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Evaluating population sizes and capture e

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Iowarth View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iowarth Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 1:06pm

Hi Steve

I think you do have the beginnings of a potential mathematical model here. Unfortunately its limitation appears to be its link to species that use refugia. I suppose to an extent, with La for example, focii might give a similar input. There is still a potential weakness in that such focii are not necessarily always found - nor do we know what proportion of a given population use them. Conversely, with the refugia using species such as Ca you have effectively eliminated variability in the surveyors so at least one of the "unknowns" has been dealt with. You also raise an interesting question regarding tin usage by herps. Is it, as you say, simply certain individuals that use them. Certainly with Lv I believe that any/all individuals will use them - albeit usually basking on top rather than underneath! Nonetheless in a recent Lv/Af rescue I estimated populations based on sightings on and under tins as 250/100 in 1 Ha in four visits in good conditions multiplying individuals see by 5. Animals actually captured and removed was 450/200 - so what the hell do I know!

Chris Davis, Site Administrator

Co-ordinator, Sand Lizard Captive Breeding Programme (RETIRED)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calumma Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 1:25pm
Steve, I would argue that your fixed factors are not actually fixed.
Tendency of species to use refugia may be dependent upon local habitat
conditions, time of day, season and weather. Population density may also
vary - especially later in the season as new recruits enter the population.

Chris, I also think that using refugia does not necessarily eliminate the
variability associated with surveyor experience. I have seen tins placed in
the most bizarre places. I guess it depends on what you are trying to
show. It would be easy to produce false negatives through inappropriate
placement of refugia...

With regard to Lv and refugia, I have found that whether animals are
found underneath or on top depends on local weather conditions and
time of day. Gemma produced a graph a while back that illustrated this
quite nicely for a site in Kent.
Lee Brady

Kent Herpetofauna Recorder | Independent Ecological Consultant



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iowarth Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 2:15pm

Hi Lee

I must agree your first point, also addressed in your final paragraph. I must admit I have so little opportunity to see Lv in the wild nowadays that the generally carefully selected weather conditions late in the season would definitely produce a bias towards refugia for additional warmth. And there is no doubt about variability of population density whatever the species.

And, indeed, I stand corrected re the surveyor experience. I only get tin placement wrong intentionally (as in I've got so many I'll chuck one here in case the lizards don't agree with me) but I must admit it would be very easy to do so. In fact, in the rescue I mentioned I had a couple of inexperienced helpers at the tin placing stage. After carefully describing and demostrating appropriate locations it felt like I spent the rest of the day saying "No, not there because.....".

Makes a helluva job with Steve's model, or indeed any other when it is filled with nothing but variables.

Chris Davis, Site Administrator

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Vicar Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 3:14pm

Cheers guys,

this is all useful debate. To be honest, I was waiting for the fixed factors comment . its supposed to represent factors beyond the control of the surveyor, the terminology is a military hangover.

As for placement of refugia, its what I mean by physical placement, although as Chris suggests, it's well worth placing tins in less obvious places. I'm a firm believer in that if you only look for targets in the places you expect to see them, then you'll only detect them where you expect. As Gemma has said before, nature doesn't follow many rules closely (or should that be absolutely?).

Part of the rationale with building any model is to expose the issues and develop understanding, its pretty much a given that any model will be wrong, its just a case of how much practical value it produces somewhere along its development.

Steve Langham - Chairman    
Surrey Amphibian & Reptile Group
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Barry Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 3:22pm
I see we are getting back to refugia again! All very relevant and worthy of more discussion but we have moved away from my original point somewhat.
If HGBI could produce guidelines all that time ago (10, 15 years?) surely with the combined knowledge we have all acumulated since then we must be capable of improving the current guidelines for say, what a medium sized population of Slow worms should be or what a suitable number of refugia would be for survey per hectare of a given habitat type and what the caveats are?
One thing's for sure , at some point a compromise will have to be made between what is achievable and what is desirable.Or should we just not bother with the current guidelines?
Here's another question : Are we just wasting energy trying to find a foolproof system for accurately predicting reptile population size? After all I've always thought that a successfull translocation is when the habitat is right and you can record some breeding in the following year (s). If you moved 465 Slow worms to a perfect receptor site and surveyed it again using the most rigourous survey technique known to man you'd still not record 465 Slow worms on the receptor site the following year, well maybe if you did a destructive search !
Barry Kemp - Sussex Amphibian & Reptile Group
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iowarth Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 3:33pm

You have a military hangover? How unpleasant!

Further to Gemma's previous comment my parallel one which I often use is "To the best of my knowledge the lizards (or snakes, or whatever) seldom read the books by the experts." Certainly most of us have come across various herps in atypical habitat and behaving in atypical manner at some time or another.

My chief frustration in this debate thus far is that it is so easy to pick holes in someone else's ideas - but I feel terribly guilty doing so because I'm damned if I can come up with anything better! Or, indeed, anything!

The single greatest problem that I can perceive is that we do need a fixed reference from which we can work. But all the potentials appear themselves to be governed by a number of variables.

