The green revolution has resulted in farmers planting fewer varieties of the crops that they grow so that they can focus on use of high yielding varieties. In addition, the varieties that are planted have been bred to a high degree of genetic uniformity within each variety. Both of these approaches are a change from past practices, in which farmers planted a large number of different, often locally-adapted, varieties, each of which generally contained a large number of different genotypes.
Thus the green revolution results in losses of genetic diversity over both the short and the long term. (Here I'm using "genetic diversity" simply to refer to the number of different genotypes in a variety or area.)
For example, in India farmers have planted 30,000 different varieties of rice over the past 50 years, with the varieties grown in a region closely matched to its soils, climate and so forth. With the advent of green revolution varieties, this has changed. It is estimated that 75% of all rice fields in India were planted to just 10 varieties in 2005!
Similar trends are found in most regions of the world in which the green revolution has been adopted.
Plant responses to many stresses, both biotic (such as pathogens or pests) and abiotic (such as drought or temperature extremes) are at least partly under genetic control (examples will follow). Thus, flexibility in response to these stresses is increased when there is relatively more genetic diversity present at the population or landscape levels. Greater flexibility means greater stability in production, as entire fields (or crops, at the landscape level) are less likely to be weakened or eliminated by pests, pathogens, or extremes of climate.
When a new crop variety is released, is usually resistant to most of the dominant current diseases. This is, of course, part of the plant breeders' strategy, as disease resistance is an important, usually genetically-controlled, trait.
In the early 1970's, a new corn genotype (new type of cytoplasm actually) was released in the US; the "Texas male sterile" (TMS). Hybrids that contained this new characteristic had many desirable properties, and growers were excited about them, planting them over miles and miles of corn acreage in US. They were, of course, bred to be resistant to the most common corn diseases. However, they were not resistant to a previously unimportant strain of a fungal disease; the southern corn leaf blight (caused by the fungus Helminthosporium maydis). Ninety percent of the corn sowed in the US in 1970 contained the TMS trait and also shared genetic susceptibility to this pathogen. The fungus encountered all this acreage of susceptible host and wiped out one fourth of the US corn crop in 1970, a loss of over one billion dollars in production! If the corn acreage hadn't been such a monoculture, the fungus wouldn't have been able to spread as rapidly, as it would have encountered barriers of genetically resistant plants.
There have been similar problems in India, Afghanistan, and W. Pakistan with epidemics in new green revolution varieties of wheat; yields have been reduced by as much as 85% in some years. Such tremendous losses were virtually unheard of when growers were still planting out the traditional hundreds of locally-adapted varieties. Overall, year-to-year fluctuations in yields have generally been much greater since the introduction of green revolution varieties; they are not as well buffered by genetic diversity against disease and climate problems as were the diverse traditional varieties. Thus, while the traditional varieties were not as high yielding as the green revolution varieties, yields from the traditional varieties were more reliable, as they were less vulnerable to being eliminated by a pathogen or climatic anomaly. In summary, with advent of the green revolution, food supplies have actually become more volatile, as large proportions of a crop fail or do very well uniformly. I tend to think of homogeneous crop varieties in light of the old nursery rhyme about "When she is good, she is very very good, but when she's bad, she's horrid!"
Reductions in crop genetic diversity have implications both over the short term and over the longer term.
(1) Over the short term, as more and more growers plant out monocultures of nearly genetically uniform stock, there are then huge acreages of plants that all respond similarly to stresses, resulting in the types of problems with instability in yields described above.
One consequence of this genetic uniformity is greater use of pesticides to protect the genetically vulnerable crops (pesticides) .
(2) Over the long term, increasing reliance on a relatively few varieties leads to the loss of well-adapted, genetically variable varieties through lack of use. Such losses are permanent. This is a crucial, but often ignored, aspect of the "biodiversity crisis."
Prior to the green revolution, different regions within countries had several favorite varieties. Traditionally, these varieties and their requirements were well understood; this one would be planted where it is a little wetter, this one on the south slope, etc. These old traditional varieties had much genetic variation for disease resistance, climatic tolerances, etc., both within and across varieties. However, the Green Revolution has been a homogenizing force.
For example, in 1903, seed catalogs in the US listed 408 edible pea varieties. Only 25 of those varieties can be found now, and by 1970, just two varieties comprised 96% of the commercial pea crop in the US. On average, across all US crops, it is estimated that 90% of the varieties grown 100 years ago are no longer in commercial production nor are they being maintained in major seed storage facilities. Similar losses in genetic diversity of livestock have occurred as well.
