Population Lecture

Population Basics

  • A population is a group of individuals of the same species living in the same time and place.
  • Subpopulations can exist in smaller locations within a larger area.
  • Focus is on a single population in a predetermined space.

Population Size Changes

  • Population sizes change due to growth and decline.
  • Increases: Births and immigration.
  • Decreases: Deaths and emigration.
  • The balance between these factors determines the rate of change.
  • A local population's space is predetermined (e.g., ground squirrels on a campus, humans in the US).

Calculating Population Growth Rate

  • Growth rate is denoted by r.
  • r = (Birth\, Rate + Immigration) + (Death\, Rate + Emigration)
  • r = (Birth\, Rate - Death\, Rate) + (Immigration\, Rate - Emigration\, Rate)
  • Growth can be positive or negative.
  • Rates are often calculated per 1,000 people, especially for human populations.

Intrinsic Rate of Growth (Biotic Potential)

  • The intrinsic rate of growth is the biotic potential.
  • It represents the maximum growth rate under ideal conditions (exponential growth).
  • This growth rate is often represented as a J-shaped curve.
  • Exponential growth occurs with unlimited resources, as demonstrated by bacteria multiplying (1 \rightarrow 2 \rightarrow 4 etc.).

Environmental Resistance

  • Environmental resistance limits population growth, preventing indefinite reproduction.
  • Factors:
    • Unfavorable food and water
    • Pollution
    • Insufficient shelter and nesting sites
    • Predation
    • Inadequate waste removal

Carrying Capacity (K)

  • Carrying capacity (K) is the maximum number of individuals an environment can support without reducing resources.
  • Factors determining K:
    • Food availability
    • Nesting site availability
    • Waste production and removal
    • Water and air quality
  • K causes a leveling off of exponential growth.

S-Shaped vs. J-Shaped Curves

  • Exponential growth (J-curve) is unrealistic long-term.
  • S-curve represents a more realistic scenario with leveling off at K.
  • Paramecia populations in a solution increase until resources are limited.
  • Growth slows as the population approaches K.

Overshooting K

  • Populations can overshoot K, leading to overuse of resources.
  • This results in a population decline or crash.
  • Example: Red deer on an island in Scotland.
    • Introduced in 1909.
    • Population grew to 2,000 by 1930 due to abundant vegetation (including seaweed).
    • Overgrazing led to resource depletion and a population crash to near zero.

Factors Affecting Population Size

Density-Dependent Factors

  • Factors that affect population change based on population density.
  • Examples:
    • Disease
    • Competition for resources (space, food, water)
    • Predation
  • Can cause boom or bust cycles.

Boom and Bust Cycles

  • Small population grows rapidly, overshoots K, leading to disease or resource competition.
  • Population crashes, followed by a period of growth when the limiting factor dissipates.
  • Example: Lynx and snowshoe hare.
    • Hare population increases, followed by an increase in lynx population.
    • Over-predation causes hare population to crash, followed by a decline in lynx population.
    • Cycle repeats.

Malthusianism

  • Thomas Malthus (late 1700s) proposed that population grows exponentially, while resources grow linearly.
  • This leads to a "Malthusian catastrophe" where living standards decline, triggering population decline.
  • Can result in war, disease, poverty, or depopulation.
  • Correction back to a sustainable level happens rapidly.

Neo-Malthusianism

  • Incorporates environmental and ecological factors.
  • Focuses on carrying capacity (K) and the impact of overshooting K on the planet.
  • Recognizes the feedback loop between the planet's health and population size.
  • Flaw in Malthus' argument: Resource growth is not strictly linear due to technological and medical advances.

Density-Independent Factors

  • Factors that affect population size regardless of population density.
  • Examples:
    • Killing frost
    • Severe blizzard
    • Giant fire
    • Tsunami

r-Selected vs. K-Selected Species

  • Species respond differently to constraints on population growth.
  • Reproductive strategies vary to maximize population growth.

r-Selected Species

  • Approach exponential growth.
  • Characteristics:
    • Early maturity
    • Asexual reproduction
    • Small body size
    • Short lifespans
    • Large broods
    • Little or no parental care
    • Low probability of long-term survival
  • Examples: Mosquitoes, dandelions, snowshoe hares, squid.

K-Selected Species

  • More responsive to environmental constraints.
  • Characteristics:
    • Smaller broods
    • Long lifespans
    • Reproduce more than once
    • Slow development
    • Large body size
    • Low reproductive rates
  • Examples: Redwood trees, humans.
  • Spectrum: Species fall along a spectrum between r and K selection.

Boom and Bust Cycles (Revisited)

  • Density-dependent factors (predation, disease, competition) can cause boom or bust cycles.
  • Example: Lemmings.
    • Population fluctuates dramatically in a cyclical pattern (approximately every four years).
    • Likely r-selected species.

Survivorship Curves

  • Type I: Most individuals live long lives, with a sharp decline at old age (e.g., humans).
  • Type II: Linear decline in population over time (constant mortality rate at all ages) (e.g., geckos).
  • Type III: High mortality rate at a young age, with some individuals living to old age.

Human Population

  • Current population closer to 9 billion (as of the time of transcription).
  • Increasing by approximately 82 million people per year.
  • Technological and medical revolutions have significantly shifted human K.

Historical Trends

  • Agricultural Revolution: Increased food production.
  • Plague: Decreased population.
  • Industrial Revolution: Further increased population growth.

Current Concerns

  • K for humans is uncertain.
  • Signs of overpopulation: Disease, food and water scarcity.
  • Severe population density issues.
  • Uncertainty about how long we can continue increasing Earth's K without degrading the life support system.

Factors Contributing to Human Population Increase

  • Agricultural revolution
  • Sanitation and control of infectious diseases
  • Expansion into diverse habitats.

Trends in Human Population Growth

  • Rate of growth is decreasing (exponential but smaller exponent).
  • Growth is uneven across the planet.
    • Decline in places like Japan and parts of Europe.
    • High growth rates in parts of Africa and Asia.
    • High growth in developing countries puts pressure on resources and coping mechanisms.

Demography

  • The study of population growth, including age class distribution.
  • Focuses on:
    • Age distribution (0-2, 2-5, 5-10, etc.)
    • Reproductive age population
    • Lifespan

Demographic Transition

  • Shift from high death rates and high birth rates to low death rates and low birth rates.
  • Changes the population structure from younger to older.

Global Growth Rate Trends

  • 1950: Growth rate between 1.5-2%.
  • 1970: Growth rate exceeded 2%.
  • Since then, a steady progressive decline in growth rate.

IPAT Equation

  • Impact (I) = Population (P) x Affluence (A) x Technology (T)
  • Indicates the impact of a population on the environment.
  • Higher affluence and technology lead to a greater impact.
  • The United States has a disproportionate impact relative to its population size.

Malthusian Growth Model and IPAT

  • Connects to the crossing of resource availability and human population growth.
  • Modern spin on Malthusianism.

Managing Population Growth

  • China: One-child policy (abolished due to demographic issues).
  • Mexico: Communication, education, counseling, and contraception (relatively successful).
  • India: Similar approach to Mexico but less successful; population recently surpassed China.

China's One-Child Policy

  • Abolished due to ineffectiveness and social consequences (imbalance of males and females).

Mexico's Approach

  • Education and access to contraceptives have slowed population growth.

Concerns and Future Directions

  • Emphasis on the interplay between K-selected and r-selected populations, logistic and exponential growth.
  • Importance of understanding how populations grow, the factors influencing growth (density-dependent/independent), and the human impact on natural ecosystems.