1. Populations Understanding Populations Measuring Populations
Human Population Growth

2. Understanding Populations
Properties of Populations
Population Dynamics

3. Learning Objectives Learning Objectives: TSW …
Explain the differences btw population size, density, & dispersion
Describe the 3 main patterns of population dispersion
Explain the importance of a population’s age structure
Contrast the 3 main types of survivorship curves

4. Properties of Populations
What is a Population?
Population Size
Population
Density
Dispersion

5. Population Group of organisms that…
…belong to the same species
…live in the same area at the same time
…are capable of interbreeding
The properties of a population differ from those of an individual organism
Ex. Population Study:
How many individuals are born?
How many individuals die?
How many individuals reproduce?

6. Population Size Number of individuals in the population
May be determined directly
Species occurs in small area
Species immobile or relatively immobile
Ex. Devils Hole pupfish in Devil’s Hole
May be estimated by sampling
Species occurs over a large area
Species is mobile
Ex. Zebras on African savanna

7. Population Density Measures how crowded a population is
Expressed as a # of individuals per unit area or volume
Ex. USA population is 30 people per km2
Population densities in other countries

8. Population Dispersion
The spatial distribution of individuals w/in a population
Clumped dispersion
Individuals are clustered (in groups)
Even (uniform) dispersion
Individuals are separated by a relatively consistent distance (even spacing)
Random dispersion
Each individual’s location is independent of the location of any other individual

9. Population Dynamics Birth Rates Death Rates & Life Expectancy
Age Structure
Patterns
of Mortality

10. Population Dynamics Birth rate = # births / time period
Death rate = # deaths / time period
Mortality rate
Life expectancy – how long an average individual is expected to live
Ex: In USA (1996), ♂ ~ 72;♀ ~ 79

11. Age Structure
BOOM!
The distribution of individuals among different ages in a population
Important factors
A large # of younger population members = higher birth rate
A large # of older population members = lower birth rate

12. Patterns of Mortality Survivorship curves
Mortality rate data for species tend to conform to one of three patterns
Survivorship curves
Type I – likelihood of dying early in life is small
Ex: humans & elephants
Type II – likelihood of dying is the same at any age
Ex: squirrels & many birds
Type III – likelihood of dying early in life is great
Ex: oysters, salmon, & insects

13. Measuring Populations
Population Growth Rate
The Exponential Model
The Logistic Model
Population Regulation

14. Learning Objectives Learning Objectives: TSW …
Describe the exponential model of population growth
Compare the similarities & differences btw the logistic model & the exponential model
Distinguish btw density-dependent & density-independent regulatory factors
List 3 reasons why small populations are more vulnerable to extinction

15. Population Growth Rate
Changes
in Population Size
Immigration
Emigration
Birth Rates
& Death Rates

16. Population Growth Rate
The amount by which a population’s size changes in a given time
Contributing Processes:
Birth
Death
Immigration – movement of individuals INTO a population
Emigration – movement of individuals OUT of a population

17. Population Growth Rate
52 births & 14 deaths per 1,000 individuals:
per capita birth rate = (52/1000)
per capita death rate = (14/1000)
Processes that ↑ population size
Birth
Immigration
Processes that ↓ population size
Death
Emigration
Demographers divide large populations into groups of 1,000 to present data “per capita”
Birth rate – Death rate
= Growth rate

18. Growth Rate Calculation
Per capita birth rate = Per capita death rate = 0.014
Population size = 50,000
Per capita growth rate
Births per capita – Deaths per capita
= Growth per capita
0.052 – = 0.038
Predicting yearly population growth
Current population size X Growth per capita
= Population increase
50,000 X = 1,900
Next year’s population will be 51,900

19. Growth Rate Calculation
Per capita birth rate = Per capita death rate = 0.014
Population size = 51,900
Per capita growth rate
Births per capita – Deaths per capita
= Growth per capita
0.052 – = 0.038
Predicting yearly population growth
Current population size X Growth per capita
= Population increase
51,900 X = 1,972.2 = 1,972
Second year’s population will be
51, ,972 = 53,872

20. Population Models The Exponential Model Predictions & Limitations
The Logistic Model
Growth
& Carrying Capacity

21. The Exponential Model Exponential Growth
The larger a population gets
The faster it grows
Predictions based on the Exponential Model
J-shaped curve
Population grows slowly when small
Population grows more rapidly as it gets larger
The rate of increase increases

22. Assumes that birth rate & death rate remain constant
The Exponential Model
Note: as density increases (space becomes limited), growth declines
Limiting Factor
Restrains the growth of a population
Limitations of the Exponential Model
Populations only grow exponentially under ideal conditions
No limit to food, space – reduced competition
Waste removed
Assumes that birth rate & death rate remain constant

