Speciation and diversity

In Ernst Mayr’s essay ”What is Darwinism”, there is a section on multiplication of species, in which Mayr notes that there still is a lot that we do not understand about speciation and how it occurs. This is interesting for a number of different reasons, but one small thing stood out to me in the essay – and that was that Mayr suggests that there are 5-10 million different species of animal and 1-2 million species of plant in the world.

Now, if this still holds it suggests an interesting question about the relative proportions of plants and animals – if these different kinds of speciation are holistically meaningful – in a biosphere. Is there an optimum relative proportion of different species, and if so – why?

It seems the overall composition of the biosphere is an intriguing question, as well as the question of what different sets of biospheres – compositionally – are viable. Is Earth an example of a rich biosphere or would alien life be as speciated as life here on earth for a reason?

If so – what is that reason, and what is the limiting condition that drives towards that kind of diversity and proportion?

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