Unit 3 – Populations

Characteristics of Generalist Species

• Broad Diet: Can eat many types of food (omnivores or varied diet within trophic level)
• Wide Habitat Range: Thrive in diverse environments and geographic areas
• High Tolerance: Withstand wide ranges of temperature, moisture, pH, pollution, etc.
• Behavioral Flexibility: Adapt behavior to changing conditions
• Large Geographic Distribution: Found across many regions and ecosystems
• Rapid Reproduction: Often have high reproductive rates (r-selected traits)
• Human-Associated: Many thrive in human-modified environments

Examples of Generalist Species

• Raccoons: Eat fruits, insects, fish, garbage; live in forests, wetlands, urban areas; active day or night
• Rats: Omnivorous diet; survive in diverse climates; highly successful in urban environments
• Coyotes: Eat rodents, fruits, carrion, garbage; adapt to forests, deserts, suburbs, cities
• White-tailed Deer: Browse on many plant species; habitat generalists
• Cockroaches: Eat almost anything; tolerate wide temperature/humidity ranges; thrive indoors
• Dandelions: Grow in many soil types and climates; reproduce prolifically
• Humans: Ultimate generalists - omnivorous, global distribution, modify environments
• Crows: Omnivorous, intelligent problem-solvers, urban adapters

Advantages of Being a Generalist

• Resilience to Change: Can survive environmental disturbances and habitat alterations
• Resource Security: If one food source declines, can switch to alternatives
• Wide Distribution: Can colonize new areas and expand range
• Climate Change Resilience: Better able to adapt to changing conditions
• Lower Extinction Risk: Multiple survival strategies reduce vulnerability
• Invasive Potential: Can successfully establish in new environments

Disadvantages of Being a Generalist

• Jack of All Trades, Master of None: Not optimally adapted to any specific environment
• Competitive Disadvantage: Outcompeted by specialists in stable, resource-rich environments
• Lower Efficiency: Less efficient at exploiting any particular resource compared to specialists
• Higher Energy Costs: Maintaining adaptability to many conditions requires energy investment

Characteristics of Specialist Species

• Narrow Diet: Rely on one or few food sources (often obligate relationships)
• Specific Habitat Requirements: Need particular temperature, moisture, vegetation, or geographic features
• Low Tolerance: Sensitive to environmental changes (pollution, temperature shifts, habitat disturbance)
• Limited Distribution: Restricted to specific regions or ecosystems
• Specialized Adaptations: Highly evolved traits for specific conditions
• Lower Reproductive Rates: Often have K-selected traits
• Indicator Species: Presence/absence indicates environmental quality

Examples of Specialist Species

• Giant Pandas: Eat almost exclusively bamboo (99% of diet); require bamboo forests in specific mountain ranges in China
• Koalas: Feed only on eucalyptus leaves; require specific eucalyptus species in Australia
• Polar Bears: Depend on arctic sea ice to hunt seals; cannot survive in warmer climates
• Spotted Owls: Require old-growth forests with dense canopy in Pacific Northwest
• Monarch Butterflies: Caterpillars feed exclusively on milkweed plants
• Coral: Require warm (20-30°C), clear, shallow, saltwater; highly sensitive to temperature and pH changes
• Snail Kite: Feeds almost exclusively on apple snails in Florida wetlands
• Trout: Require cold, clean, well-oxygenated water; sensitive to temperature increases and pollution

Advantages of Being a Specialist

• Reduced Competition: Specialized niche means fewer competitors for specific resources
• Optimal Efficiency: Highly efficient at exploiting their particular resource or habitat
• Stable Environments: Excel when conditions remain constant and predictable
• Unique Adaptations: Specialized traits allow access to resources generalists cannot use
• Resource Monopoly: Can dominate their specific niche when present

Disadvantages of Being a Specialist

• Vulnerable to Change: Environmental disturbances or habitat loss can be catastrophic
• Resource Dependency: If specialized food/habitat disappears, species cannot easily adapt
• High Extinction Risk: Especially vulnerable to climate change, habitat fragmentation, pollution
• Limited Range Expansion: Cannot colonize new areas that lack specific requirements
• Conservation Priority: Often endangered and require intensive protection efforts
• Climate Change Vulnerability: Cannot easily shift ranges or adapt to new conditions

