From the CIAT blog:
Putting true numbers on diversity loss turns out to be a complicated and contested business, with no shortage of strong opinions. One big part of the problem is that there aren’t many good ways to count the diversity that existed before it disappeared. Researchers have done some work to assess the changes in diversity in crop varieties of Green Revolution cereals, and to some degree on the genetic diversity within those varieties. The results indicate that, although diversity on farms decreased when farmers first replaced traditional varieties with modern types, the more recent trends are not so simple to decipher.
My work here is done.