23. The Logistical Model Logistic Growth
Population size is at carrying capacity
Birth rate = death rate
The logistical model accounts for limiting factors
Birth rates ↓ & death rates ↑ as a population grows
Carrying Capacity (symbol: K)
The number of individuals the environment can support over a long period of time

24. Population Regulation
Fluctuations
Perils
Of Small
Populations

25. Population Regulation
Limiting Factors
Density-independent factors
Density-dependent factors
Density-independent Factors
Reduce population regardless of its size (size-independent)
Density-dependent Factors
Reduce population because of its size (size-dependent)

26. Population Regulation
Density-independent Factors
Weather conditions
Floods
Landslides
Wild fires
Population size decreases by the same proportion regardless of how many individuals there are

27. Population Regulation
Density-dependent Factors
Competition for resources: food, space, etc.
Territoriality
Health
Predation
Intrinsic factors
Population size decreases because of competition for necessary resources or interactions w/ different species

28. Population Fluctuations
Causes of Fluctuation
Seasonal changes
Temperate climes: spring/summer -v- fall/winter
Tropical climes: rainy season -v- dry season
Environmental changes
Cycle: drought years -v- wet years
Predator & prey interactions
Snowshoe hair & lynx populations

29. Population Cycles: Ex: Dungeness Crab Population
Extreme Population Fluctuations
Due to complex interactions
Abiotic factors
Winds & ocean current carries larvae off shore
Temperature fluctuations due to storm systems
Biotic factors
Cannibalism by adult & older juvenile crabs
Competition among larvae for food
Population variation:
9,000 low – 730,000 peak

30. Population Cycles: Ex: Isle Royale Moose Population
Moderate Population Fluctuations
Due to complex interactions
Abiotic factors
Severity of winter temperatures
Duration of winter season
Biotic factors
Competition for food in winter
Wolf predation
Population variation:
2,500 peak – 500 low

31. Perils of Small Populations
Inbreeding
Mating w/ close relatives
Increases likelihood of acquiring recessive alleles
Recessive alleles often lead to deleterious conditions
Loss of genetic diversity
Other perils
Vulnerability to natural & man-caused disasters: fires, disease, floods, etc.

32. Small Populations

33. Human Population Growth
History of Human Population Growth
The Effect of Agriculture
Population Explosion
Population Growth Today

34. Learning Objectives Learning Objectives: TSW …
Explain how the development of agriculture changed the pattern of human population growth
Describe the change in human population growth that began circa 1650
Describe how growth rates have changed since WWII
Compare the general standard of living in developed countries w/ that in developing countries

35. Human Population Growth
The Hunter-gatherer Lifestyle
The Agricultural Revolution
Population Fluctuations
Population Growth
in Developed Countries
in Developing Countries

36. History of Human Population Growth
Hunter-gatherer Age
500,000 – ~11,000 B.C.E.
Characteristic small population
Due to high infant mortality
Agricultural Age
,000 – 2,000 B.C.E.
Domestication of animals & cultivation of food plants
Stabilized & increased food supply
Human Population:
10,000 years ago  million
Human Population:
2,000 years ago  million

37. History of Human Population Growth
Pre-industrial / Urbanization Age
2,000 B.C.E. – 1650 C.E.
Steady but slow population growth
Due to lower death rate
Industrial Age
1650 – 1930 C.E.
Population explosion
Due to ↑ birth rate – ↑ food supply
Due to ↓ death rate – better sanitation & hygiene
Human Population:
350 years ago  500 million
Human Population:
75 years ago  2 billion

38. History of Human Population Growth
Post WWII
1930 – 2000 C.E.
Explosive population growth continues
Due to large ↓ death rate – better sanitation & hygiene in poorer countries
Human Population:
45 years ago  3 billion
Human Population:
15 years ago  5 billion

39. Population Growth Increases Despite Lower Growth Rate
Global Population Growth
Peak – late 1960s: per capita
Current: 0.014
Due to ↓ birth rate – better sanitation & hygiene in poorer countries
1970 pop. = 3.7 billion w/ growth rate
Added 73 million people to world population
1999 pop. = 6 billion w/ growth rate
Added 84 million people to world population

40. Population Growth Today
Developed Countries
Industrialized
USA, Canada, Japan, Europe, Russia, Australia
20% of human population
Growth rates = 0.01 per capita (low)
Shrinking population in Germany, Russia, Hungary

41. Population Growth Today
Developing Countries
Non-Industrialized; agrarian
Asia (most), Mexico, Central & South America, Africa
80% of human population
Growth rates = 0.02 per capita (high)
Skyrocketing population in China, India,