Characteristics of r-Selected Species

• High Reproductive Rate: Produce many offspring in short time periods
• Small Body Size: Typically small organisms (though not always)
• Early Sexual Maturity: Reproduce at young ages
• Short Life Span: Live fast, die young strategy
• Little/No Parental Care: Offspring fend for themselves immediately after birth
• High Mortality Rate: Most offspring die before reproducing
• Rapid Development: Fast growth to maturity
• Opportunistic: Exploit temporary resources and favorable conditions quickly
• Population Fluctuations: Experience boom-and-bust cycles
• Density-Independent Mortality: Death often due to environmental factors rather than competition

Examples of r-Selected Species

• Insects: Mosquitoes, flies, aphids (thousands of eggs, minimal care)
• Bacteria: Reproduce exponentially under favorable conditions
• Weeds: Dandelions, crabgrass (prolific seed production)
• Mice and Rats: Multiple litters per year, many offspring per litter
• Many Fish: Salmon, cod (produce millions of eggs with no parental care)
• Frogs: Lay hundreds to thousands of eggs in water
• Annual Plants: Complete life cycle in one season, produce many seeds
• Oysters: Can produce millions of larvae

Environmental Conditions Favoring r-Selection

• Unstable/Unpredictable: Environments subject to frequent disturbances
• Early Succession: Recently disturbed or colonized areas
• Variable Resources: Resource availability fluctuates dramatically
• Low Competition: Populations well below carrying capacity
• Short Growing Seasons: Must reproduce quickly before conditions deteriorate

Advantages of r-Selection Strategy

• Can rapidly colonize new or disturbed habitats
• Exploit temporary resource abundances quickly
• High probability that some offspring survive unpredictable conditions
• Rapid population recovery after disturbances
• Effective in pioneer/early successional stages

Characteristics of K-Selected Species

• Low Reproductive Rate: Produce few offspring, often one at a time
• Large Body Size: Typically larger organisms (though not always)
• Late Sexual Maturity: Reproduce only after reaching older ages
• Long Life Span: Live for many years or decades
• Extensive Parental Care: Parents invest heavily in protecting and nurturing offspring
• Low Mortality Rate: High offspring survival due to parental care
• Slow Development: Extended juvenile period with learning and growth
• Competitive: Strong competitors in stable environments
• Stable Populations: Population size remains near carrying capacity with little fluctuation
• Density-Dependent Mortality: Death often due to competition and predation

Examples of K-Selected Species

• Large Mammals: Elephants, whales, gorillas, humans (1-2 offspring, years of care)
• Large Birds: Eagles, albatross, condors (1-2 eggs, extended parental care)
• Sharks: Few offspring, long gestation, some provide parental care
• Trees: Oak, redwood, sequoia (slow-growing, long-lived, few seeds that germinate)
• Bears: 1-3 cubs every 2-3 years, extensive maternal care
• Primates: Chimpanzees, orangutans (single births, years of dependency)
• Tortoises: Long-lived, slow reproduction
• Large Predators: Lions, tigers, wolves (small litters, extended care)

Environmental Conditions Favoring K-Selection

• Stable/Predictable: Consistent environmental conditions over time
• Late Succession/Climax: Mature, established ecosystems
• Constant Resources: Reliable resource availability year-round
• High Competition: Populations at or near carrying capacity
• Low Disturbance: Infrequent environmental disruptions

Advantages of K-Selection Strategy

• High offspring survival rate due to parental investment
• Better competitors in stable, resource-limited environments
• Offspring better equipped with learned behaviors and skills
• Maintain stable populations in climax communities
• Long lifespan provides multiple reproductive opportunities

Characteristics

Pattern: High survival rates throughout early and middle life, with mortality concentrated in older age groups. The curve stays relatively flat (high survival) until late in life, then drops steeply.

Shape: Convex curve (curves downward) - looks like an upside-down "J"

Mortality Pattern: Most individuals survive to old age and die from aging-related causes. Very few die young.

