The Iberian Peninsula – corridor or cul-de-sac? Mammalian faunal change and possible routes of dispersal in the last 2 million years
Introduction
The last 2.5 million years have seen enormous changes in global climate, most obviously in the Quaternary glacial/interglacial cycles. This has been particularly apparent in Europe, where a relatively small landmass contains several large mountain ranges and peninsulas that have made conditions at any one point in time differ in different regions. Iberia is almost surrounded by sea and when large quantities of fresh water were locked into ice sheets during glacial periods, the available landmass would have been larger through the expansion of the coastal shelves. Iberia also contains one of the four plausible routes for hominin dispersals out of Africa, the Strait of Gibraltar, the others being from Tunisia to Sicily, across the Sinai Peninsula and across the Bab al-Mandab Strait. In the Pleistocene, with the exception of the Sinai Peninsula, all of these routes would have involved some ability to cross water, and there is increasing interest in the potential use of coastal environments by hominins, both for resources and for the migration opportunities that they may have offered (e.g. Erlandson, 2001, Flemming et al., 2003, Turner and O'Regan, 2007). The Gibraltar Strait may have been crossed by hominins, but it is difficult to assess the fossil or archaeological evidence for hominin settlement of Iberia as the record is so sparse. However, many other animals have migrated, evolved or become extinct in Iberia during the Quaternary, and understanding what has happened to other taxa may provide a context for examining hominin dispersal and occupation. The climatic changes and geographic restrictions of the Quaternary have had a great effect on the evolution and dispersal of a myriad of different creatures, from the grasshopper (Chorthippus parallelus) (Hewitt, 1996) to the roe deer (Capreolus capreolus) (Lorenzini and Lovari, 2006). For extant animals some of these movements can be traced using phylogeographic methods (see Hewitt, 1999 for a review), but for extinct species the fossil record must be examined to see how and when creatures were moving. For much of the Quaternary Iberia was also a refugium for temperate animals restricted by the encroachment of glaciers and tundra to the north. It contains the latest occurrences of extinct species (e.g. Neanderthals (Homo neanderthalensis)), and some of the earliest finds in Europe of African taxa such as Theropithecus, and Hippopotamus, which have been used to argue for faunal dispersal across the Strait of Gibraltar (e.g. Agustí and Moyà-Solà, 1992, Agustí and Moyà-Solà, 1998). However, Iberia has a second much more obvious route for dispersal across the Pyrenees and the faunal evidence for the use of both the terrestrial (Pyrenees) and water (Gibraltar) crossings will be considered here.
Section snippets
Topography, climate and vegetation
Although a refugium for temperate plants and animals in the Quaternary glacials, the Iberian Peninsula was by no means a homogeneous temperate landscape in which warm-adapted animals could live until the glaciers retreated to the north. In fact the Iberian Peninsula itself contains a number of east–west running mountain ranges and areas of high land (over 1000 m a.s.l.), particularly the Sierra Nevada in the south, the central Sierra de Guadarrama and the northern Cordillera Cantábrica, as well
The Plio-Pleistocene large-mammal fauna of Iberia
The locations of all the sites mentioned in this discussion are shown in Fig. 1, and their approximate dates are given in Fig. 2. The mammalian fauna of Iberia was reviewed by Agustí and Moyà-Solà, 1992, Turner, 1995, Kahlke, 1999 and García and Arsuaga (2003). These reviews will not be repeated here as, overall, most of the observations made still hold true, but the main points will be summarised and some distributions will be updated. For example, the elk (Alces) has still not been found in
Routes into and out of Iberia
There are two plausible routes into and out of Iberia, the first is over the Pyrenees, while the second is the perilous crossing of the Gibraltar Strait. Initially the Gibraltar Strait looks attractive, and there is the circumstantial evidence of the early appearances of African taxa in Spain to bolster support for the idea of a crossing, but can this route be substantiated? Although narrow, the Strait is deep, and there has been a permanent body of water there since the end of the Messinian
Phylogeography and crossing the Pyrenees
The glacial periods lasted much longer than the interglacials in the Quaternary (one estimate suggests that 80% of the Quaternary was spent in glacial periods (Willis and Whittaker, 2000) while another suggests that 12% of the Quaternary was spent at full glacial maximum (van Kolfschoten, 1995), which in the Middle-Late Pleistocene Milankovic cycles of 100,000 years would make approximately 12,000 years of isolation in refugia when the glaciers were at their peak). Such isolation might well
Conclusions
Iberia represents a highly heterogeneous collection of habitats which would have fluctuated with both small scale and large scale Quaternary climate changes. Such variations would have provided the opportunity for some mammals to diversify, whilst for others it may have ultimately led to extirpation or extinction. Several taxa failed to recolonize the Peninsula after local extinction and these include such large and usually wide-ranging taxa as hyaenas and big cats, and also smaller creatures
Acknowledgements
Many of the thoughts in this paper were generated during a NERC EFCHED project grant, number NER/T/S/2002/00431. I would like to thank Anne-Marie Nuttall for helping to generate Fig. 1, and the two anonymous referees whose comments improved the manuscript.
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