Fertilisers and farming

Temperature, light and carbon dioxide levels affect photosynthesis. Farmers use fertilisers, pesticides and biological control to increase crop yields. Fish farms produce large amounts of food.

Controlling pests

Crop pests include insects and other animals that eat or damage plants. Pests can be controlled using pesticides or by introducing other species (biological control).


Pesticides are chemicals used to control pests, including:

  • fungicides (which kill fungi)
  • herbicides (which kill weeds)
  • insecticides (which kill insects)

Pesticides are effective in controlling pests. However, they are expensive and can cause environmental damage. The pests may become resistant to them, and they may also kill useful plants and animals.

Biological control

Organic vegetables including asparagus and red peppers
Organic food is grown without the use of artificial chemicals

Biological control can happen naturally, for example, ladybirds eat aphids.

Biological control can be very effective if a control species is introduced deliberately. For example:

  • parasitic wasps can control whitefly in glasshouse tomato crops
  • the myxomatosis virus was deliberately introduced to control the rabbit population in Australia

Biological control reduces the population of the pest but does not completely remove the pest.

Sometimes the control species does something unexpected.

For example, the Hawaiian cane toad was introduced in Australia to control the population of the cane beetle, a pest that damaged sugar cane crops. However, it has had little effect on the cane beetle population. Instead, cane toads have reproduced rapidly and spread across the north east of the country. They are poisonous and they have caused reductions in the populations of native toads. Consequently cane toads are causing more damage than the pest they were supposed to control.

Predator prey graph.  As the number of predators increases the number of prey descreases. As the number of predators decrease the number of prey increase.Predator-prey graph