Population Ecology

Population Ecology

  • Introduction to Population Ecology

    • Definition: Population ecology explores how biotic (living) and abiotic (non-living) factors influence the abundance, dispersion, and age structure of populations.

    • Example: The survival rate of loggerhead turtle hatchlings during their first journey to the ocean is affected by predation and light.

Key Concepts

Definition of a Population

  • A population is defined as a group of individuals of a single species living in the same general area.

  • Populations are characterized by their boundaries and size.

Density and Dispersion

Density

  • Density: The number of individuals per unit area or volume.

Dispersion

  • Dispersion: The pattern of spacing among individuals within the boundaries of the population.

Estimating Population Size

Importance of Estimation

  • It is often impractical or impossible to count all individuals in a population.

Sampling Techniques

  • Various methods can estimate population sizes, which may include:

    • Extrapolation from small samples.

    • An index of population size (e.g., number of nests).

    • Mark-Recapture Method.

Mark-Recapture Method

  • Scientists capture, tag, and release a sample of individuals (s) from a population.

  • The marked individuals are allowed time to mix back into the population.

  • A second sample (n) is captured, and researchers note how many are marked (x).

  • Population Size Formula: The estimated population size (N) can be calculated as: N = rac{s imes n}{x} where:

    • N is the estimated population size,

    • s is the number of captured individuals in the first sample,

    • n is the total number captured in the second sample,

    • x is the number of marked individuals recaptured.

Factors Influencing Density

  • Density results from the interplay between:

    • Processes that add individuals: Births and immigration.

    • Processes that remove individuals: Deaths and emigration.

Patterns of Dispersion

  • Environmental and social factors influence how individuals are spaced within populations.

Common Patterns

  • Clumped Dispersion: Individuals aggregate in patches; influenced by resource availability and social behaviors (e.g., mating, defense against predators).

  • Uniform Dispersion: Individuals are evenly distributed; often influenced by social interactions like territoriality.

  • Random Dispersion: The position of each individual is independent of others' locations; occurs without strong attractions or repulsions.

Demographics

  • Demography: The study of vital statistics (birth, death, migration rates) within populations and their changes over time.

Life Tables

  • A life table summarizes the survival and reproductive rates of populations by age.

  • Often follows the fate of a cohort (group of individuals of the same age).

  • Males may be excluded in studies of sexually reproducing species since only females produce offspring.

Example: Belding’s Ground Squirrels

  • Life table data provides:

    • Proportions of females alive at each age.

    • The number of offspring produced per female.

Life Table Data Example

  • Table 53.1: Life Table for Female Belding's Ground Squirrels

    • Age (years): 0-1, 1-2, 2-3, etc.

    • Number Alive at Start of Year: 653, 252, 127, etc.

    • Proportion Alive at Start: 1.000, 0.386, 0.197, etc.

    • Death Rate: 0.614, 0.496, 0.472, etc.

    • Average Number of Female Offspring per Female: 0.00, 1.07, 1.87, etc.

    • Source referenced: P. W. Sherman and M. L. Morton, "Demography of Belding's ground squirrel," Ecology, 1984.

Survivorship Curves

  • A survivorship curve visually represents life table data.

Types of Survivorship Curves

  • Type I: Low death rates during early and middle life, increasing death rates at older ages.

  • Type II: Constant death rate throughout the organism's life span.

  • Type III: High death rates for young individuals and lower death rates for survivors.

  • Visual Representation: Graphs illustrate these patterns.

Reproductive Rates

  • Ecologists estimate the number of breeding females using approaches such as DNA profiling for loggerhead turtles, identifying females based on genetic profiles.

Population Growth Models

Exponential Population Growth

  • Applies to idealized conditions with unlimited resources, where populations increase by a constant proportion.

  • Change in population size can be summarized as:
    ext{Change in population size} = ext{Births} + ext{Immigrants entering population} - ext{Deaths} - ext{Emigrants leaving population}

Exponential Growth Equation

  • The equation that describes exponential population growth is: rac{dN}{dt} = rN where:

    • N is the population size,

    • r is the intrinsic rate of increase, symbolizing the per capita growth rate of the population.

Characteristics of Exponential Growth

  • Results in a J-shaped curve, where the rate of increase is constant but adds more individuals over time as the population grows larger.

    • Example: The elephant population in Kruger National Park rapidly increased following a hunting ban.