Population and Community Ecology

• The population growth rate is the change in population size over time.
• Mutualism is a relationship between two organisms where both benefit from the relationship.
• Commensalism is a relationship between two organisms where one benefits while the other is unaffected.
• Parasitism is a relationship between two organisms where one benefits at the expense of the other.
• Symbiotic mutualism is where the two species live in close proximity
• Obligate mutualism is essen for both species survival
• Facultative mutualism is not essential for survival but rather an added bonus
• Ecological community


  All of the organisms in a prescribed area occurring at the same time
• Ecological community


  A set of species occurring together in space
• Population


  A group of interacting species occurring together in space
• Assemblage


  A set of species occurring together in space
• Alpha diversity


  • How species richness within a community
• Beta diversity


  • How different species richness is between communities
• Gamma diversity

  • Total species richness amongst communities
• Evenness


  The relative abundances of each species in the community
• Rank abundance curve


  The species with the highest abundance is ranked 1, the next most abundant is ranked 2 etc, a line can be drawn and the slope can be measured to determine rareness/abundance in the community
• Less steep, or flat slopes indicate more even communities
• Typical communities have a few abundant species and lots of rare species
• Diversity index

  Incorporates species richness and evenness into the same measure
• Shannon-Wiener diversity index


  Incorporates species richness and evenness
• Theory of Island Biogeography

  The species richness of islands depends on how fast new species arrive, and the rate at which they go extinct, and these immigration and extinction rates depend on two things: 1) Distance of island from the mainland, 2) Size of island
• Species richness is lower for more isolated islands, as it takes much longer for immigration events to happen in isolated islands
• Species richness is highest in larger islands
• Predicted species richness for an island

  The rate at which new species arrive on the island is matched by the rate at which species on the island go extinct
• Assemblage

  Describes the organisms
• Community

  Describes both the organisms and their interactions
• Trophic interactions


  One organism eating the other
• Food chain

  Single vertical pathway
• Food web


  Includes multiple food chains
• Trophic levels

  The position of an organism in the food chain
• Trophic levels

  • Primary producer - trophic level I
  • An organism eats 100% plants, it is trophic level 2
  • If an organism eats 100% level 2, it is trophic level 3
  • If an organism eats 50% level 1 and 50% level 2, it is trophic level 2.5
• Trophic level formula

  TL = Σ 1+ TL; DC;
• Predator-prey cycles depend on immigration events to balance out extinction of populations
• Stable predator-prey cycles could only be recreated in the lab when environmental heterogeneity was introduced
• Keystone species


  A species that is not necessarily abundant but exerts strong control on community
• Ecosystem engineers

  Change the structure of a habitat
• Top-down system

  Changes at the top of the food web dominate community structure
• Bottom-up system


  Changes at the bottom of the food web dominate community structure
• Trophic cascade


  The removal or addition of a top predator and how it affects a community
• Rank abundance curve

  The species with the highest abundance is ranked 1, the next most abundant is ranked 2 etc