BIO220: Research Skills & Info Session

• BIO220 offers optional, fun, and informative activities for interested students
• Further details and a complete schedule can be found on Quercus under Modules → Beyond BIO

Upcoming Events

• How to Find Sources (with U of T Librarians)

  • Date: Wednesday, January 29th, from 1:10–2:00 pm
  • Location: Zoom (link on Quercus)
  • Description: Learn effective strategies for finding academic sources, designed to support completing the Sources Assignment & Climate Change Report for BIO220 students

• Koffler Scientific Reserve Info Session

  • Date: Wednesday, February 5th, from 1:10–2:00 pm
  • Location: On Zoom (link on Quercus)
  • Description: Explore how to get involved with KSR, a large field research station owned by U of T near Newmarket, through an info session with the manager of KSR

BIO220 Final 6 Lectures

• Course shifts focus to humans, examining how natural processes affect humans, and vice versa
• Theme explores humanity's unique impact with the equation: Total Impact = (Per capita impact) * (# of capita)
• Draws in ecosystem perspectives and considers political, sociological, and ethical issues

Outline for Remaining Section

• Topics include:
  • Earth's carrying capacity
  • Human impact as a function of population, consumption (affluence), and technology
  • Components: population history & projections, biology of food supply, biogeochemical cycles, climate change, and biological feedbacks

Lecture 7: Human Population Ecology

• Mexico City has a population of 21.7 million
• Mexico City's public transit system has a daily ridership of 4.5 million
• The urban fraction of the global population was 3% in 1800 and is 50% today
• Per capita, cities use less energy and pollute less than rural areas

Review of population models

• If the (birth + immigration) rate is greater than the (death + emigration) rate and rates are constant, the growth trajectory is density-independent and exponential
• Model as dN/dt = rN; and the solution is N(t) = N₀e^(rt)
• There is exponential growth if r > 0
• Exponential growth can not continue unchecked, so more complex models are needed
• Simplest model of density-dependent regulation is the logistic: dN/dt = rN(K-N/K)
• Logistic model was viewed as an ecological law in mid 20th century

Historical Perspective

• "The Great APPES"
  • Authored by Allee, Emerson, Park, Park & Schmidt in 1949
  • It was the first really successful ecology textbook

Human Population Projections

• In the 1940s, Allee et al. predicted the human population would level off at 2.6 billion using a logistic equation
• In January 2025, the population was at ~8.2 billion
• UN demographers expect the world population to peak at 10.4 billion in 2086
• In 10,000 BCE, the world population was approximately 4 million
• The UN projects 9 billion people by 2036 and 10 billion by 2058
• The average growth rate from 10,000 BCE to 1700 was 0.04% per year
• Global life expectancy was less than 30 years before 1800 but is 73 years in 2023
• In the mid-14th century, the Black Death pandemic killed between a quarter and half of all people in Europe
• In the year 0, the world population was approximately 190 million

Extrapolating from Logistic

• It gave a very wrong prediction.

• Extrapolating beyond the range of data is always dangerous

• Logistic is not a law, just a simple possible hypothesis for density-dependence

• Logistic model allows no overshoots and assumes that r and K are constants

• A question to ask

  • Is K changing or are we experiencing an overshoot because of a delayed response?

Real human trajectory

• It can't be modeled simply.
• Growth (r) depends on balance of fertility and mortality rates
• Birth rates have been high, but death rates fell radically during the nineteenth-twentieth centuries
• Changes in healthcare (vaccination & hygiene and then antibiotics & insecticides) raised r
• Changes in agriculture (genetics, machines, fertilizers, biocides) raised K
• UN projects world population of 10.3 billion in 2084, which is down from previous estimates

Current Guesses about Trajectory

• In 1970, the world was growing at about 2% per year, now down to about 1.1%, a big change
• The globe is still growing exponentially!
• The world is adding 83 million per year
• Leveling off requires less birth or more death

Age Structure

• Fast-growing populations exhibit broad-based age pyramids, indicating an excess of children and a population momentum
• Populations keep growing as children reach reproductive age, even if r goes to zero at this instant

Population Momentum Explained

• Even if b = d, or fertility rates are at replacement levels (x-bar = 2.1 children per woman), populations with broad pyramids continue growing

  • This is because as a large number of children enter reproductive years, the population will grow, on average, even if they have 2.1 children

• Two populations, N = 20 age, 2 children per female, 50-50 sex ratio. It's been 20 years:

  • Most of 60-80 in Pop 1 will have died; 40-60 will be beyond reproductive age
  • Most of 0-20, 20-40 in Pop 2 will still be alive, have had children, and be of reproductive age

• Calculating carrying capacity is hard

• Antoni van Leeuwenhoek first estimated the human capacity of earth in 1679

• Estimated land area of globe, applied human density of crowded Holland:

• Room for 13.4 billion

• Historical estimates of K (carrying capacity) range over 3 orders of magnitude (1 billion to 1 trillion)

Projections Are Getting Better!

• There is a link to data about UN population projections
• Demographic Transition Model:
  • Stage 1: birth and death rates equally high, r is near zero
  • Stage 2: death rates drop (sanitation, vaccination), but birth rates stay equally high; population booms
  • Stage 3: birth rates fall (contraception, values, later marriage), growth rate slows
  • Stage 4: birth and death rates equilibrate; r is near zero again

The Demographic Transition Model

• It describes why rapid population growth is a temporary phenomenon.

• Changes in the age distribution have had profound consequences

• The note on dependency ratios:

• Selected in 1880, by Otto von Bismarck, Chancellor of Germany

  • Was designed for when not many people lived long enough to collect retirement!
  • Since then: Changes in life expectancy and age pyramids!
  • There is a required lecture reading on social consequences of population growth slowing

Demographic Transition in Sweden

• Sweden shows a clear transition through demographic stages from 1735-2000

• Spikes in death rates resulted from epidemics and famines

• Later there was an Influenza epidemic in 1919

• Subsequently, post WWI and WWII baby booms occurred, plus baby boom "echo"

• What also causes fertility to decline?

  • In some cases, the government policy (China, India)
  • In most cases, voluntary decisions by parents, motivated by: Greater survival of children, children being too expensive, society providing more security in old age, worry about overpopulation

• And made possibly by: Availability of contraception/abortion, postponement of marriage/family

• It is worth looking at:

• https://ourworldindata.org/grapher/natural-population-growth

• 30 years before, Latin America, China, India would have been in the highest category of human population growth

• Lester Brown: Real answer depends on how people will be living

• The current food production could support about 10 billion, with food consumption of countries like India

• However, if people consumed like Canada/U.S, it is about 2.5 billion with their food consumption

• But other countries aspire to consume as we do!

• The less developed world will account for more than 95% of future population growth

• 2011: 7 billion while in 1960: 3 Billion

• The age-distribution pyramids (right) show why the overwhelmingly young populations of developing centuries will produce almost all the population increase

Is there any way

• Managing population not popular with either left-wing or right-wing ideologues

• Left: Is morally wrong for Rich countries telling the poor countries to restrict reproduction

• Right: Birth control it morally wrong

• Most economists push growth as more essential; even the liberal presses agrees

• The film "Mother" suggests human fertility rates fall when women acquire more legal rights, more education, and access to contraception

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

  • Consider the film "Mother: Caring for Seven Billion," there is
  • There can be the proposition of human fertility rates falling when women acquire more legal rights, education, and access to contraception

• Recap and take-aways

  • Logistic growth; demographic translation, interpreting age pyramids ; population.
  • Momentum per capita consumption How do all those relate? Do these relate? Non-coercive ways to address population growth, resource equity