The National Geographic just published an article online about the serious problem the global community faces with the reduction in the variety of crops the world has available for food. The article shows the serious nature and the extent of loss of seed varieties with this illustration.
The full article can be read online at http://ngm.nationalgeographic.com/2011/07/food-ark/siebert-text. The top part of the tree shows what was available in 1903 and the bottom half shows whats left. The missing species are considered extinct.