Scientific context and motivation

The world is experiencing a major biodiversity crisis: rapid and accelerating loss of species and habitats (Wilson 1992). Amphibians are part of this overall loss, declining world-wide and considered the world's most endangered vertebrates (Baillie et al., 2004; Stuart et al., 2004). Amphibians are essential components of natural and semi-natural ecosystems, playing an important role in energy flow and nutrient cycling, in both terrestrial and aquatic environments (Collins and Crump 2009). Pond-breeding amphibians are particularly sensitive to habitat changes due to their complex life-cycle and need of interconnected aquatic and terrestrial habitats (Moran 1994; Gibbons 2003; Pellet et al. 2007). Temperate pond-breeding amphibians are an excellent target group for evolutionary, environmental and conservation-focused studies.

The spadefoot toads (Pelobates spp.) are highly specialized and have a narrow ecological niche. Since they are obligatory burrowing species, the landscape features have a strong influence on population spread and/or abundance (e.g. Nöllert 1990). Typical terrestrial habitats include sandy areas, heath lands and deciduous woodlands with loamy soils. The spawning habitats include a variety of permanent or semi-permanent ponds. The larval stage of spadefoot toads is of a very long duration (2 to 4 months), the larvae being much larger than other European anuran. Two related species, which differ slightly in their life history, occur in Southeastern Europe (table 1).

While their ranges are mostly disjoint, they overlap on the Balkan Peninsula (Džukić et al. 2008), along the lower course of the Danube and the western coast of the Black Sea. Both species reach the limits of their ranges here: southern limit for P. fuscus and northern limit for P. syriacus. The limits of their ranges might be restricted by competition between the related species (e.g. Tarkhnishvili et al. 2009), or by biogeographical barriers (e.g. the Iron Gates and Southern Carpathians). The area of overlap shows a high risk of desertification, making it an excellent laboratory for the study of their life history, habitat requirements and possible competition since populations at the limits of their distribution range are especially vulnerable to even slight changes of climate.

Table 1. Comparison between the two species of Pelobates with regards to life history, range and conservation status.

The contemporary range of distribution of P. syriacus seems to be much smaller: during the Pliocene its range extended much further north into Central Europe. The fragmentation of the previously contiguous range within the Balkans limited its distribution to a few restricted refugial zones (Ugurtas et al. 2002). Fossils of P. fuscusare known starting from the Middle Miocene of Central Europe (Spinar 1976; Venczel 1999). The last post-glacial invasion into Central Europe took place from the area of Southern Europe, a long-standing refugial centre (Eggert et al. 2006). Both species faced a reduction in their range during the last century in the Balkans (Džukić et al. 2005), Sweden (Nystrom et al. 2002) and Denmark (Fog et al. 1997). The range of P. syriacus has also experienced contraction recently (Delfino et al. 2007).

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