Post on 20-Dec-2015
Population Growth
Exponential growth
• No population can continue to grow indefinitely.
• At high densities, growth becomes density-dependent. (Fig. 48.3)
• All populations eventually reach the carrying capacity of their habitat. (the max number that can be supported by available resources).
Po
pu
lati
on
siz
e
Carrying capacity
Time
Figure 48.3
Case Studies Explaining How Population Size Changes Over Time
Humans exhibiting density-dependent growth (Fig. 48.5a,b)
4–5 million
10,000 B.C.
8000 6000 4000 2000 0
Year
0
1
2
3
4
5
61999: 6 billion
1900: 1.5 billion
1700: 600 million
1500: 400 million
1 A.D.: 150–200 million
2000 A.D.
Hu
man
po
pu
lati
on
(b
illio
ns)
Figure 48.5a
Historical growth
1998 Projections
1950 1970 1990 2010 2030 2050
Year
12
11
10
9
8
7
6
5
4
3
2
Hu
man
po
pu
lati
on
siz
e (b
illio
ns)
Medium
High
Low
Figure 48.5b
Recent growth
1992 Projections
Fertility rate
High
Medium
Low
Projected population in 2050
12.5 billion
10.15 billion
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.
Figure 48.5c
Population Structure
Age structure
• Developed nations have an age distribution that tends to be even. (Fig. 48.9a)
• Developing nations have an age distribution that is bottom-heavy (mostly young individuals). (Fig. 48.9b)
More-developed countries
1998 data
2050 projections
100959085
8075
706560
55504540353025201510
5
0
020 2040 4060 60
(In millions)Males Females
Figure 48.9a
Less-developed countries
1998 data
2050 projections
(in millions)Males Females
0100 100 200200 300300
0
10
1520253035404550556065
70
758085
5
9095
100
Figure 48.9b
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. (Fig. 48.10a-c)
A metapopulation is made up of small, isolated populations.
Individuals
Habitatpatches
Figure 48.10a
Although some subpopulations go extinct over time...
Figure 48.10b
…migration can restore or establish subpopulations.
Figure 48.10c
Figure 48.11
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. (Fig. 48.13a)
• Survivorship - Ix = Nx / N0
• Fecundity: the number of female offspring produced by each female in a population.
Age
Type lll
Type ll
Nu
mb
er o
f su
rviv
ors
(Nx)
0.1
1
10
100
1000
Lo
w su
rvivo
rship
High survivorship
Steady survivorship
High survivorship
Lo
w su
rvivo
rship
Figure 48.13a
Three general types of survivorship curves
Type l
Demography and Conservation
Life tables
• Contain useful pieces of information, such as survivorship, fecundity, and net reproductive rate.
Life table
Age (x) Survivorship (lx) Fecundity (mx)
0 (birth) 0.0
1
2
3
3.0
4.0
5.0
0.33
0.2
0.2
Figure 48.14a
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.
• Combine demographic models with geographic structure and rate and severity of habitat disturbance.
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. (Fig. 48.15a,b)
Box 48.1, Figure 1