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Evaluating population sizes and capture e

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Barry View Drop Down
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    Posted: 14 Aug 2005 at 1:35pm
This has been bugging me for years !
Does anyone else think it's about time we standardised all the criteria for asessing pop.size, trapping effort, key reptile sites etc etc?
You know the sort of problem : Low pop of Commom lizard (HGBI mitigatiom guidelines) = up to 20/Ha
Low pop of Commom lizard (Key Reptile sites guidelines) = up to 5/Ha.
....And why do they say a Low pop. of Adders is less than 2. surely that's 1!!!
It does my head in!
Barry Kemp - Sussex Amphibian & Reptile Group
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calumma View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calumma Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 2005 at 2:31pm
Barry,

Yes I agree that this needs some serious attention. However, the only real
way of standardising is likely to involve statistical methods that are simply
beyong most consultants.

Perhaps this would make a good workshop for the next recorders meeting??

Edited by calumma
Lee Brady

Kent Herpetofauna Recorder | Independent Ecological Consultant



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Vicar Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 2005 at 2:47pm

I agree totally !

I've been chatting this over with Chris G-O at HCT. I have a bee in my bonnet re the assessment of likely non-presence of a species at a site too. How many visits without a sighting is sufficient? My personal view is much in line with Lee, applying statistical methods to legacy records.

The only reliable method re pop estimates I have come up with so far are any of the various mark/recapture methods. But the shortcommings of this approach are manifest. Clearly there is likely to be a two-tier approach to this; a 'quick and dirty' estimation methodology (general applications/consultants) and a more rigorous (and more accurate?) approach (herpetologists ?)

I would be very keen to actively support any widespread attempt to crack these issues.

My only slight concern with a workshop is that; a significant amount of work is necessary to crack this. So long as a workshop agrees general guidance and the will in principle to reach a common goal, and perhaps the adoption method, that's fine. Agreeing on 'numbers' without this pre-work would attain consistency, but is unlikely to be particularly useful (scientifically).

Comments ?

Steve Langham - Chairman    
Surrey Amphibian & Reptile Group
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Post Options Post Options   Thanks (0) Thanks(0)   Quote herpetologic2 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 2005 at 5:23pm

 

Well there are currently mitigation guidelines being written for English Nature by Cresswell Associates - I think they may be finished - but I do know that they want to provide more information on assessing population sizes from survey results -

You left out the conservation bodies in the list of the two tier approach - there is a third lower level - where reptiles arent even bothered with - for instance in many nature conservation management plans - reptiles oh they can look after themselves......bring on the bulldozers (bovine or otherwise!)

Still wouldnt reptile surveys have to go on for several years or decades to try and answer the question - how big is the population size? wouldnt reptiles undergo the same population 'highs' and 'lows' as amphibians and other wildlife species -

The approach to the different species would be different from each other - for instance the snakes would need to be considered differently to the lizards etc

 

JC

 

PS it would make a good workshop for the next HGBI Herpetofauna Workers Meeting

 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iowarth Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 2005 at 5:34pm

Hi all

No doubt this is something that is needed. But it is appallingly complex. I have a pretty good "feel" for Common Lizards - but there's no science - just years of experience saying "habitat looks like thus, lizards seen X thus population Y"! Not a lot of use.

With Sand Lizards for example, it is far easier to spot them and to evaluate populations in heath than in dune. With intensive surveying in heath you can get to know individuals and come up with a very good estimate. With large dune complexes you count yourself remarkably lucky to ever see the same individual lizard twice or more on separate visits.

An example is in the Merseyside dunes where we are constantly increasing the estimated population - what we don't know is whether this reflects climate change extending the population, increased monitoring, improved monitoring skills or a combination of all these factors and possibly more.

Basically, every estimate we make with these populations has been blown out of the water by further monitoring - however scientific we try to make it!

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 3:51am
The problem with any population assessment is dealing with detectability.
Ideally, the method used to estimate population size would control for
differences in detectability between different species, habitats, methods,
surveyors etc.

Also, it is important to understand what is actually being assessed.
Counting slow-worms within an old allotment may give an assessment of
population. But what does a simple count of adder in a field margin
mean?

Differences in ecology between different species, surely means that we
need different assessment methods? John Baker has gone some way
towards this in devising the 'Make the adder count' project". Although it
remains to be seen how successful counting individuals as they emerge
from hibernation will be - it is at least a start.   
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 4:04am

Precisely what I was trying to indicate by example, Lee.

There are simply so many variables to take into account - we don't even know how to evaluate them. For example surveyor "skill" - what baseline do we measure from, how do we set a scale on it, how do we apply it to an individual (without causing internecine warfare ) and so on.

Inevitably some species/habitats are easier than others - the adder hibernaculum being a case in point - but even then - how confident can we be of its accuracy? Having said this almost every method we currently use can be out by several orders of magnitude - even getting +/- 50% would be a huge improvement in many cases.

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 Vicar Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 7:14am

I have a feeling that pop estimation methods can't progress much further without some confidence in the ground truth for a selection of sites.

Were it possible to know the population by species, for even a handfull of sites; then various methods could be tested (by species) and appropriate calibration or estimations applied to more simplified methodologies. Clearly, the more sites where this can be achieved the better, but even a small number initially would provide a good working premise.

Next comes the thorny question of HOW to we obtain such a ground truth? My thinking is that it is likely to involve detailed work over a number of years, which is why veterans' work, such as Tony Phelps(no offence intended Tony ), where sites have been monitored closely over a good number of years is so important.

I can't see a 'quick-fix' for this one.

Steve Langham - Chairman    
Surrey Amphibian & Reptile Group
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jim Finnie Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2005 at 8:09am

I'm new to this, so apologies if this is old hat:

Imagine a bag containing an unknown number of white marbles. Now add, for example, 5 black marbles which are otherwise identical to the white ones. Next spend some time withdrawing, noting the colour of the withdrawn marble, then putting it back in the bag. You can make an increasingly accurate estimate of the total population from the emerging ratio of drawn black and white marbles. If, for example, after 100 withdrawals and replacements the tally is 50 black and 50 white, we can conclude that there are probably 5 white marbles.

The key is to have a known number of recognisable individual reptiles or black marbles!. The withdrawals and replacements are of course sightings.

Hope this helps .

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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 8:21am

Jim,

Good standard scientific approach that ! Unfortunately, it relies somewhat upon the assumption that 'detection' probability is similar, and that you WILL detect every marble, given enough time. (although I take your point about the estimate improving over time). The other issue in my mind is how many surveys are sufficient ? Adders are relatively easy to distinguish between individuals, but I have a harder time with some of the other species.

Some of our little blighters (I'm thinking of Ca), I personally find very difficult to detect visually, so tins are needed. We do see many reptile species under tins, but are these the individuals who 'like' tins ? what is the proportion of individuals who use tins ?

Nevermind the negative tone above, I actually agree whoeheartedly with the principle you suggest. Yes this is a tough nut to crack, but far from impossible to improve upon what we currently have.

Detection is key, just to fuel the debate, I offer something I've been working on, and would be grateful for comment. (this is just the headlines - lots of hidden subtleties). I'm trying to generate a simple mathematical model to assist with detection.



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Steve Langham - Chairman    
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