The following exercise represents a modification of Miller's exercise that I obtained from a D.C. Heath & Co. advertisement promoting Biology:Discovering Life.. The important difference is that the original used a complete cookbook approach and did not consider the situation where a prey population grows in the absence of a predator. This seemed an important observation for students to make first, prior to predicting what will happen in the presence of a predator. So my modification was simply to put the observational horse in front of the terminological cart, or, in other words, a reorganization of the exercise to reflect the process of science and allow students to do some thinking. Rather than expounding on terminology prior to the exercise, introduce the necessary terminology afterwards, when they have active concepts of population dynamics. This reorganization represents the instructional change being promoted by BIOLAB. What is taught remains the same, but how it is taught changed.
Dept. of Biological Sciences, Illinois State University, Normal IL 61761, jearmstr@ilstu.edu
Shallow, flat-bottomed (4-6" min bottom diam) bowls Navy or Pea beans, or 1/4" beads Soup Spoons
Since the original exercise was distributed in an advertisement, this modification may be freely copied and disseminated for all non-commercial educational activities provided that appropriate credit is given to the author, this source (BIOLAB) and its NSF support. Its use is explictly permitted in laboratory manuals and compiled class handouts sold at or below the cost of printing or duplication.
Fair usage allows modification to suit the user's needs and specific situations. The BIOLAB SYSOPs request that major modifications and improvements to this exercise be resubmitted so they may be shared.
Darwin argued that "there is no exception to the rule that every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair." Populations of organisms have the biological capacity to grow into huge numbers, but something must keep populations' sizes in check. What types of factors might limit the grow and size of populations?
In this laboratory exercise you will simulate populations of two different types of organisms, a prey organism and a predator, to observe and interpret their interactions.
1. The prey organism, a simulated mouse, lives in a simulated meadow community. For purposes of this exercise, the biology of the simulated mouse is defined as follows.
a. Initially the meadow was populated by 10 immigrants, and anytime the mouse population falls below 10, another 10 will immigrate into the meadow. So if the population falls to 4, another 10 immigrate into the meadow, making 14 total.
b. Each mouse averages 2 offspring that constitute the breeding population of the next generation, so with the death of the parents, the population doubles generation to generation. That is the same as 4 offspring per pair, but the sex of simulated mice is so difficult to detemine that it makes things easier to ignore sex and mating, and just assume it is taking place. Ex. A population of 28 mice results in 56 mice in the next generation (28 x 2 = 56).
c. Any time the mouse population in the meadow goes over 500, 98% of the population dies. So if the population were 540, the next generation would be 11. (540 x 0.02 = 11)
2. Assume you are starting with an empty meadow. From the initial 10 immigrants, calculate the mouse population growth through 10 generations. Fill in (Table 1) to show the number of mice in the meadow population in each generation. Graph the mouse population growth through 20 generations.
3. Although the reproduction rate is constant (2N), how would you describe the population growth? Does it grow slower or faster as the population get bigger? Write out your answer in your notebook or lab report.
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Table 1: Geometric growth of mouse population
Trials Trials
1 2 1 2
_______________ _______________
Generation Number Number Generation Number Number
1 _______ _______ 11 _______ _______
2 _______ _______ 12 _______ _______
3 _______ _______ 13 _______ _______
4 _______ _______ 14 _______ _______
5 _______ _______ 15 _______ _______
6 _______ _______ 16 _______ _______
7 _______ _______ 17 _______ _______
8 _______ _______ 18 _______ _______
9 _______ _______ 19 _______ _______
10 _______ _______ 20 _______ _______
4. How many times does the mouse population crash in 20 generations? What do you think might be causing the population to crash when it gets to a certain size? How many reasons can you think of? List all of the reasons the class thought of.
5. What types of resources in the enviroment might limit the grow of the mouse population?
6. Biologists have developed the concept of environmental CARRYING CAPACITY to describe the number of organisms, the size of the population, that a given environment can support on a sustained basis. What is the carrying capacity of our simulated meadow for simulated mice?
Mice play the role of 1st consumer, eating plants to obtain energy and grow. When there are too many mice and not enough plant food, space, or other resource, most of the mice die.
Predators consume prey to obtain energy to grow & reproduce, so in this example some of the mice in the population will become food for a predator, reducing the number of mice reproducing. However, predator populations grow too, and there must be a carrying capacity for them as well.
1. How might predators affect the growth of the mouse population?
2. With respect to the carrying capacity, do you predict that the crashes of the mouse population will be bigger or smaller with predation? Do you predict that the crashes will be more frequent or less frequent with predation? Why?
