This International Herald Tribune article explains how the extinction of plants and loss of genetic diversity threatens our food supply:

"Historically, humans utilized more than 7,000 plant species to meet their basic food needs, Esquinas says. Today, due to the limitations of modern large-scale, mechanized farming, only 150 plant species are under cultivation, and the majority of humans live on only 12 plant species, according to research by the Food and Agriculture Organization."