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