04 Population Mini Labs

Ecology Unit Mini-Laboratory Exercises in Population Demographics
Exercise A: Patterns of population dispersion

Three general patterns of spacing of individuals (dispersion) are observed in natural
settings: Uniform spacing, randomly-dispersed clumps, or random distribution of individuals.
Dispersal patterns are indicative of resource distributions within an ecosystem and may reflect
the social behavior of the species under study.
Procedure:
Ecologists often divide an area used for field studies into quadrats and do counts of individuals
of different species for each quadrat. We are going to pretend that each row of desks is a
transect in a research field site, and each of the desk areas in the room is a quadrat of a field
study area. The different colors of buttons found within each desk 'quadrat' are each entirely
different species of organisms (i.e. green might be elephants, blue might be daylilies, etc.) You
are to count the number of each type of species in each quadrat. Record this data in the
table—if you need more lines, add them on.
Unique Buttons Button Color Number in Quadrat
1
2
3
4
5
6
7
Analysis:
a. Create a graphical representation that shows at a glance how the
different species were distributed over the coordinates of our 'field
study site.' One such graphical representation is shown below, but you
may use a graphical representation of your own design if it shows 'at a
glance' the pattern of distribution of species in your 'field site.' The
example graphic shows distribution of one species throughout a
research plot, but you should include different shapes or colored
symbols to show multiple species on the same plot. Include a key.
b. For each of your species (each color of button is a species) determine which type
of distribution is indicated by your data.
c. What factors may contribute to each type of distribution found in nature?
Exercise B-1: Exponential Population Growth
Procedure:
To examine the biotic potential of a growing population, we will imagine a population
of bacteria that reproduce asexually using binary fission. Our population begins with a single
individual who produces one offspring. Each individual in succeeding generation also produces
one offspring in each generation. The population doubles in each generation. The growth of
this population can be seen in the following series. Calculate the missing values for the
generations below.
Generation
Population Size
1 2 3 4 5 6 7 8 9 10
1 2 4 8 16 32 64 128 256 512

Analysis:
1. Compare the number of individuals at the beginning of generation 1 through 3 with
the number of individuals in generations 6 through 8. What general trend do you notice?
I notice an exponential growth trend and it doubles every time.
2. Is the gain in population size between all generations the same?
No the gain is not the same because the growth is exponential which means that it has an increasing growth rate.
3. Does the population grow at a constant rate per individual?

The population does grow at a constant rate per individual.
4. Why would exponential population growth be more likely for an asexually reproducing organism?
As there are less requirements, reproduction is easier and can occur more frequently. Moreover, one individual
is required which means that no one else is required for reproduction which makes reproduction take less time.
Exercise B-2: Biotic Potential of Natural Populations

Procedure: The exercise just completed is not realistic since it does not include the population's death rate.
Redo this exercise a second time, but assume a 25% death rate in each generation. The first two lines of the
table have been completed for you; fill in the rest of the table. (Note: round down fractions to nearest
whole numbers.)
4 # at Start #after Loss due to 25% # for next

of Generation Doubling death generation
1 1 2 0 2
2 2 4 1 3
3 6 1 5
3
5 10 2 8
4
8 16 4 12
5
12 24 6 18
6
18 36 9 27
7
27 54 13 41
8
nalysis:
Analysis
Make a graph below for the population data generated in both part 1 and 2 above.
Include a key. Place generation number along the horizontal axis and population numbers on the
vertical axis. Then, examine these curves to answer the following questions.
Biotic Potential of Natural Populations
Key: Dots are the points of the generation and the # at the start of each generation
1. Does including the effects of death rate change the shape of the curve?
It does change the shape of the curve.
2. What effect do death rates have on population growth?

