Rucete ✏ AP Environmental Science In a Nutshell
3. Populations
This unit focuses on population dynamics, including how species are classified by reproductive strategies, growth curves, limiting factors, human population trends, and how populations are analyzed through demographic data. Understanding these concepts helps explain how organisms and human societies interact with their environments.
3.1 Generalist and Specialist Species
Generalist Species
• Can thrive in a wide range of environmental conditions and use various resources.
• Adaptable to environmental change; broad niche.
• Higher tolerance to environmental stressors.
• Less likely to go extinct.
• Example: humans, raccoons.
Specialist Species
• Thrive in a narrow set of environmental conditions with limited resource use.
• Lower adaptability; narrow niche.
• More vulnerable to extinction.
• Example: pandas, koalas.
3.2 K-Selected and r-Selected Species
r-Selected Species
• High reproductive rates, many offspring, low parental care.
• Short lifespan, rapid maturation, population size fluctuates (booms & busts).
• Affected by density-independent factors (e.g., weather, disasters).
• Examples: insects, rodents, algae, bacteria.
K-Selected Species
• Low reproductive rate, few offspring, high parental care.
• Long lifespan, slow maturation, stable population near carrying capacity.
• Affected by density-dependent factors (e.g., competition, predation).
• Examples: elephants, humans, cacti, sharks.
3.3 Survivorship Curves
Type I: Late Loss
• Low mortality in early life, high survival into old age.
• Example: humans, sheep, elephants.
Type II: Constant Loss
• Fairly constant death rate across age groups.
• Example: rodents, songbirds, perennial plants.
Type III: Early Loss
• High mortality among young; few survive to adulthood.
• Example: oysters, sea turtles, fish, parasites.
3.4 Carrying Capacity
Definition
• Carrying capacity (K) is the maximum number of individuals of a species that an environment can support sustainably over time.
• Exceeding K results in population crashes due to limited resources.
Regulating Factors
• Sunlight availability
• Food supply
• Nutrient levels
• Oxygen (in aquatic systems)
• Space
Population Dynamics
• Populations below K tend to grow; those above K decline.
• Overpopulation beyond K leads to resource depletion and population crashes.
3.5 Population Growth and Resource Availability
Dispersal Patterns
• Clumped: individuals gather where resources are concentrated (e.g., elephants, lions).
• Random: individuals are spread unpredictably (e.g., wind-dispersed plants like dandelions).
• Uniform: evenly spaced to reduce competition (e.g., creosote bushes with allelopathy).
Biotic Potential
• Maximum reproductive rate under ideal conditions.
• Influenced by adaptation, migration, resistance to disease, food supply, habitat quality, defense mechanisms.
• Environmental resistance limits population growth (e.g., predation, disease, poor conditions).
J-Curve (Exponential Growth)
• Rapid population increase in a new environment.
• Population crashes when environmental resistance becomes significant (e.g., algal blooms, locust outbreaks).
S-Curve (Logistic Growth)
• Population growth slows as it approaches carrying capacity.
• Reflects stable, self-regulating systems with feedback mechanisms.
Feedback Loops
• Positive: amplifies change (e.g., more births → larger population → more births).
• Negative: stabilizes systems (e.g., predator-prey dynamics).
3.6 Age Structure Diagrams
Structure & Interpretation
• Show distribution of individuals by age and gender.
• Three main groups: pre-reproductive, reproductive, and post-reproductive.
Diagram Shapes
• Pyramid: rapid growth (e.g., Nigeria); wide base, high birth rate.
• Bell: slow/stable growth (e.g., United States); more even distribution.
• Urn: declining growth (e.g., Germany); small base, aging population.
3.7 Total Fertility Rate
Definition
• Total Fertility Rate (TFR) is the average number of children a woman is expected to have during her lifetime.
• A TFR of 2.1 is considered replacement-level fertility in most countries.
Examples of TFRs
• Niger: 7.63 (highest in the world)
• United States: 1.87
• China: 1.60
• Japan: 1.41
• World average: 2.59
Effects of Low TFR
• Population decline and aging if not offset by immigration.
• Increased burden on working-age population to support the elderly.
Factors That Decrease TFR
• Access to healthcare and contraception
• Higher female education and workforce participation
• Urbanization and higher cost of living
• Government incentives and social acceptance of smaller families
• Cultural changes delaying marriage and childbirth
3.8 Human Population Dynamics
Population Growth Trends
• Driven largely by declining death rates, not increased birth rates.
• Advances in agriculture, medicine, sanitation, and water supply improve survival.
Historical Milestones
• Tool use, fire, and agriculture allowed slow early population growth.
• Industrial and medical revolutions caused explosive growth in the last 200 years.
Worldviews
• Earth Wisdom: Respect for natural limits and sustainability.
• Frontier/Planetary Management: Humans control and manage Earth’s resources for our benefit.
Future Projections
• Expected to reach ~9.8 billion by 2050.
• Growth will mostly come from developing countries with young populations.
3.9 Demographic Transition
Definition
• Demographic transition is the shift from high birth and death rates to low birth and death rates as a country industrializes and develops economically.
• It typically progresses through 4–5 stages.
Stage 1: Pre-Industrial
• High birth and death rates → little population growth.
• Harsh living conditions, low medical care, food scarcity.
• Example: very few modern countries; some regions with extreme poverty or disease.
Stage 2: Transitional
• Death rates decline due to medical and sanitation advances.
• Birth rates remain high → rapid population growth.
• Examples: sub-Saharan Africa, Afghanistan, Guatemala.
Stage 3: Industrial
• Birth rates decline due to urbanization, female education, and economic change.
• Growth slows as birth rate nears death rate.
• Examples: India, Mexico, South Africa.
Stage 4: Post-Industrial
• Birth and death rates both low → population stabilizes or slightly declines.
• High standard of living; aging populations.
• Examples: United States, Australia, Canada.
Stage 5: Declining (Sub-Replacement Fertility)
• Birth rate falls below death rate → population shrinkage unless offset by immigration.
• Examples: Japan, Germany, South Korea.
In a Nutshell
Population dynamics are shaped by reproductive strategies, environmental resistance, and resource availability. Generalist and specialist species respond differently to environmental change, and K- vs. r-selected strategies influence population growth patterns. Human population trends are analyzed through TFR, growth rates, and age-structure diagrams. The demographic transition model explains how societies evolve from high to low growth. Ultimately, understanding population trends helps address sustainability challenges and resource management.