6.3.1 Ecosystems

• What is an ecosystem?


  All the organisms living in a certain area and all the non-living conditions found there.

  
  

  It's a dynamic system - this means it's changing all the time.

  
  

  Includes both biotic and abiotic factors.
• What are biotic factors?


  The living features of an ecosystem, for example, the presence of predators or food.
• What are abiotic factors?


  The non-living features, such as the temperature, rainfall, shape of the land and soil nutrient availability.

  
  

  In an aquatic ecosystem these may also include the pH and salinity (salt content) of the water.
• Give an example of how a biotic factor affects a rock pool ecosystem.


  Seaweed can be a food source for consumers such as limpets that graze on this producer.

  
  

  Intense competition for food such as seaweed can limit the number of organisms that are present in a small rock pool ecosystem.
• Give an example of how an abiotic factor affects a playing field ecosystem.


  Rainfall and sunlight affect the growth of the producers in the ecosystem. In a very wet year, the soil may become waterlogged, making it difficult for plants to grow.

  
  

  Poor plant growth may decrease the number of consumers the ecosystem is able to support.
• What is the main route by which energy enters an ecosystem?

  Photosynthesis.

  
  

  Some energy enters sea ecosystems when bacteria use chemicals from deep sea vents as an energy source.
• What are plants called in an ecosystem?

  Producers.

  
  

  They store energy as biomass.
• What is biomass?

  The mass of living material.

  
  

  After producers store energy as biomass, you can then think of the following energy transfers through ecosystems as biomass transfers.
• How is energy transferred through the living organisms of an ecosystem?

  By organisms eating other organisms.

  
  

  Producers are eaten by organisms called primary consumers.

  
  

  Primary consumers are then eaten by secondary consumers and secondary consumers are eaen by tertiary consumers.
• What species are these consumers normally?


  Primary consumers are generally herbivores whereas secondary and tertiary consumers are generally carnivores.
• What are food chains and food webs?


  They show how energy is transferred through an ecosystem.

  
  

  Food chains show simple lines of energy transfer, and food webs show lots of food chains in an ecosystem and how they overlap.
• What is a trophic level?


  A stage in a food chain that's occupied by a particular group of organisms, e.g. producers are the first trophic level in a food chain.
• What are decomposers?


  Organisms that break down dead or undigested organic material.

  
  

  Energy locked up in the things that can't be eaten gets recycled back into the ecosystem by decomposers.
• Why is 60% of availible energy never taken in by the organisms in the first place?


  Plants can's use all the light energy that reaches their leaves (e.g. some is the wrong wavelength)

  
  

  Some sunlight can't be used because it hits parts of the plant that can't synthesise (e.g bark)

  
  

  Some parts of food (e.g. roots or bones) aren't eaten by organisms so the energy isn't taken in - they pass to decomposers

  
  

  Some parts of food are indigestible so pass through organisms and come out as waste (e.g. faeces - also passes to decomposers)
• What is the rest of the availible energy called?


  This 40% is called the gross productivity..

  
  

  But not all of this is availible to the next trophic level either. 30% of the total energy availible (75% gross productivity) is lost to the environment when organisms use energy produced from respiration for movement or body heat.
• What is this energy lost through respiration called?


  The respiratory loss.
• With all this in mind, how much of the total availible energy becomes biomass?

  10%

  
  

  This is called the net productivity.
• What is the net productivity?


  The amount of energy that's availible to the next trophic level.

  
  

  The flow of energy transfer continues at the next trophic level - the process starts again from the beginning.
• How can you work out net productivity?


  net productivity = gross productivity - respiratory loss
• How can you find out the efficiency of energy transfer?


  (Net productivity of trophic level / net productivity of previous trophic level) x 100
• How does efficiency of energy transfer change further up the food chain?

  Generally effciency increases but energy is still lost so the more stages there are in a food chain, the more energy is lost overall.

  
  

  The energy loss limits the number of organisms that can exist in a particular ecosystem.
• How can you measure the amount of energy in organisms?


  By measuring its dry mass (its biomass).
• How do you measure the dry mass?


  You first need to dry the organism out - this is done by heating it up to 80 degrees Celcius until all the water in it has evaporated.

  
  

  You then weigh the organism and multply the results from the sample to get an estimate of the energy in one trophic level.
• What are the problems with this method of measuring dry mass?

  For example, consumers might have taken in energy from sources other than the producer measured.

  
  

  This means the difference between the two figures calculated wouldn't be an accurate estimate of the energy transferred between only those two organisms.

