Energy Flow, Food Chains, Food Webs & Pyramids

• What is the conservation of energy principle?

  Energy cannot be created or destroyed but only changed from one form into another.
• Since energy flows through an ecosystem, where does it get its energy from + what happens to it?

  Energy flows through an ecosystem so for an ecosystem to exist, it must have an external source of energy, this is provided through sunlight, and this energy must then be harnessed and converted into other forms.
• What do plants do with light energy + how does this benefit the ecosystem? (2)

  - Plants trap the light energy using chlorophyll and convert it into chemical energy by producing organic molecules from inorganic.

  
  

  - They therefore provide the energy for all other organisms in the ecosystem.
• What are photoautotrophs?


  They are producers that make their own food from inorganic molecules, water and carbon dioxide using sunlight as an energy source.
• What are chemoautotrophs? (3)


  - They are producers that use chemical energy to make their own food. (e.g. some species of bacteria that live in deep cave systems with no available light).

  
  

  - They make organic substances using the minerals in the rock as a source of chemical energy.

  
  

  - Also nitrifying bacteria are chemoautotrophs.
• Why have communities in these ecosystems been able to survive?


  Communities in these ecosystems have only been able to develop and survive as the producers have evolved an energy source other than light.
• What do autotrophs provide?

  Autotrophs ultimately provide all the food for other species in the ecosystems either directly or indirectly.
• What are heterotrophs?


  - They cannot make their own food therefore rely on other living organisms to provide them with organic molecules.

  
  

  - They are consumers since they must obtain organic food 'ready - made'.
• What do food chains indicate? (3)

  - Feeding relationships in an ecosystem

  
  

  - Energy flow through an ecosystem (hence the direction of the arrows).

  - Food energy is transferred from one trophic level to another.

  
  

  - Each of these feeding levels is known as a trophic level.

  - Producers are always the 1st trophic level, primary consumers the 2nd trophic level and so on.

  
• How many trophic levels do most food chains have?

  Most food chains have 3 or 4 trophic levels but in aquatic ecosystems there may be as many as 6.

  
• What happens to energy between each trophic level? (3)

  - Energy is lost at each level so that there is less available for transfer to the next.

  
  

  - The chemical energy contained in the bodies of organisms is passed from one organism to another along the food chain.

  
  

  - This energy is then passed to detritivores and decomposers.
• What do some food chains start with? (2)

  - Some food chains start with dead organic matter - these are detritus food chains, and the dead plants and animals are fed upon by detritivores and decomposers.

  
  

  - e.g. dead grass, woodlice (detritivores), dead grass, fungi (decomposers)
• What do food chains give + why is it inaccurate?

  - However, food chains give only a limited impression of the feeding relationships in a community.

  
  

  - Most organisms feed on more than one kind of organism.
• What are food chains linked with?

  Food chains in an ecosystem are therefore often linked with each other to form complex food webs.
• What is the nature of food webs in complex ecosystems?

  In complex ecosystems such as the climax community stage, food webs tend to be very complex and involve many different species.
• What are pyramids drawn to represent?

  Pyramids can be drawn to represent the flow of energy and energy losses at each of the trophic levels in a food web or food chain.
• What should you remember when drawing a pyramid? (4)


  - The producers (trophic level 1) are always at the base of the pyramid

  
  

  - Pyramids are drawn symmetrical about the centre

  
  

  - Bar width must be in proportion to the numbers/biomass of the organisms involved at each trophic level.

  
  

  - The bars must always be the same depth
• What is a pyramid of numbers?

  A pyramid of numbers is a bar diagram indicating the relative numbers of organisms in a food chain, or total numbers of all organisms at each of the trophic levels in a food web.
• How is data for pyramids of number obtained? (2)

  - To obtain the data all organisms are counted in theory, but usually in practice, a small area is randomly sampled for the organisms under study and multiplied up to give an estimate of the total population size.

  
  

  - This may involve quadrat analysis, the use of nets or humane trapping.

  
• What is the difference between predator + prey in pyramids of number? (2)

  - Predators are normally larger than their prey, so as we go up the pyramid, the organisms are usually larger and so smaller in number.

  
  

  - However not all food chains give a pyramidal shape.

  
• What are the problems associated with pyramids of numbers? (5)

  - When very large numbers are involved at any trophic level, it is nearly impossible to scale the bars accurately.

  
  

  - Although giving quantitative information, pyramids of number do not give any indication of the relative mass of organisms at each trophic level.

  
  

  - For example, both an oak tree and a blade of grass count as one organism.

  
  

  - This limitation can result in some unusual shaped pyramids.