Life History Strategy: Strongly associated with K-selected species

Key Features

• High Parental Investment: Extensive care and protection of offspring
• Few Offspring: Low reproductive output per breeding event
• High Infant Survival: Most young survive to adulthood
• Long Lifespan: Individuals live many years
• Late Sexual Maturity: Begin reproducing at older ages
• Death Cause: Primarily senescence (aging) and age-related diseases

Examples

• Humans (developed countries with modern healthcare)
• Large Mammals: Elephants, whales, gorillas, rhinoceros
• Large Birds: Eagles, albatrosses, swans
• Domestic Animals: Well-cared-for pets, livestock
• Some Marine Mammals: Dolphins, orcas

Interpretation: Type I curves indicate species that invest heavily in each offspring, ensuring high survival rates. Most individuals reach their maximum lifespan. This strategy works in stable environments with low predation and requires significant resources for parental care.

Characteristics

Pattern: Relatively constant mortality rate throughout life. Individuals have approximately equal probability of dying at any age.

Shape: Linear/diagonal line (straight diagonal from upper left to lower right)

Mortality Pattern: Death occurs at a fairly steady rate across all age groups. No particular age is more vulnerable than others.

Life History Strategy: Intermediate between r and K selection

Key Features

• Moderate Parental Care: Some protection but not extensive
• Moderate Offspring Number: Neither extremely high nor low reproduction
• Equal Vulnerability: Young, middle-aged, and old face similar mortality risks
• Random Death: Mortality often due to unpredictable events (accidents, predation, disease)
• Death Cause: Environmental hazards, predation affecting all ages equally

Examples

• Many Birds: Songbirds, robins, gulls
• Small Mammals: Squirrels, rabbits, mice (in wild populations)
• Some Reptiles: Lizards, some snakes
• Rodents: Prairie dogs, chipmunks
• Some Invertebrates: Hydra, certain mollusks

Interpretation: Type II curves indicate species where environmental hazards (predation, accidents, disease) affect individuals randomly regardless of age. This is actually relatively rare in nature - most species show some age-specific mortality patterns. Type II is most common in species with consistent predation pressure across all life stages.

Characteristics

Pattern: Very high mortality in early life stages, but individuals that survive to adulthood have high survival rates thereafter. The curve drops steeply at first, then levels off.

Shape: Concave curve (curves upward) - looks like a "J"

Mortality Pattern: Most individuals die very young (often >90% mortality in juvenile stages). Those that reach adulthood tend to survive longer.

Life History Strategy: Strongly associated with r-selected species

Key Features

• No/Minimal Parental Care: Offspring left to fend for themselves immediately
• Many Offspring: Extremely high reproductive output
• Low Infant Survival: Vast majority die before maturity
• Vulnerable Young: Juveniles are easy prey or susceptible to environmental stresses
• Lottery Strategy: "Spray and pray" - produce so many that some survive by chance
• Death Cause: Predation, starvation, environmental conditions affect young; adults die from various causes

Examples

• Most Fish: Salmon, cod, tuna (millions of eggs, <1% survival to adulthood)
• Marine Invertebrates: Oysters, sea urchins, corals (broadcast spawning)
• Insects: Most species (thousands of eggs, high larval mortality)
• Amphibians: Frogs, toads (hundreds of eggs, few tadpoles survive)
• Plants: Most species (thousands of seeds, few germinate and reach maturity)
• Sea Turtles: Lay many eggs on beaches; hatchlings face extreme predation
• Many Invertebrates: Spiders, crustaceans

Interpretation: Type III curves indicate species that produce vast numbers of offspring with minimal investment in each. This "numbers game" strategy compensates for extremely high juvenile mortality. Common in species that reproduce in unpredictable environments, have many predators, or whose young must disperse widely.

