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GCN terrestrial habitat range

Printed From: Reptiles and Amphibians of the UK
Category: Herpetofauna Native to the UK
Forum Name: Great Crested Newt
Forum Description: Forum for all issues concerning Triturus cristatus
URL: http://www.herpetofauna.co.uk/forum/forum_posts.asp?TID=4405
Printed Date: 28 Mar 2024 at 10:39pm
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Topic: GCN terrestrial habitat range
Posted By: Matt Harris
Subject: GCN terrestrial habitat range
Date Posted: 05 Nov 2012 at 1:46pm
We often read things like "the majority of animals probably stay within about 250m of the breeding pond" or "the vast majority of animals are found within a few hundred metres of the breeding pond" and that juveniles can variously disperse 900m, 1Km, 1.3 Km etc from natal pond, but I wondered if there were evidence-based percentage figures attached to the 250, 500, 1Km radii that are often advised, e.g. do 95% of terrestrial animals occur within 250m of a breeding pond, 99% within 500m etc?

Clearly at the level of the individual pond, there will be a range of factors such as suitability of habitat, presence of barriers/dispersal corrifdors etc, but does anyone know of research that has looked at the average dispersal distance from breeding ponds?

Or did someone just pluck 250 metres out of the air?



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Local Authority Ecologist



Replies:
Posted By: GemmaJF
Date Posted: 05 Nov 2012 at 5:10pm
Probably someone just picked 250 metres out of the air Matt. Logically as they only have little legs one might expect much higher concentrations around the breeding pond and this reducing with distance from the pond(s) used for breeding. So it is rule of thumb stuff.

No doubt though there are lots of modifiers and it isn't a rule. It would depend on the proximity of suitable foraging habitats, etc etc. I have little doubt from my own observations of native amphibians that many juveniles disperse widely from breeding ponds. After all most species take two to three years to become sexually mature and not all will return to ponds during that time. I have often observed for example concentration at the edge of motorways, along borders with arable fields which suggest they move some distance which stops when reaching a barrier. So one could find a high concentration of animals an appreciable distance from an obvious breeding pond if such circumstances existed within say a development site.

I'm sure we have also all encountered odd individual GCN at times turning up in ponds which do not normally support them and this again suggests distance dispersal.

Of course the problem is if someone says 250m or 500m in guidelines it becomes dogma. I certainly know of cases where ponds falling just outside this range have not been surveyed and also cases where consultants have requested data to that sort of limit of a site boundary, rather than a more pragmatic request such as within 2Km of the site.

Clearly though concentration of animals will be expected to reduce. If you draw a circle 250m around a pond it has a much smaller area than a 500m one and a substantially smaller area than one 2Km so detectabiilty will become an issue. Though there is nothing at all to say that one couldn't find a high concentration of animals within that larger area at all.


Posted By: Mark_b
Date Posted: 05 Nov 2012 at 6:36pm
Hi Matt,
 
I have emailed you an extract from The Crested Newt – A Dwindling Pond Dweller
by Robert Jehle, Burkhard Thiesmeier and Jim Foster (2011).
 
If you wish to buy the book, look here for details....
 
http://www.thebhs.org/Documents/Newt%20book%20flyer.pdf" rel="nofollow - http://www.thebhs.org/Documents/Newt%20book%20flyer.pdf


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http://www.wgarg.co.uk/">


Posted By: Matt Harris
Date Posted: 06 Nov 2012 at 10:02am
Ok thanks for your responses; I asked the question more in hope than expectation really as I suspect the answer lies among the many things we don't know about species such as this. I did wonder if the 250m and 500m distances that are so often regurgitated were based upon any particular studies, but from the few examples quoted in the extract from Jehle, which Mark has kindly sent to me, it does seem that an average of 250m is too small a radius to encompass a reasonable majority of the population.

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Local Authority Ecologist


Posted By: Noodles
Date Posted: 06 Nov 2012 at 11:28am
I was about to recommend the same book, it has good chapters on home range/dispersal and a thorough reference section listing all the latest scientific papers if you want to go further. 

One of the studies (mentioned in the book) found that the vast majority of the population sheltered within 50 m of the pool, although this is certain to depend on the availability of suitable habitat etc etc. If you talk to most people involved in GCN mitigation, this capture distance is common in many of the more 'typical' cases.

In a UK study, Baker and Halliday found that new ponds were not colonized within 400 m of existing breeding pools, although this is never going to be a set rule. You can get the paper from the BHS if you are a member. 