Furthermore, the Green Revolution varieties are sometimes not well matched to local circumstances. A particularly acute example of a mismatch involves Africa. Hunger is commonplace in sub-Saharan Africa; of the 600 million inhabitants, 200 million are chronically undernourished. Yet Green Revolution crop varieties and practices have done little to help here. The green revolution crop varieties have been bred to channel more of their photosynthate into grain, as we've seen before, and this increases production IF water and fertilizer are applied. However, such allocation decreases allocation to other useful traits, such as vigorous deep roots, sturdy stems, and ability to compete with weeds. As Conway and Sechler argued in an article in Science (Vol 289; page 1685):
"Asking African farmers to invest in Green Revolution technology meant asking them to invest in fragile plants in a harsh landscape."
Few plant breeding or biotechnology efforts are aimed at helping to enhance properties (agricultural and nutritional) of crops that can be grown in much of Africa, as most such efforts are devoted to private sector, economically lucrative enterprises. Yet the native biodiversity in African crops is a valuable resource that could be utilized to enhance food production there, along with providing access to sustainable agricultural practices, including water harvesting, zero tillage, legume rotations, crop-livestock systems, and so forth. A recent exception, however, is the new hybrid "Rice for Africa," which was bred to produce well on uplands and produces ~ 50% more yield than other varieties using traditional rainfed systems without added fertilizer (Science  302: 1917-1919). Note that this was created using conventional breeding, rather than using genetic engineering. Another potentially bright spot for African agriculture came with the announcement that the Gates and Rockefeller Foundations are donating $150 million to increase agricultural productivity in Africa (Science Sept. 25, 2006). Funds will be used both to breed hardy varieties of regionally-appropriate crops and also to develop efficient means of fertilizing soils, which are often inherently low in fertility and have also become severely nutrient depleted and compacted owing to the cropping practices that have been used there.
We are in danger of losing the genetic diversity housed in the old varieties. This diversity could turn out to be very important, for example under conditions of changed climate. Some efforts are being made to preserve them. The seed of some varieties is kept in cold storage, or small plots of them are planted out to maintain them. However, the attempts are unlikely to be successful over the long term, unless funding for preservation of old varieties increases. (It is expensive to keep seed in cold storage, to maintain records of when each variety should be taken from storage and planted out to generate new seed (seed doesn't last forever in cold storage and must be replanted periodically to produce fresh seed), and to do the necessary planting and harvesting.)
A huge seed storage facility, dubbed the "Doomsday Vault" or "Noah's Ark for Agriculture" opened in Norway in ~ 2008. It is in Svalbard Norway, a frigid archipelago about 620 miles from the North Pole. It includes a cold storage facility that is almost half the size of a football field, all buried deep in permafrost (several hundred feet of ice and permafrost overtop it), and was built to withstand earthquakes, nuclear war or asteroid strikes. It is intended to store millions of seeds from over 100 nations in the world; gene banks within nations make deposits here, much like a big central bank. Also like a central bank, nations can deposit seed there without charge, and can withdraw them if the need arises -- just as one can take things out of and make deposits into a safety deposit box in a bank. Many of the varieties stored there are no longer grown, and risk extinction if their seeds aren't stored properly. Temperatures are maintained at about - 10 to - 20 degrees C. The hope is that the seeds within can be maintained for over 1,000 years, many will need to be planted out to produce new seeds every century or so; it is the responsbility of the nation that donated the seed to do this planting and harvesting of new seed. OSU has sent seeds from many horticultural crops to be stored there, including seed from various varieties of blueberries and strawberries, hops, pears, mints, hazelnuts....In most cases, at least 500 seeds per variety will be stored, to increase the range of genetic diversity that is encompassed. Over 4 million collections are stored in this facility! The project was initiated by the Global Crop Diversity Trust.
The Global Crop Diversity Trust, which is an international noprofit organization dedicated to preserving crop diversity for global food security is now also addressing the challenges that will be imposed by global climate change. That change will cause increased temperatures, changed patterns of precipitation with increased drought in many regions of the world, rising sea levels with intrusion of salt water into freshwater aquifers used for irrigation (and other uses) and altered pest and disease dyamics. They have invested major funding in the screening of worldwide crop collections to try to find varieties or traits that might better tolerate these changes so that they can get on what is referred to as "race against time." It typically takes 10 years to breed a new variety and global climate change is happening fast.
The combination of the problems described above means that our food supply is increasingly vulnerable, both over the short and long terms.
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Page maintained by Patricia Muir at Oregon State University; last updated Nov 18, 2011