Topic 5- Energy and Food production

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Agricultural Ecosystems

These are made up of plants and domesticated animals which are used to produce food for humans

Energy is lost as each stage in the food chain and humans are often the third of fourth trophic level

 its important for agricultural ecosystems to try and reduce the amount of energy lost at each stage and inrease productivity

Net productivity is the energy stored within new biomass of the organism so is available to us as consumers

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Comparison of Natural and agricultural ecosystems

Energy input:

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Comparison of natural and agricultural ecosystems


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agricultural farming practices can increase produc

An increase in productivity is usually brought about by increasing the efficiency of energy conversion

Ways of doing this:

  • Using fertilisers
  • Using chemical pesticides, biological agents or integrated systems to control pests
  • Intesive rearing of domestic livestock- Belgian Blue Cow (more muscle, cant walk, less energy lost, more productivity)
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Plants need light and CO2 to photosythesise and produce carohydrates. They also need inorganic ions to make other compounds such as proteins.

In a natural ecosytem ions are recycled but in agricultural ecosystems the crops are harvested and removed so there is less recycling of ions

As the levels of inorganic ions decreses the productivity is reduced, this is because Nitogen is required to produce proteins and DNA, both of which are needed for growth.

It is important to replace these lost ions. This can be done by using fertilisers

There are two types:

Organic/Natural- waste materials such as manure

Inorganic/ Artificial- manufacured powders or pellets produced in factories containg Nitrogen, Phosphorous and Potassium

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Advantages and Disadvantages of Fertilisers


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Limiting Factors

a) As amount of fertiliser increases yield increases, ion conc. is a limiting factor

b) At a certain point, increase in fertiliser doesnt give an increase yield, ion conc no longer a LF

c) After a certain point the increase in fertiliser causes decrease in yield, ions are such high conc. could lower water potential of soil causing loss of water from roots via osmosis. Level of ions is now toxic (Plants grow to quickly they can outcomptete for light. Also, too much Nitrogen may make another ion a LF e.g. Phosphorous

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Environmental Impact of fertilisers


Build up of algae in water

light cant penetrate water

plants under water cannot photosynthesise and die

increase in decomposers to decompose dead plants

decomposers respire and reduce o2 conc

o2 depleats and death of other aerobic organisms occurs

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Environmental impacts of Fertilisers

Reduced species diversity

Plants which are adapted to soils high in Nitrates out comptete other species  


Inorganic fertilisers result in more ions applied to the soil than are actually taken up by the plants. This leads to leaching, the ions dissolve in soil water and are washed from the soil into streams, rivers and lakes

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Pests and Pesticides

A pest is an organisms which may dcrease the yield of a crop plant either directly or indirectly 

Indirect reduction- pest eat leaves so reduces plants  cant photosythesise so no carbohydrates are produced and so cannot respire and therefore growth is reduce

Weeds- Compete for light, nitrates and water ∴ decrease yield (interspecific competition)

Insects- Feed on leaves of plants. Yield decreased directly id insect damages part of crop used by humans

Fungi- Infections may reduce yield

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Pests and Pesticides

Why are crops grown in moncultures susceptible to pests?

  • Monoculture makes it very easy for disease to spread and also for pests to travel from plant to plant.
  •  A very large area can be affected in a short space of time

Pests can affect domestic farm animals by:

  • Growth rate may be affected which will reduce yield
  • disease may mean the animal is unfit for human consumption
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Pests and Pesticides


  • May get into water and kill organisms
  • Many pests are food for other organisms
  • Some pests can survive and reproduce they become resistant to the pesticide

Effective pesticides are:

1) Specific- specific chemicals that kill only the pest

2)Biodegradable- they dont stay in the environment, break down into hramless substances

3)Do not accumilate- doesnt build up in specific parts of an organism and along the food chain (BIOACCUMILATION)

4) Cost Effective- cost of developing new pesticides are high. Pests may develop resistance so pesticides may only be uselful for a limited period of time

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Pests and Pesticides

Mode of action of pesticides:


  • Applied directly to plant
  • Herbicides absorbed directly through leaves
  • insecticides absorbed through spiracles, insect has to come into direct contact with chemical

Systemic- becomes a apart of plants system

  • Herbicides taken in by leaves and transpoted around plant, more effective than contact as chemical reaches all parts of plant
  • insecticides taken in when insect is feeding


  • Sprayed onto the soil
  • can be used to treat seeds before planting
  • remain in soil- kill larvae etc
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Pests and Pesticides

Biological Control

Use of natural predators

predator is a CONTROL AGENT

Reduces levels of pest so that they are no longer harmful

Aim is to achieve a balance at a level where pest is not harmful


  • Reduce pest and keep it low
  • only needs one application
  • Specific to one pest
  • can be used in organic farming
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Pests and Pesticides

Biological Control continued


  • Cost of treatment
  • takes along time to act
  • pest not completely removed
  • may become a pest itself
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Pests and Pesticides

Integrated Systems

Use of both biological control agents and chemical pesticides to reduce pest levels

Levels of pests on crop are monitored and control agents used to keep levels low. If levels so high  that they start to cause economic damage then pesticides are used

Prevents resistance to pesticides

Other methods used in integrated systems are:

  • Using pest resistant GM crops
  • crop rotation
  • weeding- lack of interspecific competition for nitrates, sunlight etc
  • deep ploughing- brings everything to surfac so birds can eat it
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