Limiting Resources

• Food Supply: Availability and quality of nutrition sources
• Water: Access to clean, fresh water
• Space/Territory: Physical area for living, breeding, and foraging
• Shelter: Protection from weather and predators (nesting sites, den locations)
• Light: For plants and photosynthetic organisms
• Nutrients: Minerals and chemical elements (nitrogen, phosphorus for plants)

Environmental Factors

• Climate: Temperature and precipitation patterns
• Habitat Quality: Condition and suitability of the environment
• Predation: Pressure from predators
• Disease: Prevalence of pathogens and parasites
• Competition: Both intraspecific (within species) and interspecific (between species)
• Waste Accumulation: Toxic waste products that inhibit growth

Density-Dependent Factors

These factors become MORE limiting as population density INCREASES. They regulate population growth and enforce carrying capacity:

• Competition for Resources: More individuals = more competition for limited food, water, space
• Predation: Higher prey density makes hunting easier for predators
• Disease Transmission: Pathogens spread more easily in crowded populations
• Parasitism: Higher host density increases parasite transmission
• Stress: Crowding increases physiological stress, reducing reproduction and survival
• Waste Buildup: Toxic metabolic waste accumulates in high-density areas

At Carrying Capacity (N = K)

• Zero Population Growth: Birth rate = Death rate
• Stable Population Size: Number of individuals remains relatively constant
• Resource Balance: Resources are being consumed at the rate they're replenished
• Maximum Sustainable Population: Highest number that can persist long-term

Above Carrying Capacity (N > K) - Overshoot

When a population temporarily exceeds carrying capacity:

• Resource Depletion: Consumption exceeds regeneration rate
• Environmental Degradation: Habitat damage reduces future carrying capacity
• Increased Mortality: Starvation, disease, stress cause death rates to rise
• Decreased Reproduction: Stressed individuals have fewer offspring
• Population Crash: Rapid decline to below K (potentially well below due to habitat damage)

Example: Deer populations without predators can overshoot carrying capacity, overgraze vegetation, damage habitat, then crash from starvation.

Below Carrying Capacity (N < K)

• Abundant Resources: More than enough food, water, space
• Low Competition: Reduced intraspecific competition
• Population Growth: Birth rate > Death rate
• Trend: Population increases toward K

Factors That INCREASE Carrying Capacity

• Increased food supply (better rainfall, more prey)
• Habitat expansion or restoration
• Reduction in predators or competitors
• Improved environmental conditions (favorable climate)
• Disease control or eradication
• For humans: Agricultural technology, medicine, sanitation, resource extraction

Factors That DECREASE Carrying Capacity

• Habitat destruction or degradation
• Resource depletion (overfishing, deforestation)
• Pollution and environmental contamination
• Climate change altering suitable habitat
• Introduction of competitors, predators, or diseases
• Drought or natural disasters
• Fragmentation reducing effective habitat area

Key Question: Are we above, at, or below Earth's carrying capacity for humans?

Arguments vary based on assumptions about technology, consumption patterns, and what quality of life constitutes "sustainable." Signs of potential overshoot include: resource depletion, biodiversity loss, climate change, ocean acidification, and ecosystem degradation.

Characteristics

Definition: Population growth that occurs when resources are unlimited and the population can grow at its maximum rate (biotic potential). Growth accelerates over time, producing a J-shaped curve.

Pattern: Population size increases slowly at first, then faster and faster as the population grows larger. The larger the population, the faster it grows (positive feedback loop).

Shape: J-shaped curve - starts flat, then curves upward steeply

Exponential Growth Equation

Key Point: Growth rate is proportional to population size. As N increases, growth rate increases proportionally. This is why the curve gets steeper over time.

Conditions for Exponential Growth

• Unlimited Resources: Abundant food, water, space, shelter
• Ideal Environmental Conditions: Optimal temperature, moisture, pH
• No Predators or Diseases: Minimal mortality from external factors
• No Competition: Population well below carrying capacity
• No Emigration: Individuals don't leave the population

When Exponential Growth Occurs

• Colonization: Species entering a new habitat with no competition
• Recovery: Population rebounding after a major decline
• Temporarily: After disturbance clears area (fire, flood)
• Laboratory Cultures: Bacteria in fresh nutrient broth
• Invasive Species: Non-native species in new environments without natural predators
• Human Population: Historically experienced near-exponential growth with agriculture and technology

Limitations of Exponential Growth

Critical Point: Exponential growth cannot continue indefinitely in nature. Eventually, limiting factors (resource depletion, waste accumulation, disease, predation) slow growth.

Reality: Exponential growth is usually temporary - a brief phase that transitions to logistic growth as resources become limited.

Characteristics

Definition: Population growth that slows as the population approaches carrying capacity (K). Growth is initially exponential but decelerates as resources become limited, eventually stabilizing at K.

