1. Chapter 20
POPULATIONS

2. 20.1 Understanding Populations
As the human population grows, the population of other species declines
Population: a group of organisms that belong to the same species and live in a particular place at the same time

3. Population Size The actual population size can be difficult to measure
Individuals can be too abundant, too widespread, or too mobile to be easily counted

4. Sampling a Population
When it isn’t possible to count every member of the population, a sample is taken
From that sample, the population can be estimated

5. Population Density Measures how crowded a population is
Expressed as the number of individuals per unit of area or volume
Estimates are calculated for the total land area

6. Dispersion The spatial distribution of individuals within a population
Clumped: clustered together
Even: separated by the same amount of space
Random: no pattern

7. Clumped Distribution Food or living space may be clumped
Social behavior (herds)
Flocking behavior

8. Even Distribution Social interactions Colonial nesting behavior
V-pattern in flight (waterfowl migration)

9. Random Distribution Results from seed dispersal by wind or by birds
Example: forests or a field of wildflowers

10. Population Dynamics
Birth rate: the number of births occurring over a certain period of time
Death rate/mortality rate: the number of deaths over a period of time
Life expectancy: how long an individual is expected to live (the average life span)

11. Age Structure
Shows the distribution of individuals in different age groups in a population
Populations with a higher percentage of younger individuals will tend to have rapid growth

12. Patterns of Mortality
Survivorship curves: show the likelihood of survival at different ages throughout the life of an organism
Type I, II, or III

13. Types of Survivorship Curves
Type I: the chance of dying is small
until late in life
Type II: the chance of dying is the same regardless of age
Type III: the chance of dying while
young is great

14. 20.2 Measuring Populations
Growth Rate: the amount by which a population changes size in a given amount of time
Depends on birth, death, emigration and immigration
Birth rate – death rate = growth rate

15. Immigration vs. Emigration
Immigration: movement INTO a population
Emigration: movement OUT OF a population

16. The Exponential Model
Describes a population that increases rapidly after only a few generations
The larger the population gets, the faster it grows
J-shaped curve
Will become limited by not
enough food and too much
wastes (limiting factors)

17. The Logistic Model
Describes a population that becomes stable at the carrying capacity
Carrying capacity: the number of individuals the environment can support over a long period of time
S-shaped curve

18. Population Regulation
Density-independent factors: reduce a population regardless of the population size (weather, natural disasters, fire)
Density-dependent factors: the higher the population, the more organisms that are affected (food, territory, nest sites)

19. Population Fluctuation
Populations can fluctuate due to environmental changes
Example: population cycle of the snowshoe hare and the lynx

20. Perils of Small Populations
Small populations are at risk of extinction
Vulnerable to inbreeding, disease, environmental disturbances, etc.
Inbred offspring have shorter life spans, decreased genetic variability, and are more susceptible to diseases

21. 20.3 Human Population Growth
Early human populations lived in small nomadic groups and obtained food by hunting and gathering
Had a high mortality rate, especially among infants and children

22. The Agricultural Revolution
10,000 years ago, humans domesticated animals and cultivated plants
Agriculture increased the available food supply
Human population began to grow faster

23. The Industrial Revolution
After the Middle Ages, 25% of the human
population in Europe died due to the bubonic plague
The industrial revolution began around 1780 when there was a shift to using machinery and burning fossil fuels
Improvements in hygiene and in
health care led to population growth

24. Developed Countries
Developed countries: modern, industrialized areas with better health care, longer life expectancies, but lower population growth
Examples: U.S., Japan, Canada, Russia, Germany, Australia
Includes only 20% of the world’s population, but uses 75% of all fossil fuels!

25. Developing Countries
Developing countries: poorer areas, lower life expectancy, but higher population growth
Examples: Central America, South America, Mexico, Africa
Includes 80% of the world’s population, but uses only 25% of the fossil fuels globally

26. Human Population Explosion
By 2020, the human population could be as high as 12.5 billion
Will Earth be able to sustain that
amount of people or will the
human population reach its
carrying capacity?