Lecture 5- Population Ecology

Population Ecology Overview

Definitions

  • Organism: A single living individual.

  • Population: A group of the same species living in the same place and time. Measurement of population size is crucial for various ecological studies.

  • Ecosystem: The interaction between living organisms and nonliving components in an area.

  • Community: All populations that occupy the same region.

Key Concepts in Population Ecology

1. Habitats and Population Density

  • A habitat is the physical location where members of a population live.

  • Population Density: Refers to the number of individuals of a species per unit area or volume.

    • For example, saguaro cacti thrive only in the arid deserts of Southwestern US.

2. Population Distribution Patterns

  • Uniform Distribution: Evenly spaced individuals, often due to competition.

  • Clumped Distribution: Individuals grouped in patches, often around resources.

  • Random Distribution: Individuals are spread randomly in an area, not influenced by competition or resources.

3. Measuring Population Size

  • Various methods used:

    • Sampling: Counting individuals in small quadrats and scaling up.

    • Mark-Recapture: Capturing animals, marking them, then recapturing to estimate total population size.

    • Relative Measures: Using hunting success or camera trap surveys.

Population Growth Dynamics

1. Factors Affecting Population Size

  • Additions: Births and immigration increase population.

  • Subtractions: Deaths and emigration decrease population.

2. Population Growth Rate (r)

  • Birth Rate (b): Number of individuals born per individual per time unit.

  • Death Rate (d): Number of individuals dying per individual per time unit.

  • Formula:r = b - d

    • Example:

      • If birth rate (b) = 0.3 and death rate (d) = 0.1, then r = 0.2.

  • Growth Increment (G):G = rN

    • Represents the number of individuals added or lost in population in one time unit.

3. Population Trends Over Time

  • Next Year's Population Size: N(t+1) = N(t) + G

    • Example: In a population of 10 with 2 births and 1 death → G = (0.3 - 0.1) * 10 = 2 → N(1 year) = 10 + 2 = 12.

Exponential vs. Logistic Growth

1. Exponential Growth

  • Characterized by:

    • Unlimited resources.

    • Produces a J-shaped curve on a graph.

    • Growth accelerates rapidly when conditions are ideal.

2. Logistic Growth

  • Characterized by:

    • A Carrying Capacity (K): Maximum number of individuals an environment can support based on resources.

    • S-shaped (sigmoid) curve representing growth slowing as the population approaches K.

    • Growth factor decreases as N gets close to K, such that G approaches 0 when N = K.

Density-Dependent and Density-Independent Factors

1. Density-Dependent Factors

  • Affect population density, such as competition for food, space, and mates, and the spread of disease.

2. Density-Independent Factors

  • Do not depend on population density, e.g., natural disasters which can drastically alter population sizes regardless of current density.

Application in Conservation

  • Understanding life history traits helps in addressing population dynamics in conservation efforts, influencing strategies to manage endangered species effectively.