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Evaluating population sizes and capture e

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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 7:17pm

Just to throw a spanner into the works, and for my own personal education.

Why are absolute population figues useful ?

I'd suggest..(OK playing Devil's advocate quite heavily here ), that for monitoring purposes, if the goal is to determine how well an individual population is responding, to say a management practice, why not have a sucession of similar surveys (monitoring). If sightings go up (adjusted for suitable conditions etc), then the trend is healthy, conversely, sommit's wrong.

The current froglife numbers representing relative population densities could still be fit for purpose in determining key reptile sites, even though they may be arbitary figues.

I say this, as I have mapped the current (old) La SAP to a proposed action plan, and actually a relatively small percentage of goals actually require population numbers, and even when required, I suspect that trend analysis would do just as well.

Tho it is getting late and I'm noticing senility creeping in recently, just thought I ought to ask the 'stupid/obvious question'.

Steve Langham - Chairman    
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calumma Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2005 at 3:34am
Steve, in principal what you propose is quite valid and would represent a
relative abundance that can be monitoring over time using the same
methodology.

However, the problem comes back down to detectability. If the
management work simply alters a species detectability, any changes in
relative abundance may not actually represent changes in actual
population.

Calculating absolute population figures are important, since they help us
to determine the detectability baseline upon which estimates of
population can subsequently be derived.

With appropriate models, it is possible to estimate density in a way that
controls for detectability. However, existing methods (e.g. capture/
recapture) require considerable effort.

Chris has highlighted another important issue by reminding us that
reptiles tend not to be evenly distributed across any given site. Animals
tend to be patchy in their distribution. A surveyor who randomly samples
a very patchy population may underestimate density (although this will be
overcome with increasing effort), while an experienced surveyor who only
targets focii could overestimate density for the site as a whole.

Edit: I should add, that as surveyors we tend to fall in the latter category -
concentrating our effort in those areas where we expect to find animals.
Not unreasonable from a presence/likely absence survey perspective, but
hopelessly biased when using the simple counts that are derived for
estimating relative populations.

Edited by calumma
Lee Brady

Kent Herpetofauna Recorder | Independent Ecological Consultant



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Post Options Post Options   Thanks (0) Thanks(0)   Quote rhysrkid Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2005 at 2:01pm

This is great stuff.

IĈm keen to know how these ideas work with sites where animals immigrate and emigrate.  With the sites that I work on that support N natrix I have been undertaking a capture-mark-recapture study (using photos of belly markings).  I tend to find that some individuals stay (good reliable folk!) whilst others appear for a few weeks before never being seen again.  Of course this can be down to mortality but given the species habit of having large home ranges it is just as likely that they have simply moved away from the study area.  Similarly, many new individuals are seen with each survey.  This could be due to high recruitment rates or alternatively immigration of individuals from other areas.  When determining the population size how big does the survey area have to be in order to ascertain an accurate popn size?  Would you have to survey an area that covers the maximum distance individuals would travel? If the number of individuals recorded is high is this a true reflection of the actual number relying on the site as key habitat or simply a reflection of many individuals passing through? We work within geographical boundaries such as site borders, development area or counties for example.  As others have mentioned, herps care little for our way of thinking and tend to go where ever they want. Forgive me if this is not relevant, if it has already been mentioned or is straying from the main thrust of maths and models!



Edited by rhysrkid
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calumma Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2005 at 2:08pm
Rhys, the most robust population estimate models should control for
emmigration and immigration. Most of the more basic techniques
unfortunately assume a closed population. The problem with expanding
survey areas to cover an individuals range is that you will encounter new
individuals and have to increase your area to include all of their range, with
the reult that you will encounter new indivi.......   
Lee Brady

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Vicar Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2005 at 5:15pm

heh ! having started to develop a model (and reviewing similar models), yes some do cater for migrant specimens. Mine certainly won't for the first few passes !...its difficult enough as it is !

