0.12 Biogeographic diversity  (Page 22/5)

Biogeography is "the study of the distribution of organisms in space and throughtime". Analyses of the patterns of biogeography can be divided into the two fields of historical biogeography and ecologicalbiogeography ( Wiley, 1981 ).

Historical biogeography    examines past events in the geological history of the Earth and usesthese to explain patterns in the spatial and temporal distributions of organisms (usually species or higher taxonomicranks). For example, an explanation of the distribution of closely related groups of organisms in Africa and South Americais based on the understanding that these two land masses were formerly connected as part of a single land mass (Gondwana). Theancestors of those related species which are now found in Africa and South America are assumed to have had a cosmopolitandistribution across both continents when they were connected. Following the separation of the continents by theprocess of plate tectonics, the isolated populations are assumed to have undergone allopatric speciation    ( i.e. , speciation achieved between populations that are completely geographically separate). Thisseparation resulted in the closely related groups of species on the now separate continents. Clearly, an understanding of thesystematics of the groups of organisms ( i.e. , the evolutionary relationships that exists between the species)is an integral part of these historical biogeographic analyses.

The same historical biogeographic hypotheses can be applied to the spatial and temporal distributions of marine biota. Forexample, the biogeography of fishes from different ocean basins has been shown to be associated with the geological evolution ofthese ocean basins (see Stiassny and Harrison, 2000 for examples with references). However, we cannot assume that all existing distribution patterns are solely theproduct of these past geological processes. It is evident, for example, that the existing marine fauna of the Mediterranean isa product of the complex geological history of this marine basin, involving separation from the Indian and Atlantic Oceans,periods of extensive desiccation followed by flooding and recolonization from the Atlantic ( Por, 1989 ). However, there is also good evidence that the eastern end of the Mediterranean has been colonized morerecently by species that have dispersed from the Red Sea via the Suez canal.

Thus, the field of ecological biogeography first examines the dispersal of organisms (usually individuals or populations) and the mechanisms thatinfluence this dispersal, and then uses this information to explain the spatial distribution patterns of theseorganisms. For further discussion see the module on "Biogeography" and see Wiley, 1981 , and Humphries and Parenti, 1999 .