Ecosystems

• Ecosystem - All the living organisms that interact with one another in a defined area and all the physical factors present in that area
• Biotic factors:

  • interactions between living organisms
  • e.g. competition over food, space, breeding partners
• Abiotic factors:

  • non-living/physical factors within an ecosystem
  • Light
  • Temperature
  • Water Availability
  • Oxygen Availability
  • Edaphic (soil) factors
• Light:

  • plants are directly affected by light levels
  • in low light, certain plants have adapted to cope e.g. larger leaves, photosynthetic pigments that require less light
• Temperature:

  • effects enzyme controlled reactions
  • can cause migration and hibernation in animals, and leaf-drop, dormancy and flowering in plants
• Water Availability:

  • lack of water can lead to stress
  • stress can lead to death
• Oxygen Availability:

  • Aquatic Environments:
  • fast flowing cold water has high concentrations of oxygen
  • Water logged soil:
  • air spaces in the soil are not full of water therefore less oxygen available for the plantsn
• Edaphic factors:

  • different soils have different particle sizes. this affects what lives in them:
  • clay - fine particles, easily waterlogged, forms clumps when wet
  • loam - different-sized particles, retains water but does not become water logged
  • sandy - coarse, well-separated particles that allow free draining, does not retain water and is easily eroded
• the sun is the source of energy for nearly all ecosystems on earth due to photosynthesis
• Trophic level - each stage in a food chain
• producer - first trophic level (organism converts sunlight into energy by photosynthesis)
• consumers - found in the subsequent trophic levels after producers, they feed on other organisms
• decomposers - break down dead organisms releasing nutrients back into the ecosystem
• Biomass:

  • the mass of living material present in a particular place or in a particular organism
  • it can be equated to the energy content of a trophic level
  • can be represented by a pyramid of biomass
  • biomass is calculated by working out the dry mass of the organism (organism is killed and placed in an 80 degree oven to remove all water)
  • measured in grams per square meter for areas of land and grams per cubic metre for areas of water
• as trophic level increases, biomass and energy content decrease (mostly)
• ecological efficiency - the efficiency with which biomass or energy is transferred from one trophic level to the next
• producers only convert 1-3% of the sunlight they receive into chemical energy because:

  • 90% of solar energy is reflected, some is transmitted through the leaf, some is an unusable wavelength
  • photosynthesis limited by other factors
  • some energy is used to drive photosynthetic reactions
• Gross production - the total solar energy that plants convert to organic matter. 20-50% of this is used in respiration, the rest is converted to biomass
• net production = gross production - respiratory losses
• primary production - net production in a producer
• secondary production - net production in a consumer
• consumers at each trophic level convert 10% at most of the biomass in their food to their own organic tissue because:

  • not all biomass is eaten (bones)
  • some energy is transferred as metabolic heat
  • some parts of the organism aren't digested
  • some is lost in excretory products e.g. urine
• only 0.001% of the total energy present in the incident sunlight will become biomass in the tertiary consumer
• ecological efficiency = energy or biomass available after the transfer/ energy or biomass available before the transfer x100
• farming involves manipulating environments so that plants and animals can thrive and high yields can be achieved:

  • competition is removed e.g. kill weeds
  • protect organisms from predators e.g. fences
  • watering plants
  • provide warmth e.g. sheds/stables
• Farming creates short food chains which reduces energy loss:

  • producing crops for humans only involves two trophic levels
• decomposition - a chemical process where a compound in broken down into smaller molecules or its constituent elements
• decomposer - organism that feeds on and breaks down dead organic matter. they turn organic compounds into inorganic compounds that are then available to photosynthetic organisms
• Saprobiotic nutrition - an organism that obtains their energy from dead or waste organic matter
• saprotrophic nutrition - food is digested externally by the secretion of enzymes
• detritivores - speed up the decaying process by feeding on dead and decaying matter. this process breaks down the material into smaller pieces, increasing the SA for decomposers
• Nitrogen - essential for making amino acids; 78% of the air is nitrogen
• nitrogen fixation:

  • performed by nitrogen fixing bacteria e.g. Azotobacter (free-living) and Rhizobium (root nodules, symbiotic with plant)
  • the enzyme nitrogenase combines atmospheric nitrogen with hydrogen to produce ammonia
• Nitrification:

  • performed by nitrifying bacteria
  • ammonium compounds are converted into nitrites then nitrates
  • oxidation reaction; only occurs in well-aerated soils

  Nitrosomonas + ammonium --> nitrites

  Nitrobacter + nitrites --> nitrates
• Denitrification:

  • lack of oxygen
  • Denitrifying bacteria (pseudomonas aeruginosa) convert nitrates in the soil back into nitrogen gas
  • the bacteria use the nitrates as a source of energy for respiration causing nitrogen gas to be released
• Ammonification:

  • decomposers convert nitrogen-containing molecules in dead organisms, faeces and urine into ammonium compounds
• Carbon:

  • 0.04% of the atmosphere is carbon
  • levels of CO2 fluctuate between day and night and seasons due to the availability of light and leaf loss
• • CO2 levels have increased significantly due to increased burning of fossil fuels
  • deforestation results in fewer trees to perform photosynthesis and take in CO2
  • the increase in sea temperatures means that less CO2 is dissolved in the sea water thus more CO2 is released in a positive feedback loop
• Succesion - the progressive replacement of one dominant type of species or community by another in an ecosytem until a stable climax community is reached
• primary succession - occurs in an area that is newly formed or exposed such as bare rock. there is no soil/organic matter to begin with:

  • volcanoes erupt, depositing lava which cools into igneous rock
  • sand is blown by wind or deposited in the sea to create dunes/banks
  • silt and mud are deposited at river estuaries
  • glaciers retreat depositing rubble and exposing rock