Mario Schweiger wrote:Maybe some kind of competition with najadum or other species, microclimate, food?
Jeroen Speybroeck wrote:Maybe collaris copes not us well with harsh winters. Maybe collaris used to be more widespread and was displaced
These are indeed some possibilities. I think microclimate = milder winters might be a bigger factor than competition or food. If there is competition it would be with the other bigger colubrids like Dolichophis and Malpolon that are found where collaris is (as well as further inland). These also feed on smaller snakes too but still collaris seems to cope with that. There's nothing special in the diet of collaris or the distribution of prey in that area, so I'd rule that out too.
Jeroen Speybroeck wrote: I believe there is a gap between najadum and collaris (= an area where neither occurs).
Yes, that's what the books show.
Jeroen Speybroeck wrote:Rivers may act as biogeo barriers.
For snakes? They don't drown very easy Unless it's something of the scale of the Danube I don't think it would stop them.