1. Population Dynamics

2. Population characteristics
All populations show characteristics that help scientists predict their future dynamics
Population size = the number of individual organisms present at a given time
Numbers can increase, decrease, cycle or remain the same

3. Population characteristics
Population density = the number of individuals in a population per unit area
Large organisms usually have low densities
They need many resources and a large area to survive
High densities make it easier to find mates
But increase competition and vulnerability to predation
Increased transmission of diseases
Low densities make it harder to find mates
But individuals enjoy plentiful resources and space

4. Population characteristics
Population distribution (dispersion) = spatial arrangement of organisms
Random = haphazardly located individuals, with no pattern
Uniform = individuals are evenly spaced
Territoriality, competition
Clumped = arranged according to availability of resources
Most common in nature

5. Population characteristics
Sex ratio = proportion of males to females
In monogamous species, a 1:1 sex ratio maximizes population growth
Age distribution (structure) = the relative numbers of organisms of each age in a population
Age structure diagrams (pyramids) = show the age structure of populations
In species that continue growing as they age
Older individuals reproduce more (i.e. a tree)
Experience makes older individuals better breeders

6. Birth and death rates
Survivorship curves = the likelihood of death varies with age
Type I: more deaths at older ages
Type II: equal number of deaths at all ages
Type III: more deaths at young ages

7. Four factors of population change
Natality = births within the population
Mortality = deaths within the population
Immigration = arrival of individuals from outside the population
Emigration = departure of individuals from the population
Births and immigration add individuals
Deaths and emigration remove individuals
Crude birth (death) rates: number of births (deaths) per 1000 individuals per year

8. Population growth rate
Natural rate of population growth =
(Crude birth rate) – (crude death rate)
Population change due to internal factors
Population growth rate =
(Crude birth rate + immigration rate) – (Crude death rate + emigration rate)
Net changes in a population’s size/1000/year
Growth rate as a percent =
Population growth rate * 100%
Populations of different sizes can be compared

9. Exponential population growth
Exponential growth
A population increases by a fixed percent
Graphed as a J-shaped curve
Exponential growth cannot be sustained indefinitely
It occurs in nature with a:
Small population
Low competition
Ideal conditions

10. Limiting factors restrain population growth
Limiting factors = physical, chemical and biological attributes of the environment
They restrain population growth
Environmental resistance = all limiting factors taken together
Stabilizes the population size
Space, food, water, mates, shelter, suitable breeding sites, temperature, disease, predators
Aquatic systems: salinity, sunlight, temperature, etc.

11. Carrying capacity
Carrying capacity = the maximum population size of a species that its environment can sustain
Limiting factors slow and stop exponential growth
An S-shaped logistic growth curve
Many factors contribute to environmental resistance and influence a population’s growth rate and carrying capacity

12. Perfect logistic curves aren’t often found

13. Reproductive strategies vary among species
Biotic potential = an organism’s capacity to produce offspring
K-selected species = species with long gestation periods and few offspring
Have a low biotic potential
Stabilize at or near carrying capacity
Good competitors
r-selected species = species which reproduce quickly
Have a high biotic potential
Little parental care, populations fluctuate greatly

14. Our world at seven billion
Populations continue to rise in most countries
Particularly in poverty-stricken developing nations
Although the rate of growth is slowing, we are still increasing in numbers
It would take 30 years, counting once each second, to count to a billion!
It would take 210 years to count to 7 billion!

15. The human population is growing rapidly
Our population grows by over 80 million each year
It took until 1800 to reach 1 billion
In 1930 (130 years later) we reached 2 billion
We added the most recent billion in 12 years
Due to exponential growth, even if the growth rate remains steady, population will continue to grow

16. Rates of growth vary from region to region
At today’s 1.2% global growth rate, the population will double in 58 years (70/1.2 = 58)
If China’s rate had continued at 2.8%, it would have had 2 billion people in 2004

17. Is population growth a problem?
Technology, sanitation, medication, and increased food increase population
Death rates drop, but not birth rates
Population growth was seen as good
Support for elderly, a larger labor pool
Thomas Malthus’ An Essay on the Principles of Population (1798)
Humans will outstrip food supplies
War, disease, starvation reduce populations

