Human Population Growth Chapter 4 (and Section 3.3)

1. Human Population GrowthChapter 4 (and Section 3.3) • Dynamics of human population growth • Human populations are like other pops. • Natality factors • Mortality factors • Age structure • Demographic transition • Future human population

2. What is a population?(Chapter 3 – Section 3.3) • Population = a group of organisms of the same species that occurs in a particular area • Boundaries somewhat arbitrary • Can potentially interbreed

3. Attributes of populations: • Abundance - total numbers • Density - #s per unit area • Dispersion - distribution of individuals in space • Structure - makeup of individuals (age, sex, size, )

4. Members of a population may be dispersed as: • Random - Individuals are not affected by each other • Uniform - Individuals tend to repel each other • Clustered - Individuals tend to attract each other Fig. 3.23

5. Dynamics of Population Growth • Exponential Growth - Growth at a constant rate of increase per unit time (= Geometric)=“J-shaped” growth≠Arithmetic Growth (Increases at constant amount per unit time)

6. Population grows as a function of its intrinsic growth rate (r) Population growth (DN) is just Births - Deaths over time (Dt)But, Births & Deaths obviously depend on NSo to compare pops. of different sizes we use percapita Birth & Death rate (B & D, respectively)Then, we let B - D = r,And thus, for a given time (Dt), pop. growth is: DN = r • NDt

7. How does population change over time?Nt = N(t-1) ert • If r > 0, pop. grows exponentially • Doubling Time of a population ≈ 70 / r (%) • When a pop. increases at its maximum r, it is growing at its Biotic Potential

8. Pop. increases as a % of its current size (like compound interest) • Function of its geneticpotential • Varies with species

9. Doubling times at various compund interest rates

11. Whooping crane population at Patuxent Wildlife Refuge, Maryland

12. Human Population Growth and Demography Fig. 4.3

13. B. Logistic Growth • No pop. can grow exponentially forever—> ultimately limited by its environment= Environmental Resistance or Feedback • Logistic Growth = Growth rates are regulated by environmental factors • “S-Shaped” growth curve • Growth rate slows as population approaches its Carrying Capacity

14. See Fig. 3.20

15. Growth of some laboratory populations fits the S-shaped curves fairly well

16. Logistic model provides a basis to which we can compare real populations • Natural populations tend to oscillate over time

17. Population Oscillations (3 stages): • Malthusian Growth (Irruptive Growth) • Population explosions driven by biotic potential • Overshoot - Population exceeds carrying capacity of its environment • Dieback - Negative growth • Severity of dieback is related to the extent of overshoot

19. Note that Overshoot of carrying capacity may degrade it over time Figure 3.18

20. Severe overshoot of Carrying Capacity may reduce K in future (caribou on Pribilof Islands)

21. Population Success Involves Tradeoff between Biotic Potential vs. Survival at K • Evolutionary strategies that favor increased biotic potential (r) • Rapid growth • Early maturity • Many small offspring • Little parental care • Adapted to: • Unstable environment • Colonizers after disturbance • Niche generalists

22. Human population growth history • ~100 million people on Earth in 1000 B.C. • Took 1800 years to reach 1 billion (1800) • Reached 2 billion in 1930 (130 yr doubling time) • Reached 4 billion in 1975 (45 yr doubling time) • Reached 6 billion in 1999 (~60 yr doubling time) • Human population tripled during the 20th century ! * November, 2012: World population passes 7 billion

23. Current World Population Growth: Birth and Death Rates • Every second: about 4 children are born, while about 2 other people die • Net gain: ~2.35 humans added to the world population every second, ≈75 million added every year

24. World Population Growth Rate • In exponentially growing populations: • Doubling Time = 70 / (% growth rate) • So, % growth rate = 70 / doubling time • Population doubled from 1960 to 2000: • Growth Rate = 70/40 = 1.8% • Population is currently growing at 1.14% • Doubling Time = 70/1.14% = 61 years

