Discussion

Although the Holdhaus line was introduced more than a half century ago, it is still up-to-date. The brief overview of the most recent literature on European cave beetles (including the comprehensive faunistic investigations in Northern Italy and Southern France) shows that only a few amendments, but no basic corrections, had to be made to the Holdhaus line.

Of what importance is the Holdhaus line for the identification of refugial areas for other endemic taxa? Recent studies of endemic-rich taxa indicate that the Holdhaus line is not only a remarkable Northern border for the distribution of blind cave- or soil-dwelling beetles, but is also congruent with the Northern distribution limit of several endemics (e.g., from the genus Leistus, Assmann and Heine 1993).

If highly specialized endogeic species were able to survive the glacial period(s) in Southern France and Northern Italy, i.e., in regions close to the Holdhaus line, it is likely that other species which are currently more widely distributed in Europe would also have had a survival chance in these "Massifs de Refuge." Molecular analyses of populations of the ground beetle genus Carabus are a suitable method to localize former refuge areas, as shown for Carabus solieri, an endemic ground beetle species with a small postglacially recolonized area and a hybrid zone formed after a secondary contact of individuals from different glacial refuge areas (Garnier et al. 2004).

In our test species, C. auronitens, we found genetically rich but strongly diverging populations - rear edge populations according to Hampe and Petit (2005) and therefore of special importance for nature conservation. Moreover, these populations are obviously still situated close to their assumed glacial refuges South of the Holdhaus line (Fig. 5). Our analysis confirms the earlier ideas of multiple glacial refuges of C. auronitens in the Pyrenees, the Montagne Noire, close to Rodez, and in the Cevennes (Assmann et al. 1994; Assmann and Weber 1997; Reimann et al. 2002). Some of these refuge areas coincide with the putative refuges of the beech (Fagus sylvatica), as shown by a combination of palaeobotanical and genetic data (Magri et al. 2006). This example clearly illustrates the importance of the Holdhaus line in separating populations living in former glacial refuges from populations inhabiting postglacially recolonized areas. The Holdhaus line may also help to localize glacial refuge areas of other species, such as the fire salamander (Salamandra salamandra, Steinfartz et al. 2000), which prefers cold and humid conditions, and which recolonized parts of Europe postglacially.

The results of the genetic differentiation of C. auronitens do not mirror the sub-specific taxonomy of C. auronitens as there are at least two subspecies described (C. a. auronitens and C. a. festivus, Turin et al. 2003) which are not separated genetically. The morphometric analysis by Terlutter (1991) does not support this taxonomic system either. For C. auronitens, there are numerous taxonomic works in which several subspecies are described (e.g., Breuning 1932; Deuve 1994) which are partly contradictory and seem to lack any objective criteria. Thus, we strongly recommend renouncing the use of taxonomic units below species level for C. auronitens (for a general discussion, see Assmann et al. 2008).

Our results show that populations that are situated North of the line are genetically poorer than those located South of it. These populations display - from a genetic point of view - only a section of the refuge populations (with the exception of the populations from the French-German-Swiss border triangle, see below), which is in concordance with Reinig's (1938) idea of stepwise allele elimination in the course of a set of postglacial expansion processes.

As an exception, we even found populations in the Southwestern part of Central Europe (Jura, Vosges, and Black Forest) which contain private alleles. It seems improbable that these alleles originate from one of the putative refuge populations in Southern France as the populations situated in between these areas are genetically impoverished and display only a section of the allelic richness of the populations from the Cevennes. Surprisingly, a set of endemic taxa was described for this Southwestern part of Central Europe just after Holdhaus had published his compendium. It is noteworthy that some of these species share the same habitat with C. auronitens: five diplopod species (Spelda 1991), one carabid species (Huber and Molenda 2004), and Lumbricus badensis (Kobel-Lamparski and Lamparski 2004). All these species prefer humid and cold habitats. Furthermore, the Black Forest region is explicitly named as a potential glacial refuge area for the vole Microtus arvalis (Jaarola and Searle 2002).

Normally, there is a striking connection between blind beetles and certain bedrock or soil types. Although limestone bedrocks are not colonized exclusively, there is a main occurrence of blind beetles in these soils. Therefore, the lack of endogeic or troglobitic beetles in den Black forest region point to a certain shortcoming of the Holdhaus line.

Refuge areas on non-limestone bedrocks can thus be overlooked due to the lack of this special group of indicator species. It seems, then, that the limits demarcated by the Holdhaus line are further South than the possible geographic position of some glacial refuges for species of cold and humid habitats. Holdhaus seems to have recognized part of this problem as he explicitly named the records of blind troglobitic beetles in the Jura which caused him to draw an isolated enclave North of the continuous Northern distribution limit of blind troglobitic beetles. The putative glacial refuge area in the Black Forest is situated close to this enclave.

The data for C. auronitens as well as for many other species of cold-humid habitats (review by Schmitt 2007) further support Holdhaus' idea (Holdhaus 1906, 1954) that glacial refuge areas are located in the extra-Mediterranean. These results are in line with the findings of Willis et al. (2000) and Willis and van Andel (2004) who provide evidence for the existence of trees in many parts of Central Europe throughout the cold stages of the full and late glacial.

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