INDIVIDUALS AND POPULATIONS
NAME: _____________________________ P:_____
Purpose: To introduce the population and population density concept and to describe some factors which
affect populations.
If an ecologist or an ecology student wanted to study the organisms in a certain area such as the East Gallatin
Recreation Area, he or she would have two choices. He could study each deer, each grass plant, or each
specific shrub one by one. In that case he would be studying individuals. It would be easy to do this if the
subjects were deer, but it would be more difficult to separate and study each individual grass plant.
His second choice would be to study all of the deer, all of the grass plants of a specific kind, and all of a
certain type of shrub in the area at the same time of study. To investigate a population of organisms, a
group of the same kind of individuals in the same space at the same time, would be the ecologists usual
choice. The time element is important, for a population might differ at various times of the day, during the
different seasons or from year to year. Two examples of populations are: The number of crows within the
Belgrade city limits and the number of dandelions in the front lawn at our middle school.
Procedure:
1. What if you had a fish aquarium at your house and there were 5 goldfish and 3 guppies in it; could
you identify individual fish of the same species? _________
How could you calculate how many of the same species lived in the tank?
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If you wanted to study these fish as a population of fish in a pond, how might you go about it?
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2. In one of the future labs you will learn how to sample populations as wildlife biologists do. Before
doing this, however, you have to learn more about population dynamics.
3. Populations are usually measured in terms of density. Ecologists calculate density (D) by counting
the number of individuals in the population (N) and dividing this number by the total units of space
the population occupies. The formula for calculating population density thus becomes:
D = N/S (read D equals N divided by S)
4. When studying population density on land, ecologists use the dimensions of length and width when
measuring the occupied space. They deal in square units. When studying aquatic environments they
use length, width and depth. Space is measured in cubic units.
5. Let’s look at one example of density so that we may apply the formula D=N/S. When ecologists
study forest populations they often use a space of definite size. This space is called a quadrant. The
size of the quadrant is usually 10 meters square (10 m X 10 m). 10 m2 plots are marked off with
twine. Within each quadrant the tree species can be plotted and counted. Suppose you were in a
forest counting Ponderosa Pine seedlings in a quadrant. You counted 40 seedlings in a particular
quadrant. What is the density per square meter? D=N/S or D=40 seedlings/100 square meters (m2)
or D = .04 pine seedlings per square meter.
6. When studying population density, ecologists realize the density may change, often quite swiftly and
dramatically. If a lumber company cut 1000 trees in a forest, this would decrease the density. On the
other hand, germination and growth of last season’s seeds would, in time, increase density on that
same forest. Both births and deaths cause changes in living populations. Ecologists call the birth rate
of a population natality and the death rate mortality.
7. In animal populations, two factors besides natality and mortality can affect density. For example,
with a flock of geese on a wildlife refuge, the density could vary as more birds migrated in or as
others left the flock for better feeding grounds elsewhere. The arrival of new individuals to a
population is termed immigration. The leaving of individuals from a population is called
emigration.
Questions:
1. Define a population.
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2. What is the relationship of density to a population?
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3. In calculating density, why is time important?
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__________________________________________________________________________ Why is
space important?
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4. Using a good definition of a population for a guide, explain why the following do not belong to a
good, single population.
a. Horses in Texas and horses in Montana
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b. The ducks in the refuge during spring of 2001
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c. The mallard ducks in the wildlife refuge in June and in September
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d. The plants on the lawn outside
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5. Why is the following statement a good example of a population? All the dandelion plants in the lawn
outside the cafeteria in May, 2013.
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6. Assume there are 40 deer in 10 acres of the refuge in October of 2013. What is the density of this
deer population per acre?
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7. Assume there is a population of 300 sparrows within a 30 block community. What is the density per
block? _________________________________________________________________
8. Which two of the following factors could increase the density of the turkey population on the game
range? (natality, mortality, immigration or emigration)
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9. List two of those factors from the previous question that could decrease the turkey population.
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