The Value of Genetic Diversity for Agriculture
In agriculture and forestry, genetic diversity can enhance production. Several varieties can be planted in the same field to minimize crop failure, and new varieties can be bred to maximize production or adapt to adverse or changing conditions. The Massa of northern Cameroon (who cultivate five varieties of peral millet), the Ifugao of the island of Luzon in the Philippines (who identify more than 200 varieties of sweet potato by name), and Andean farmers (who cultivate thousands of clones of potatoes, more than 1,000 of which have names) all use highly diversified farming systems–the first approach.
The second approach is also widely used. In the United States from 1930 to 1980, plant breeders’ use of genetic diversity accounted for at least one-half of a doubling in yields of rice, barley, soybeans, wheat, cotton, and sugarcane; a threefold increase in tomato yields; and a fourfold increase in yields of corn, sorghum, and potato.
As important a genetic diversity is to increasing yields, it is at least as important in maintaining existing productivity. For example, crop yields can be increased by introducing genetic resistance to certain insect pests, but since natural selection often helps insects quickly overcome this resistance, new genetic resistance has to be periodically introduced into the crop just to sustain the higher productivity.
Pesticides are also overcome by evolution, so another important agricultural use of genetic diversity has been to offset productivity losses from pesticide resistance. Indeed, the record shows that pesticides only temporarily conquer pests. Over 400 species of pests now resist one or more pesticides, and the proportion of U.S. crops lost to insects has approximately doubled–to 13 percent–since the 1940s, even though pesticide use has increased. Even more significantly, as crop yields increase, so must efforts to sustain the gains. In general the easy gains come first.
Genetic Engineering
Until the advent of genetic engineering, the spectrum of genetic resources available for plant breeding ranged from other varieties of the crop to wild relative so the species. Wild relatives of crops have contributed significantly to agriculture, particularly in disease resistance. Thanks to wild wheats, domesticated wheat now resists fungal diseases, drought, winter cold, and heat. Rice gets its resistance to two of Asia’s four main rice diseases from a single sample of rice from central India.
The spectrum of genetic resources available to breeders is now expanding to still more distantly related species, thanks to genetic engineering techniques. However, much more work remains to be done before the potentials of these techniques are realized. Moreover, most new genetic engineering techniques will at least initially involve single-gene modifications of species, and in many cases such modifications are less useful than the multiple-gene changes that result from traditional breeding programs. A transfer of a single gene to a crop plant may confer resistance to a certain pest, but that resistance is often relatively quickly overcome by the pest. In only one species–tobacco–has single-gene resistance been useful for extended periods of time.
Genetic Diversity and Livestock Breeding
Genetic diversity has also been of significance in breeding programs for species other than edible plants. It is becoming increasingly important in forestry and fisheries, and the use of genetic resources in livestock breeding has markedly increased yields. The average milk yield of cows in the United States has doubled over the past 30 years, and genetic improvement accounts for more than 25 percent of this gain in at least one breed.
Still, for several reasons, genetic diversity has been less useful in livestock breeding than in crop breeding. First, whereas one major use of the genetic diversity of crops has been in the development of strains resistant to specific pests and diseases, livestock husbandry has relied largely on vaccines since animals (unlike plants) can develop immunity to disease. Second, maintaining livestock germplasm is tougher logistically than maintaining the genetic material of plants: since domesticated animals do not go through dormant stages comparable to the seed stage of plants, long-term storage is a problem. Finally, many of the closest relatives of domesticated animals are extinct, endangered, or rare, and thus unavailable for breeding.
Genetic Improvement of Forest Species
For different reasons, genetic improvement of forest species has also received less attention than crop improvement. Until recently, most timber harvested has been wild, so breeding programs seemed unnecessary. In addition, because trees are so long-lived, the rate of genetic improvement of tree species is quite slow. Tests and measurements of growth characteristics have been made for some 500 species (primarily conifers) over the years, but less than 40 tree species are being bred. Yet, impressive gains have been made with these species. In intensive breeding programs, a 15 to 25 percent gain in productivity per generation has been attained for trees growing on high-quality sites without inputs of fertilizer, water, or pesticides.
Aquaculture
Most of the fish that humanity eats or converts to livestock feed are wild, so breeding has not been widely utilized in fisheries to enhance yields either. An exception is aquaculture. In one case, the domestic carp (Cyprinus carpio) was bred with a wild carp in the Soviet Union to enhance the cold resistance of the domestic species and allow a range extension to the north.
Risks of High-Yielding Crop Varieties
The development of high-yielding varieties of crops has greatly benefitted some segments of society, but some risks attend use of these new varieties. With the advent of modern plant breeding techniques, a trend toward genetically more uniform agriculture has developed in areas suited to the high-yielding, high-input modern varieties. Whereas traditional mixed farming systems produce modest but reliable yields, planting a single modern crop variety over a large area can result in high yields but the crop may be extremely vulnerable to pests, disease and severe weather. In 1970, for instance, the U.S. corn crop suffered a 15 percent reduction in yield and losses worth roughly $1 billion when a leaf fungus (Helminthosporim maydis) spread rapidly through the genetically uniform crop. Similarly, the Irish potato famine in 1846, the loss of a large portion of the Soviet wheat crop to cold weather in 1972, and the citrus canker outbreak in Florida in 1984 all stemmed from reductions in genetic diversity.
To stabilize production, breeders use one of several tactics to maintain a genetically diverse crop array. Typically, varieties are replaced with higher-yielding relatives after four to ten years even if they still resist disease or pests. In effect, the spatial diversity of traditional agriculture is replaced with a temporal diversity created by a continuous supply of new cultivars. In the United States, the average life-time of a cultivar of cotton, soybean, wheat, maize, oats, or sorghum is between 5 and 9 years.
Newer strategies for stabilizing production involve the use of varietal blends (a mix of strains sharing similar traits but based on different parents) or multilines (varieties containing several different sources of resistance). In each case, the crop represents a genetically diverse array that can better withstand disease and pests. Despite these efforts, genetic uniformity still places some crops at risk of disease outbreaks and in some regions that risk is considerable. Some 62 percent of rice varieties in Bangladesh, 74 percent in Indonesia, and 75 percent in Sri Lanka are derived from one maternal parent.