Pattern: S-shaped (sigmoidal) curve with three phases:

1. Lag Phase: Slow initial growth (small population, low reproduction)
2. Exponential/Log Phase: Rapid growth (abundant resources, low competition)
3. Stationary/Plateau Phase: Growth slows and stabilizes at carrying capacity

Shape: S-shaped curve - slow start, steep middle, levels off at top

Logistic Growth Equation

Key Point: The term [(K - N)/K] is the key difference from exponential growth:

• When N is small, (K - N)/K ≈ 1, so growth is approximately exponential
• As N approaches K, (K - N)/K approaches 0, so growth slows
• When N = K, (K - N)/K = 0, so growth stops (ΔN/Δt = 0)

When Logistic Growth Occurs

• Natural Populations: Most real-world populations follow logistic patterns
• Resource Limitation: When food, space, or other resources become scarce
• Stable Environments: Mature ecosystems near carrying capacity
• Density-Dependent Regulation: Competition, predation, disease increase with density
• K-Selected Species: Populations adapted to crowded conditions

Factors That Slow Growth at K

• Resource Competition: Insufficient food/water/space for all individuals
• Increased Predation: Higher prey density attracts more predators
• Disease Spread: Pathogens transmit more easily in crowded populations
• Stress: Crowding causes physiological stress, reducing reproduction
• Territoriality: Dominant individuals exclude others from breeding
• Waste Accumulation: Toxic metabolites inhibit growth

Components

• Horizontal Axis: Percentage or number of individuals in each age group
• Vertical Axis: Age groups (typically 5-year cohorts: 0-4, 5-9, 10-14, etc.)
• Left Side: Males (conventionally)
• Right Side: Females (conventionally)
• Bottom (Wide Base): Young population (pre-reproductive age: 0-14 years)
• Middle: Reproductive age adults (15-44 years)
• Top (Narrow): Older population (post-reproductive age: 45+ years)

Three Key Age Groups

• Pre-Reproductive (0-14 years): Children not yet able to reproduce; future reproductive potential
• Reproductive (15-44 years): Adults capable of reproduction; current birth rates depend on this group
• Post-Reproductive (45+ years): Older adults past prime reproductive years; require healthcare and social services

Rapid Growth (Expanding) Pyramid

Shape: Wide base (pyramid shape) - many young people, fewer old people

Indicates: High birth rate, short life expectancy, young population

Prediction: Population will continue to GROW rapidly as large cohorts of young people reach reproductive age

Characteristics:

• Large proportion (>35%) under age 15
• Small proportion (<5%) over age 65
• High fertility rate (>3 children per woman)
• High infant mortality rate
• Short life expectancy

Examples: Most developing countries - Nigeria, Afghanistan, Niger, Democratic Republic of Congo, Mali

Economic/Social Implications:

• Need for schools, education, child healthcare
• High dependency ratio (many dependents per working adult)
• Future job creation challenges as youth enter workforce
• Potential for rapid population momentum

Slow Growth (Stationary) Column

Shape: Relatively uniform width (column or box shape) - similar numbers across age groups

Indicates: Low birth rate, low death rate, long life expectancy, stable population

Prediction: Population will remain STABLE with little growth or decline (replacement-level fertility)

Characteristics:

• Moderate proportion (~20-25%) under age 15
• Growing proportion (10-15%) over age 65
• Fertility rate near replacement level (~2.1 children per woman)
• Low infant mortality
• High life expectancy

Examples: United States, Canada, Australia, China (recently)

Economic/Social Implications:

• Balanced dependency ratio
• Aging population requiring increased healthcare
• Stable workforce
• Social security and pension sustainability concerns

Negative Growth (Declining) Inverted Pyramid

Shape: Narrow base, wider middle/top (inverted pyramid or urn shape) - fewer young, more elderly

Indicates: Very low birth rate, very low death rate, very long life expectancy, aging population

Prediction: Population will DECLINE as small cohorts of young people cannot replace larger cohorts dying

Characteristics:

• Small proportion (<15-18%) under age 15
• Large proportion (>15-20%) over age 65
• Fertility rate well below replacement (<1.5-2.0 children per woman)
• Very low infant mortality
• Very high life expectancy (>80 years)

