Population Ecology Overview

Overview

This lecture introduces population ecology, focusing on how populations change over time, factors affecting their growth, and models used to describe these changes.

Population Characteristics

• Population size changes through births, deaths, immigration (entry), and emigration (exit).
• Density is the number of individuals per unit area.
• Distribution describes how individuals are spread (random, uniform, or clumped).
• Sex ratio is the proportion of males to females.
• Age structure refers to the distribution of individuals among age groups.

Factors Affecting Population Growth

• Intrinsic growth rate (r) measures how fast a population grows under ideal conditions.
• Density-dependent factors (e.g., food, water, shelter, disease) limit growth as density increases.
• Density-independent factors (e.g., floods, fires) affect populations regardless of density.
• Carrying capacity (K) is the maximum population size an environment can support.

Population Growth Models

• Exponential growth model: population increases rapidly (J-shaped curve), equation: Nₜ = N₀ * e^(r * t).
• Logistic growth model: population grows rapidly, then levels off at carrying capacity (S-shaped curve).
• Exponential growth assumes unlimited resources; logistic growth includes limits (K).

Life History Strategies

• K-selected species have few offspring, high parental care, and populations stabilize near K (e.g., humans, whooping cranes).
• r-selected species have many offspring, little parental care, and populations show boom-and-bust cycles (e.g., Arctic hares).

Survivorship Curves

• Type I: High survival early, most die late (K-selected, e.g., humans).
• Type II: Constant death rate (e.g., some birds).
• Type III: High early mortality, few survivors (r-selected, e.g., acorns).

Key Terms & Definitions

• Intrinsic Growth Rate (r) — the natural rate at which a population increases under optimal conditions.
• Carrying Capacity (K) — the largest population size an environment can support.
• Density-dependent Factor — factors whose effects increase with population density.
• Density-independent Factor — factors affecting population regardless of density.
• Exponential Growth — rapid, unlimited population increase (J-curve).
• Logistic Growth — population increase slows as it nears carrying capacity (S-curve).
• K-selected Species — species with stable populations, few offspring, and high parental care.
• r-selected Species — species with fluctuating populations, many offspring, and low parental care.
• Survivorship Curve — graph showing the percentage of individuals surviving at each age.

Action Items / Next Steps

• Practice population growth calculations using the given formulas.
• Review examples of K-selected and r-selected species.
• Understand and be able to interpret survivorship curves.