In contrast to other weak dispersers that survived the ice ages in Southern refugia and remained largely confined to these areas ever since, C. variolosus expanded its range subsequent to postglacial climate changes despite being flightless. As a stenotopic woodland species specialised on water margins, C. variolosus must have suffered considerably from the climate changes of the glacial periods. Given the species' current habitat preference (e.g. Sturani 1962), its confinement to lowland or lower montane altitudes (e.g. Breuning 1926; Barloy et al. 2004) and its strict stenotopy established by the habitat models (Matern et al. 2007), it is likely that populations survived the ice ages in sheltered, wet deciduous woodlands. These habitats may have persisted during the Last Glacial Maximum in Iberia, Italy and the Balkans (Zagwijn 1992), and in the Carpathians (Kotlik et al. 2006). Deffontaine et al (2005) furthermore suggest the existence of mixed coniferous-deciduous forests in the river systems present near the Alps and in the Hungarian plain and Slovakia. Paleobotanical and genetic evidence also corroborates the survival of beech populations as far North as Southern France, the Eastern Alps, Slovenia, Istria and possibly Southern Moravia-Bohemia (Magri et al. 2006), but so far no refuges of forest-forming tree species are known from further North. Recent faunistic records of C. variolosus, however, exist from numerous regions North of the Alps such as the presence of C. v. variolosus at the foothills of the Sudetic Mountains, Upper Silesia and the Lublin Upland in Southern Poland (Pawlaczyk et al. 2004) and as far as the Hamburg area, where C. v. nodulosus occurred until about 100 years ago (Breuning 1926).
The homogeneous mtDNA in the Central European populations of C. v. nodulosus is consistent with a recent colonization of the region North of the Alps. Under the standard molecular clock for insect mtDNA of approximately 2.3% divergence per million years (Brower 1994), all Central European populations of C. v. nodulosus and the Southern Slovenian lineage separated no more than 0.3 Myrs ago (0.7% pairwise average sequence divergence). Likewise, the Central European populations and haplotypes of the subspecies C. v. variolosus separated approximately 0.5 Myrs ago (1.2% divergences). The existence of these lineages which likely predate the most recent glacial cycles may suggest the existence of more than one glacial refuge or source area within or near to the investigated area.
Post glacial colonization processes are not reflected in allozyme variability. We did not observe a directional loss of alleles according to the allele elimination hypothesis (Reinig 1938; Hewitt 1999) nor any regionally specific sets of allozymes and their combination in intermediate populations, which could suggest potential contact zones. The largely random geographic structure of allozyme variants, the lack of private alleles, and the lack of correlation in allele frequencies with geographic distance (Matern et al. 2009) suggests that the variation in Central European populations was more similar at first, but that populations lost variability subsequently due to stochastic processes. Hence, the case of C. variolosus could represent an example where postglacial colonization, in combination with subsequent geographic isolation, produced genetic differentiation of a similar magnitude as that resulting from survival in different glacial refuges. For example, the postglacial C. variolosus populations from Germany and the Vosges display an FST of 0.45 (7 polymorphic enzyme loci). This is similar to values obtained for 29 populations of Carabus punctatoauratus in the Pyrenees that survived in different glacial refugia (mean FST of 0.23, enzyme loci, Assmann and Weber 1997) or to values found in C. solieri, another flightless silvi-colous ground beetle, that remains largely confined to its glacial refuges in the Southwestern Alps close to the border between France and Italy (FST of 0.34, microsatellite markers, Garnier et al. 2004). Although FST values obtained with different makers are not directly comparable, the latter example also supports the depth of population separation within the Northwestern lineage of C. variolosus.
It is likely that the present interpopulation differentiation of C. v. nodulosus is only partly due to low dispersal power of these flightless beetles, but was exacerbated by small population sizes. Ecological knowledge helps to understand better what affects the recent population history of C. v. nodulosus: its high habitat specificity not only increases the barrier function of the landscape structure surrounding suitable habitat patches but may also result in very small populations, as demonstrated for the above two populations in Northwestern Germany (Matern et al. 2008). Descriptions of patch sizes in additional populations of C. v. nodulosus suggest that these may exist or have existed under similar spatial conditions (e.g. Gersdorf and Kuntze 1957; Perraudin 1960; Morati and Huet 1995). While the investigated populations do not appear to be inbred, they are genetically impoverished in comparison with other carabid beetles (Matern et al. 2009). This renders them highly susceptible to genetic stochasticity that is associated with extinction vortices, in particular in the face of expected climatic changes.
While C. v. nodulosus was distributed much more widely until about a century ago, it has since suffered from a marked decline and nowadays only persists in very localised relict populations (Breuning 1926; Turin et al. 2003; Matern and Assmann 2004). Intensive human land use with its massive impact on the natural hydrologi-cal budget is thought to be responsible for the decline (e.g. Perraudin 1960; Weber and Weber 1966; Pavicevic and Mesaros 1997; Turin et al. 2003). The resulting destruction and degeneration of suitable habitat has aggravated the geographical separation of headwater areas and pristine forest swamps that already affected
C. variolosus populations under natural conditions but is now increasing the effect of habitat fragmentation. Currently, C. v. nodulosus is considered endangered throughout its entire range (Turin et al. 2003), and its highly restricted distribution is further contracting. Therefore, it is unquestionably a relict in the sense of Fryxell (1962) meeting also further specified criteria such as a reduced variability and narrow ecological tolerance, which potentially indicate a reduction of adaptability, and may ultimately be a candidate for extinction. While the beginning of the decline of C. v. nodulosus cannot be clearly identified, the relict status of C. v. nodulosus in Central Europe, however, is relatively recent, as the beetle successfully colonized this area post glacially. While the situation of C. v. variolosus apparently is less alarming (Turin et al. 2003; Pawlaczyk et al. 2004), this subspecies is also characterized by a narrow ecological tolerance and a restricted range that is further declining in some regions (Turin et al. 2003).
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