Berislav Horvatic wrote:Jeroen Speybroeck wrote:I'm not up to speed with the research results, but you may want to consider disturbance as one variable,
especially with vipers.
You mean disturbance by US, humans? That' a general argument, but I know my viper "hotspots", I know
the amount of (visible) disturbance to the habitats (actually, not much), but the decline in the number
of vipers seems just not proportional to the amount of disturbance. Of course, who am I to judge how
much of that might be too much for THEM, but still... I see what I see, and I just don't like it.
Around here I have no reason to believe there is any significant negative trend in viper numbers. I
do suspect human disturbance can be a signinficant factor in terms of sightings, however.
In general I have no problems finding new localities pretty much wherever habitat seems appropriate. This is not becoming harder and numbers I find at new localities are not decreasing.
There is only one area I've visited frequently enough over a sufficiently long period of time to have anything even approaching significant data in terms of the development of individual populations. This is a spot with three small subpopulations in the suburbs of Copenhagen.
Two of them are in woods where logging and reforestation with fast growing pines forces the adders to move around at lot and must have done so for decades, if not centuries.
One of these spots had a seemingly dense population when I discovered it, seemingly, because they were found in an grove of still young pines where a few years earlier they probably would have been spread out across a much larger area. Some 10-14 years ago the trees had grown to a size where sunlight only reached the forest floor along two narrow paths causing the rather few adders to congregate there. A couple of years later as the trees grew larger still and the canopy closed they gradually disappeared. I have no reason not to expect they simply dispersed to other more open areas in the forest as the have probably done many times before, yet to my surprise and disappointment I have not been able to find them. I have heard of a couple of sightings of individuals in this forest over the past couple of years, though.
The second spot, in another nearby forest, had a den some 6-9 years ago with a high concentration of adders in the immediate vicinity in a small clearing right next to a wide path with rather heavy traffic of dog walkers and the like. It became quite well known in the local community and received probably too much attention. This was not negative attention in the sense that the general public showed a positive interest in the adders rather than the opposite. That does not change the fact that the continued disturbance of basking adders by curious humans (myself included, I have become more careful since...) and the trampling down of the ferns around the den may well be the reason why the adders all but disappeared. Increased shading from upgrowth of newly planted trees across the path may also be a factor but in this case it seems to me there should still be enough light so I doubt it. I no longer see quite as many adders in the area as I did back then but I do still see them only now mostly at other, more difficult to reach, spots. I saw rather few this year but I expect that's just an anomaly. Otherwise, numbers have been stable over the past five years or so.
The third subpopulation is in an open area between these two forests. I have followed this population over the past 6-7 years where the habitat has not changed visibly. Numbers have not gone down, in fact it seems I'm seeing more adders now than I was in the beginning but that's probably just down to my increased familiarity with the site. What's interesting, though, is that the adders are moving around - and not just according to season but from year to year - even in this absence of visible habitat change. Again, I can only (albeit speculatively) put this down to increased disturbance, most likely from human observers. I first searched the area after hearing about the population from an acquaintance. At the spot - again, next to a heavily trafficked path - where he had regularly seen good numbers of adders just a couple of years before they were now gone. I then started finding them a couple of hundred meters away at a den also near the path and along the slope south of it. After a couple of years the adders started to disappear from this area as well, moving to the other side of the hill, and north along the railroad. I suspect my liberal attitude towards sharing locality info for probably the easiest to reach from Copenhagen adder population once again meant that the attention from curious humans disturbed the adders enough to cause them to move.
From friends around Denmark I hear complaints about dwindling adder numbers - yet only from the most well known spots with the highest densities. I suspect - though I don't have data to substantiate this - that this is nothing more than a case of the adders relocating from excessive disturbance.
In short, disturbance need not be habitat disruption. In some cases, our actions as herpers may also affect adder populations even if we take nothing but photographs, leave nothing but footprints. This effect may be innocuous but to the extent that we cause the snakes to move to - in other respects - lower quality habitats our actions as observers may also be detrimental to individual populations.