Overview
This lecture explains population growth curves, specifically exponential and logistic growth, their phases, key terminology, and how factors like resources and environmental resistance affect population size.
Population Growth Curves: Types & Characteristics
- Populations exhibit two main growth curves: exponential (J-shaped) and logistic (S-shaped).
- Exponential growth occurs when resources are abundant and populations increase rapidly after a slow start.
- Logistic growth includes an initial exponential phase, then levels off as resources become limited and carrying capacity is reached.
Key Concepts: Carrying Capacity & Environmental Resistance
- Carrying capacity is the maximum number of individuals an ecosystem can support sustainably.
- Environmental resistance refers to factors (e.g., space, food, water) that limit population growth.
- Environmental resistance determines carrying capacity; as resources decrease, carrying capacity goes down, and vice versa.
Phases of Population Growth (Logistic Curve)
- Lag Phase: Population growth is slow due to few breeding individuals.
- Exponential Phase: Rapid population increase due to plentiful resources and many breeding pairs.
- Transitional (Decelerating) Phase: Growth slows as carrying capacity is approached and resources become scarce.
- Plateau (Stationary) Phase: Population stabilizes, fluctuating around carrying capacity as births and deaths balance.
Examples and Interpretations
- Unicellular organisms (like bacteria) show rapid population cycles in days; larger animals (like fur seals) take years.
- Population graphs' shapes and steepness depend on lifespan, reproductive rate, and environmental conditions.
- Overshoot occurs when populations exceed carrying capacity, leading to a crash or extinction if the environment cannot recover.
Growth Curve Events and Extinction
- Short-term overshoots are sometimes followed by recovery and stabilization.
- Permanent decline (extinction) happens if carrying capacity drops due to events like resource depletion or disasters.
Key Terms & Definitions
- Carrying Capacity — Maximum population size an environment can sustainably support.
- Environmental Resistance — Limiting factors that prevent indefinite population growth.
- Exponential Growth — Rapid, unchecked population increase when resources are abundant.
- Logistic Growth — Population growth that slows near carrying capacity, forming an S-shaped curve.
- Lag Phase — Initial slow population growth due to few reproductive individuals.
- Exponential Phase — Period of rapid population growth.
- Transitional/Decelerating Phase — Growth rate slows as resources become limited.
- Plateau/Stationary Phase — Population size stabilizes at carrying capacity.
- Overshoot — Population exceeds carrying capacity before declining.
Action Items / Next Steps
- Practice drawing, labeling, and explaining exponential and logistic growth curves.
- Review and memorize key terms for use in exam explanations.
- Watch upcoming lecture/video on predator-prey population cycles.