Mangrove Ecosystems 19 Figure 10 World distribution of mangroves in relation to 20 1C isotherms Reprinted from Duke NC (1992) Mangrove floristics and biogeography In: Robertson AI and Alongi DM (eds.) Tropical Mangrove Ecosystems, pp 63–100, with permission from American Geophysical Union and the author Eastern Pacific Western Atlantic West Africa East Africa IndoMalesia Australasia Figure 11 Comparison of the mangrove flora in six geographical subregions Because of the recent closure of the Isthmus of Panama, the eastern Pacific and western Atlantic (including Caribbean) are most similar in species composition Note also the separation between Atlantic–Caribbean–eastern Pacific (ACEP) and Indo-West Pacific (IWP) regions Further dispersal barriers, including the Isthmus of Panama, open ocean, and arid coasts unsuitable for mangrove occupation, divide the major regions into smaller subregions, each with a more or less distinctive mangrove flora (Figure 11) Only one species occurs in all six subregions: the mangrove fern Acrostichum aureum Two genera, Avicennia and Rhizophora, are common to both IWP and ACEP regions All other genera are found exclusively in either the IWP or the ACEP, although the close similarity between Laguncularia (ACEP) and Lumnitzera (IWP) suggests a recent separation of these two genera The traditional explanation of mangrove species distribution is of a center of origin and of diversification in Southeast Asia, followed by dispersal restricted by physical barriers This clearly makes little sense in relation to the current dispersal barriers Fossil evidence of mangroves is widespread and reveals a much wider distribution during the Eocene and earlier epochs: Fossil Nypa, Avicennia, and Rhizophora pollen and other remains, for instance, have been identified in Eocene and Miocene deposits that now form part of North and South America, Europe, and North Africa as well as south and east Asia At the time, these locations were connected by the Tethys Sea, continuous through what is now the Mediterranean and Indian Ocean Subsequently, this pantropical distribution was partitioned as a consequence of continental movements Cosmopolitan genera such as Avicennia and Rhizophora were separated into regional populations by the approach of Africa to Asia 30–35 million years ago which closed the Tethys Sea, and separation of the sister genera Laguncularia and Lumnitzera followed the widening of the Atlantic barrier The emergence of modern species ensued within the isolated subregions Closure of the Isthmus of Panama was geologically very recent (a mere or million years ago) so that differences between eastern Pacific and Caribbean species are slight One species (Pelliciera rhizophorae) is found on both sides of the Isthmus, presumably reflecting a separation into two populations too recently for allopatric speciation to have occurred An originally pantropical mangrove distribution was therefore partitioned into regions and subregions, with subsequent evolutionary divergence Climatic conditions then eliminated mangrove species from areas such as southern Europe and the Mediterranean fringes The current distribution pattern results from a combination of large-scale geographical factors and more regional climatic ones