3b

• Community


  A collection of species or populations, and interactions between these collections over time, and in a given space
• No two communities will be exactly alike even if we have the same number of organisms of each species of the same size and age in the same position etc.
• The challenge is to be able to generalise to the point can make useful comparisons / understand systems, without being so sweeping as to eliminate the subtlety
• Communities are more of a continuum than a discreet entity
• Communities as groups of species


  A useful definition only in that it is relatively simply to define, but it ignores the components of space
• Communities as a collection of populations

  Plant community data typically looks at numbers of individuals within species whereas animal data focuses on the presence or absence of species
• Abstract vs concrete community

  The concrete form refers to it at a given point, or components of it that exist. The abstract community is the more generalised version
• Ways communities may show organisation (structure)


  • Species composition
  • Temporal
  • Trophic
• Species composition


  At any given time a community will have a pool of species represented, with dominant and keystone species
• Keystone species


  Those on which others (or even the community as a whole) depends, and their loss will lead to the loss of others
• Indicator species


  Those which provide information about a community and can be used as a proxy or marker for the likely condition of the community
• Temporal structures


  • Diurnal
  • Nocturnal
  • Crepuscular
  • Seasonal
• Communities change over time (succession) and can be long term (millions of years) or daily (intertidal zones) and individuals may live for days or centuries
• Different species occupy landscapes at very different scales though may still interact (krill vs whales)
• Predation


  The complete consumption of one organism by another, where one is harmed and one benefits
• Parasitism


  Consumption of part or all of an organism by another, where one is harmed and one benefits, usually lives on / in the host
• Mutualism


  An interaction where both organisms benefit
• Food web


  Illustrates relationships (what eats what) between species or populations within a community, and represents energy transfer
• Energy enters ecosystems as light, and leaves primarily as heat
• Trophic levels


  • Primary producers
  • Primary consumers
  • Secondary consumers
• Definitions of trophic levels are not as clear cut as we would like, as some organisms can be in multiple levels
• On average, only 10% of the energy makes it from one trophic level to the next
• Ways to represent transfer of energy or trophic levels

  • Pyramids of numbers
  • Pyramids of biomass
  • Food chains or webs
• Food chains are typically short (few have more than 3 or 4 links) or the system would run out of energy to support higher trophic levels
• Selection may potentially increase and decrease food chain lengths
• Guilds


  Aggregations of single species or populations into similar functional types for simplicity
• Compartmentalisation


  Webs can be compartmentalised into groups where interactions are strong, but between which interactions are weak
• Ways to trace food webs

  • Observational data of feeding
  • Examination / analysis of waste
  • Isotope tracking
  • Ecotron
• Food webs can be used to help build models of interactions and predict effects of changes, which is increasingly important for conservation and with problems like invasive species