Where do you find the greatest diversity on earth and why?

Answer 1

The greatest biodiversity is found in tropical rainforests of the humid tropics, and in coral reefs in the ocean. These ecosystems each occupy a small proportion of the terrestrial and marine realms but each contains over half the species of those realms.

On a broader scale biodiversity globally is high in the Tropical World

of the humid tropics and in the extremely infertile Unenriched World

of the ancient arid landmasses of Australia and Southern Africa. Biodiversity is much lower in the geologically young and cool Enriched World

which basically corresponds to the extratropical northern and western hemispheres, plus New Zealand (which can very easily be thought of as part of the Western Hemisphere though it is on the other side of the International Date Line).

Although it is popularly though that high biodiversity relates to the more “benign” and less demanding environment in hot climates, recent research by Michael Huston of the University of Texas in “Biological diversity, soils, and economics” and “Precipitation, soils, NPP, and biodiversity: resurrection of Albrecht's curve”, Australian ecologist Tim Flannery in The Future Eaters

plus Jason Weir and Dolph Schluter in “The latitudinal gradient in recent speciation and extinction rates of birds and mammals” show that these are misconceptions.

Flannery and Huston both demonstrate that owing to the roughly four orders of magnitude greater age of their soils (except in the volcanic regions of the Pacific Rim), the productivity of the Tropical and Unenriched Worlds is very much less than those of the Enriched World of the extratropical northern and western hemispheres. Weir and Schluter show that speciation rates are much lower in the Tropical World than in the depauperate fauna of the Enriched. One would presume that with their extreme geological stability the Unenriched World of Australia and Southern Africa would have still lower speciation rates than the humid tropics

. This is especially true when one considers that many birds and mammals in Australia and Southern Africa require so much labour for reproduction that most adults must serve as “helpers at the nest” rather than reproduce on their own, which acts as an extremely severe limiter on potential dispersal.

All these indicators point to the higher diversity of the Tropical and Unenriched Worlds as being due to reduced interspecific competition

compared to the young and highly productive Enriched World. Soils of an infertility universal in all of the Unenriched and most of the Tropical World are exceedingly rare in the Enriched World - occurring only in a few areas of exceptionally nutrient-poor parent materials

like ultrabasic rocks (serpentines, peridotites) - and then only outside of glaciation limits within which intrazonal parent materials are converted to highly fertile zonal soils. (In this context, it’s notable that the most northerly major biodiversity hotspot in the Klamath Basin is one of the major occurrences of serpentines in the world, as is the very rich tropical hotspot of New Caledonia).

In the Tropical and Unenriched World resources are so scarce in unfertilised environments that co-operation rather than competition tends to be the rule to allow plants to obtain the minimal nutrition possible on these soils. The absence of competition reduces extinction rates to a fraction of the level observed throughout the Enriched World: in essence, the creation of biodiversity occurs in the Enriched World, but it is rapidly pooled into reservoirs in the Tropical World, and the Unenriched retains older species (marsupials, mousebirds) dating from periods when its ecological conditions were globally general.