Lecture 7: Human Population Ecology

Question 1

Review of population models

Answer 1

• If rates are constant and if (birth + immigration) > (death + emigration), growth trajectory is density independent and exponential

Question 2

Exponential growth cannot continue unchecked, need more complex models

Answer 2

The simplest model of density-dependent regulation is a logistic
- For human projection, many errors: assumes all rates are constant

Question 3

Extrapolating from logistic gave very poor prediction

Answer 3

• Statistical:
  
  • Extrapolating beyond range of data is always dangerous
  • Logistic is not a law, just very simple possible hypothesis for density dependence
• Biological:
  - Logistic allows no overshoots
  - Logistic assumes r and K to be constants

Question 4

Real human trajectory cannot be modeled simply

Answer 4

• Growth (r) depends on balance of fertility rates (bx) and mortality (inverse of Lx)
• Birth rates have been high for much of human history, but death rates dropped radically during 19th-20th centuries
• Changes in health care raised r
  - Vaccination and hygiene, then antibiotics and insecticides
• Changes in agriculture raised K
  - Genetics, machines, fertilizers, biocides

Question 5

Current guesses about trajectory

Answer 5

• UN: projected to reach 10.3 billion in 2084
• In 1970, growing about 2% per year, now down to 1.1%, big change
• Still growing exponentially
• Leveling off requires less birth or more death

Question 6

Age structure affects trajectory

Answer 6

• Fast growing populations have broad-based age pyramids, excess of children
• Even if r goes to zero, populations keep growing as children reach reproductive age

Question 7

Population Momentum Explained

Answer 7

• If b=d, or fertility rates are at replacement levels (x-bar = 2.1 children per woman)
• Populations with broad pyramids continue growing
  
  • As large number of children enter reproductive years, even if they have 2.1 children on average, population will grow because there are more people that have children

Question 8

Demographic Transition Model

Answer 8

• Stage 1: pre-industrial birth and death rates both high, r near zero
• Stage 2: death rates drop (sanitation, vaccination, etc.) but birth rates stay high; population booms
• Stage 3: birth rates fall (contraception, changing values, later marriage), growth rate slows
• Stage 4: birth and death rates equilibrate, r near zero again

Question 9

What causes fertility to decline

Answer 9

• Voluntary decisions by parents
  
  • Motivated by greater survival of children
  • Children too expensive
  • Society providing more security in old age
  • Worry about overpopulation
• Made possible by:
  
  • Availability of contraception/abortion
  • Postponement of marriage/family

Question 10

Is there any way to address population growth that is not morally offensive

Answer 10

• Proposition: human fertility rates fall when women acquire more legal rights, more education, and access to contraception
• Standard of living goes up