1. How Populations Grow

2. How Populations Grow
Three important characteristics of a population are:
1) Population density – number of individuals per unit area
150 bullfrogs = 50 bullfrogs per square kilometer
3 sq. kilometers
– 2) Geographic distribution – range – area inhabited by population
(where do you find the population on Earth?)
– 3) Growth rate – net gain or loss to number of individuals in the population
Birth rate > death rate; population increases (↑)
Birth rate = death rate; population equalizes (=)
Birth rate < death rate; population decreases (↓)
– 4) Other factors affecting population growth – migration/movement of organisms
Immigration vs Emigration
(movement into a population) (movement out of the population)
think in
think exit

3. Exponential Growth – aka “J-curve”
- Occurs when the individuals in a population reproduce at a constant rate.
* Under ideal conditions, with unlimited resources (no limiting factors), a population will grow exponentially.
= infinite population size – BUT in reality, this doesn’t continue in nature…

4. Limits to Growth
Limiting Factors – factor that causes population growth to decrease
Competition, predation, parasitism and disease, drought and other climate extremes, human disturbances

5. Logistic Growth – aka “S-curve”
Occurs when a population’s growth slows or stops following a period of exponential growth
Logistic population growth occurs when the growth rate decreases as the population size approaches carrying capacity.
Carrying capacity (aka “k”) – largest number of individuals that a given environment can support.
Resources become less available – slow or limit population growth

6. Logistic Growth of Yeast Population
Carrying capacity
Number of Yeast Cells
Time (hours)

7. Regulation of Population Growth
Density-Dependent Factors & Density-Independent Factors

8. Density-Dependent Factors
A limiting factor that depends on population size
Affect large populations the most
Competition – food, water, space, mates (birds for nesting sites)
interspecies – between different species
intraspecies – within the same species
Predation (moose and wolves)
Parasitism and disease (wasp cocoons and larval sphinx moth)

9. Population Cycles - Predation60
2400 50
2000 40
1600 30
1200 20
800 10
400
1955
1960
1965
1970
1975
1980
1985
1990
1995
Moose
Wolves

10. Density-Independent Factors
Affect all populations in similar ways, regardless of the population size
- unusual weather, natural disasters, seasonal cycles, human activities (damming rivers, clear-cutting forests, etc.)

11. Human Population Growth
Human population increasing over time
3 people per second, 7 billion now, 9 billion in your lifetime
Exponential growth following improvements in: medicine, sanitation, agriculture, energy, technology
Human population cannot continue to grow exponentially – at some point, it will slow – but when?
Around 1800s – English economist Thomas Malthus predicted exponential growth could not occur forever – war, famine, and disease would control human populations
* Birth rates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly

12. Human Population Growth
Industrial
Revolution
begins
Agriculture
begins
Bubonic
plague
Plowing
and
irrigation

13. Demographic Transition
Demography – study of human populations
Demographic transition – a change in birth and death rates
Throughout history, birth rates = death rates; populations were stable
Modern countries’ advances in nutrition, sanitation, and medicine cause exponential growth; birth rate exceeds death rates; populations grow
As countries modernize, birth rates fall, population growth slows
Transition completes when birth rate falls to meet death rate and population growth stops

14. Age Structure
These graphs show the number of people in different age groups
Age structures help predict a population’s future growth.
Age structure diagrams may also indicate social conditions. An expanding population needs
schools,
employment, and
infrastructure.

15. Age Structure Diagrams
U.S. Population
Rwandan Population
Males
Females
Males
Females
U.S. age structure predicts slow, but steady growth rate for future = Developed country
Rwanda age structure predicts a population to double in 30 years due to many young people = Underdeveloped country

16. Age Structure
The following figure shows the estimated and projected age structures of Mexico’s population in
1985,
2010, and
2035.

17. Age 1985 2010 2035 Male Female Male Female Male Female
75–79
Male
Female
Male
Female
Male
Female
60–64
45–49
30–34
15–19 5 4 3 2 1 1 2 3 4 5 5 4 3 2 1 1 2 3 4 5 5 4 3 2 1 1 2 3 4 5
Population in millions
Total population: 76,767,225
Population in millions
Total population: 112,468,855
Population in millions Total population: 139,457,070

18. Age Structure
The following figure shows the estimated and projected age structures of the United States population in
1985,
2010, and
2035.

19. Population in millions Population in millions Population in millions
Age
1985
2010
2035
Birth years
Male
Female
Birth years
Male
Female
Birth years
Male
Female
75–79
1906–10
1931–35
1956–60
60–64
1921–25
1946–50
1971–75
45–49
1936–40
1961–65
1986–90
1946–50
30–34
1951–55
1976–80
2001–05
1956–60
1981–85
1961–65
1986–90
15–19
1966–70
1991–95
2016–20 12 10 8 6 4 2 2 4 6 8 10 12 12 10 8 6 4 2 2 4 6 8 10 12 12 10 8 6 4 2 2 4 6 8 10 12
Population in millions
Total population: 238,466,283
Population in millions
Total population: 310,232,863
Population in millions
Total population: 389,531,156