Category: Genetic diversity

Don’t you just hate it when a striking message from an elegant model is complicated by, well, facts? I may have Nibbled a press release on a recent modeling study from Wageningen University. The crux of the results was that as species migrate north due to climate change, they shed diversity from the central, most diverse part of their distribution, which is bad for their ability to adapt.

Plant and animal species can lose their ability to adapt as a result of climate change. This is shown by research performed by Marleen Cobben with which she hopes to obtain her doctorate at Wageningen University (part of Wageningen UR) on 17 April 2012. Cobben used computer calculations to illustrate how the genetic base of plants and animals is seriously deteriorating due to climate change. The smaller genetic base makes species more vulnerable to problems such as diseases. Moreover, the fragmentation of landscapes and the loss of wildlife areas is accelerating this decline.

This was interesting to me because we routinely, and perhaps somewhat blindingly, these days say that climate change will lead to shifts in the distributions of species. Crop wild relatives, say. Shift that will absolutely require germplasm collecting and ex situ conservation. Nothing else will do. Forget about in situ, ex situ it must be. That’s because, when added up, these shifts in the distributions of individual species will result in profound alterations in the geographic patterns of species diversity. Some hotspots will disappear, some diversity-poor areas will be enriched. Difficult to plan in situ conservation under these conditions. Ergo, need to collect. Also, the distributional shifts required for a species to track the climate will in most cases surely be faster than the rate of migration of the species, leading inexorably to its extinction. Need to collect, and quick. I mean, what can a poor species do under climate change besides move or perish? Need to collect, I tell you.

Well, adapt, of course, that’s what it can do. And collecting is not going to help with that. Need to do in situ, maybe assisted migration, you clod.

So a study which suggests that climate change is likely to also result in a decrease in genetic diversity within species would seem to push the pendulum further towards ex situ. Without being able to delve into the particularities of the model, the results seemed plausible to me, assuming that the highest diversity was indeed found in the central part of the distribution. Genetic erosion ensues. Won’t be able to adapt. Need to collect!

I can’t remember if I did nibble it, but I certainly sent the link to the Crop Wild Relatives mailing list. And it elicited an interesting, skeptical reply from Prof. Jonathan Gressel of the Weizmann Institute of Science in Israel. The professor pointed to a possible mechanism by which climate change could conceivably increase genetic diversity.

Unfortunately it is common for modelers to to say that their research “shows” (in this case), demonstrates or even proves something. As a sometime modeler (first model on herbicide resistance published in the Journal of Theoretical Biology in 1978), the best models can do is suggest priorities for experimentation to validate them. Ignoring (or not knowing) one important parameter can skew the model. My mathematician colleague always kept mumbling at me: “Garbage in, Garbage Out”. I would hazard a guess that one parameter was left out of the simulations: the fact that sub-lethal stresses increase mutation rates. Thus, climate change stress will increase mutational diversity in pre-existing genes. For a discussion of this, see: Pest Management Science 67:253-257, 2011.

Oh no, you mean we have to do both ex situ and in situ? Well that won’t do at all. While I naturally hope Marleen Cobb successfully defended her PhD last week, I hope that when she comes round she’ll tweak her model and help us decide once and for all.

The Southern Corn Leaf Blight epidemic that struck the US in 1970 is usually seen as a canonical example of the dangers of genetic uniformity. I use it that way myself, often. Certainly yield losses in 1970 seemed very high, higher than the average 12% “expected from all diseases of corn”. But could we all be wrong? A commenter thinks so.

[W]as 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 ((I don’t know whether he really means year on year increase; somehow, I doubt it. What would be the point?)) in maize in the USA for about 70 years‚ but leaving off the actual years. … I challenged the audience to identify the blight year (1970). Nobody could. … Try this on colleagues and students.

I did, and it is true, 1970 does not look all that extraordinary against the trend.

A more interesting graph is this one, in which the rising trend in average yield is removed from the actual yield each year.

Now 1970 is a little more visible, though I agree it still doesn’t look catastrophic. I mean, compare that with 1988 and 1993. There is one huge difference. In 1988 drought was widespread, while in 1993 floods devastated many farms and yields in the northwest corn belt. Weather in 1970 was just fine, thank you. Weather is clearly a very important factor in annual yields, and it interacts with pests and diseases in complex ways, but it seems pretty clear that the yield loss of 1970, while not as drastic as in other years, was certainly not the result of wayward weather.

The commenter asked “are we making too much of a fuss about the Leaf Blight”? I don’t think so, obviously, so I asked Professor Darrel Good, of the University of Illinois. He knows more about maize yields than almost anyone (and is responsible for the graphs above). He said:

I have not seen any specific analysis of 1970, but am pretty sure that the decline in corn yield was in fact attributed to the outbreak of southern corn leaf blight. Hard to quantify that impact relative to weather. It is a similar phenomenon as the aphid damage to the soybean crop of 2003. ((A pest recently arrived in the US from the soybean’s native China, and a rabbit hole I am not now going to explore.)) These rare events are not captured in our models.

In some respects, pests and diseases are as unpredictable as weather. In industrialized agriculture, genetic diversity within a crop is unlikely to provide much protection against the vagaries of weather. ((Subsistence agriculture is almost certainly different.)) But genetic diversity definitely can protect against unpredictable pests and diseases, not just in maize, and not just against Southern Corn Blight.