Basic Terms

Population: a group of individuals from the same species
that all live in the same area at the same time.
• Ecology: the study of how populations interact with their
environment.
Changes in whooping crane population at
Wood Buffalo Park
200
Number of cranes
160
120
80
40
0
1940
1950
1960
1970
Year
1980
1990
2000
Population Growth

Births and immigration add individuals to any population,
while deaths and emigration remove them.
Population Growth

Basic models of population growth
• Continuous growth can be represented by the equation:
Nt = N0er t
800
Discrete
Growth
(Nt = N0t)
 = 1.6
600
Population size (N)
400
Continuous
Growth
(Nt = N0ert)
200
0
0
1
2
3
4
3
4
800
r = 0.47
600
400
200
0
0
1
2
Time (t)
Population Growth

Exponential growth
• Growth continues indefinitely and is density-independent.
• Can occur for short intervals, but cannot be sustained.
Exponential growth
High r
500
Moderate r
Population size (N)
400
300
200
Low r
100
Very
low r
0
0
1
2
3
4
5
Generations
6
7
8
9
10
Population Growth

Exponential growth
• No population can continue to grow indefinitely.
• At high densities, growth becomes density-dependent.
• All populations eventually reach the carrying capacity of their
habitat.
Population size
Carrying capacity
Time
Case Studies Explaining How Population Size
Changes Over Time

Humans exhibiting density-dependent growth

Recovery from trauma: The Exxon Valdez oil spill
Historical growth
6
1900: 1.5 billion
5
1700: 600 million
4
1500: 400 million
3
1 A.D.: 150–200 million
2
4–5 million
10,000
B.C.
8000
1
6000
4000
Year
2000
0
0
2000 A.D.
Human population (billions)
1999: 6 billion
Recent growth
12
10
High
Medium
1998 Projections
9
8
Low
7
6
5
4
3
2
1950
1970
1990
2010
Year
2030
2050
Human population size (billions)
11
1992 Projections
Fertility rate
Projected population in
2050
High
12.5 billion
Medium
10.15 billion
Low
7.8 billion
The 1992 projections for 2050 are higher than those from 1998 primarily because the
earlier projections did not account for the impact of AIDS.
Persistent oil in mussel beds…
…led to slow recovery in other species.
…led to slow recovery in other species.
Population Structure

Age structure
• Developed nations have an age distribution that tends
to be even.
• Developing nations have an age distribution that is
bottom-heavy (mostly young individuals).
More-developed countries
100
95
90
85
80
75
1998 data
2050 projections
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
60
40
20
0
20 40
60
(In millions)
Males
Females
Less-developed countries
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
300
200
100
1998 data
2050 projections
0
(in millions)
Males
100
200
300
Females
Population Structure

Geographic structure
• Many species exist as a metapopulation.
• Small, isolated populations, even those on nature reserves,
are unlikely to survive over the long term.
A metapopulation is made up of small, isolated populations.
Individuals
Habitat
patches
Although some subpopulations go extinct over time...
…migration can restore or establish subpopulations.
Demography and Conservation

Demography: the study of factors that determine the size
and structure of populations through time.
Demography and Conservation

Life tables
• Summarize the probability that an individual will survive and
reproduce in any given year over the course of its lifetime.
Three general types of survivorship curves
1000
100
Low survivorship
10
Low survivorship
Number of survivors (Nx)
High survivorship
1
Type lll
High survivorship
0.1
Age
Demography and Conservation

Life tables
• Contain useful pieces of information, such as survivorship,
fecundity, and net reproductive rate.
Life table
Age (x)
Survivorship (lx)
0 (birth)
Fecundity (mx)
0.0
1
0.33
3.0
2
0.2
4.0
3
0.2
5.0
Demography and Conservation

Life tables
• Can be used to make population projections and guide
conservation programs.
Demography and Conservation

Population viability analysis (PVA)
• A model that estimates the likelihood that a population will
avoid extinction for a given time period.
Demography and Conservation

Population viability analysis (PVA)
• Populations are considered viable if they have a 95%
probability of surviving for at least 100 years.
Demography and Conservation

Population viability analysis (PVA)
• Currently being used by natural resource managers.