RAUK - Archived Forum - Response to habitat fragmentation

Hi guys,

Im currently looking to test the role of habitat fragmentation in influencing sub-population size and structure of the adder on some of my sites this soming spring. However, when predicting a species response, I have found that I have come to a bit of a cross roads.

According to population genetic theory, small, fragmented populations are at high risk of extinction due to genetic drift and inbreeding potential. I would expect the carrying capacity of patch habitats to become reduced with increased fragmentation, and therefore inndividuals to disperse and colonise sub-optimal habitats. However, is this a justified prediction given that the species is well known for its high degrees of site fidelity?

HELP!!

Thanks for your input Steve. My study sites feature predominantly on a local AONB, where by the majority of the established sub-populations are within SSSI's. so I am lucky in that there is immediate protection from development. So from what I have observed over the past years, is that it is habitat disturbance,manifested from recreational activity and inappropriate habitat management, that are considered the foremost threats here. I am currently working with the local landowners and managing authorities to provide increased security to some of the more vulnerable habitats (e.g. hibernacula and lying out areas. Certainly there is an issue of potential isolation of sub-populations here, but Im still conducting baseline surveys here to map the distribution of spring assemblages for the area.

What is interesting in particular is a small sub-population that became isolated on a single linear remanant fragment following heath burns being implemented on the site in 2004. I have not observed any female activity on this site for over three years, and so I may have a "bachelor" group of males on my hands here. I suspect that many individuals have dispersed and recolonised new areas as the heath has gradually regenerated over the past five years.

Best

Oops!! Forgot to mention Steve that Im thinking mainly  along the lines of reduced carrying capcity as a function of habitat quality and quantity!

Hmmm not a bad idea!! I already have all sightings over the past two years mapped on satellite images using GIS, so it would actually be realatively simple to investigate correlations between sightings and habitat. I could also use the same GPS and satellite images to map heath fragments that are utilised by sub-populations to estimate edge area and interior habitat area. Certainly there is a scope to do this kind of thing. I will keep you updated on developments as it would be useful to have your input Steve.

Best

Anecdotal observation, I think adder attempt to carry on 'business as usual' in these situations. I have first hand observations of them attempting to cross entirely barren soil scrapes using what limited habitat remained as cover. My gut feeling is that this leads to higher mortality rates and the population crashes. I don't see any evidence of them 'moving' to suitable habitat elsewhere, simply some individuals become more and more sedentary in what existing habitat remains, the others being lost to predation as they attempt to carry on as normal. I have also observed this with spring dispersal and the males setting up surface dens, they continued to utilise the habitat they had tradionally used that remained, even in cases where it was a tiny fragment of what existed before management. Other 'apparently' suitable habitat nearby being ignored.

Be interesting if your studies support my pessimistic view Kevin.

Aye, my own observations would support what you have seen GF. I have previously tracked a male for over an hour on one occassion, including across totally barren landscapes. Observations by TP confirm the idea that habitat distrubance, particularly within hibernacula and surface den sites may lead to increased mortality rates within males due to their basking habits. This also suggests that adders indeed just carry on with their regular behavioural patterns. So it is a possibility for adders to respond in the same way with regards to dispersal.

Thanks for your input GF

[QUOTE=Vicar]Time of year will also be a factor to consider, as males in mate-search mode seem oblivious to pretty much anything, including available cover.
[/QUOTE]

Quite true, I watched a male in May 1985 search all around a busy car park next to the heathland, on one occasion I had to step in to stop a car parking on him, my footfall or movement attracted him toward me at great speed.

This location was also at least 200m away from the nearest colony, quite bizaare.

Interesting stuff chaps!! Jon do you know of a link to this evidence, or is it yet to be published? But anybody have any ideas of how best to measure degrees habitat fragmentation. I could (I guess) combine many variables i.e. amount of edge, area of interior habitat, distance between frags  (using GIS) etc. Any other suggestions??

Grateful for the input as always

Kev 

Thanks for your comments Lee. At this point I would like to add that I have offered this project as a dissertation title to some degree students with the University of Chester, so hopefully they will be able to commit to longer field seasons than myself (dam work commitments!!). I agree that comparing presence data with site specific fragmentation co-efficient would be the way forward for the purpose of this study. My data thus far dates back 3-4 years depending on the sub-population, but hopefully my students will work on developing this over the coming years. I am fortunate in that landowners and habitat managers alike are keeping an open mind with regards to their methods of habitat management, and they are keen to take greater consideration of adders in the application of their methods. I would certainly be interested in discussing this further. Will you be attending the ARC-BHS Joint Scientific Meeting in December? Otherwise I could always email.

Best

Kev

I have only recently moved up North, and have found little time to familiarise myself with such upcoming events, so probably not Im afraid. BUT just in case, do you have the dates and venue etcetc?? I should be at the ARC/BHS meeting though, so may well see you there!