What I am already finding is...even rough data, based upon extemely simple assumptions just isn't there, but then one of the 'benefits' of creating models is that it always leads to applied research requirements.

Out of interest; I'm dabbling with the concept of distribution probabilites of a given population, chance of encountering, say, refugia, and detection probability. I very much doubt this will result in a method for estimating a population from survey results, but it will force me to consider,and quantify (where possible) the variables together with their relationship. This could be a complimentary approach to that being followed by Cresswell.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote administrator Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2005 at 5:18pm

One can't help thinking of Einstein on his death bed attempting to mathematically model the behaviour of sub-atomic particles.. Albert.. it is chaos!!

I was pondering the same problem as Rhys. Consider a site that lies in the base of a valley. I survey the 40 acre area in the early spring and record a small number of adder regularly under refugia. I determine a likely hibernation area. I consult my database of records and run the numbers through my mathematical model and conclude that I am dealing with a low-density population.

I plan mitigation, I erect exclusion fencing in June. During August I start to remove 8 adders a day from the site and after 2 weeks there is no sign that the numbers are reducing. My receptor site is totally inadequate I am now panicking and wondering why on earth my survey data did not reveal the numbers on site.

Of course I overlooked that the site at the base of a valley was a foraging area serving 6 separate locii located on the escarpment 1km away and large number of animals moved onto the site before the exclusion fence was erected but after I completed my presence absence surveys

Probably a very extreme example, but I wonder how a mathematical model could cope with this sort of fluctuation? Clearly if the surveys had extended throughout an entire season it is likely that the true nature and importance of the site to adder would have been established.. but how many consulultancy based surveys extend for the entire season? One salient point arose during my time working with mathematical models for fluid flow, rubbish in, rubbish out!

Which leads to the bottom line, we need to look at standardising survey techniques, in my opinion presence absence is inadequate to base a mitigation on and not conclusive in many circumstances.



Edited by GemmaJF
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iowarth Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2005 at 5:30pm
Your final line sums it up Gemma - to quote Chris G-O " The absence of evidence is not the same as the evdience of absence"
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calumma Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2005 at 3:55am
Gemma, for your example substitute gcn for adder and breeding pond for
focii - would that be considered acceptable?

I really think that we need different techniques that adequately address
the very real differences between different species. This is especially
important when considering species such as adder, that may display
significant temporal differences in habitat occupancy and subsequently
local distribution. In the example you quote, I would argue that different
population estimates should be generated (where possible), one for each
component of the metapopulation.

It's interesting that for amphibians, we generally only estimate
populations from breeding adults in ponds. Yet for reptiles, we assume
that simple counts are a true reflection of population - even when some
of the same metapopulation dynamics may be at work...

Alas, I also don't think that much of this debate is relevant for
consultancy - at least not at the moment. The level of legal protection
that widespread species receive (and consequent lack of mitigation
licensing - even for adder), means that consultants can meet their client's
legal requirements without worrying too much about any of this stuff.

If we as practicing herpetologists (conservationists and consultants) can
help to coordinate a large-scale project that attempts to address some of
these issues, then perhaps we can help to influence change?
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: 17 Aug 2005 at 9:01am
I suspect, in this context, it refers to gaining appropriate evidence of the likely non-presence of a species. Not seeing a species on one trip is probably insufficient evidence to suggest its likely absence. This then leads to the question of how many site visits in suitable conditions are necessary before we make the judgement that a particular species is probably not present.

This should be a fairly easy one to crack using analysis of historical evidence. Number of visits are likely to be different by species, especially concering Ca ? But existing records should provide a distribution of number of site visits until a particular species shows up, so putting a figure on this with a degree of confidence (in general terms) should be cake compared to some of the issues on this thread :P
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Post Options Post Options   Thanks (0) Thanks(0)   Quote administrator Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2005 at 10:51am

Population dynamics and spatial distribution of the adder Vipera berus in southern Dorset, England

Tony Phelps

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