Population Ecology and the Effects of Density

Population: A group of individuals of the same species living in a specific area.
- Population Ecology: Studies the factors that affect population size and the reasons for changes over time.

Density: Refers to the number of individuals per unit area.
- Can be determined by:
- Direct counting (rarely used due to practical challenges).
- Sampling techniques (count individuals in a smaller area, average to estimate total size).
Knowledge of population density helps understand the relationship between a population and its resource utilization.

Dispersion: The arrangement of individuals within a population.
- Clumped: Individuals gather in patches (e.g., resources are clustered).
- Uniform: Evenly spaced, often due to territoriality.
- Random: Unpredictable spacing; not common in nature.

Population size is dynamic and influenced by:
- Births and deaths.
- Immigration and emigration.
Demography: The study of population vital statistics and their changes over time.
- Life Table: An age-specific summary of survival patterns, represented by a Survivorship Curve:
- Type I Curve: Low death rate in early/middle life; high death at older ages.
- Type II Curve: Constant death rate throughout organism's life.
- Type III Curve: High early-life death rate; lower death rate for those that survive youth.

Per Capita Rate of Increase: Calculated by
ext{Change in population size} = ext{Birth rate} - ext{Death rate}

Two primary models for population growth:
- Exponential Growth Model: Occurs under ideal conditions (ample resources, minimal competition).
- Logistic Growth Model: Population growth slows as it approaches carrying capacity.

In this model, the population grows rapidly and continuously at a constant rate:
- Represented by a J-shaped curve.
- Calculated using the formula:
  ext{Change in population size} = r_{max} imes N
- Where $r_{max}$ is the maximum per capita growth rate and $N$ is the population size.
Practice Problem: For a bunny population with growth rate $r = 1.5$ and size $N = 3,000$:
- Annual increase = 4500 bunnies.

Approaches zero growth as the population nears its carrying capacity (K):
ext{Change in population size} = rac{r_{max} imes N imes (K - N)}{K}
Practice Problem: For a population below carrying capacity (1,200 with $K = 2,000$ and $r_{max} = 1.0$):
- Growth rate = 480 individuals.

Influenced by natural selection and environmental factors:
- Life History Traits: Affect reproductive schedules and survival, including:
- Timing of reproduction.
- Frequency of reproduction.
- Number of offspring per reproductive event.
K-selection: Traits sensitive to population density, prevalent near carrying capacity.
R-selection: Traits that maximize reproductive success under low-density conditions.

Density-dependent Regulation:
- Growth impacted by factors like competition, predation, and disease as population increases.
Density-independent Regulation:
- Environmental factors such as weather and natural disasters affect population size without changing birth/death rates.