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Lots of interesting material with, as Maggie describes, varying degrees of compellingness. What I found mself particularly interested in is the question of what existing diversity gradients have to tell us about the significance of biodiversity itself. Clearly there are, in our biosphere at the moment and, as far as one can tell have always been, locations of higher and lower biodiversity. What this tells us, at a minimum, is that it is naive to think that "natural" corresponds to the highest possible biodiversity or to take "biodiversity" as a monotonic measure of biological well-being.

Existing biodiversity gradients seem to me to hold out the possibility of a more nuanced appreciation of biodiversity, and particularly of the question of whether biodiversity is actually an important functional characteristic of life, one that to some degree at least can be used as a measure of biological "health" or is instead simply a consequence of the tendency of biological systems to explore all possibilities. Perhaps biological diversity, at any given time, is simply a reflection of the combination of resource availability and the amount of spatial/temporal environmental variation, and so will be low when those factors are low and high when those factors are high?

My own inclination is to think that there is more than this to biodiversity, that there is actually some value of diversity to living systems, perhaps characterizable at fixed points in time (in terms of productivity or ... ?) but at least measurable in terms of the ability of systems to respond adaptively to unpredictable change (cf Grobstein, 1989, 2008). Looking forward to reading/talking more along these lines.