Download Population Growth - San Diego Miramar College

Population Growth
The purpose of this exercise is to investigate the growth of populations, attempting to answer
these questions: Does the growth of a population have a basic pattern? If so, what are the
characteristics of that pattern?
I.
Illustration of Population Growth: the White-footed Deer Mouse
The white-footed deer mouse, Peromyscus maniculatus, is one of the most widely
distributed mice in North America. Found in a variety of habitats, these nocturnal mice
build nests in a variety of places including burrows in the ground, and in trees. Seeds,
nuts, acorns, insects, etc., serve as food sources. The deer mice, in turn, are part of the
diet of many bird and mammal predators. The breeding season is normally FebruaryNovember, although it varies with latitude as well as elevation. Young often become
sexually mature as early as 5-6 weeks of age.
A.
Setting up the Model
We will structure a hypothetical population of deer mice and in so doing,
simplify nature by making a series of assumptions.
Let us assume that our population is on a small island in the San Diego
River. The population begins in the spring of 1995 when a male and female pair
of deer mice arrive accidentally aboard a floating piece of wood.
Assumptions
1. Each pair of mice produce 10 offspring – 5 males and 5 females, each
breeding season (spring).
2. Each winter all parent mice die before the next spring.
3. Each year all the offspring live and breed during the next breeding
season.
4. No further immigration or emigration occurs on the island.
B.
Using the Model to Predict the Growth of the Deer Mouse Population
According to assumption 1, during the 1995 breeding season our original
breeding pair produces a total of 10 new deer mice (5 pairs). According to
assumption 2, the 1995 parent pair will die before the spring of 1996. Thus, the
1996 parent population will consist of 10 breeding mice, or 5 breeding pairs.
Using similar reasoning, predict the deer mouse population size for the beginning
of the breeding seasons of 1996, 1997, 1998, 1999, and 2000. Use your
predictions to fill in Table 1 on the following page.
Table 1
Year:
Predicted Deer Mouse
Population Size
II.
1995
1996
1997
1998
1999
2000
Factors Which Can Limit Population Growth
Every population has a natural tendency to increase in size. This tendency is called its
biotic potential.
On the other hand, environmental factors, which tend to limit population growth, are
referred to collectively as environmental resistance. Such factors may be physical or
biological.
Examples of physical factors are light, space, availability of oxygen or carbon dioxide
and other chemical substances such as nitrogen, calcium, phosphorus and water.
Examples of biological factors are competition among organisms for food and
nesting space. Other examples include parasitism and predation.
Thus, long-term stability in population size results from a fairly even balance between
biotic potential and environmental resistance.
One consistent phenomenon found in nature is the continued tendency to reach
equilibrium or a “steady state.” Many plant and animal populations evidence this
tendency, sooner or later. A population in equilibrium appears fairly constant in size over
time (with minor fluctuations). The tendency toward equilibrium is the result of all the
factors that can affect the survival and reproductive success of the individuals in the
population. This equilibrium is known as carrying capacity, or K. K is the number of
individuals in a population that an environment can sustain.
Example:
A 20-acre island in a California river was colonized in the 1990’s by a mated pair of deer
mice. Observations of the population size were made over a period of years. Some of
the data are seen in Table 2 on the following page.
Table 2
Actual Deer Mouse
Population Size
1st year
2
2nd year
8
3rd year
42
4th year
196
5th year
210
6th year
204
Graph both sets of data. Label the first graph “Model Deer Mice Population; the
second “Actual Deer Population Growth.” Answer the following questions:
1.
Does the model predict that the deer mouse population will continue to grow
indefinitely?
2.
Describe the shape of the growth curve for the model deer mouse population.
3.
Does the data from Table 2 show the pattern of population growth predicted by
the “model” growth curve?
4.
Describe the shape of the growth curve for this deer mouse population.
5.
For the actual population, do you think the biotic potential, environmental
resistance or both change as the population grows during the six-year period?
Our population increased relatively slowly until 1650. But with advent of better nutrition,
sanitation, and medical care, the world population has experienced unprecedented growth.
6. What type of growth curve is seen in our species?
7. What factors will most probably limit growth in the years to come?
8. What is meant by carrying capacity?
9. What is the Earth’s carrying capacity for our species?
10. What is an ecological footprint?
Visit the following website to determine own ecological footprint. Print the results and attach
them to this page. Visit www.myfootprint.org