Some of the NE research reports, based on capture returns from mitigation cases, are very useful and free to upload from the NE website; in particular try the Cresswell report ENRR 576 and the DICE stuff. Just a few ideas; although there are countless more. 

The standard 250 m is accepted as the extent of a 'typical' home range (maybe larger in a non-isolated meta-population) and there are plenty of studies to support similar distances used (greater or lesser). Doesn't mean you are always going to find many adult animals beyond 100 m or so of a pond though! Also, typical home range or meta-population dispersal should be viewed differently from the distance an individual animal COULD or sometimes might travel......500 m, 1 km, 1.3 km, 2 km etc etc! If i remember rightly, Jehle et al suggest the maximum extent of nightly migration to be 100 m (based on several studies), which indicates the potential distance an animal could disperse during an active season, if desired. The fact that most animals appear not to do so is more telling.

Also home ranges/dispersal distances are always going to vary depending on habitat quality/availability and distribution, yearly or seasonal environmental changes, habitat losses/gains, animal age/sex/health, seasonal changes in resources, hibernation migrations etc etc. It should be possible to analyse at least some of these things when looking at a site and form a reasonable hypothesis as to what is going on (in the absence of a detailed survey).



Posted By: Caleb
Date Posted: 06 Nov 2012 at 2:24pm
Originally posted by Noodles Noodles wrote:

One of the studies (mentioned in the book) found that the vast majority of the population sheltered within 50 m of the pool, although this is certain to depend on the availability of suitable habitat etc etc. 

If it's the same one I'm thinking of, then you can get the full paper here:  http://www.rana-internet.de/media/Sonderheft4_Muellner.pdf" rel="nofollow - http://www.rana-internet.de/media/Sonderheft4_Muellner.pdf

The implication of the paper seems to be that adults will leave for known good habitat, and stop as soon as they find it (so the distance they travel will depend on how close that habitat is), but that juveniles will keep on wandering.


Posted By: GemmaJF
Date Posted: 06 Nov 2012 at 3:14pm
I was looking for that paper the other night Caleb so thanks for the link. I think one could see why in practical terms mitigation would concentrate on the adult population and for sure I would believe that the bulk of the breeding population would most likely be within 250 m of the pond. I think one of the other points made in the summary was the importance of terrestrial habitat. I doubt a factor anyone on here would overlook but it is amazing how often one comes across consultants who simply don't get that it is at least if not more important than the pond. If a pond dried up one year it would not wipe out a population, if the terrestrial habitat is destroyed, it probably would!


Posted By: Richard2
Date Posted: 06 Nov 2012 at 3:48pm

This Spring I happened upon some survey work taking place on the Isle of Purbeck. Chatting to the surveyor, who runs a business providing this and other ecological services, I was surprised to hear that GCNs were apparently moving between this pond and another about half a mile away across rough pasture, during the one breeding season. Individuals had been trapped in both ponds in the same season. I didn't know they did this. Sorry the information is so inexact.

Richard


Posted By: herpetologic2
Date Posted: 07 Nov 2012 at 11:15am
Originally posted by Matt Harris Matt Harris wrote:

Ok thanks for your responses; I asked the question more in hope than expectation really as I suspect the answer lies among the many things we don't know about species such as this. I did wonder if the 250m and 500m distances that are so often regurgitated were based upon any particular studies, but from the few examples quoted in the extract from Jehle, which Mark has kindly sent to me, it does seem that an average of 250m is too small a radius to encompass a reasonable majority of the population.

Basically the figure of 500m was the halfway point that the maximum distance of a newt has been tracked to in various studies - one in particular a female newt was logged over 1km away from her breeding pond. The SNCO and advice 10 or so years ago came down on up to 500m from breeding ponds.

The 250m figure comes from consultancies who are looking at ponds across a large area of land - the argument being that the ponds within 250m of a developement were more important for survey than the ones 500m or 1km away as a way of cutting down on the number of surveys required to assess a road or similar development. As there is often a requirement of 6 survey visits on each pond with gcn presence you can see where this would never end if you happen to have ponds within 100 metres of each other but no significant barriers like roads and large rivers etc

There is research on this by Cresswell et al in the UK - 'An assessment of the efficiency of capture
techniques and the value of different habitats for the great crested newt Triturus cristatus
English Nature Research Reports 576'

In the summary this can be seen
'By far the most captures were recorded within 50m of ponds and few animals were captured at distances greater than 100m.'

It would be down to the assessment of the habitats on the ground. The further away from a recruiting breeding pond you go the less newts will be found - is that fair to say?