For what its worth we loosely estimate Sand Lizard populations on heath as around three times the number of individuals seen in at least four visits in otimal conditions. Even more loosely, in dune, experience so far suggests that the factor should be more like seven. But even that is difficult because the nature of the habitat is such that we might never be able to confirm our estimate.

It does, however, seem to me, that if we could assemble and analyse the results of various rescues, together with all possible details from preceding surveys of the rescue site, habitat type etc we might have something to work on. After all this is the only occasion on which we can be sure that a reasonably substantial plot of land actually surrenders all - or at least the majority - of its animals.

Chris Davis, Site Administrator

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Post Options Post Options   Thanks (0) Thanks(0)   Quote calumma Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 3:43pm
In fact Barry, I don't think that things have progressed very much at all.
Personally I feel that simple counts (whether collected through the use of
refugia or otherwise) mean very little with regard to reptile populations.

There are some interesting models being developed that attempt to relate
counts to population estimates but there is a long way to go before these
are likely to be used in the UK.

I'm reasonanly confident that existing techniques allow one to reliably
determine likely presence, but I agree that we really need to push for
more research projects in this area.


Lee Brady

Kent Herpetofauna Recorder | Independent Ecological Consultant



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Post Options Post Options   Thanks (0) Thanks(0)   Quote calumma Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 3:51pm
Chris, it sounds like you developing a 'detectability constant'. Perhaps this is
a good starting point for where we go from here.

I agree that rescue type projects may be the best option for attempting to
determine absolute counts. However, most such projects lack the luxury of
time for collecting data in a sufficiently rigorous manner.
Lee Brady

Kent Herpetofauna Recorder | Independent Ecological Consultant



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Post Options Post Options   Thanks (0) Thanks(0)   Quote herpetologic2 Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 5:04pm

Dear all

I would agree that rescue sites seem to be the only real opportunity to try and answer these questions - but would the secretive reptile mitigation industry give up their records or do we need to start up a reporting scheme for consultants to report their data and findings.......ohhh another part of the NARRS project??

There are several projects looking at reptile mitigation - Cresswell Associates are looking to compare survey results with total captures - along with habitat criteria - I also know that the numbers of questionaires being returned is quite low - incidently not many reptile records or even crested newt records are passed onto local recorders - not even the BRC!!! - does make you wonder where the records are?

I feel that this debate is needed to try and push reptile conservation rather than 'protection' - translocating thousands of reptiles is clearly not conservation - rather the focus should be on habitat protection, creation, enhancements and monitoring - Research should also be included in reptile mitigation schemes - an agreed format of recording data across the country would help - Reptile habitat suitability index, standard survey methods etc

JC

 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iowarth Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 5:33pm

Barry, obviously you fired up an interesting debate but you are right, we have strayed from your initial question. Returning to your specific point while what you say sounds good in theory I am unsure it holds good in fact. I am, for example, unsure of the origin of the HGBI figures you quote. If you hunt around enough you will find density figures here and there BUT most of them are either informed guesses, specific to only one habitat type/climate etc or simply quotes of the other two.

Now, having said this, from nothing but personal experience (informed guess!) I would not disagree greatly with any of the figures you quote. Less than 20 Common Lizards per hectare is certainly a very low density population - and, by definition this includes 5 (which is a very, very low density!). So far as Adders are concerned these are found at much lower densities and over wider areas. Thus, 15 adders over 10 hectares represents 1.5 per hectare - certainly both a low density and less than 2 !

Conversely most of those 15 adders might be stuffed into 1 hectare and most of the 20 lizards stuffed into 20 x 20 metres within the hectare. This is one respect in which even measuring density as animals per hectare can be grossly misleading. Clearly, if 15 of the Common lizards were in prime habitat of 400 sq metres we would have a population density there of 375 per hectare which represents a reasonably dense population (and yes, I know a population fairly near me that does broadly match those figures).

I am, in fact, less than convinced that we can improve significantly on these specific definitions. The real problem is that which this thread has people trying to address - not what constitutes as low/medium/high density - but how, other than by destructive search can we calculate density.

So, I think that we are actually moving towards what you want - or at least trying to. If we had a reliable method of assessing populations (and note I say populations rather than population density) then we would be in a position to re-define low/medium/high density - if necessary.

Lee, you are quite correct in that I am seeking a "detectability constant". Still a variable constant but one that varies within definable parameters so could become an absolute in each specific locale. I recognise that it might not be truly scientific and disciplined - but nonetheless better than what we have.

Sadly, as Jon says, there do seem to be a number of members of the mitigation/consultancy business who do not seem to be prepared to share this information. Generally, and unsurprisingly, these seem to be the ones who regard it as money for old rope and have little care or concern for the animals themselves. So, while I think my proposal has some validity, as Cresswells are discovering it may still not be wholly practical.

Chris Davis, Site Administrator

Co-ordinator, Sand Lizard Captive Breeding Programme (RETIRED)
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