  
  

  For an accurate estimate you'd have to include all the individual organisms at each trophic level.
• How do herbicides make energy transfer more efficient?


  They kill weeds that compete with agricultural crops for energy.

  
  

  Reducing competition means crops recieve more energy, so they grow faster and become larger, increasing productivity.
• How do insecticides make energy transfer more efficient?

  They kill insect pests that eat and damage crops.

  
  

  Killing insect pests means less biomass is lost from crops, so they grow to be larger, which means productivity is greater.
• How do fungicides make energy transfer more efficient?


  They kill fungal infections that damage agricultural crops.

  
  

  The crops use more energy for growth and less for fighting infection, so they grow faster and become larger, increasing productivity.
• How do natural predators make energy transfer more efficient?

  Natural predators introduced to the ecosystem eat the pest species, e.g ladybirds eat greenfly.

  
  

  This means the crops lose less energy and biomass, increasing productivity.
• How do fertilisers make energy transfer more efficient?


  Fertilizers are chemicals that provide crops with minerals needed for growth, e.g. nitrates.

  
  

  Crops use up minerals in the soil as they grow, so their growth is limited when there aren't enough minerals.

  
  

  Adding fertiliser replaces the lost minerals, so more energy from the ecosystem can be used to grow, increasing the efficiency of energy conservation.
• How does rearing livestock intensively make energy transfer more efficient?

  Involves controlling the conditions they live in and when they're slaughtered, so more of their energy is used for growth and less is used for other activities - the efficiency of energy conversion is increased so more biomass is produced and productivity is increased.
• Give some examples of intensive rearing.


  Animals may be kept in warm, indoor pens where their movement is restricted. Less energy is wasted in keeping warm and moving around.

  
  

  Animals may be given feed that's higher in energy than their natural food. This increases the energy input, so more energy is availible for growth.

  
  

  Animals may be slaughtered before they reach adulthood. Young animals use a greater amount of their energy for growth, so this means more energy is transferred into their biomass.
• What are the benefits of these methods?

  More food can be produced in a shorter space of time, often at a lower cost.
• What are the disadvantages of these methods?


  Raise ethical questions.

  
  

  Some people think the conditions cause the animals pain, distress or restricts their natural behaviour, so it shouldn't be done.
• What is the carbon cycle?


  How carbon moves through living organisms and the non-living environment.

  
  

  The cycle includes processes that involve organisms, respiration and also chemical and physical processes such as combustion and weathering.

  
• What is the role of photosynthesis in the carbon cycle?


  Carbon, in the form of CO₂ from air and water, is absorbed by plants when they carry out photosynthesis - it becomes carbon compounds in plant tissues.

  
  

  Carbon is passed on to primary consumers when they eat the plants. It's passed on to secondary and tertiary consumers when they eat other consumers.
• What is the role of decomposition in the carbon cycle?

  All living organisms die and are broken down by microorganisms.

  
  

  Decomposers secrete enzymes which break down the carbon compounds in dead organic material.

  
  

  The decomposers then absorb the products of digestion for use in respiration.

  
  

  Feeding on dead, organic matter is called saprobiontic nutrition.
• What is the role of respiration in the carbon cycle?


  Carbon is returned to air as all living organisms, including the decomposers, carry out respiration, which produces CO₂.
• What is the role of combustion in the carbon cycle?

  If dead organic matter ends up in places where there aren't any decomposers, e.g. deep oceans, its carbon compounds can be turned into fossil fuels over millions of years.

  
  

  The carbon in fossil fuels is released when they're burnt - this is called combustion.
• What is the role of the release from volcanoes in the carbon cycle?


  As well as coal, other types of rock can be formed from dead organic matter deposited on the sea floor.

  
  

  Rocks such as limestone and chalk are mainly composed of calcium carbonate. This comes from marine organisms like crabs, mussles, sea urchins and coral that utilise this compound in their development, e.g. to form shells.

  
  

  One way carbon can be returned to the atmosphere from these rocks is by them being drawn down deep into the Earth's crust by the movement of tectonic plates.

  
  

  There they undergo chemical changes and release carbon dioxide, which is returned to the atmosphere by volcanoes.
• What is the role of weathering in the carbon cycle?


  The rocks can eventually become land, which is then weathered.

  
  

  This can happen chemically by rainwater and physically by plant roots, animals etc.

  
  

  Chemical weathering causes mineral ions and bicarbonate ions to be released from the rock into solution and enter groundwater, from where they are transported into rivers and the oceans.

  
  

  There they combine to form carbon-containing compounds such as CaCO₃.