Influence of Pleistocene Glacial Refugia to Current Patterns of Mediterranean Biodiversity

Pleistocene climatic cycles have profoundly affected the biogeographical footprint of several Mediterranean species, notably in the Mediterranean Basin, California, and Chile. Recent researches combining genetics and biogeo-graphy (i.e., phylogeography) and paleocology underline that glacial refugia represent crucial areas for the long-term persistence and dynamics of modern biodiversity in temperate regions. Glacial refugia constitute territories sheltered from the strong climatic deteriorations during Ice Ages, and where species survived the drastic consequences of severe cold and aridity. A major and noteworthy glacial event is the Last Glacial Maximum (LGM) that occurred c. 20 000 years BP. First, there exists a clear influence of Pleistocene climatic cycles on patterns of species-richness and endemism. Second, full-glacial refugia have also had a powerful influence in shaping current patterns of genetic diversity in several temperate and Mediterranean ecoregions. Finally, these refugia played an important role on vegetation dynamics during previous interglacial periods of the Pleistocene; these areas contributed to the forest recolonization process that started approximately 13 000 years ago in the Mediterranean Basin and lasted throughout the Holocene. Once climatic conditions became truly favorable, the expansion of a highly diversified deciduous forest could happen rapidly over large territories, such as in the Pindos mountains in northwestern Greece where over 20 deciduous woody plants could already be found in around 10 000 yr BP.

There is growing evidence, both at a global-scale and for Mediterranean-type ecosystems, indicating that biodiversity hot spots coincide generally with areas that were buffered against climatic extremes. Reduced impacts of Milankovitch climate oscillations and smaller-amplitude climatic changes during the LGM constitute the best descriptors to explain both the survival of paleoendemics and the speciation ofneoendemics. Therefore, the climate stability-diversity pattern accounts for the location of most of the endemic-rich Mediterranean areas, and this pattern appears clearly when we compare two ecoregions with distinct historical biogeography, the Mediterranean Basin and SW Africa.

Around the Mediterranean, the alternation of humid and hyperarid phases in North Africa or interglacial and glacial episodes in Europe have induced profound shifts in the evolution and geographical distribution of species lineages, resulting in, respectively, expansion from Mediterranean refugia or extinction-reduction of populations during unfavorable periods. The major Mediterranean areas where temperate and thermophilous species survived are the three Iberian, Italian, and Balkan peninsulas, but also the largest Mediterranean islands and the submontane and mountain margins of North Africa, Turkey, and Catalonia-Provence. The glacial events induced the extinction of several subtropical lineages and paleoendemics, and speciation by radiation was quite reduced.

In South Africa, glacial climates of Pleistocene were moister in western Cape and drier in eastern Cape than present. It seems that the western Cape elements have extended northward into the present succulent karoo and Namib desert during glacial episodes, whereas drier conditions in the east have induced the restriction of Cape vegetation and species to some mesic refugia. Due to the peninsula configuration surrounded by ocean, southwestern Africa was one of the mildest continental landmasses of the world, and Pleistocene climates there were exceptionally stable favoring higher rates of speciation and lower rates of extinction compared to eastern Cape or to northern Mediterranean ecoregions.

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