Yields of the world's major crops have generally increased steadily during this century, thereby helping to feed the burgeoning human population. This stepped-up agricultural production has been achieved by the release of improved varieties, the spread of irrigation, the application of fertilizers, pesticides, and herbicides, and the expansion of tilled land.
Yet while the yield ceiling of crops has been rising, the genetic diversity of our crops has been shrinking. Instead of a mosaic of different crop varieties covering a region, which has prevailed for most of the period since plants were first domesticated some 10,000 years ago, cropping patterns are becoming more uniform. Genetic erosion of crops is especially advanced in the industrial countries and in the more productive agricultural areas of the third world. In the last 40 years, for example, 95 percent of Greece's wheat races have been abandoned by farmers adopting modern varieties, and virtually all of the indigenous sorghum varieties of South Africa disappeared after the introduction of high-yielding Texas hybrids.
A patchwork quilt of genetically different crops provides a buffer against catastrophic attack by diseases, pests, or sudden changes in weather. But enormous areas planted to a single variety can be risky. The Irish potato famine of the last century is the most famous example of the danger of going out on a limb with a handful of varieties. In 1970 US farmers received a costly lesson in genetic vulnerability when a previously minor fungal threat mutated and cut corn production by an average of 15 percent. Whereas such setbacks in relatively wealthy countries are serious, a sudden downturn in food production in most developing countries could dramatically increase the incidence of malnutrition.
To counteract this crumbling of the genetic base of the main crops, gene banks are being built or extended to store samples representing the rich diversity of varieties. The fact that neither diseases nor pests have seriously undermined food production on the scale of Ireland's potato blight can be attributed to the establishment of gene banks where samples have been evaluated by scientists for their useful properties. In short-term gene banks, breeders constantly tap material to upgrade crops, just as checking accounts are designed for frequent use. In long-term gene banks, samples are frozen and rarely disturbed; their function is similar to that of savings accounts.
Crop gene banks have brought generous payoffs. The US corn crop was able to rebound the year following the 1970 fungal attack because of extensive and well-tested collections of corn held by private seed companies and public institutions. Breeders draw on gene banks to make new varieties more resistant to a broad spectrum of diseases and pests. Wild species have been particularly helpful to breeders trying to combat the enemies of crops. A weedy potato from Mexico, for example, provided genes that helped the common potato to withstand the blight that triggered the mass emigration from Ireland.
The value of gene banks is especially evident when they contain material that has vanished on the outside. The gene bank maintained by the International Rice Research Institute (IRRI) in the Philippines contains the world's largest rice collection with over 60,000 accessions; it has helped numerous developing countries. In Kampuchea, for example, many unique rice varieties were lost during the 1970s when war disrupted farm life. Seeds of numerous rice strains were eaten or rotted, so the lines died out. Fortunately, IRRI's gene bank holds rice samples that were collected in Kampuchea before fighting broke out, and some of these have been successfully reintroduced to the country. In 1981, IRRI sent 36 Khmer varieties to Kampuchea and a further 103 indigenous rice races have been requested for 1982.
The industrial nations have a stake in conserving the genetic resources of crops as well. Most of the important food crops, such as wheat, rice, and corn, were domesticated in the lower latitudes. The developed countries have a vested interest in the maintenance of gene banks in the third world to ensure that a wide range of crop varieties is preserved. More natural areas need to be set aside and protected so that wild species that may prove useful to breeders are not lost. The bewildering array of breakfast cereals on display in supermarkets is greater than the number of crop varieties used to produce them.
More clearly needs to be done to collect, conserve, and evaluate crop germplasm so breeders will have an adequate arsenal at their disposal to overcome future challenges to agricultural productivity.