The effect of the death rates is that the population growth will continue to increase, but at a slower
rate.
3. Do you believe any population can exhibit the kind of growth (biotic potential) exhibited
by the data in Exercise B-1? If so, under what conditions? if not, why not?
I believe that that a population can exhibit the kind of growth exhibited by the data in Exercise B-1 if
there is more than enough food/water, plenty of resources are present, there is low predation, and there is
a lot of space so that the carrying capacity of an environment is not reached.
Exercise C: Logistic growth rates

The biotic potential of a population is held in check by some form of "environmental
resistance". This might include limits on food, water or habitat, or waste accumulations. In most
cases, populations tend to level off at a certain size, which is known as the carrying capacity of
the environment. This population size is characteristic of the environment in which the organism
lives and will vary among populations and among environments.
Procedure:
Use a paper with a marked grid, a pair of dice, and a container of markers. The squares
on the grid represent an "environment" and the markers are "organisms." In this simulation,
each organism is unable to move and occupies a unit of the environment indicated by the
quadrats on the marked grid. Follow the rules indicated here to simulate logistic growth:
1. Every individual alive at the start of a generation will produce one offspring
during that generation.
2. The new individuals of each generation are placed in quadrats by rolling dice (For
example, if you roll a "3" and a "6", place an "organism" in the sixth square of the third row--marked
"36".)
3. First place 6 organisms. If random assignment places more than one organism
in a quadrat, continue placing additional organisms until you have occupied 6 grid
spaces.
4. These 6 (or more) organisms will all reproduce, each producing one offspring.
Leave the original first generation organisms in their place. Count out the number of offspring
produced and place in a pile off to the side of your workspace.
5. Roll the die to assign these 6 (or more) new offspring to a grid space. Keep going
until you have 12 spaces occupied with at least 1 organism. You may need to place additional
offspring to get to 12 occupied grid squares.
5. Then apply the following rule: At the end of a generation (beginning with the end
of generation 2), if more than one organism are present in a quadrat, all individuals die of
starvation/resource depletion and are removed from that quadrat.
6. Record your observations for each generation in the table below.
Generatio Starting Population Ending Population Offspring To Individuals That

n Add Die
1 6 9 9
2 18 20 20 23
3 40 17 17 19
4 34 15 15 13
5 30 17 17 22
6 34 12 12 11
7 24 13 13 12
8 26 14 14 15
Analysis:
After you complete the exercise, plot a graph in your lab notebook. Use generation
number along the horizontal axis, and population size on the vertical axis. Plot one line showing
"# at start of generation" (use solid circles for each data point). Then plot another line
showing the population size for the first 5 generations if the population were to realize its
biotic potential (i.e. 6, 12, 24, and 48, 96, etc.) (use open circles for biotic potential).
1. Does the population grow at an exponential rate at any point in this curve?
The population does not grow at an exponential rate at any point in the curve.
2. What is the carrying capacity for the "environment" in this exercise?

The carrying capacity for the environment is 40 for this exercise.
3. What does the difference between biotic potential for this population and the
carrying capacity of this environment represent?
The biotic potential does not contain any environmental factors or changes and represents the
growth with perfect circumstances while the carrying capacity is the actual amount of living things
that an environment can sustain based on environmental conditions and availability of resources
and necessities.
4. What sorts of things might provide "environmental resistance" to exponential growth

in natural populations?
Some things that provide environmental resistance are natural disasters, predators, abrupt
changes in temperature and climate, and physical intervention like the habitat of birds
being chopped down to build homes for people.

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Ecology Unit Mini-Laboratory Exercises in Population Demographics

Exercise A: Patterns of population dispersion

c. What factors may contribute to each type of distribution found in nature?

Exercise B-1: Exponential Population Growth

1 2 4 8 16 32 64 128 256 512

I notice an exponential growth trend and it doubles every time.

2. Is the gain in population size between all generations the same?

3. Does the population grow at a constant rate per individual?

Exercise B-2: Biotic Potential of Natural Populations

4 # at Start #after Loss due to 25% # for next

Biotic Potential of Natural Populations

It does change the shape of the curve.

2. What effect do death rates have on population growth?

Exercise C: Logistic growth rates

Generatio Starting Population Ending Population Offspring To Individuals That

2. What is the carrying capacity for the "environment" in this exercise?

4. What sorts of things might provide "environmental resistance" to exponential growth

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