AQA A Level Biology复习笔记5.3.10 The Effect of Farming

• Farming practices can increase the efficiency of energy transfer in order to increase the total amount of energy available for human consumption
• This can be achieved by:
  • Reducing the energy lost by crops to other organisms (e.g. pest species that feed on crops)
  • Reducing the energy lost by livestock (e.g. cattle) through respiration

   

Reducing energy loss to other organisms

• In the food web below, the weed, the harvest mouse and the caterpillar are all considered pests because they reduce the energy available for the growth of the crop (corn)
  • This means they reduce the net primary production (NPP) of the corn
  • This, in turn, reduces the amount of energy available to humans (who consume the corn)

   

• Corn farmers can simplify the food web below by getting rid of the pest species
  • The reduces the energy lost by the corn to these organisms
  • This will cause the NPP of the corn to increase

   

Energy that would otherwise contribute to the net primary production of crops can be lost to pest species either directly (i.e. to organisms that feed on the crop) or indirectly (i.e. to organisms that compete with the crop)

Removing pest species

• Farmers can simplify food webs using pest control
• One method of pest control is to use chemical pesticides
  • Insecticides are chemicals that kill insect pests that eat and damage crops, reducing the NPP of the crops
  • Herbicides are chemicals that kill weeds (unwanted plants) that compete with crops for sunlight and water, reducing the NPP of the crops

   

• Another method of pest control is to use biological agents to reduce the number of pests (this is known as biological control). For caterpillar pests, for example:
  • Toads that consume caterpillars can be used
  • Parasitic wasps that lay their eggs inside caterpillars can be used (when the larvae hatch they consume and eventually kill the host caterpillar)
  • Pathogenic bacteria and viruses that kill caterpillars can be used

   

• Often, farmers will use both these methods (chemical and biological) together to reduce pest numbers more effectively

Reducing energy loss through respiration

• Farming practices can increase the net production of livestock (e.g. cattle)
• This can be achieved by carefully controlling the conditions that the livestock are raised in so that the energy lost through respiration is minimised and the energy available for growth is maximised. For example:
  • Respiration rate is increased when animals move, so keeping livestock in pens where their movement is restricted lowers energy loss through respiration
  • Respiration rate is increased when animals need to generate body heat to keep warm, so keeping livestock indoors and in heated pens also lowers energy loss through respiration

   

• These farming practices ensure that more chemical energy is stored as biomass in the livestock, increasing their net production
  • This means more energy is available for human consumption
  • It also means that in a given period of time, a greater amount of food can be produced, sometimes even at a lower cost
  • However, some people think it is unethical to keep livestock in cramped, unnatural conditions where their movement is restricted and they may have a lower quality of life