More on tropical trees:
As you know the evidence that population sizes in animals are regulated by the fact that more distant kinship reduces fertility. The principles that underlie this are very general. All you need to assume is evolution, speciation and reproduction through sexual fertilization. So it ought also to be true of trees.
But the evidence for trees is rather sparse. The only thing I can put my finger on is that in tropical forest, which are very old, there is a large number of species. That suggests that they have learned to limit population size so any species is dispersed and there is plenty of room in the sun for other species. At the same time the relatively young northern forests have not developed all those different species. But if you look at the distribution of the forests, they consist of little bands following river beds. There is not the unbroken canopy typical of a tropical forest, or of a temperate zone forest for that matter. So their effective population size is already limited. This is evidence of a sort, but there ought to be more.
There is now some new information. (Asymmetric Density Dependence Shapes Species Abundances in a Tropical Tree Community, Liza S. Comita, Helene C. Muller-Landau, Salomón Aguitar and Stephen P. Hubbell SCIENCE vol. 329 no. 5989 July 16, 2010 page 330) It turns out that if you take a seedling in the jungle of Panama and look at the trees nearby, the seedling is more likely to grow into a tree if there are few or no neighbors of the same species than if there are many. At the same time, the number of other trees of different species seems to have little or no effect on the seedling’s chances.
From my perspective of course that is a slam dunk. All those species are limiting their own density and thus limiting the size of the population among which they exchange genetic information for reproduction. Were the world not utterly mad, we could all go home now. The principle is established and anyone concerned about the survival of any population or the fertility of an individual ought to take it into account.
And this new information does not overlap at all with the mass of data I have already found and written about.
But alas, the world is quite mad, and this will probably pass unnoticed like every other dire warning.
But there is another point. Different species vary in their ability to tolerate neighbors of their own kind. The rare ones are the least able to live close to their own kind. This means they are more dispersed. This means that in any area there must be fewer of them, which means they are rare.
And that provides an opportunity. It is all very well and good to look at a result and say, “See. I could have told you that would happen.” It is another thing to say, “Look at your data again. This will be true of your data, and you did not notice it.” That in fact is what happened with the second paper from Denmark about marital radius. I wrote the lead author and pointed out that he needed to look at greater radii than he had. He did so. I was right. He published.
In this case, the question is of pollen and seed dispersal. If population size is as important among trees as it is among animals, then those that broadcast their pollen and/or seeds the farthest need to have the most stringent restrictions on growing up together.
Rare trees will be sending their pollen and seeds farther.
There. I’ve made the prediction. They’ve already done the hard work. All they need now to do is compare mobility with rarity. I shall hasten and make the suggestion.
Don’t hold your breath.
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