Manipulation of the food species

• Optimum livestock/crop density is an example of population control. Increasing population density can increase the total yield, although yield per individual may be reduced due to inter-species competition. A high population density can increase the risk of the rapid spread of disease.
• Monocultures are an example of population control. A monoculture involves cultivating a single species, often over a large area. This can make cultivation easier by allowing the use of larger machinery but pests and diseases can spread rapidly if they colonise the field. The removal of hedgerows and other habitats to create larger fields can also increase pest damage as natural predator habitats are lost.
• Asexual reproduction in plants is an example of genetic control. In natural asexual reproduction, the offspring develop from the parents plant rather than from seeds. Artificial asexual reproduction involves cuttings where new plants are produced from sections of leaf, stem or root tissue from the parent plant.
• The advantages of asexual reproduction in plants is the offspring are genetically identical to the parent plant, so their characteristics are predictable and survival rates are high.
• The disadvantages of asexual reproduction of plants is there is no genetic variation in the offspring so their characteristics cannot be improved and fewer offspring are produced than by sexual reproduction.
• Asexual reproduction in animals is an example of genetic control. Cloning is an artificial asexual reproduction technique for livestock that is still being developed. The aim is to produce offspring that are genetically identical to a selected individual with desirable characteristics.
• The method of cloning is to remove cells from the donor and grown in culture. An egg is removed from a female. The nucleus of the egg is removed and replaced by the nucleus from one of the donor cells. The egg is implanted into a surrogate female where it develops, during a normal pregnancy. When it is born, it has the characteristics of the donor.
• The potential applications of asexual reproduction in agriculture are: valuable animals that die can be replaced with genetically identical individuals; herds that are culled during a disease outbreak can be replaced with genetically identical individuals; large numbers of individuals with desirable characteristics could be produced.
• Selective breeding is an example of genetic control. This involves the production of offspring from parents that were chosen because of their genetic characteristics. Breeding between genetically similar individuals may produce offspring with similar characteristics, but there is an increased risk of inbreeding.
• Crossbreeding is an example of genetic control. Crossbreeding between two different parental breeds may produce a combination of desirable characteristics with ‘hybrid vigour’ and lower risk of inbreeding. An example is crossbreeding between Ayrshire and Zebu cattle to produce cattle that can tolerate heat and produce high milk yield. Crossbreeding also reduces the problems caused by homozygous recessive genes that are often found in inbred varieties.
• Improved breeding techniques is an example genetic control. This can be done through: artificial insemination, where semen is collected from a male with desirable characteristics to impregnate females - more offspring can be produced and offspring can still be produced after the male has died through freezing the sperm; embryo transfer, where FSH is used to stimulate ovulation and releas eggs in females, which are then fertilised by males with desirable characteristics and implanted into surrogates - more offspring can then be produced.
• Genetic modification allows the introduction of single characteristics from one species to another. A range of GM varieties have been developed but concerns over environmental and human health impacts have prevented may of them entering commercial cultivation.
• The advantages of GM production are: individual desirable characteristics can be introduced without associated unwanted characteristics, as can occur in normal selective breeding; genes may be introduced from other species that could never have been achieved by normal selective breeding; GM techniques can increase resistance to specific insects reducing the amount of pesticides used to protect a crop; GM techniques can increase pathogen resistance reducing disease in a crop.
• The disadvantages of GM production are: claimed to increase food allergies; potential gene transfer from GM find to cells of the body or to bacteria in the gastrointestinal tract; migration of genes from GM plants into conventional crops and plants; transfer through the food chain; genes that provide resistance to antibiotics are often inserted with desired genes by acting as marker genes, and there is concern antibiotic resistance genes may transfer to pathogenic bacteria; increased cost to farmers; concerns in LEDCs that GM crops will reduce the local native crop diversity.