2. UNIT 2: ECOLOGY POPULATIONS
How Populations Grow 5-1
Limits to Growth 5-2
Human Population Growth 5-3

3. 1. How Populations Grow Characteristics of Populations
Three important characteristics of a population are:
Geographic Distribution
Density
Growth Rate

4. Geographic Distribution (Range)
= the area inhabited by a population

5. Population Density = the number of individuals per unit area

6. Population size (blue)
Population Growth
Populations can stay the same (=), grow (+), or decrease (-) in size
from year to year
Three factors can affect population size:
The number of births
The number of deaths
The number of individuals that enter or leave the population
Population Size = # of Births - # of Deaths
Births (olive)
Deaths (red)
Population size (blue) ? ?

7. Characteristics of a Population
Population - individuals inhabiting the same area at the same time
Population Dynamics: Population change due to
Population Size - number of individuals
Population Density - population size in a certain space at a given time
Population Dispersion - spatial pattern in habitat
Age Structure - proportion of individuals in each age group in population

8. Population Size Natality Mortality
Number of individuals added through reproduction
Crude Birth Rate - Births per 1000
Total Fertility Rate – Average number of children born alive per woman in her lifetime
Mortality
Number of individuals removed through death
Crude Death Rate Deaths per 1000

9. Population Density
Population Density (or ecological population density) is the amount of individuals in a population per unit habitat area
Some species exist in high densities - Mice
Some species exist in low densities - Mountain lions
Density depends upon
social/population structure
mating relationships
time of year

10. Immigration = the movement of individuals into an area
Emigration = the movement of individuals out of an area
Which causes an increase in population size? Decrease?  

11. Exponential Growth
Exponential Growth = occurs when the individuals in a population reproduce
at a constant rate
Under ideal conditions with unlimited resources
“J-shaped curve “

12. E. coli
Reproduce once/20 minutes
20 minutes = 2 2n
40 minutes = 4
n = # times reproduced
60 minutes = 8
80 minutes = 16
100 minutes = 32
120 minutes = 64
140 minutes = 128
160 minutes = 256
180 minutes = 512
200 minutes = 1024
220 minutes = 2048
240 minutes = 4096
4 hours?
5 hours = 32,768
6 hours = 262,144
8 hours = 2,097,152
24 hours?
274
What if “Unchecked?

13. Logistic Growth
Logistic Growth = occurs when a populations growth slows or stops following a period of exponential growth
What factors decrease?
What factors increase?
Birthrate and Immigration
Death rate and Emigration
“S-shaped” Curve

14. Carrying Capacity (K)
Exponential curve is not realistic due to carrying capacity of area
Carrying capacity is maximum number of individuals a habitat can support over a given period of time due to environmental resistance (sustainability)

15. Carrying Capacity = the largest number of individuals (within a species)
that a given environment can support
More Natural
Growth rate equals zero

16. Age Structure
The age structure of a population is usually shown graphically
The age structure of a population dictates whether it will grow, shrink, or stay the same size

17. Age Structure Diagrams
Positive Growth Zero Growth Negative Growth
(ZPG)
Pyramid Shape Vertical Edges Inverted Pyramid

18. 2. Limits to Growth Limiting Factors
Limiting Factor = a factor that causes population growth to decrease
Primary Productivity
Limiting Nutrients

19. Density-Dependent Factors
Density-Dependent Limiting Factor = a limiting factor that depend on
population size
These factors operate most strongly when a population is large and dense

20. Competition When populations become crowded, organisms
compete with one another for food, water,
space, sunlight, and other essentials

21. Environmental Resistance
Biotic Potential
factors allow a population to increase under ideal conditions, potentially leading to exponential growth
Environmental Resistance
affect the young more than the elderly in a population, thereby affecting recruitment (survival to reproductive age)

22. Exponential and Logistic Population Growth: J-Curves and S-Curves
Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off.
Figure 8-4

23. Exponential and Logistic Population Growth: J-Curves and S-Curves
As a population levels off, it often fluctuates slightly above and below the carrying capacity.
Figure 8-4

