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- Title: Global Biogeography – Developments in Palaentology and Stratigraphy
- Author: John C. Briggs
- Publisher: Elsevier
- Pages: 473
Five hundred million years (Ma) ago, a diversity of higher (metazoan) life forms was present in the sea. One hundred Ma later, an invasion of the land was well underway. Since then, there has been a generally increasing trend in the species diversity (richness) of both habitats. The trend has been interrupted, most notably in the marine environment, by a series of extinction episodes during which species diversity was temporarily reduced. None of these episodes occurred with catastrophic suddenness, consequently most biotic communities were able to accommodate in an ecological and evolutionary sense. The time spans occupied by the extinctions and the subsequent recovery of diversity
were on the order of millions to tens of millions of years.
For the past 65 Ma, until the 20th Century, the multiplication of species continued until Cenozoic diversity became about double that of the preceding Mesozoic. Most of the increase may be attributed to the evolutionary success of vascular plants and the animals associated with them. Human beings became a distinct species about 3 Ma ago, thus arriving on a planet with a rich biota of some 12 million or more species. One important facet of biogeography is the study of the geographic distribution of diversity. The superior species diversity of the tropics has been known since the days of Charles Darwin and Alfred Russel Wallace. But, it is only recently that the true global pattern has begun to
emerge.
As Wilson (1991) has noted, there appears to be a consensus among many tropical biologists that the rain forests probably hold 10 million or more species. Briggs (199 l a, 1994) made rough global estimates of about 12 million metazoan species on land, as opposed to less than 200 000 in the sea. Rain forests occupy only about 6% of the land and, of the surface of the entire globe, about 2%. This leads to the astonishing conclusion that more than 80% of all living species may be concentrated in the rain forests that occupy only 2% of the earth’s surface.