1. Food supply as a limiting factor
Chapter 37

2. Today’s lesson
Understand the concept of natural succession, land overuse, & deforestation
Discuss the use of chemicals to increase food production
Discuss how selective plant breeding & DNA technology has potential benefits for food production
Discuss the effects of food shortages on the world’s population

3. Natural Succession Bare rock colonised by a pioneer community
- e.g lichen
Other species would follow
- e.g moss, soil, plants, grasses
Eventually a climax community would form
- e.g. deciduous trees
Human land use ensures that vast natural forests are cleared for agriculture etc.
Natural succession very rare these days

4. Developing countries
Population increase leads to land overuse (soil fertility drops)
Demand for fertile land rises dramatically
Forests cleared – prevents natural succession
Causes deforestation – often irreversible
Often cleared land used for cash crops, not food (v risky)
Often wood is needed for fuel
Use of marginal land can accelerate desertification

5. Use of chemicals to increase food production
Monoculture – vast cultivation of one identical type of crop
E.g. wheat, maize, rice, potatoes
Fertilisers raise nutrient supply to soil
Eliminate need for natural cycling of chemical nutrients
- by decay & nitrification
Supports continuous use of land for growth of a crop
Disadvantages
- Doesn’t substitute humus – poorer soil aeration
- Nitrates can get washed into water supplies
- Eutrophication – makes a waterway over-rich (algal blooms)

6. Herbicides/Pesticides
Herbicides eliminate weeds which may compete with crops for nutrients etc.
Can be selective or non-selective
In a monoculture, pests & parasites have unlimited food
Insecticides wipe out invertebrate pests
- e.g. nematode worms, slugs, insects
Without their use 25-45% of cereal crops could be lost
Fungicides kill fungi (e.g. mildew)
Sprayed onto crop plants or grains covered in spores
- require repeated applications
All pesticides must be specific, short-lived & safe
Genetically engineered pesticides now widely developed

7. Selective plant breeding
To produce food plants with desirable characteristics
E.g. higher yields, disease resistance, faster growth etc
Plants can be inbred (self-pollination)
Maintains uniformity in future generations
Can lead to inbreeding depression
Plants also can be outbred (cross-pollination)
Benefit – hybrid viguour
Disadvantage – new strains not guaranteed to have desirable characteristics
These have led to a Green Revolution
Risks
– if all plants are identical, could all suffer from one disease strain
- require vast amounts of fertiliser (V. costly)

8. Genetic manipulation
Recombinant DNA technology allows DNA from one species to be transferred to another
E.g lectin transferred from pea to potato plants
Somatic fusion allows non-sex cells from different species to be fused together
Can form a hybrid protoplast
This has a mixture of parental plant traits
E.g. potato leaf roll disease control

9. Effects of food shortage
Famine – spell of food shortage
Balanced diet – supply of proteins, fats & carbs
- min. of 9500kJ/day
STARVATION:
1. Undernutrition
– failure to receive enough food
- tissue death, emaciation, death
2. Malnutrition
– lack of a balanced diet
Leads to a deficiency disease e.g. kwashiorkor (lack of protein)
World food distribution very uneven (40 million die every year)
Food production exceeds population growth
Excess food often stored, rather than be shared to the needy
Overeating & irregular food chains prevalent in developed countries