population 1

• Population


  A group of organisms of the same species which occupy a particular geographical area at a given time
• Genetic exchange

  Can occur between individuals in a population
• Population changes

  1. Gain individuals through birth (B) and immigration (I)

  2. Lose individuals through death (D) and emigration (E)
• When the population is stable and in dynamic equilibrium, B + I = D + E
• Population density

  The total number of individuals (or total biomass of individuals) per unit area or volume
• Estimating population density


  1. Total count

  2. Quadrat sampling

  3. Marking and recapture method

  
• Quadrat sampling


  • Requires uniformity in environment, uniform population distribution, and counting animals before they move between quadrats
• Marking and recapture method

  Assumes the proportion of marked animals in the entire population is the same as the proportion in the captured animals
• Crude density


  Number of individuals per unit of total space
• Ecological density


  Number of individuals per unit of habitable space
• Dry season

  Ecological density can be higher than crude density
• Natality


  The number of individuals added to the population per unit time
• Maximum natality


  The theoretical maximum number of individuals which can be added to the population per unit time under ideal conditions
• Realized/actual natality


  The actual number of individuals being added to the population per unit time under specific environmental conditions
• Mortality


  The number of individuals being lost from the population per unit time
• Minimum mortality


  The theoretical minimum number of individuals lost from the population per unit time, due only to end of physiological longevity
• Realized/ecological mortality


  The actual number of individuals being lost from the population per unit time, due to ecological longevity
• Age distribution


  • Ratio of pre-reproductive, reproductive, and post-reproductive age groups determines reproductive status, natality, and mortality
• Determining animal ages

  • Annual growth rings in trees, fish scales, mollusc shells, and tooth development in mammals
• Population pyramid


  A graphical illustration showing the distribution of various age groups in a human population

  
• If a population is growing, the population pyramid has a high natality and more individuals in the pre-reproductive phase
• If a population is stable, the population pyramid has an equal number of individuals in the pre-reproductive and reproductive phases
• If a population is in decline, the population pyramid looks like an inverted one, with fewer individuals in the pre-reproductive phase and more in the post-reproductive phase
• The replacement rate for the human population was 2.1 globally before the pandemic
• Cohort


  A group of organisms from one population which are all approximately the same age
• Biotic potential


  The increase potential of a population, given by the quantity 'r' which is characteristic and specific for a species
• Strategies for reproduction

  • Semelparity (one massive reproductive effort followed by death) and iteroparity (repeated reproductive efforts throughout lifetime)
• Fecundity increases with size and age in plants and poikilothermic animals, but the relationship is not as clearly defined in homeotherms
• 1. selected species


  Produce a large number of offspring to increase chances of survival in fluctuating, uncertain environments
• 1. selected species


  Produce a limited number of larger offspring that are better competitors in stable environments
• Survivorship curves


  Graphs showing the number of individuals surviving per thousand of the population through each phase of life

  
• Types of survivorship curves

  • Type I (low mortality in infancy, high mortality near end of lifespan), Type II (relatively constant death rate), Type III (high mortality in infancy followed by lower, constant loss)

  
• Populations rarely exhibit idealized survivorship curves, often displaying a composite of multiple types
• To compare survivorship of species with different lifespans, percentage of lifespan survived is plotted on the x-axis instead of age
• Some organisms display a "stair step" type of survivorship curve with sharp changes in transition
• Survivorship curves


  • Composite of more than one type
• Percentage of life span survived

  Age up to which individual has lived/maximum age limit (theoretical or from cohort size)
• Survivorship curve of Dall mountain sheep


  • Certain amount of mortality in the young
  • Curve steepens at the high age range as older animals show a high death rate
  • Steeper curves are usually due to predation, particularly by wolves which are more likely to catch immature and old animals
  • Adult sheep in their prime, which are fully grown and healthy, can avoid predators better

  
• Survivorship curve

  • Can have a "stair step" type where the survival rate undergoes sharp changes in transition from one life history stage to another
  • Composite nature, e.g. insects which undergo metamorphosis may have different rates of survival in egg, larval, pupal and adult stages
• Type I survivorship curve


  Species produce few offspring, offspring are well cared for so their survival rate is high