One of the great problems in talking about biodiversity is that it has so many different meanings. ((At which point the average lazy English writer would insert the Humpty Dumpty quote and move on. Next would come the Samuel Johnson quote, and then we can get back to the story.)) That makes it easy to misunderstand one another, and to make mistakes.
So, for example, we pointed out that having great diversity in your pedigree does not confer any diversity in the here and now if all the offspring of those highly diversified matings are genetically identical. Our friend Mike Jackson was quick to point this out, and he recently followed up with a link to an IRRI annual report for 1997-98. Delivering Diversity to the Field amplifies the confusion, talking about the huge increase in the number of ancestors in a variety’s pedigree ((Confusingly calling them all “parents”.)) and telling us that “these varieties have also increased the danger of genetic vulnerability to major disease or insect pest outbreaks”. Modern varieties, then, are more diverse because they have many “parents” and more vulnerable because they are more uniform. ((No point, here, in picking more holes in that article; we leave that as an exercise for the reader.))
All this is important because it addresses something that Bruce Chassy brought up in response to our criticism of his effort to debunk an anti-GMO paper by bringing biodiversity into the picture. Chassy wrote:
Biodiversity has taken on an almost spiritual meaning. I do not mean to in any way diminish the importance of biodiversity but sometimes when people start believing they stop thinking.
Amen to that. Chassy goes on to illustrate, as follows:
The American Chestnut comes to mind as species that was diverse and widespread through diverse ecosystems in the US. Three billion trees, 25-30% of all trees in the Eastern US, were nonetheless wiped out by a single disease, Asian bark fungus (Cryphonectria parasitica) in a few short decades. Just a few scattered individuals remain of this once common tree. Biodiversity did not save them.
On the other hand, the Quaking Aspen (Populus tremuloides) is propagated by root and shoots. A grove of these Aspen is essentially a giant monoclone; the specie itself appears to have very little biodiversity. They have survived for perhaps 100s of millions of years and have been called the largest living organisms since all the trees in a grove are connected by roots and can be thought of as part of one distributed organism. Lack of diversity appears not to have hurt this specie.
A number of common crops are propagated clonally as well. Naval oranges, bananas, potatoes, pineapples, apples are examples. Why doesn’t biodiversity matter to these crops? Because they were produced by plant breeders and are propagated commercially before being planted by farmers.
Where to begin?
At the beginning. Biodiversity can exist at several different levels. A landscape contains different species from different kingdoms. Populations of a single species will differ from one another. And individuals in a population will differ genetically from one another. So all those chestnuts may have been “diverse” but not where it counted, in their susceptibility to Chestnut blight, although at least one resistant individual has been found. As for Quaking Aspen, some groves are indeed large clones (as are groves of many other trees) but groves differ greatly from one another, and aspens will reproduce sexually, for example after fire, increasing the genetic diversity among stands. Aspens are also threatened by many pests and diseases.
I could make the same points with regard to vegetatively propagated crops and obligate out breeders. Clubroot, anyone?
One measure of diversity is the probability that two individuals chosen at random will share some specified quality, but it really matters what that quality is. Could be something as simple as a specific sequence of DNA. Could be a specific protein coded by the DNA (which can be identical even if there are differences in the DNA). Could be something as complicated as susceptibility to a disease, or resistance to drought. It’s complicated. But that’s no excuse for wilfully misusing an important concept.
The levels of biodiversity you mention are spatial, in a sense.
However, as Donald Duvick pointed out, there is also temporal diversity. So yes, a breeding programme may produce just a few varieties, but over time they work their way through a diverse array of materials looking for ancestors.
For instance, US maize varieties collectively combine genes from almost all geographical sources except the tropical highlands. New varieties may differ quite a bit from the previous generation.
So, instead of spreading risk in space, you spread it in time, betting that plant breeders can solve any upcoming problem in the future using the huge diversity they can easily access (in the case of maize). In 1970-1 they showed they could. The question is if the benefits of this approach to diversity outweigh the costs and risks.
Of course you are right Jacob, that diversity in time is also important. Indeed, one can consider standard crop rotation as a form of temporal diversity that both reduces the impact of pests and diseases and improves the use of soil resources.
The response to Southern Corn Blight was indeed rapid. Other threats might not be so easy to respond quickly to.
Ah, yes, the space vs time thing: https://agro.biodiver.se/2012/04/two-things-about-agricultural-biodiversity/#comment-1049553. I wonder whether we can demolish that meta-narrative one of these days.
It’s about the unknown unknowns, as a lazy English writer would say.
The Souther Corn Leaf Blight is normally cited as showing the dangers of crop uniformity. But was it a major problem? Over twenty years ago I gave a seminar at CIMMYT. I had prepared a slide showing the year on year average yield increase in maize in the USA for about 70 years – but leaving off the actual years. Although the trend line was uniformly upwards, there were yearly hiccups up and down around the trend.
I challenged the audience to identify the blight year (1970). Nobody could. There was an obvious fall that year, an increase above the trend the following year, and a fall well below the trend the year after (this nothing to do with the Blight). But the drop in yield, while recognizable, was in no way exceptional as the yield jigged up and down each year. It was impossible to identify the Blight year. Try this on colleagues and students.
Did it matter? The yield in maize in the US is partly influenced by market predictions. Predicted low prices will cause farmers to apply lower inputs (herbicides, fertilizer) to save costs: it seems partly this, rather than disease, that causes major variations in yield (and also rainfall and other abiotic determinants of yield). Are we making too much of a fuss about the Leaf Blight?