Food supply as a limiting factor

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 deforestation

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.glectin 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