Organisms and the environment

• 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
• Producers


  Organisms that produce their own organic nutrients, usually using energy from sunlight. Plants are producers as they carry out photosynthesis to make glucose.
• Herbivore


  An animal that gets its energy by eating plants
• Carnivore


  An animal that gets its energy by eating other animals
• Primary Consumers


  Herbivores - they feed on producers (plants)
• Secondary Consumers


  Predators that feed on primary consumers
• Tertiary Consumers


  Predators that feed on secondary consumers
• Decomposers


  Bacteria and fungi that get their energy from feeding off dead and decaying organisms and undigested waste (such as faeces) by secreting enzymes to break them down
• Food Chain


  1. Producer

  2. Primary Consumer

  3. Secondary Consumer
• 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 Web


  A network of interconnected food chains
• Food webs are more realistic ways of showing connections between organisms within an ecosystem as animals rarely exist on just one type of food source
• Pyramid of Numbers


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


  Secondary consumer
• Insects


  Primary consumer
• Pyramids of numbers are not always pyramid-shaped
• The size of the organism is also important - one large organism, like the oak tree in the pyramid above, contains enough energy to support many smaller organisms (the insects)
• Rules to remember when drawing a pyramid of numbers


  You cannot change the trophic level of the organisms - they must stay in the same order as in the food chain with producers on the bottom, followed by primary consumers, then secondary consumers, then tertiary consumers
• Generally, the larger an individual organism is, the less of them there are
• 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
• The mass of organisms has to decrease as you go up a food chain - if we take our first food chain as an example, it's impossible to have 10kg of grass feeding 50kg of voles feeding 100kg of barn owls
• Sparrowhawk


  Tertiary consumer
• Thrush


  Secondary consumer
• Snail


  Primary consumer
• Pyramids of biomass provide a much better idea of the quantity of the plant or animal material at each level of a food chain and therefore are a better way of representing interdependence within the food chain
• Pyramids of biomass are generally pyramid-shaped, so they are simple to draw. Some ecosystems, such as an aquatic ecosystem may show a different shape or inverted pyramid but it is very unlikely you will see this in an exam.
• Pyramids of number can be any shape - so make sure you learn the rules for drawing a pyramid of numbers as it is more common to see unusual shapes in the exam.
• Energy transfer in food chains

  1. In order for the energy to be passed on, it has to be consumed (eaten)

  2. Not all of the energy grass plants receive goes into making new cells that can be eaten

  3. The same goes for the energy the vole gets from the grass, and the energy the barn owl gets from the vole

  4. Only the energy that is made into new cells remains with the organism to be passed on

  5. Even then, some of this energy does not get consumed - for example, few organisms eat an entire organism, including roots of plants or bones of animals - but energy is still stored in these parts and so it does not get passed on

  6. The majority of the energy an organism receives gets 'lost' (or 'used') through making waste products eg (urine), movement, heat (in mammals and birds that maintain a constant body temperature), and undigested waste (faeces) that is removed from the body and provides food for decomposers
• This inefficient loss of energy at each trophic level explains why food chains are rarely more than 5 organisms long
• In the example above, something that preyed regularly on the barn owl would only get 0.1J of energy from each barn owl it ate
• In order to survive, it would have to either eat a huge number of them every day to get the amount of energy it needed to survive (are there that many barn owls close together?) or not expend much energy itself hunting them (is this likely?)
• Trophic levels


  • Producer
  • Primary consumer
  • Secondary consumer
  • Tertiary consumer
• 10% of energy is available to the next trophic level to make new biomass
• 90% of energy is lost as heat and in undigested materials