Population Ecology and the Effects of Density

AP Topics 8.3 & 8.4: Population Ecology

• Population: A group of individuals of the same species living in an area.
  • Population Ecology: Analyzes factors affecting population size and changes over time.

Key Concepts in Population Dynamics

• Density: The number of individuals per unit area.

  • Methods to determine density:
  • Direct counting (rarely practical).
  • Sampling techniques: Count small areas, average them to estimate total population size.

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

  • Types of Dispersion:
  • Clumped: Individuals gather in patches.
  • Uniform: Evenly spaced due to territoriality.
  • Random: Unpredictable spacing, not common.

• Population Size: Is not static and can be influenced by:

  • Births and deaths.
  • Immigration and emigration.

• Demography: The study of vital statistics of populations and changes over time.

  • Life Table: An age-specific summary of survival, represented by a survivorship curve.

Survivorship Curves

• Type I Curve: Low death rate in early/middle life; high death rate in late life.
• Type II Curve: Constant mortality rate throughout the lifespan.
• Type III Curve: High death rate early in life; survivors have lower death rates later.

Change in Population Size

• Per Capita Rate of Increase Formula:
  • $\Delta N = (Birth Rate - Death Rate) \times N$
  • Where $\Delta N$ is the change in population size.

Growth Models

• Two Models of Growth:
  • Exponential Growth: Unrestricted population growth under ideal conditions.
  • Logistic Growth: Growth that levels off as population approaches carrying capacity.

Exponential Growth

• Characteristics:
  • Occurs in ideal conditions (ample resources, no competition).
  • J-shaped growth curve.
• Formula:
  • $\Delta N = r_{max} \cdot N$
• Example: A population of bunnies, where $r_{max} = 1.5$ and initial population size $N = 3000$.
  • Annual increase = 4500 bunnies.

Logistic Growth

• Characteristics:
  • Growth slows as population nears carrying capacity.
  • Carrying capacity (K) is the maximum population size that the environment can sustain.
• Formula:
  • $\Delta N = r_{max} \cdot N \left(1 - \frac{N}{K}\right)$
• Example: A population with carrying capacity $K = 2000$, current size $N = 1200$, and $r_{max} = 1.0$ leads to a growth of 480 individuals as it is below carrying capacity.

Population Dynamics

• Influenced by natural selection and environmental factors.

• Life History: Affects an organism’s schedule of reproduction and survival based on:

  • Timing of reproduction.
  • Frequency of reproduction.
  • Number of offspring per reproductive event.

• K-selection (Density-dependent): Traits that are sensitive to population density, observed near carrying capacity.

• R-selection (Density-independent): Traits that maximize reproductive success in low-density populations.

Regulation Mechanisms

• Density-dependent regulation: Growth slows with increased population size due to:

  • Competition.
  • Predation.
  • Toxic waste.
  • Disease.

• Density-independent regulation: Birth/death rates remain constant regardless of population size, influenced by:

  • Weather.
  • Climate.
  • Natural disasters.