3. How will the carrying capacity of the predator compare to the carrying capacity of the prey?
Now you will introduce a simulated fox into the simulated meadow. The biology of the fox is as follows.
a. For each generation of mice, if no other foxes are present, one fox will immigrate into the meadow.
b. The fox must capture and eat at least 4 mice to survive, and 5 mice to produce 1 offspring. For example: If 1 fox captures 4 mice = fox survives but no offspring (Next generation Foxes = 1), 8 mice = 1 offspring (Next generation Foxes = 2), 10 mice = 2 offspring (Next generation Foxes = 3).
Beans will be the simulated mice. A spoon will be the simulated fox. A bowl will be the simulated meadow.
For each generation of mice, each fox gets to forage across the meadow ONCE, which is accomplished by the fox looking away, gently shaking the bowl with one hand to distribute the mice around, and then running the fox (spoon) through the meadow (bowl), once, with the other hand.
To start, place 10 mice (beans) in your meadow (bowl). The single fox gets to forage in the meadow.
a. Record the number of mice each fox captures in Table 2.
b. From this data determine if the fox survives and if so, if and how many offspring will be added to the fox population. Fill in the appropriate numbers in Table 3.
c. Record the number of mice surviving predation and calculate the number of mice (2n) in the population on Table 3. Double the number of beans remaining in the bowl to produce the next generation.
d. For the next generation of mice and foxes, continue by letting each fox forage across the meadow once to capture mice. Remember if the only fox dies, another immigrant will take its place in the next generation, and if the mouse population declines below 10, another 10 will immigrate to the meadow. If the fox captures 4 mice, it survives, and for every 5 mice captured, it produces 1 offspring.
e. The class will do the first 4 generations together, and although similar, everyone will not have the same results. Then continue the simulation through 20 generations.
f. Graph the mouse and fox populations by generation and compare with the graphs generated by just the mouse population.
1. Which of your predictions were true? Which were false? Why? How are the results different from what you expected?
2. How do the graphs of the mouse population differ with and without predation?
3. How does predation affect the mouse population with respect to the meadow's carrying capacity?
4. In what generations does the mouse population reach maximum and minimum numbers? In what generations does the fox population reach maximum and minimum numbers? Do they agree? If not, can you detect a pattern?
5. Which population, predator or prey, was most likely to drop to zero? Which population, predator or prey, is most likely to be affected by a reduction in the size of the meadow?
6. What do you estimate the meadow's carrying capacity is for foxes?
7. Not all of the meadows are of similar size. Compare the results from your meadow to other meadows. Does the size of the meadow have any impact? Can you detect any pattern?
8. If different size meadows support different size populations of prey and predators, then what would this suggest to biologists about the concept of a carrying capacity?
9. What ecological "rules" can were illustrated in this exercise?
10. Look up definitions of FOOD CHAIN and TROPHIC LEVEL. Do the biologies of the simulated organisms seem appropriate for their roles in this exercise?
11. This simulation only involved 2 dynamic populations, a prey species and a predator species. Suppose the simulation got a lot more complicated. Plant populations must also fluctuate. How would that affect the meadow community? What if there were a couple of dozen plant species, 6 or 7 herbivores, and 2 predator species? Which community will on the whole have the greatest organismal stability, the simple 3 member community of our simulation or the much more complicated community? In other words, is the a relationship between biological diversity and community structure and stability?
12. What does the interaction between predator and prey populations suggest to be a problem for conservation efforts? Which populations in a community will be the most difficult to maintain? How will the size of a conservation area affect the difficulty of maintaining species diversity?
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Table 2: Field Data for Mouse Predation by Foxes (Record # of
mice captured by each fox.)
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Generation of Mice
__________________
| # of Foxes present
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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1 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
2 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
3 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
4 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
5 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
6 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
7 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
8 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
9 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
10 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
11 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
12 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
13 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
14 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
15 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
16 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
17 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
18 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
19 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
20 ___|___|___|___|___|___|___|___|___|____|____|____|____|____|____|
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Table 3. Summary of Mouse & Fox Populations
Generations 1 2 3 4 5 6 7 8 9 10
# of Mice
# Mice Capt.
Survivors
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# of Foxes
Survivors
Offspring
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Generations 11 12 13 14 15 16 17 18 19 20
# of Mice
# Mice Capt.
Survivors
___________________________________________________________
# of Foxes
Survivors
Offspring
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