Summary Species diversity, here defined simply as the total number of species in a community, represents the historic processes by which additional species enter the community and species disappear from it. The additions arise chiefly through speciation by geographic isolation followed by a congregation of the new species formed. The mechanism is by reversible geologic events such as glacial periods, orogenies, and dispersal corridors, the first two affecting organisms as reversible climatic changes. These events cause speciation on a grand scale, acting simultaneously on species in all seral stages of the biome. On this basis, the input side of the species diversity of a community is a combined function of (1) the geologic longevity of its ecological conditions and (2) the number of times the community area undergoes geographic disjunctions and reconnections.
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Herbert H. Ross (1972) studied this question.