Although there are at least 20,000 species of edible plants worldwide, fewer than 150 are cultivated widely. This creates a great deal of vulnerability to plant diseases which could ravage the commonly used species. A major epidemic of plant disease striking any of the major cereal crops (wheat, rice, and maize) would result in famine on a massive scale. Research grants, however, are not available for crops which may offer alternatives, but in which few are currently interested.
Not only is genetic diversity declining on farms, but many of the areas that are home to the wild relatives of our food crops are also under serious threat. Most of these crops were domesticated in temperate and subtropical zones and in tropical highlands. Domesticated varieties are also under threat as a result of the homogenization caused by large-scale agriculture and the demand for uniform varieties that can withstand the rigours of shipping and storage.
Thousands of traditional crop varieties were also dropped from cultivation with the advent of the Green Revolution, which promoted use of a limited number of high-yielding varieties. In developing countries, these high-yielding varieties are now used on 52 percent of the agricultural land planted in wheat, 54 percent of land planted in rice, and 51 percent of land planted in maize.
The loss of crop varieties, although less publicized than the loss of wild species, has severe implications for global food security. Thousands of species and many more thousands of varieties (subspecies) have been grown since the development of agriculture 12,000 years ago.
It is reported that of the 150 cultivated species, only 22 species provide the vast majority of protein for the world's population. An alternative estimate is that 95% of human food come from 30 plants, and 75% from only 8 of these thus restricting the species to an extremely narrow dietary base.
A survey of fruit and vegetable varieties revealed that up to 96 percent of the commercial vegetable varieties listed by the U.S. Department of Agriculture in 1903 are now extinct. Of more than 7,000 apple varieties in use between 1804 and 1904 in the United States, 86 percent have been lost; of 2,683 pear varieties 88 percent are no longer available.
In an amazing display of genetic uniformity, nearly all the coffee trees in South America are descended from a single tree growing in an Amsterdam botanical garden 200 years ago, prompting a serious situation when a new disease began attacking coffee trees. The origin of that Coffee arabica tree was the forests of southwest Ethiopia, which have now virtually disappeared.