Population Growth Notes

How Populations Grow

Key Concepts

  • Population: A group of organisms of a single species that lives in a given area.

Learning Objectives

  • Explain the ways ecologists study populations.

  • Identify the factors that affect population growth.

  • Explain what happens during exponential growth.

  • Explain what happens during logistic growth.

Describing Populations

  • Geographic range: The area inhabited by a population.

  • Density: The number of individuals per unit area.

  • Distribution: How individuals in a population are spaced out across the range of the population. Types include random, uniform, and clumped.

  • Growth rate: Whether a population is growing, shrinking, or stable.

  • Age structure: Distribution of ages within the population.

Geographic Range

  • The geographic range is the area inhabited by a population.

Density and Distribution

  • Density: Refers to the number of individuals per unit area.

  • Distribution: Refers to how individuals in a population are spaced out across the range of the population.

    • Random

    • Uniform

    • Clumped

Growth Rate

  • Growth \space rate = 0: Population size is unchanged.

  • Growth \space rate > 0: Population size is growing.

  • Growth \space rate < 0: Population size is decreasing.

  • Example: A bacterial population with plenty of resources likely has a growth rate greater than zero.

Population Growth

  • Populations can grow if more individuals are born than die (birth rate > death rate).

  • Immigration: Individuals move into a population's range from elsewhere, causing the population to grow.

  • Emigration: Individuals leave a population, causing it to decrease.

Exponential Growth

  • Under ideal conditions with unlimited resources, a population will grow exponentially, leading to rapid increases in population size.

  • Conditions that promote exponential growth: abundant food and space, protection from predators and disease, and removal of waste products.

Logistic Growth

  • Logistic growth occurs when a population's growth slows and then stops, following a period of exponential growth.

  • Phases of Logistic Growth

    • Phase I: Exponential Growth.

    • Phase II: Growth slows down.

    • Phase III: Growth stops; population stabilizes.

Carrying Capacity

  • The carrying capacity is the maximum number of individuals of a particular species that a particular environment can support.

  • Population stabilizes at carrying capacity.

Age Structure

  • Factors to consider:

    • Ratio of males to females

    • Breeding age of the population

    • Gestation time

    • Number of offspring