Population Size
Dana Desonie, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2015 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: February 15, 2015
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Population Size
1
Population Size
• Describe the factors that regulate population size.
How many geese are the right number for this area?
As many as can survive and have healthy offspring! The figure above shows a flock of snow geese at the Kenai
National Wildlife Refuge. Their population will tend to grow as big as it can for the resources it needs. Once it is
too large, some of its members will die off. This keeps the population size at the right number.
How Populations Grow
A population usually grows when it has what it needs. If there’s plenty of food and other resources, the population
will get bigger. Refer to the table below ( Table 1.1); it shows how a population of bacteria grew. A single bacteria
cell was added to a container of nutrients. Conditions were ideal. The bacteria divided every 30 minutes. After just
10 hours, there were more than a million bacteria! Assume the bacteria population keeps growing at this rate. How
many bacteria will there be at 10.5 hours? Or at 12 hours?
TABLE 1.1: Growth of a Bacterial Population
Time (hours)
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Number of Bacteria
1
2
4
8
16
32
64
128
1
www.ck12.org
TABLE 1.1: (continued)
Time (hours)
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10
Number of Bacteria
256
512
1,024
2,048
4,096
8,192
16,384
32,768
65,536
131,072
262,144
524,288
1,048,576
Population Growth Rate
The population growth rate is how fast a population is growing. The letter r stands for the growth rate. The growth
rate equals the number of new members added to the population in a year for each 100 members already in the
population. The growth rate includes new members added to the population and old members removed from the
population. Births add new members to the population. Deaths remove members from the population. The formula
for population growth rate is:
r = b - d, where
b = birth rate (number of births in 1 year per 100 population members)
d = death rate (number of deaths in 1 year per 100 population members)
If the birth rate is greater than the death rate, r is positive. This means that the population is growing bigger. For
example, if b = 10 and d = 8, r = 2. This means that the population is growing by 2 individuals per year for every
100 members of the population. This may not sound like much, but it’s a fairly high rate of growth. A population
growing at this rate would double in size in just 35 years!
If the birth rate is less than the death rate, r is negative. This means that the population is becoming smaller. What
do you think might cause this to happen?
Carrying Capacity
A population can’t keep growing bigger and bigger forever. Sooner or later, it will run out of things it needs. For a
given species, there is a maximum population that can be supported by the environment. This maximum is called
the carrying capacity. When a population gets close to the carrying capacity, it usually grows more slowly ( Figure
1.1). When the population reaches the carrying capacity, it stops growing.
Summary
• A population will grow as long as it has what it needs.
• The carrying capacity of an environment is reached when the number of births equal the number of deaths.
• A limiting factor determines the carrying capacity for a species.
2
www.ck12.org
Chapter 1. Population Size
FIGURE 1.1
A population can’t get much larger than
the carrying capacity. What might happen
if it did?
Explore More
Use the resource below to answer the questions that follow.
• Population Growth at http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Population
Growth
1.
2.
3.
4.
5.
6.
7.
What is population growth?
What is the normal pattern of population growth?
What internal factors can limit population growth?
What external factors can limit population growth?
What are limiting factors? List examples.
What is carrying capacity?
What can cause carrying capacity to change?
Review
1. What happens if a population exceeds its carrying capacity?
2. Explain how a single bacterium can grow to a population of more than a million 10 hours later.
3. What happens if the birth rate in a population is more than the death rate? What happens if it is less?
References
1. Hana Zavadska. A logistic growth curve . CC BY-NC 3.0
3