Examples: Japan, Italy, Germany, South Korea, Russia, much of Eastern Europe

Economic/Social Implications:

• High dependency ratio (many elderly per working adult)
• Shrinking workforce and tax base
• Unsustainable pension and healthcare costs
• Labor shortages
• Economic stagnation potential
• Need for immigration to maintain workforce

Population Momentum

Definition: The tendency for population growth to continue even after fertility rates decline to replacement level, due to a large cohort of young people entering reproductive age. Even if each woman has only 2 children, the large number of women reproducing causes continued growth for 1-2 generations.

Dependency Ratio

Definition: The ratio of dependents (children 0-14 and elderly 65+) to working-age population (15-64). High ratios strain economies as fewer workers support more dependents. Calculated as: [(population 0-14 + population 65+) / population 15-64] × 100

Sex Ratio

Definition: The proportion of males to females in a population. Normal ratio is approximately 1:1 (105 males born per 100 females, but evens out due to higher male mortality). Significant deviations indicate sex-selective practices (abortion, infanticide) or migration patterns.

Replacement-Level Fertility

Definition: The TFR required for a population to exactly replace itself from one generation to the next, resulting in zero population growth (assuming no migration).

Value:

• Developed Countries: ~2.1 children per woman
• Developing Countries: ~2.3-2.5 children per woman (higher due to higher infant/child mortality)

Why 2.1 and not 2.0?

• Two children replace the two parents
• The extra 0.1 accounts for mortality (some children die before reproducing)
• Also accounts for slightly higher male births (105:100 ratio)

TFR Ranges and Population Trends

• TFR > 2.1: Population will GROW (above replacement)
• TFR = 2.1: Population will STABILIZE (at replacement level)
• TFR < 2.1: Population will eventually DECLINE (below replacement)
• TFR > 5.0: Very rapid growth (doubling time <30 years)
• TFR 3.0-5.0: Rapid growth
• TFR 2.1-3.0: Moderate growth
• TFR < 1.5: Rapid aging and potential population crisis

Global TFR Examples (Approximate Current Values)

• Highest TFR: Niger (~6.8), Somalia (~6.0), Chad (~5.8)
• High TFR (>4.0): Sub-Saharan Africa, Afghanistan
• Moderate TFR (2.5-4.0): India, Pakistan, some Middle Eastern countries
• Replacement Level (~2.1): United States, France, New Zealand
• Below Replacement (<2.1): China (~1.7), Canada (~1.5), Australia (~1.7)
• Lowest TFR (<1.5): South Korea (~0.9), Singapore (~1.0), Italy (~1.3), Japan (~1.3), Spain (~1.3)

Factors That INCREASE TFR (Pronatalist)

• Economic: Children as economic assets (agricultural labor, old-age security)
• Cultural/Religious: Traditions favoring large families, religious teachings
• High Infant Mortality: Parents have more children expecting some will die
• Lack of Education: Limited knowledge about family planning
• Low Status of Women: Limited autonomy in reproductive decisions
• Limited Contraception: Poor access to birth control
• Early Marriage: Longer reproductive window
• Rural/Agricultural Economy: Children needed for farm labor

Factors That DECREASE TFR (Antinatalist)

• Women's Education: More educated women have fewer children, delay childbearing
• Women's Employment: Career opportunities compete with childrearing time
• Urbanization: Children are economic burdens in cities (housing, education costs)
• Contraceptive Access: Availability of birth control methods
• Economic Development: Higher living standards, less need for child labor
• Low Infant Mortality: Confidence that children will survive reduces need for "extras"
• Pension/Social Security: Government support reduces need for children as old-age security
• Later Marriage: Shorter reproductive window
• Child-Rearing Costs: High costs of raising children (education, healthcare)
• Government Policies: Family planning programs, education campaigns

Major Milestones

• 10,000 BCE: ~5 million (pre-agricultural, hunter-gatherers)
• 1 CE: ~300 million (agricultural revolution established)
• 1804: 1 billion (first billion milestone)
• 1927: 2 billion (123 years for second billion)
• 1960: 3 billion (33 years)
• 1974: 4 billion (14 years)
• 1987: 5 billion (13 years)
• 1999: 6 billion (12 years)
• 2011: 7 billion (12 years)
• 2022: 8 billion (11 years)
• Projected 2037: 9 billion

