Variation In Peripheral Areas

The special nature of isolated or marginal and peripheral areas with regard to variation and evolution was first discussed in relation to tropical ants and gave rise to the recognition of the phenomenon known as ecological release (Wilson, 1959). Areas studied included savannas, monsoon forests, sunny margins of lowland rainforests and salt-lashed beaches. In ants, the expanding dominant species became adapted to such marginal areas, in which the relatively small numbers of (ant) species that occurred had fewer competitors. Consequently, species in marginal areas are more flexible as the places in which they live have less competition and therefore according to Wilson the species are ecologically released and show increased variation. This argument was subsequently pursued in the Island Biogeography theories developed jointly with MacArthur (MacArthur & Wilson, 1967).

During periods of climatic instability, as have occurred in polar regions both during and after the Pleistocene, it can be expected that there will have been many extinctions and re-immigrations of plants in peripheral areas. Consequently, the relatively recent end of the Pleistocene glaciations has suggested to some investigators that the plant populations that now inhabit arctic regions are largely the result of Holocene migrations from areas that supported peripheral populations during periods of Pleistocene glacial advance. Application of the classical age and area concept (Willis, 1922) would associate areas of high diversity with a long period of occupation and conversely areas which have only recently been colonized would have less diversity. In addition, the many climatic oscillations that have taken place at high latitudes will have resulted in numerous extinctions and reimmigrations. The genetic consequences of these Ice Age climatic fluctuations would have been to cause a reduction of variability, as during phases of northward expansion recolonization will take place mainly from individuals spreading out from the periphery of southern populations (Hewitt, 1996). For species that survived the Pleistocene glaciations in southern Europe such as grasshoppers, hedgehogs, oak, common beech, silver fir and black alder trees, brown bears, newts, shrews, water voles, and house mice, molecular data confirm a reduction in diversity from southern to northern Europe in the extent of allelic variation and species subdivision (Hewitt, 1999). However, other molecular data have also shown for plants that there is an ancient arctic flora that survived the Pleistocene glaciations at high latitudes (see also Section 6.7). Even in the Arctic, subspecies diversity varies in relation to glacial history. In one of many studies showing this effect clonal diversity in four species of Carex was found to be lower in areas that had been deglaciated in the last 10 000 years as compared with areas that had either become deglaciated 60 000 years ago or not glaciated at all during the Weichselian (Stenstrom et al., 2001). In species that have a long uninterrupted history of survival in polar regions there is emerging a considerable body of evidence for a high level of genetic variability (Section 6.7) even at the most northern limits of species distribution (Grundt et al., 2006; Crawford, 2004).

Both these contrasting views of species variability in relation to latitude may therefore be correct depending on the particular area and the biogeo-graphical history ofthe species under discussion. Time for variation to take place is obviously necessary. Hence, areas that have not had their biological history drastically interrupted by the Pleistocene glaciations usually have higher numbers of species than in equivalent areas which lay under ice during the last glaciation.

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