HL ecology

• Normal pyramid of numbers

  • The number of organisms falls as you ascend each pyramid
  • The body size of the organisms increases as you ascend each pyramid
• Pyramids of numbers that are not standard

  • The size of the organisms can change the standard shape
  • It may not be possible to represent large numbers of organisms correctly
• Population


  All the members of a species living in an area
• Factors controlling population size

  • Competition
  • Predation
  • Parasitism
  • Symbiosis
• Competition


  Two or more organisms seek a scarce resource
• Intra-specific competition


  Competition between members of the same species
• Inter-specific competition


  Competition between different species
• Types of competition


  • Contest competition (one organism gets the resource, the other is left without)
  • Scramble competition (all competing get some, but often not enough, of the resource)
• Predation


  The catching, killing and eating of another organism
• Predation


  Initially increases the number of predators and decreases the number of prey
• Numbers of predators and prey

  Often show repeated cycles of rising and falling numbers
• Parasite


  An organism that feeds from and harms another organism
• Exoparasites


  Live on the outside of the host
• Endoparasites


  Live inside the host
• Parasites sometimes reduce the numbers in a population, but often have little effect on host numbers
• Symbiosis


  Two organisms from different species live (and are usually obliged to live) in close association for the benefit of at least one of the organisms
• Parasitism


  One organism benefits but the other is harmed
• Mutualism


  Both organisms benefit
• Population dynamics


  Factors that cause changes in population numbers
• Availability of food


  Increases predator numbers when high but reduces them when low
• Concealment


  Some prey survive by hiding from the predators
• Movement of predators


  Predators move to new areas when prey numbers are low
• Human population growth has continued to rise rapidly since the 1900s because of falling death rates
• Factors affecting human population numbers

  • War
  • Famine
  • Contraception
  • Disease control
• War


  Normally reduces population numbers temporarily
• Famine


  Reduces population numbers
• Contraceptives


  Increased use has reduced the birth rate and the rate of population growth, especially in developed countries
• Disease control methods


  Improved methods have reduced the death rate and caused an increase in human numbers