Energy and feeding relationships

• Transfer of Energy


  • The Sun is the principal source of energy input to biological systems
  • Energy flows through living organisms
  • Including light energy from the Sun and chemical energy in organisms
  • Energy is eventually transferred to the environment eg. as heat
• Food Chains


  • A food chain shows the transfer of energy from one organism to the next, starting with a producer
  • The source of all energy in a food chain is light energy from the Sun
  • The arrows in a food chain show the transfer of energy from one trophic level to the next
  • Energy is transferred from one organism to another by ingestion (eating)
• Food Webs


  • A food web is a network of interconnected food chains
  • Food webs are more realistic ways of showing connections between organisms within an ecosystem
  • Food webs give us a lot more information about the transfer of energy in an ecosystem
  • They also show interdependence - how the change in one population can affect others within the food web
• Pyramid of Numbers


  • A pyramid of numbers shows how many organisms we are talking about at each level of a food chain
  • The width of the box indicates the number of organisms at that trophic level
  • A pyramid of numbers doesn't always have to be pyramid-shaped
• Pyramid of Biomass


  • A pyramid of biomass shows how much mass the creatures at each level would have without including all the water that is in the organisms (their 'dry mass')
  • Pyramids of biomass are ALWAYS pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like
• Pyramid of Energy


  • Only the energy that is made into new cells remains with the organism to be passed on
  • The majority of the energy an organism receives gets 'lost' (or 'used') through making waste products, movement, heat, and undigested waste
  • This inefficient loss of energy at each trophic level explains why food chains are rarely more than 5 organisms long
• Humans eat wheat


  More energy available to humans than if they eat cows that eat the wheat
• It is more energy efficient within a crop food chain for humans to be the herbivores rather than the carnivores
• Energy Transfer in a Human Food Chain

  1. Humans are omnivores, obtaining energy from both plants and animals

  2. Humans have a choice of what they eat

  3. Choices have an impact on what we grow and how we use ecosystems
• Food chains involving humans

  • wheat → cow → human
  • wheat → human
• If humans eat the wheat there is much more energy available to them than if they eat the cows that eat the wheat
• We often feed animals on plants that we cannot eat or that are too widely distributed for us to collect
• Nutrients


  Carbon and nitrogen are not endless resources, they need to be recycled
• The Carbon Cycle


  1. Carbon is taken out of the atmosphere by photosynthesis

  2. It is passed on to animals and decomposers by feeding

  3. It is returned by respiration; in plants, in animals and in decomposing microorganisms

  4. It is returned (in increasing amounts) by combustion of fossil fuels
• Nitrogen


  Neither plants nor animals can absorb it from the air as N2 gas is very stable
• Nitrogen Fixation


  1. Nitrogen fixing bacteria take N2 gas and change it into nitrates in the soil

  2. Lightning can 'fix' N2 gas, splitting the bond between the two atoms and turning them into nitrous oxides

  3. Plants absorb the nitrates they find in the soil and use the nitrogen in them to make proteins

  4. Animals eat the plants (or other animals) and get the nitrogen they need from the proteins

  5. Waste (urine and faeces) from animals sends nitrogen back into the soil as ammonium compounds

  6. Decomposers break down proteins into ammonium compounds and put them back into the soil

  7. Nitrifying bacteria convert the ammonium compounds to nitrites and then to nitrates, which can then be absorbed by plants

  8. Denitrifying bacteria take the nitrates out of the soil and convert them back into N2 gas
• Population


  A group of organisms of one species, living in the same area at the same time
• Community


  All of the populations of different species in an ecosystem
• Ecosystem


  A unit containing the community of organisms and their environment, interacting together
• Factors controlling population growth

  • Food supply
  • Predation
  • Disease
• The Population Growth Curve

  1. Lag phase

  2. Log phase (exponential phase)

  3. Stationary phase

  4. Death phase
• Organisms in a natural environment are unlikely to show population growth like a sigmoid growth curve because they are affected by many other factors