Summary
Highlights
Survivorship curves illustrate the percentage of a population surviving at different life stages. Type 1 (K-strategists) shows high survival until old age. Type 3 (R-strategists) indicates high mortality early in life. Type 2 (some K-strategists like birds) exhibits a consistent mortality rate throughout the lifespan.
Mr. Smees introduces Unit 3, which focuses on population dynamics in both natural ecosystems and human societies. The first five topics cover natural populations, while the latter four focus on human populations. Timestamps are provided for easy navigation.
Specialist species, like pandas, rely on a narrow niche and specific resources, making them vulnerable to ecosystem changes. Generalist species, such as raccoons, have a wider tolerance for food and habitat, making them more adaptable to disturbances. Generalists thrive in changing environments, while specialists are advantaged in stable ecosystems.
R-strategists (e.g., insects) prioritize quantity over quality, producing many offspring with little parental care, leading to high population growth rates. K-strategists (e.g., mammals) prioritize quality, having few offspring but investing heavily in their care, resulting in slower growth rates and populations near carrying capacity. R-strategists are prone to being invasive species, while K-strategists are more susceptible to invasive R-selected species.
Carrying capacity is the maximum population an ecosystem can sustain, determined by limiting resources like food, water, and shelter. Natural populations often experience 'overshoot and die-off' cycles when they exceed carrying capacity. Density-dependent factors (competition, disease) and density-independent factors (natural disasters) influence population growth regardless of density.
Species exhibit different distributions: uniform (e.g., seabirds), random (e.g., trees), or clumped (e.g., herds for protection). Exponential growth, a J-shaped curve, occurs with abundant resources but is unsustainable. Logistic growth, an S-shaped curve, models more realistic growth that slows as it approaches carrying capacity.
Age structure diagrams visualize the proportion of a country's population in pre-reproductive (0-14), reproductive (15-44), and post-reproductive (45+) age cohorts. A large pre-reproductive cohort indicates high birth rates and rapid growth, often seen in industrializing nations with agrarian economies. Equal pre-reproductive and reproductive cohorts suggest slow growth and nearing stable population size in industrialized nations. A smaller pre-reproductive cohort than reproductive indicates negative growth in highly developed countries.
TFR, the average number of children a woman has, is influenced by the average age of first childbirth. Early childbearing increases TFR. Economic, social, and cultural factors play a role: agrarian economies value child labor, while limited education for women can restrict financial independence and family planning choices. Cultural and religious values can also encourage larger families.
Developed nations have lower TFRs due to economic development, improved healthcare, and increased educational and career opportunities for women, leading to delayed childbirth. Access to family planning resources also plays a significant role. Governments may intervene to influence birth rates through education campaigns or economic incentives.
Crude birth rate (CBR) and crude death rate (CDR) are births/deaths per 1,000 people. Percent growth rate is calculated as (CBR - CDR) / 10. The Rule of 70 (70 / growth rate %) estimates population doubling time. Malthusian theory, which predicted famine due to exponential population growth surpassing linear food production, was challenged by agricultural advancements like the Green Revolution.
Access to clean water impacts infant mortality (high rates lead to higher TFR as parents have 'replacement children'). Replacement level fertility is the TFR needed for a stable population (around 2.1 in developed nations, higher in developing). GDP, a measure of national wealth, correlates with better healthcare, education, and lower TFRs.
The Demographic Transition Model describes how countries industrialize, leading to falling death rates, then falling birth rates, and eventual population stabilization or decline. Stage 1 (Pre-industrial) has high birth and death rates, little growth. Stage 2 (Industrializing) sees falling death rates due to improved healthcare, but birth rates remain high, leading to rapid growth. Stage 3 (Industrialized) has declining birth rates as education and opportunities for women increase, slowing growth. Stage 4 (Highly Developed) has TFR below replacement level, potentially leading to negative growth.