Unit 3 Biotic potential Tap / Space to flip
Unit 3 Maximum reproductive rate of a population under ideal conditions (no resource limits, no predators).
Unit 3 · 10–15% of exam
Populations
Population dynamics, R/K selection, demographic transition, age-structure pyramids, human population. The priority deep dive has the worked Rule-of-70 problems.
Must-know content
- r-selected vs. K-selected: r-strategists produce many small offspring with little parental care, short lives, unstable habitats (insects, weeds, frogs). K-strategists produce few large offspring with much care, long lives, stable habitats (elephants, humans, oaks).
- Survivorship curves:
- Type I — humans, K-selected; low youth mortality, high old-age mortality.
- Type II — birds, lizards; constant mortality across ages.
- Type III — fish, plants, r-selected; very high early mortality, few survive to adulthood.
- Carrying capacity (K): max population an environment can sustain. Limited by resources, predators, disease, space.
- Exponential growth — J-curve, dN/dt = rN. Unconstrained.
- Logistic growth — S-curve, dN/dt = rN[(K−N)/K]. Slows as N → K.
- Population growth rate: r = (births + immigration) − (deaths + emigration).
- Doubling time = 70 / r% — Rule of 70.
- Total Fertility Rate (TFR): average lifetime children per woman. Replacement TFR ≈ 2.1 (developed nations); higher (~2.3) in nations with high infant mortality.
- Demographic transition model — 4 (or 5) stages. See the priority page for the full breakdown.
- Age-structure pyramids: wide base = expanding (Niger), rectangle = stable (USA), narrow base = declining (Japan).
- Factors that lower TFR: education of women, family-planning access, urbanization, low infant mortality, economic opportunity for women.
- Density-dependent factors (disease, competition) intensify with crowding. Density-independent factors (weather, fire) act regardless of density.
Example questions
MCQ A country has a growth rate of 2%. What is the doubling time? (A) 14 yr (B) 35 yr (C) 70 yr (D) 140 yr
Answer: B. Rule of 70: 70 / 2 = 35 years.
FRQ Identify TWO factors that lower TFR in developed countries.
Answer: Any two of: (1) Increased education of women, leading to delayed childbearing and careers. (2) Greater access to family planning and contraception. (3) Low infant mortality, which reduces the need to have "extra" children as insurance. (4) Urbanization — children become an economic cost rather than farm labor. (5) High cost of raising children in industrialized economies.
MCQ A pyramid-shaped age-structure diagram with a wide base indicates a population that is: (A) Stable (B) Declining (C) Rapidly growing (D) Aging
Answer: C. A wide base means many young people who will reach reproductive age — strong future growth.
Drill flashcards
Unit 3 Carrying capacity (K) Tap / Space to flip
Unit 3 Maximum population size an environment can sustain. Limited by resources, predators, disease, space.
Unit 3 Exponential growth Tap / Space to flip
Unit 3 Population grows at a constant rate (J-curve). Occurs only when resources are unlimited.
Unit 3 Logistic growth Tap / Space to flip
Unit 3 Growth slows as N approaches K, producing an S-curve. dN/dt = rN[(K-N)/K].
Unit 3 r-selected species Tap / Space to flip
Unit 3 Many small offspring, little parental care, short lives, unstable habitat (e.g., insects, weeds, frogs).
Unit 3 K-selected species Tap / Space to flip
Unit 3 Few large offspring, much parental care, long lives, stable habitat (e.g., elephants, humans, oak trees).
Unit 3 Survivorship curves Tap / Space to flip
Unit 3 Type I (humans, late mortality), Type II (birds, constant), Type III (fish, very high early mortality).
Unit 3 Density-dependent factor Tap / Space to flip
Unit 3 Limiting factor whose effect intensifies as population density rises (disease, competition).