Key Events Accelerating Growth

• Agricultural Revolution (~10,000 BCE): Domestication of plants/animals allowed food surplus, permanent settlements, population increase
• Industrial Revolution (1750s-1800s): Improved food production, sanitation, medicine reduced death rates dramatically while birth rates remained high
• Medical/Public Health Revolution (1900s): Vaccines, antibiotics, clean water, sewage systems drastically reduced mortality, especially infant/child deaths
• Green Revolution (1960s-1970s): High-yield crops, fertilizers, pesticides increased food production supporting larger populations

Current Trends

• Global Growth Rate: Declining from peak of ~2.1% (1960s) to current ~1.0% (2023)
• Growth Slowing: Still growing but rate of growth is decreasing
• Regional Variation: Africa growing rapidly; Europe declining; Asia stabilizing
• Peak Projection: Expected to peak at ~9-10 billion around 2080-2100, then potentially decline

Doubling Time

Definition: The number of years it takes for a population to double in size at its current growth rate.

Importance: Helps visualize exponential growth and understand the urgency of population/resource challenges.

The Rule of 70

Doubling Time (years) = 70 / Growth Rate (%)

How to Use:

1. Find the annual population growth rate as a percentage
2. Divide 70 by that percentage
3. Result = years to double

Examples:

• Growth rate = 1% per year: 70 / 1 = 70 years to double
• Growth rate = 2% per year: 70 / 2 = 35 years to double
• Growth rate = 3% per year: 70 / 3 ≈ 23 years to double
• Growth rate = 0.5% per year: 70 / 0.5 = 140 years to double
• Growth rate = 7% per year: 70 / 7 = 10 years to double

Important Notes

• Rule of 70 works for ANY growth rate (population, economy, investment, pollution)
• Negative growth rates indicate declining populations (no doubling, but can calculate "half-life")
• Small differences in growth rate = BIG differences in doubling time
• Assumes constant growth rate (rarely true in reality, but useful approximation)

Stage 1: Pre-Industrial (High Stationary)

Birth Rate: HIGH | Death Rate: HIGH | Population Growth: LOW/STABLE

Characteristics:

• Birth Rate: Very high (~40-50 per 1,000) - no family planning, children needed for labor, high infant mortality
• Death Rate: Very high (~30-50 per 1,000) - disease, famine, poor sanitation, limited medicine
• Life Expectancy: Short (~25-35 years)
• TFR: Very high (>6 children per woman)
• Population Growth: Slow - birth and death rates roughly balance
• Age Structure: Wide base pyramid but high mortality in all age groups

Reasons for High Birth Rate: No contraception, cultural/religious norms, high infant mortality, children as economic assets, women's low status

Reasons for High Death Rate: Disease (plague, malaria, tuberculosis), famine, poor hygiene, contaminated water, no modern medicine

Examples: No countries currently in Stage 1 (all have experienced at least some development). Historically: pre-industrial Europe, pre-colonial societies

Stage 2: Developing/Early Transitional (Early Expanding)

Birth Rate: HIGH | Death Rate: RAPIDLY DECLINING | Population Growth: VERY HIGH

Characteristics:

• Birth Rate: Remains high (~35-45 per 1,000) - cultural lag, traditions persist
• Death Rate: FALLS RAPIDLY (~15-25 per 1,000) - improved healthcare, sanitation, nutrition
• Life Expectancy: Increasing (~40-55 years)
• TFR: High (4-6 children per woman)
• Population Growth: VERY RAPID - large gap between birth and death rates (POPULATION EXPLOSION)
• Age Structure: Very wide base (youth bulge)

Why Death Rate Declines: Improved sanitation/clean water, vaccines and antibiotics, better nutrition/food supply, basic healthcare access, reduced infant mortality

Why Birth Rate Stays High: Cultural inertia (traditions change slowly), lack of family planning access, women's limited education/empowerment, children still valued as labor/old-age security

