August 10, 2010

Scott A. Mangan
Department of Biological Sciences
University of Wisconsin
Milwaukee, Wisconsin 53201

Dear Scott Mangan:
I read your article (Negative Plant-soil Feedback predicts tree-species Relative Abundance in a Tropical Forest, NATURE vol. 466 no. 7307 August 5. 2010 page 752).  I had been aware that not only are tropical forests more species rich than far northern forests but a seedling planted near a tree of the same species is unlikely to thrive when other things being equal it would thrive near a tree of a different species.  You now have shown that this effect is, at least in part, mediated by microbes.  A tree will be accompanied by microbes in the ground that will inhibit the development of another tree, and these microbes are specific to the species.

I confess I am surprised.  I should not have thought microbes would spread so far.  A hundred meters must be a long way for a microbe, and if I were walking through a forest and saw a particular species of tree every one hundred meters, I would not be disposed to call it rare.  But your work seems above reproach.

The reason I am interested is that it has been shown in animals, including humans, that adequate fertility requires mating a reasonably close relative.  This is tantamount to saying that there is a factor that limits population size in animals, and the factor is unrelated to the environment.  Here are three critical papers in case you want to check it out:
On the Regulation of Populations of Mammals, Birds, Fish and Insects, Richard M. Sibly, Daniel Barker, Michael C. Denham, Jim Hope and Mark Pagel SCIENCE vol. 309 no. 309 July 22, 2005 page 607.

An Association Between Kinship and Fertility in Human Couples, Agnar Helgason, Snaebjörn Pálsson, Daniel Guöbjartson, Pórdöur Kristjánsson and Kári Stefánson, SCIENCE vol. 319 no. 319 February 8, 2008 page 813.

Comment on “An Association between Kinship and Fertility of Human Couples”, Rodrigo Labouriau and António Amorim SCIENCE vol. 322 no. 5908 December 12, 2008 page 1634.

At first blush this seems odd.  Why should evolution make such an arrangement?  The best explanation I can offer has to do with speciation.  Say, for example, that some form of animal will undergo speciation after 10,000 generations.  Separate a sub population and allopatric speciation will occur more or less on schedule.  Looked at from a chromosome’s eye level, this means that two homologous chromosomes that have not been in the same zygote for 10,000 generations just can’t do business when they get together.  Then consider a population of only 10,000 randomly mating members.  The two chromosomes will get into trouble and the population will crash in about the same time as speciation takes.  And this would be true even of an enormous population.  This could destroy a common and well adapted species, were the members able to mate randomly.  The effect of evolution has been to forestall this by limiting population sizes. 

The trouble with this explanation is that it ought to apply equally well to trees.  But things get difficult with trees.  It is easy enough to watch salmon fighting their way up streams and finding cousins there to mate with.  It is easy to go to a park and notice that where the pigeons are fed, they are fat.  Malthus would have us think that the population should rise to the point where food is exhausted, and they should be starving.  The fact that Malthus does not apply to people is glibly put down to birth control and high tech agriculture.  But pigeons have neither.  So you can see population size being controlled right before your eyes.

But trees.  Unless you do, as you have, meticulous work, it looks like they just sit there.  (They asked the old Swede what they did during the long winters, and he said, “Sometimes we used to sit and think.  But mostly we just sat.”) 

The fact that trees will not grow near others of the same species in tropical forests serves to limit population size.  Since this effect is mediated by microbes, you could say that the microbes are symbiotes – providing a service to the trees – rather than enemies in the long run.  This seem rather ironic. 

Since some species of trees are more sensitive to conspecifics than are other species, a test comes to mind.  Trees that broadcast their pollen and seeds more widely should be more sensitive to others of the same species than trees that broadcast more locally.  It would not be difficult for a knowledgeable person to put that to the test. 

If there is correlation I would find it rather comforting.  Things would make sense.  Of course that ultimately would prove nothing.  The challenge could be made that trees with more aggressive microbes at their roots have been forced to evolve means to reach out farther for reproductive purposes.  It might be possible to figure that microbes evolve faster than trees and thus are more likely to be secondary, but as I am far from my own turf and have no data except yours and that of Lisa S. Comita, I shall give it a rest.

Incidentally I have written Lisa Comita about this as well.  I generally post letters regarding kinship and fertility and anything else I find relevant on my website


M. Linton Herbert MD   

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