4-Population Ecology 1

Population Ecology I: Demography and Growth

• Population ecology is the study of births, deaths, and the factors affecting population size and dynamics within a specific area.
• A population is a group of interbreeding individuals of a single species living in the same general area.
• Key characteristics of describing populations include:

Spatial

• Density: The number of individuals per unit area or volume.
• Dispersion: The pattern of spacing among individuals within the population's boundaries. Common patterns are clumped, random, and spaced (also known as over-dispersed).
  • Spaced dispersion relates to competition or territoriality
  • Clumped dispersion may indicate social patterns or resource distribution
  • Random dispersion patterns are less common.

Temporal

• Birth rates: Patterns and factors influencing birth rates.
• Death rates: Patterns and factors influencing death rates.
• Life history traits: Characteristics like age at first reproduction, reproduction frequency/duration, total offspring number, and life expectancy. These are products of natural selection.
  • Semelparous species: Organisms that reproduce only once in their lifetime (e.g., Pacific salmon, agave)
  • Iteroparous species: Organisms that reproduce multiple times throughout their life (e.g., most vertebrates, shrubs, and trees)
• Survivorship curves: Illustrate patterns of survival across the lifespan. Include Type I, II, and III patterns.
  • Type I: High survival early and middle, and death rates increase in old age.
  • Type II: Uniform death rate across the lifespan
  • Type III: High mortality early in life, with survival rates improving later in life.

Measuring Population Size

• Mark-recapture techniques: Used to estimate population size, especially for mobile organisms
  • Capture, tag, release animals
  • Recapture animals, record number of marked and unmarked animals in the second capture
  • Calculate population estimate: (Number marked × total of capture 2) / number of marked found in the second capture
• Other methods: Counting individuals outright, or counting signs of an individual (foot prints etc) to get relative density estimates. Remote sensing technology has improved counting of individuals.

Population Growth Models

• Exponential growth: Describes population growth in ideal (unlimited resource) environments.

  • Growth rate is constant (rmax).
    • The growth rate stays the same, but the numbers of individuals change.

• Per capita birth rate: Number of births per individual

• Per capita death rate: Number of deaths per individual

• Per capita rate of increase: Per capita birth rate minus per capita death rate. A positive value indicates population growth, and a negative value indicates population decline.

• Formula for population growth (change in population size / given time): change in N / change in T = rN (r = per capita rate of increase, N = pop. size)

• Important Note: The exponential growth model assumes unlimited resources. In reality, populations rarely undergo exponential growth for long stretches of time. Environmental limitations such as resources and competition eventually influence population growth.