Lecture 16

• the biosphere is the sum of all ecosystems
• there are some places that do not follow the larger patterns of biomes
• it is not only the climate that determines where ecosystems occur:

  • topology
  • soils (e.g. nutrients, texture)
  • history and chance (e.g. fire, volcano)
  • species (physiology, ecology)
• ecosystem concept

  'the relationships among organisms (biota) in a given area and their interactions with the physical (abiotic) environment'
• an ecosystem is a community of living organisms in conjunction with the non-living components of their environment, interacting as a system. These biotic and abiotic components are linked together through nutrient cycles and energy flows
• external factors:

  • climate
  • parent material
  • topography
• energy input and conversion:

  photosynthesis is the key (and only) biological process that can harvest energy derived from the sun
• energy flow:


  food webs are constructed to describe species interactions (direct relationships) but can be used to illustrate indirect interactions among species, too.
• decomposition:

  • leaching
  • fragmentation
  • chemical alteration
• In the United Kingdom (UK) nearly 50 percent of ancient woodland has been lost since 1947. Between 1979 and 1990 there was a 14 percent loss of vegetation species richness in woodland, the highest for any habitat in the UK. This was because dead wood within existing semi-natural woodland was poorly represented and decaying wood communities have become more isolated.
• nutrient cycling:


  soil conditions controlling nitrification rates

  • substrate availability
  • aeration (availability of O2)
  • well-drained soils with 60% soil moisture
  • pH (near neutral)
  • temperature (best 20-30 degrees C)

  nitrification is seasonal and affected by land use practices
• the nitrogen cycle is often regarded as one of the biggest nutrient cycles

  • ubiquity
  • global distribution
  • multiple forms
  • biological fixation
  • nitrification and denitrification
  • human influence
  • interactions with the carbon cycle
• payment for ecosystem sevices