February 14, 2012
to be posted on nobabies.net

Robert E. Ricklefs
Department of Biology
University of Missiouri – St. Louis
St. Louis, MO 63121-4499

Dear Robert Ricklefs:
I read your article (R. L. Ricklefs and Susanne Renner Global Correlations in Tropical Tree Species Richness and Abundance Reject Neutrality SCIENCE vol. 335 no. 6067 January 27, 2012 page 464) with great interest.  I am far from my usual field of interest here, so take almost anything I say here with proper skepticism since it is very tentative.

What I do believe beyond reasonable doubt is that among animals fertility is related to kinship of a mating couple.  This has best been demonstrated among humans in Iceland (An Association between Kinship and Fertility of Human Couples Agnar Helgason et al. SCIENCE vol. 329 no. 5864 February 8, 2008 page 813 - 816).  If you have any doubt, please go to nobabies.net and look at the notes I prepared for the Orlando meeting or just look at the copy I shall attach.  The Sibly paper indicates that the phenomenon is widespread in the animal kingdom.

There ends my assurance.  However when I look at the numbers I find that fertility falls below replacement if a mating pair are very distant kin.  This should mean that a large randomly mating population faces an inevitable population crash, possibly going extinct.  On this point I have had little or no agreement.  The logic seems, however, to be air tight.  And I believe it.

But there is a problem when we come to plants.  The logic of why it happens, as laid out in the Orlando notes, is that a large randomly mating population is ultimately doomed by speciation effects.  Nature forestalls this by applying a mechanism – presumably epigenetic since it takes hold much more rapidly than DNA change could – that limits population size to a safe level. 

And so it should be for trees.  It is a commonplace that species diversity in the tropics, where forests are very old, is much greater than in high latitudes, where they date back only to the last ice age.  There has not been time to establish tropic style richness of species. 

A few moments with Google Earth will show what you already know, that tropical forests tend to form unbroken canopies while boreal forests confine themselves to river beds.  Both of these limit population size of any one species, in the tropics the members of a species are dispersed among other species while in the north the population size is limited by the size of the forest along one dimension.  Of course the first reaction must be, “Of course.  It’s not so cold and dry down there.”  And that may be it.  But the breadth of the forest seems to be rather constant over vast distances.  At least sometimes it looks like that to me.  Sometimes it doesn’t. 

Your own paper observes that one explanation for species richness in the tropics has simply been that species appear and vanish at random.  In this case abundance of species in a plot on one continent should not correlate with abundance on another continent.  And – and here is where my mind starts to spin – the abundance of families of trees on one continent should not correlate with the abundance on another continent.  But, after much meticulous work, you find that the correlation is there.  So the random appearance and disappearance model simply cannot be true.  There is some mechanism that sets diversity at the family level at a more or less constant value.

You point out that a possible cause might be that a forest of all one type of tree would be a sitting duck for parasites, which presumably evolve faster than trees do.  And so it might be.

Following that logic, since all members of a family of trees must be descended from a single species, then a long long time ago species evolved the means to limit their population density.  That would result in a collection of families.  If the cause for that limitation is universal, then it comes as no surprise that the richness of diversity at the family level is rather constant.  Against this I have no argument. 

On the other hand, the notion that population size is limited by some epigenetic mechanism works the same way.  And indeed, referring to the Vergeer study in the Orlando paper, there does seem to be in plants an epigenetic cause of inbreeding depression.  It would seem plausible to wonder whether there is an epigenetic cause for what one might call outbreeding depression. 

It’s just a thought.  I would not be willing to go to the wall on it.  But there might just be a way to work out a testable hypothesis so as to distinguish between a pathogen driven diversity and a diversity limiting population size to avoid the long term threat of extinction by speciation effect.

What do you think?


M. Linton Hebert MD

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