18. Paul Ehrlich’s Population Bomb (1968)
Neo-Malthusians: population growth will increase faster than food production
Population growth causes famine and conflict
Civilization would end by the end of the 20th century
Intensified food production fed more people

19. Population growth will affect quality of life
Population growth has caused famine, disease, conflict
Prosperity, education, gender equality reduce birth rates
Cornucopians (e.g., economists) say new resources will replace depleted ones
But some resources (e.g., species) are irreplaceable
Quality of life will suffer with unchecked growth
Less space, food, wealth per person

20. Population growth affects the environment
The IPAT model: I = P × A × T × S
Total impact (I) on the environment results from:
Population (P) = individuals need space and resources
Affluence (A) = greater per capita resource use
Technology (T) = increased exploitation of resources
Sensitivity (S) = how sensitive an area is to human pressure
Further model refinements include the effects of education, laws, and ethics on the formula

21. Population growth: causes and consequences

22. Demography
Demography: the application of population ecology to the study of change in human populations
All population principles apply to humans
Environmental factors limit population growth
Humans raise the environment’s carrying capacity through technology
How many humans can the world sustain?
1–33 billion: prosperity to abject poverty
Population growth can’t continue forever

23. Demography Demographers study: Population size
Density and distribution
Age structure
Sex ratio
Birth, death, immigration, and emigration rates

24. Population size and density
The UN predicts 9 billion by 2050
Increased density impacts the environment
But relieves pressure in less-populated areas
Highest density: temperate, subtropical, tropical biomes
Cities
Lowest density: away from water

25. Population distribution
Humans are unevenly distributed around the globe
Unpopulated areas tend to be environmentally sensitive (high S value in the IPAT equation)
Vulnerable to humans (e.g., deserts, arid grasslands)

26. Age structure affects population size
Age structure diagrams (population pyramids) show age structure
Wide base = many young:
High reproduction
Rapid population growth
Even age distribution:
Remains stable
Births = deaths

27. Age structures: Canada vs. Madagascar
Canada’s age structure is balanced
Madagascar’s age structure is heavily weighted toward the young

28. Changing age structures pose challenges
China’s age structure is changing
In 1970, the median age was 20
By 2050, it will be 45
By 2050, over 300 million will be over 65
Fewer people will be working to support social programs

29. Many populations are aging
Many populations are getting older
They will need care and financial assistance
Taxes will increase for Social Security and Medicare
But fewer dependent children means lower crime rates
The elderly can remain productive

30. Sex ratios Human sex ratios at birth slightly favor males
For every 100 females born, 106 males are born
Chinese females are selectively aborted
120 boys were reported for 100 girls
Cultural gender preferences
The government’s one-child policy
The undesirable social consequences?
Many single Chinese men
Teenage girls are kidnapped and sold as brides

31. Factors in population change
Whether a population grows, shrinks, or remains stable depends on rates of birth, death, and migration
Birth and immigration add individuals
Death and emigration remove individuals
Technological advances caused decreased deaths
The increased gap between birth and death rates resulted in population expansion
Natural rate of population change = due to birth and death rates alone

32. Immigration and emigration
War, civil strife, and environmental degradation cause people to flee their homes
Each year, 25 million refugees escape poor environmental conditions
This movement causes environmental problems
No incentives to conserve resources

33. Falling growth rates do not mean fewer people
Slower rates of growth do not mean a decreasing population—population size continues to increase

34. Factors affecting total fertility rate
Total fertility rate (TFR) = the average number of children born to each female
Replacement fertility = the TFR that keeps the size of a population stable (about 2.1)
Causes of decreasing TFR:
Medical care reduces infant mortality
Urbanization increases childcare costs
Children go to school instead of working
Social Security supports the elderly
Educated women enter the labor force

35. Life expectancy is increasing
In countries with good sanitation, health care, and food, people live longer
Life expectancy = average number of years that an individual is likely to continue to live
Increases with reduced rates of infant mortality
Urbanization, industrialization, and personal wealth reduce infant mortality rates
Demographic transition = a model of economic and cultural change
Explains the declining death and birth rates in industrializing nations

36. The demographic transition
A stable pre-industrial state of high birth and death rates changes to a stable post-industrial state of low birth and death rates
As mortality decreases, there is less need for large families
Parents invest in quality of life
Death rates fall before birth rates
Resulting in population growth