  
  

  - A top heavy pyramid is said to be inverted.
• What is a pyramid of biomass?

  A pyramid of biomass will show the mass of living organisms, at a particular trophic level, per unit area or volume, at a particular time.
• What does biomass measure + what problem do we not encounter with it?

  When biomass is measured at each trophic level, rather than numbers, it is one way of overcoming the problem of the size of organisms. Biomass is the mass of living material present at a given time.
• How is biomass measured? (3)

  - To measure this, random quadrats have all organisms harvested and weighed.

  
  

  - This is the wet mass or fresh mass.

  
  

  - An average mass is calculated for each trophic level, which is then multiplied by the number of organisms.
• How is dry mass calculated? (2)

  - Some scientists prefer the dry mass.

  
  

  - This is obtained by drying the organisms until a constant mass is achieved, and is more accurate than wet mass, but much more time consuming, and means that the organisms are killed in the process.
• What are the problems associated with pyramids of biomass? (4)

  - Both pyramids of number and biomass only show the numbers of organisms present at any one time.

  
  

  - With biomass this is called the standing crop.

  
  

  - This doesn't take into account the changes in biomass over a period of time and can therefore lead to inverted pyramids.

  
  

  - The classic example is found in marine ecosystems.

  
• Give an example (in an aquatic environment) where pyramids of biomass would be inaccurate or difficult to use. (3)

  - In the waters around our islands in spring, the biomass of zooplankton exceeds the biomass of phytoplankton.

  
  

  - This happens because the primary production of the phytoplankton is so high and they are reproducing so rapidly that they can support, temporarily, a larger biomass of zooplankton feeding on them.

  
  

  - However, over the whole year, the biomass of the phytoplankton will far exceed that of the zooplankton.

  
• Give an example (with trees) where pyramids of biomass would be inaccurate or difficult to use. (2)

  - Another problem can be seen when looking at an oak tree food chain and pyramid of biomass.

  
  

  - It is difficult to obtain either the wet or dry mas of an oak tree, and a single oak tree can provide food for millions of leaf eating insects, but should the value in the pyramid be the entire mass of the tree of just the edible leaves?
• What are pyramids of energy? (2)

  - This type of pyramid takes into account productivity (how much new material is produced) for each level in the ecosystem, during a fixed period of time.

  
  

  - If this time is one year, it will take into account annual changes, and units are therefore kJ.m-2.y-1.

  
• How do pyramids of energy differ from pyramids of number + biomass? (2)

  - Energy content of different tissues varies and therefore energy pyramids give more accurate information than biomass, but the values are more difficult to obtain.

  
  

  - Energy pyramids are never inverted, and are particularly useful in comparing ecosystems.
• What is the sun in terms of ecosystems?

  The sun is the ultimate energy source for all ecosystems, and photosynthesis is the route by which energy is made available to the ecosystem.
• How much energy do primary producers use from sunlight?

  Primary producers only utilise approx. 0.5 - 1% of the incipient energy from sunlight, converting it into chemical energy by photosynthesis.
• Why is so little energy from the sun used?

  This is because, not all the solar energy falling on a plant is used to make new tissue.
• In which ways is solar energy unavailable and so plants lose this energy? (5)

  - Some wavelengths cannot be absorbed by the chlorophyll

  
  

  - Some energy is transmitted through the leaf (miss the chloroplasts)

  
  

  - Some light is reflected from the leaf surface

  
  

  - Some is lost from the photosynthetic reactions (these are quite inefficient), in the form of heat

  
  

  - Some energy is lost in the evaporation of water.

  
• Give the equation to find photosynthetic efficiency.

  
• What is gross primary production?

  Gross Primary Production (GPP) is the energy in organic compounds produced by plants in photosynthesis (units kJm-²yr-¹ or sometimes biomass per metre squared per year).

  
• What amount of energy is available for the next trophic level? (2)


  - This energy is not all available for the next trophic level as some (up to 50%) is used by the plant in respiration (R).

  
  

  - This leaves the net primary production or NPP.
• What does the NPP represent? (2)

  - NPP represents the rate of production of new biomass which is then available for consumption by heterotrophic organisms (i.e. the only energy source in an ecosystem for the next trophic level).

  
  

  - However, plants may die in which case the NPP in the form of dead leaves, flowers or other parts will form detritus.
• What are GPP + NPP indicative of?

  GPP & NPP are indicators of the productivity of an ecosystem, and in terrestrial ecosystems, may be limited temperature and moisture.
• What is trophic level efficiency?

  Trophic efficiency is the percentage of the energy at one trophic level which ends up in the next trophic level.