Energy Transfer

Question 1

Discuss the efficiency of energy transfers between trophic levels

Answer 1

• 60 % of the available energy is never taken in by organisms
- plants can’t use all the light energy that reaches their leaves (some is the wrong wavelength, some is reflected, some passes straight through the leaves)
- some sunlight can’t be used because it hits part of the plant that can’t photosynthesise e.g. Bark
- some parts of food such as roots and bones aren’t eaten by organisms so the energy is not taken in
- some parts of food are indigestible so pass through organisms and come out as waste
• the rest of the 40% available energy is taken in (absorbed) - this is called gross primary productivity
- 30% of the total energy available (75% of the gross productivity) is lost to the environment when organisms use energy produced from respiration for movement or body heat, this is called respiratory loss
• 10% of the total energy available (25% of the gross productivity) becomes biomass (stored or used for growth, this is called net productivity

Question 2

What is net productivity?

Answer 2

• it is the amount of energy that’s available to the next trophic level
• net productivity = gross productivity - respiratory loss

Question 3

How can energy transfers between trophic levels be measured?

Answer 3

• you can directly measure the amount of energy in organisms by burning them in a calorimeter, the amount of heat given off tells you ho much energy is in them
• you can indirectly measure the amount of energy in the organism ps by measuring their dry mass (biomass) it is created using energy so it is an indicator of how much energy an organism contains
• efficiency of energy transfer = energy in trophic level 2
—————————— X100
energy in trophic level 1

Question 4

Explain how human activities can manipulate the flow of energy through ecosystems (primary productivity)

Answer 4

• herbicides kill weeds that compete with agricultural crops for energy
- reducing completion means crops receive more energy so they grow faster and become larger, increasing productivity
• fungicides 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
• insecticides 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
• natural predators introduced into the ecosystem eat the pest species
- crops lose less energy and biomass, increasing productivity
• fertilises are chemicals that provide crops with minerals needed for growth
- 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 conversion

Question 5

Explain how human activities can manipulate the flow of energy through ecosystems (secondary productivity)

Answer 5

• rearing livestock intensively involves controlling the conditions they live in 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
• e.g. Animals may be kept in warm, indoor pens where their movement is restricted, less energy is wasted keeping warm and moving around
• e.g. Animals may be given feed that’s higher in energy than their natural food so this increases energy input so more energy is available for growth

Question 6

Describe how energy is transferred through an ecosystem

Answer 6

• the main route by which energy enters an ecosystem is by photosynthesis - plants (producers) convert sunlight energy into a form that can be used by other organisms
• energy is transferred through the living organisms of an ecosystem when organisms eat other organisms (e.g producers eaten by primary consumers, primary consumers eaten by secondary consumers, secondary consumers eaten by tertiary consumers)
• food webs show how energy is transferred through an ecosystem - show lots of food chains in an ecosystem and how they overlap, arrows represent the flow of energy between organisms
• energy locked up in things that can’t be eaten (bones, faeces) gets recycled back into the ecosystem by decomposers)