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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.

If you consider this true - "Europe is generally divided from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting the Black and Aegean Seas" I guess Ommatotriton should be high on the European salamander rare list.

If you take in consideration that in ancient times (but not that long ago in terms of evolution and species distribution) the region of the Black sea looked quite different with much lower water levels and the sea even being a lake. And you look at the distribution of both species of Platyceps in Turkey and the middle east. It doesn't seem so strange to me that a relict population of collaris lives in Bulgaria trapped between the coast and the Strandzha hills. I've been there and I think you can speak of a micro climate in that region.
I find it a bit stranger that najadum doesn't occur in the same place. In Turkey najadum has a much broader distribution inland (= higher altitudes) than collaris. Najadum is also believed to be more common in the north of its distribution area while with collaris it is the other way around. So you would think it also could concur Strandzha. Maybe too forested for their likings?

If only we could look into the past...

or the future... who knows one day because of climate changing both species will have found their way al the way to Belgium

we'll never know

Bert Vandebosch wrote: It doesn't seem so strange to me that a relict population of collaris lives in Bulgaria trapped between the coast and the Strandzha hills. I've been there and I think you can speak of a micro climate in that region.

Yes that makes sense.

Bert Vandebosch wrote:In Turkey najadum has a much broader distribution inland (= higher altitudes) than collaris. Najadum is also believed to be more common in the north of its distribution area while with collaris it is the other way around. So you would think it also could concur Strandzha. Maybe too forested for their likings?

Yes, same in Bulgaria, najadum is found in Pirin mountain at significantly higher altitude than Strandzha and the coastal collaris population. It is also quite forested in Pirin though, so maybe the reason for the absence of najadum in Strandzha is something else....or maybe we just haven't found it there yet. Strandzha is not that well explored.