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Slow-worm densities |
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Matt Harris
Senior Member Joined: 03 Jun 2003 Location: United Kingdom Status: Offline Points: 233 |
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Posted: 16 Sep 2004 at 3:37pm |
A consultant's report has quoted slow-worm as reaching densities of 600 - 2000 per hectare, but provided no reference for this. Before I embarrass them by asking where they got this from, has anyone else come across this piece of data, or anything similar on slow-worm densities? Ta.
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Local Authority Ecologist
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administrator
Admin Group Joined: 01 Jan 2007 Status: Offline Points: 10 |
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It doesn't sound too far out Matt, though I do not know the exact reference it came from. Though similar values are given by Platenberg and Langton in a paper called Slow-worms in Kent:estimates of population density and post-translocation monitoring, this was published in English Nature Science No.27 Reptile Survey Methods. I don't know if Lee Brady could tell you more about this paper and the results. An estimated density of 1050 per hectare is given. Edited by administrator |
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Matt Harris
Senior Member Joined: 03 Jun 2003 Location: United Kingdom Status: Offline Points: 233 |
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What method did Lee, or anyone else for that matter, use to estimate pop. densities? I know Betty used recognition of individuals from head markings etc, but is there any way of estimating from straight tin data, without individual recognition/marking?
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Local Authority Ecologist
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calumma
Senior Member Joined: 27 Jun 2003 Location: United Kingdom Status: Offline Points: 375 |
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The density estimate of ~1000 slow-worm per ha comes from multiple studies. These include the the work of Nick Smith (1990, MPhil Thesis), Anne Riddell (1996, Diploma in Ecology Thesis) and Betty Platenberg (1999, PhD Thesis). Miguel Vences has also published a paper that estimated slow-worm density on an isolated Spanish population. Extrapolating from his results a density of 960 individuals per ha is achieved.
Nick, Anne and Betty used head and chin patterns to recognise individual animals. One thing to be aware of with reptile densities is that although estimated densities can be very high indeed in specific areas, calculated density depends upon the size of the survey area. Although obvious many consultants don't take this into consideration. It is rare for all parts of a site to offer excellent habitat, densities are usually much lower than the quoted figures. Consider the situation of a development site that consists of an arable field with a narrow linear headland. The overall density of reptiles may be very low (if the arable field is included in the estimate). However, the *density* of reptiles within the headland could be very high (yes even 1000 per Ha) - yet the overall *abundance* within the headland may be relatively low. Population estimates and the terminology used to describe them need to be interpreted with a great deal of caution. Beware of consultants who claim the 1000 - 2000 animals per ha as justification for dumping large numbers of animals into a small receptor area. A common tactic by the ignorant or unscrupulous... There is absolutely no way of estimating population density from basic tin data without individual recognition or marking. Statistical models (e.g. capture recapture etc) that control for detectability are required. |
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Robert V
Senior Member Joined: 06 Aug 2004 Location: United Kingdom Status: Offline Points: 1264 |
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I'd have to agree with you on that one Lee, 100%! I'd be suprised if the actual figure of slow worms in an area of say a Hectare was any larger than 30! That is any given hectare where they are known to thrive. That should cause a stir. R
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RobV
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calumma
Senior Member Joined: 27 Jun 2003 Location: United Kingdom Status: Offline Points: 375 |
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I have data available and slow-worm numbers can easily exceed 1000 animals per ha. I have a site that supports a closed population of slow-worm. The site is about 2 ha in size that has had over 1000 animals removed from it. Not all areas within the site offered suitable habitat and the work remains uncompleted (only ~60% of the site has been cleared to date). Obviously delays in progressing the works cause problems when attempting to calculate density/abundance in schemes such as this, since recruitment into the population may occur. No reptile fence is perfect and animals do breed!
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Matt Harris
Senior Member Joined: 03 Jun 2003 Location: United Kingdom Status: Offline Points: 233 |
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OK thanks for your replies
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Local Authority Ecologist
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nils
Member Joined: 18 Jan 2011 Status: Offline Points: 1 |
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I was wondering if anyone of you could send me published papers where these slow worm densities are mentioned. I am writing a dutch article about translocation of a slow worm population in the Netherlands. We translocated 151 slow worms out of 1 ha grasland that became a building site (it used to be a developmental site, but building was delayed for years). This number has, as far as I know, never been found in the Netherlands. Only estimations have been made. I am pretty astonished about the numbers you are discussing. I could use some references. Need more information, let me know!
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sussexecology
Senior Member Joined: 30 Sep 2010 Location: United Kingdom Status: Offline Points: 411 |
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For slow worms, consultants assess the population density figure based on the maximum peak count of adults. For slow worms, you would normally only see approx 10% of the population at any one time. So if you saw 10 adult slow worms during one survey session, this would be 100 slow worms on the site, 20 adults would be 200 etc. it's a rough estimate but you should only base these counts on adults, not juveniles.It would depend on how many ACO you have put on the site though. There are some consultants who quote the population density of slow worms in the number of animals seen and they don't use the 10% figure. So if they see 20 slow worms, they indicate that it is a low population - whereas if they used the 10% figure, they would realise that actually it's a very good population. If i find a reference to the 10% figure, I'll PM you. Edited by sussexecology - 17 Feb 2012 at 12:42am |
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sussexecology
Senior Member Joined: 30 Sep 2010 Location: United Kingdom Status: Offline Points: 411 |
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Would have to agree with you Lee on this one. Working that way is just madness, because the receptor site should always be bigger and better habitat quality than the development site. I think some consultants under-estimate the amount of time to move a large population of animals, esp slow worms. Edited by sussexecology - 17 Feb 2012 at 12:42am |
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