Long term hybrid zone dynamics in red- and yellow-bellied toads estimated from environmental data at allopatric and parapatric scales

Aim: Aims of the study are to identify environmental parameters underlying the mutual distribution in a species pair engaged in a long and winding hybrid zone, to reconstruct pattern and process of species range developments following the Last Glacial Maximum (LGM), and to open research lines for the study of hybrid zone dynamics in a model system. Location: Central and Eastern Europe, Croatia. Taxon: The red-bellied toad Bombina bombina and the yellow-bellied toad Bombina variegata. Methods: The construction of two-species distribution models (TSDM) at allopatric and parapatric scales and the reconstruction of geographical clines across a low mountain range, in transects with and without extensive lowland forest cover. Results: It is confirmed that B. variegata is a mountain species and B. bombina is a lowland dweller, with hybrid populations in between, but traditional distribution models are oversimplistic. Environmental parameters selected with TSDM at both allopatric and parapatric scales are elevation and forestation. At transects with lowland forestation hybrid zones are positioned further away from elevated areas than in the absence of lowland forest. Bombina variegata stronghold areas are characterized not just by elevation but also by forestation. Out of 17 historical Bombina studies that are evaluated for promise in the study of hybrid zone dynamics several qualify for extended research. Conclusions: The overall species mosaic with isolated mountain strongholds suggests that B. variegata was displaced from the surrounding lowlands upon its counterparts northward advance, following post-LGM climate amelioration. Initial hybrid zone formation will have been along the lower Danube River, distant from present-day positions. Because lowland forestation constitutes a buffering effect to species replacement, current hybrid zones may not have reached equilibrium conditions, depending on deforestation history and other characters of the landscape. Future genetic work on B. variegata enclaves may shed light on the pattern and process of hybrid zone movement and species replacement, though timing and the size and environmental signature of the enclaves will affect genetic introgression in ways that may be hard to disentangle.