37. The demographic transition’s four stages
Population growth is seen as a temporary phenomenon

38. Is the demographic transition universal?
It has occurred in Europe, the U.S., Canada, Japan, and other nations over the past 200–300 years
But it may or may not apply to developing nations
The transition could fail in cultures that:
Place greater value on childbirth or
Grant women fewer freedoms
For people to attain the material standard of living of North Americans, we would need the natural resources of four and a half more Earths

39. Birth control: key to controlling growth
The greatest single factor slowing population growth
Birth control = controlling the number of children born
Reducing the frequency of pregnancy
Contraception = deliberate prevention of pregnancy through a variety of methods
Family planning = affects the number and spacing of children
Clinics offer advice, information, and contraceptives
Hindered by religious and cultural influences
Rates range from 10% (Africa) to 90% (China)

40. Contraceptive methods for birth control

41. Empowering women reduces growth rates
Fertility rates drop when women gain access to contraceptives, family planning programs, and educational opportunities
Women with little power have unintended pregnancies
Two-thirds of the world’s illiterate are women
Educating women reduces fertility rates, delays childbirth, and gives them a voice in reproductive decisions

42. We are a long way from gender equity
Over 60% of the world’s poor are women
Violence against women remains shockingly common
Many men resist women’s decision making
The gap is obvious at high levels of government
We are a long way from achieving gender equality
The U.S. lags behind the world in proportion of women representatives

43. Policies and family planning work
Many countries provide incentives, education, contraception, and reproductive health care
Funding and policies that encourage family planning lower population growth rates in all nations
Thailand’s educational-based approach to family planning reduced its growth rate from 2.3% to 0.6%
Brazil, Mexico, Iran, Cuba, and other developing countries have active programs
1994’s UN population conference in Cairo, Egypt called for universal access to reproductive health care

44. Family planning reduces fertility rates
Blue = with family planning
Red = without family planning

45. Poverty and population growth are correlated
Poorer societies have higher population growth rates
Consistent with the demographic transition theory
They have higher fertility and growth rates, with lower contraceptive use
99% of the next billion people added will be born in poor, less developed regions that are least able to support them

46. Correlation of poverty and population
Poverty exacerbates population growth
Population growth exacerbates poverty
In 1960, 70% of all people lived in developing nations
As of 2010, 82% live in these nations
99% of the next billion will be born in these nations

47. Poverty causes environmental degradation
Population growth in poor nations increases environmental degradation
Farming degrades soil in arid areas (Africa, China)
Poor people cut forests, deplete biodiversity, and hunt endangered species (e.g., great apes)
Africa’s Sahel and western China are turning to desert

48. Wealth also impacts the environment
The population problem is not only in poor countries
Affluent societies have enormous resource consumption and waste production
People use resources from other areas, as well as from their own
Ecological footprints are huge
We are living beyond our means
One American has as much environmental impact as 4.5 Chinese or 10 Indians or 19 Afghans

49. The Earth can’t support our consumption
Biocapacity = the amount of biologically productive land and sea available to us
Ecological deficit = ecological footprint > biocapacity
Ecological reserve = ecological footprint < biocapacity
We are running a global ecological deficit
Humanity’s global ecological footprint surpassed Earth’s capacity in 1987

50. The wealth gap and population growth cause conflict
The contrast between rich and poor societies causes social and environmental stress
The richest 20% use 86% of the world’s resources
Increasing tensions between “haves” and “have-nots”

51. HIV/AIDS impacts African populations
The AIDS epidemic is having the greatest impact since the Black Death in the 14th century
Of 33 million infected, two-thirds live in sub-Saharan Africa; 3,800 die/day
Low rates of contraceptive use spread the disease
HIV is established and spreading quickly around the world

52. AIDS has severe effects
AIDS undermines the ability of poor nations to develop
Millions of orphans are created
Fewer teachers and workers to fill jobs
Families and communities break down
Income and food production decline
Debt and medical costs skyrocket

53. Demographic fatigue
Demographic fatigue = governments face overwhelming challenges related to population growth
With the added of stress of HIV/AIDS, governments are stretched beyond their capabilities
Problems grow worse and citizens lose faith
Good news: HIV transmission has slowed recently
Decreased AIDS deaths are due to policy, collaboration, research, NGOs, and grassroots efforts