Examples: Afghanistan, Niger, Mali, Somalia, Angola, sub-Saharan Africa

Stage 3: Developed/Late Transitional (Late Expanding)

Birth Rate: RAPIDLY DECLINING | Death Rate: LOW | Population Growth: SLOWING

Characteristics:

• Birth Rate: FALLS RAPIDLY (~15-25 per 1,000) - family planning, economic changes
• Death Rate: Low and stable (~10-15 per 1,000)
• Life Expectancy: Continues increasing (~60-75 years)
• TFR: Declining (2.5-3.5 children per woman)
• Population Growth: Still growing but RATE SLOWING - gap between birth/death rates narrowing
• Age Structure: Base narrowing, middle widening

Why Birth Rate Declines: Urbanization (children costly in cities), women's education and employment, access to contraception, shift from agricultural to industrial economy, reduced infant mortality (fewer "replacement" births needed), government family planning programs, rising cost of raising children

Examples: India, Brazil, Mexico, Indonesia, South Africa, much of Latin America and Southeast Asia

Stage 4: Post-Industrial (Low Stationary)

Birth Rate: LOW | Death Rate: LOW | Population Growth: ZERO or SLOW

Characteristics:

• Birth Rate: Low (~10-15 per 1,000)
• Death Rate: Low (~8-12 per 1,000), may rise slightly as population ages
• Life Expectancy: High (~75-85 years)
• TFR: At or near replacement level (~2.0-2.5 children per woman)
• Population Growth: Zero or very slow - birth and death rates balanced
• Age Structure: Column shape, aging population

Why Birth and Death Rates Both Low: High quality healthcare, advanced medicine, excellent living conditions, widespread contraception use, women's empowerment, career focus, delayed marriage/childbearing, high cost of children, urbanized lifestyle

Examples: United States, United Kingdom, France, Australia, Canada, China (recently transitioning from Stage 3 to 4)

Stage 5: Post-Industrial (Declining) - Debated/Emerging

Birth Rate: VERY LOW | Death Rate: LOW-MODERATE | Population Growth: NEGATIVE (DECLINE)

Characteristics:

• Birth Rate: Very low (~8-10 per 1,000) - falls below death rate
• Death Rate: Low but may exceed birth rate (~10-13 per 1,000), rising slightly due to aging
• Life Expectancy: Very high (~80-85+ years)
• TFR: Well below replacement (<1.3-1.7 children per woman)
• Population Growth: NEGATIVE - population shrinking
• Age Structure: Inverted pyramid, elderly dominated

Challenges: Aging population, shrinking workforce, pension/healthcare burden, labor shortages, economic stagnation concerns, need for immigration

Why Birth Rate So Low: Extreme urbanization, very high cost of living and childcare, career prioritization, delayed marriage/childbearing, individualism, lack of family-friendly policies

Examples: Japan, Italy, Germany, South Korea, Spain, Russia, much of Eastern Europe

Strengths of the DTM

• Helps predict population trends based on development
• Shows demographic transition is linked to economic development
• Generally describes pattern most developed countries followed
• Useful for policy planning and resource allocation

Limitations of the DTM

• Based on European/Western development - may not apply universally
• Doesn't account for government policies (China's one-child policy)
• Ignores migration effects (major factor in some countries)
• Some countries may "stall" at Stage 2 or 3 (poverty trap)
• Timelines vary - some countries transition faster than others
• Stage 5 was added later - original model had only 4 stages

✓ Must-Know Concepts

• Generalist vs. specialist species
• r-selected vs. K-selected traits
• 3 survivorship curve types
• Carrying capacity (K) definition
• Exponential vs. logistic growth
• Age structure diagram interpretation
• TFR and replacement level (2.1)
• Rule of 70 calculation
• 5 demographic transition stages

⚠️ Common Mistakes to Avoid

• Confusing generalist/specialist with r/K
• Type I = concave (FALSE! It's convex)
• Replacement level = 2.0 (FALSE! It's 2.1)
• K is constant (FALSE! K can change)
• J-curve is sustainable (FALSE! Temporary)
• Wide base pyramid = declining (FALSE!)
• TFR = birth rate (different metrics!)
• Forgetting Rule of 70 formula
• Stage 2 has slow growth (FALSE! Fastest!)