Density-dependent (disease, competition) intensify with population size; density-independent (natural disasters) don't.
Age Structure Diagrams
Wide base = rapid future growth. Rectangular shape = stable population. Used to predict a country's population trajectory.
Total Fertility Rate
Average children per woman. Replacement level ≈ 2.1 in developed countries — needed to keep population stable.
Demographic Transition Model
Four stages as countries develop: high birth/death rates → falling death rates (rapid growth) → falling birth rates → low birth/death rates (stable).
The key terms you must know
Carrying capacity (K) — the maximum population an environment can sustain indefinitely.
Exponential growth — unrestricted, accelerating growth producing a J-curve.
Logistic growth — growth that slows as resources become limiting, producing an S-curve.
Survivorship curve — a graph of mortality patterns across a species' lifespan (Types I, II, III).
Age structure diagram — a population pyramid showing age/sex distribution, used to predict growth.
Total fertility rate (TFR) — average children per woman over her lifetime.
Replacement-level fertility — the TFR (~2.1) needed to keep a population stable.
Demographic transition model — the four-stage shift in birth/death rates as countries develop.
Rate of natural increase — crude birth rate minus crude death rate.
Density-dependent vs. density-independent limiting factors — whether the factor's effect scales with population density.
Key themes to remember
No population grows exponentially forever. Resources eventually limit growth, producing logistic growth in the real world.
Life history strategy predicts survivorship. r-selected species cluster around Type III curves; K-selected species cluster around Type I.
Age structure predicts the future, not just the present. A wide-based pyramid today means rapid growth tomorrow, even if current growth looks modest.
Human population growth follows the same logic as other species — with added social factors. Education, healthcare, and economic development shift birth and death rates predictably.
The demographic transition model explains global population trends. Most developed countries are in Stage 4; many developing countries are in Stage 2 or 3.
Common exam traps
Don't confuse r-selected with "bad" or K-selected with "good." Both are valid evolutionary strategies suited to different environments.
Carrying capacity is not fixed forever. It can change if resource availability or environmental conditions change.
TFR below 2.1 means a population will eventually shrink (absent immigration) — don't assume any population is automatically growing.
Read age structure diagrams carefully. A pyramid that's wide at the base AND narrow at the top indicates high growth potential; don't just glance at the overall size.
Stage 2 of demographic transition has the fastest population growth — not Stage 1 (high birth AND death rates) or Stage 4 (both low).
Exponential vs. logistic — know which curve shape matches which scenario in a data/graph-based question.