast ships or invisibility cloaks when alone?
11. Why do prey animals prefer herding to fending for themselves?
12. Why do other prey animals prefer more secluded lives and spread out?
13. Give 3 benefits and 2 disadvantages of herding behaviors
14. How do you expect predators that hunt herding animals to be different from those who hunt secluded animals?
15. In Part D, when did disease spread faster? How do you know? Why?
16. Discuss how your pre-lab predictions fared against the results
17. Describe how the activity modeled the significance of limited resources in regards to a species’ population.
18. List some possible sources of errors and how they could affect the lab results.
19. Make suggestions for improving the lab to make the simulation more accurate and effective.
20. Suggest a way to make this kind of study in real environments.
21. Ecologists have found that no organism can experience indefinite exponential growth, yet Homo Sapiens have experienced exponential
growth for hundreds of years. How have humans modified the limiting factors of our population growth? Will this work forever?
Extension:
 Online Activity 35.2: Population Growth of Two Different Species www.biology.com Students plot exponential growth of bacteria as
they view a video. Then they plot and analyze data showing changes in a hypothetical population of grizzly bears. Then they
compare exponential growth to population limited by environmental factors.
 Gizmo: Rabbit Population by Season