24. Predator- Prey Relationship
“Inverse” relationship

26. Parasitism

27. Density-Independent Factors
Density-Independent Limiting Factors = a limiting factor that does not
depend on the density of a population
They affect all populations in similar ways, regardless of the population size
Pop <
then >

28. Natural Disasters
Human Disturbance

29. 3. HUMAN POPULATION GROWTH

30. Population Growth Population growth depends upon
birth rates
death rates
immigration rates (into area)
emigration rates (exit area)
Pop = Pop0 + (b + i) - (d + e)

31. 5-3 Human Population Growth
Demography- the study of human populations.

32. Historical Overview
Like the populations of many other living organisms, the size of the
human population tends to increase with time
For most of human existence, the population grew slowly
Life was harsh, and limiting factors kept population sizes low
Food was scarce and diseases were rampant
Until fairly recently, only half the children in the world survived to adulthood
Families would have many children to offset the death rate
About 500 year ago, the human population began growing more rapidly
Why?

33. Case Study: Fertility and Birth Rates in the United States
In 2006, the total fertility rate in the United States was slightly > 2.0
Figure 9-5

34. Replacement Level Baby boom (1946–64)
Births per woman
Replacement
Level
Baby boom
(1946–64)
Figure 9.5
Total fertility rates for the United States between 1917 and Use this figure to trace changes in total fertility rates during your lifetime. QUESTION: How many children do you plan to have? (Data from Population Reference Bureau and U.S. Census Bureau)
Year
Fig. 9-5, p. 175

35. Case Study: Fertility and Birth Rates in the United States
The baby bust that followed the baby boom was largely due to delayed marriage, contraception, and abortion.
Figure 9-6

36. Agriculture and industry
Improved sanitation, medicine, and health
Death Rate?
Birth Rate?
Experienced exponential growth

37. 1650 = ~ 500 million
1850 (200 years) = doubled  1 billion
1930 (80 years) = doubled  2 billion
1975 (45 yeas) = doubled  4 billion
Today = + 6 billion
Increasing ~80 million/year
214,000 people/day = Amarillo population
Weekly = + San Antonio
3 years = + United States
2025 = 7.8 billion
Indefinite Growth?

38. Limited Resources
The question is, when and how will our population growth slow?

39. Patterns of Population Growth
Demography = the scientific study of human populations
Examines the characteristics of human populations and attempts
to explain how those populations will change over time
Examine: birthrates, death rates, and the age structure of a population
people per km²

40. Over the past century, population growth in the U.S., Japan, and much
of Europe has slowed dramatically (Developed Countries)
Hypothesis: These countries have completed
the demographic transition

41. Throughout most of history, human societies have had high death rates
and equally high birthrates
With advances in nutrition, sanitation, and medicine, more children survive
to adulthood and more adults live to old age
These changes lower death rates and begin the demographic transition

42. 5-3
The shift from high birth and death rates to low birth and death rates is called the demographic transition.

43. The demographic transition has three stages
The demographic transition has three stages. Write this in off to the side.
In stage 1, there are high death rates and high birthrates.
In stage 2, the death rate drops, while the birthrate remains high. The population increases rapidly.
In stage 3, the birthrate decreases, causing population growth to slow.

44. Demographic transition graph

45. Demographic Transition = a dramatic change in birth and death rates
BR = DR

46. 5-3 Stage III Japan Most European countries United States Stage II
African countries
South and Central American countries
Many Asian countries

47. Much of the population growth today is contributed by only 10 countries,
with India and China in the lead, where birthrates remain high

48. Age Structure:
Age-Structure diagrams (Population Profiles) = graph the numbers of
people in different age groups in the population
Used to predict future growth

50. Future Population Growth
Current projections suggest that by 2050, the world’s population may reach
more than 9 billion people

51. Most ecologists suggest that if this growth does not slow down even more,
there could be serious damage to the environment as well as to the
global economy
On the other hand, many economists assert that science, technology, and
changes in society will control those negative impacts on the environment
and economy