Analysis of habitat determinants in contact zones of parapatric European salamanders

Parapatry is a remarkable distributional pattern where the ranges of two species come into contact but only narrowly overlap. Theory predicts and empirical data suggest that parapatric range margins are most likely to form along environmental gradients when there is interspecific competition. Here, we study the ecology of the narrow contact zones of two parapatric European land salamanders, Salamandra salamandra and Salamandra atra. Previous research showed that abiotic conditions determine parapatric range margins of these two species. However, in contrast to other parapatric salamander species and theoretical predictions, there is no evidence for competitive interactions in the two Salamandra species. To study whether competition restricts these species' ranges and to understand local syntopic co-occurrence within their contact zones, we used site-occupancy models (1) to assess species–habitat relationships and (2) to test whether there is evidence for competition. We found that the two salamanders show dissimilar species–habitat relationships. The slope of the site positively affected the site-occupancy probability of S. salamandra, while none of the habitat characteristics explained the occupancy probability of S. atra. The local presence of one species had no effect on the occupancy probability of the other, suggesting that there is no effect of competition on local occurrence or that competition does not lead to spatial segregation. To fully understand the mechanisms that determine the parapatric range margins between the salamander species and to unravel the role of interspecific interactions, it is necessary to further study species' functional traits.