25. World population growth rate 1950-2010 2010 Projected

26. I. Human Population Demography • Demography - vital statistics about people, such as births and deaths • Includes underlying causes of population growth and distribution • Population age structure • Socio-economic conditions • Two Worlds: • Less-developed counties represent 80% of the world population, and more than 80% of projected growth • Richer countries that tend to have low or negative growth rates

28. Population Growth - Opposing Factors:Natality & Mortality • Factors Increasing Population: • Natality (Births) • Crude Birth Rate - # of births / year per 1000 • Total Fertility Rate - # of children born to an average woman in a population during her life • A good way to project population growth • Also Includes Immigration - movement of individuals into a population

29. Factors decreasing population size • Mortality (Death) • Crude Death Rate - # of deaths per 1000 persons in a given year • Life expectancy - Probable number of years of survival for an individual of a given age • Life Span - Longest possible lifetime • Also Includes Emigration - Movement of individuals out of a population • Zero population growth (ZPG) - occurs when births + immigration just equal deaths + emigration

30. Life expectancy Fig. 4.8

32. II. Population Age Structure & Demographic Transition • How population is changing (and will change in the future depends on current Age Structure • Growing vs. declining populations have different proportions of individuals in various age classes

33. Rapidly expanding populations have many pre-reproductive individuals (high pop. momentum) • Age-Structure diagram is a pyramid • Stationary Populations have balanced natality and mortality • Age-Structure shows no bulges • Diminishing Populations have fertility below replacement level (< ~2.1) • Age-Structure shows bulge in upper age classes

34. Age Structure Diagrams Fig. 4.9

36. Industrialization & Demographic Transition • Death rates and birth rates decline as economic development improves living conditions (4 phases) • Pre-Developed Country —> Death rates high, thus birth rates are correspondingly high • Economic Development —> Better conditions, thus death rates fall (High Pop. growth rate!) • Eventually, birth rates decline • Developed Country - Population is in equilibrium

38. Replaces Figure 4.13

39. Demographic Transition in U. S. Figure 4.12

40. III. Current World Situation: • Global population growth rate is ~1.14 % • Net gain: 2.3 humans added to the world population every second, 72 million added every year • These numbers do not indicate anything about global variability !

41. Two Demographic Worlds • First is wealthy, old, and mostly shrinking • North America, Western Europe, Japan • Average age is about 40 • Populations are declining or expected to decline • Second is poor, young, and rapidly growing • Less-developed countries • Africa, Asia, Latin America • Contain 80% of world population, and will account for 90% of projected growth

42. Population Growth - Opposing Factors: Rich vs. Poor Countries • Total Fertility Rate: • 5.5 in poor countries vs. • 1.6 in wealthy countries • 2.7 worldwide average • Crude Death Rate: • > 20 per 1000 in poor versus • < 10 per 1,000 in wealthy

43. iveFigure 04.01

44. Figure 4.18

45. IV. Future of Human Population • Most demographers believe the world population will stabilize sometime during this century • Projections of maximum population size: • Low 8 billion • Medium 9.5 billion • High 13 billion • Ultimate pop. size depends on “Carrying Capacity” of Earth for humans • But what is that??

46. Some demographers suggest Carrying Capacity is 10-12 billion Fig. CL 4.2

47. Population Growth - Opposing Factors:Malthus vs. Marx • Thomas Malthus (1798) argued human populations tend to naturally increase exponentially —> inevitably outstrip food supply and resources —> poverty & social decline—> populations crash—> cycle repeats (usually in a new location) • But how does this apply to the whole Earth?

48. Karl Marx (1845) argued that population growth is a symptom rather than a root cause of poverty and other social problems • The way to slow population growth and alleviate exploitation and oppression is through social justice

50. Technological optimists argue that Malthus was wrong b/c he failed to account for scientific progress • Current pop. growth was stimulated by the scientific and industrial revolutions • Human ingenuity and intelligence increase with numbers • More people mean larger markets, more efficiency