 




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Posted By: GemmaJF
Date Posted: 07 Nov 2012 at 3:10pm
It's fair to say if one assumes totally uniform habitat Jon. 

As I mentioned above it is possible to find high concentrations of newts away from breeding ponds. These most likely represent animals that 'wandered' as juveniles and eventually reached a barrier. In all cases the location of suitable terrestrial foraging habitats will act as a modifier.

I think capture rate would decrease away from the pond. Clearly even if the majority of animals traveled further than 50m it would be far more difficult to detect them in the area of a 100m circle than one of 50m.

That's what worries me, people don't seem to see things like that. 'We caught the majority of the population within 50m' - could be 'we found newts far more detectable at 50m' couldn't it? In fact the largest portion of the population could have been at 50m + or 250m + but much more spread out and therefore far more difficult to detect.

There is a lot of assumption here that the 'population' represents the breeding animals, it's much like the discussion on the other thread, an inward looking site by site analysis which on many occasions misses the wider picture.


Posted By: Matt Harris
Date Posted: 07 Nov 2012 at 4:32pm
Originally posted by GemmaJF GemmaJF wrote:



I think capture rate would decrease away from the pond. Clearly even if the majority of animals traveled further than 50m it would be far more difficult to detect them in the area of a 100m circle than one of 50m.

That's what worries me, people don't seem to see things like that. 'We caught the majority of the population within 50m' - could be 'we found newts far more detectable at 50m' couldn't it? In fact the largest portion of the population could have been at 50m + or 250m + but much more spread out and therefore far more difficult to detect


As you double the radius of search area, the area of search is quadrupled so detectability would be quartered, assuming even distribution?

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Local Authority Ecologist


Posted By: GemmaJF
Date Posted: 07 Nov 2012 at 6:58pm
That is how I would figure it Matt. 

It seems that people are looking at the linear distance but not allowing for increase in search area and subsequent decrease in detectability simply due to it being a significantly larger area. 

Getting away from the uniform habitat model, in the real world one could also easily have a 'hot spot' due to significantly better habitat or a barrier beyond the easy to search 50m circle that is not considered.


Posted By: Noodles
Date Posted: 08 Nov 2012 at 11:58am
Originally posted by GemmaJF GemmaJF wrote:

That is how I would figure it Matt. 
Getting away from the uniform habitat model, in the real world one could also easily have a 'hot spot' due to significantly better habitat or a barrier beyond the easy to search 50m circle that is not considered.

One of the of the findings of the Cresswell report is that capture rates do increase beyond 50 or 100 m if foci exist e.g. hedgerows. Since it was a study of past mitigation schemes, capture figures were obtained from trap densities spaced evenly throughout sites. The findings therefore do show a typical spatial distribution in relation to the pond, regardless of the size of search area. 


Posted By: GemmaJF
Date Posted: 08 Nov 2012 at 3:51pm
Originally posted by Noodles Noodles wrote:

[QUOTE=GemmaJF]Since it was a study of past mitigation schemes, capture figures were obtained from trap densities spaced evenly throughout sites. The findings therefore do show a typical spatial distribution in relation to the pond, regardless of the size of search area. 

I would consider that to be mathematically a contradiction of terms, as the trap density would have to be increased the further away from the pond you were to give a true indication of spatial distribution as detectability reduces by a factor of 4 each time the radius of the search area is doubled as discussed. 

If the trap density is kept constant than all one is showing is that detectability reduces with increase in search area, which was the point I was making.

That aside it's not to say that I don't think the number of animals reduces the further away from the pond one gets, as at most sites it is logical that one would find that. I'm just dubious of so called 'scientific' data that contain flaws in basic reasoning being quoted as proof of anything!

At the very least this methodology would exaggerate the results significantly, the bulk of the population could well have been further out than the results suggest for example.


Posted By: MancD
Date Posted: 08 Nov 2012 at 4:24pm

It is difficult to compare trapping data between sites for a host of reasons. There is a minimum trap density required based on the level of population at the site (small/medium/large), and in some cases ecologists deliberately lay more traps around and close to ponds because they believe there will be more newts there. I've been on sites where they are a 2 metres apart.

On the flip side, you could argue that trapping density doesn't really matter. If a newt has to walk 5 metres along a fence line before it hits a trap, over the course of that evening the likelihood is that it would walk an extra 5 metres and fall in the next one so it would be captured anyway.
 
Drift fencing is also a problem when comparing sites. Some people put huge amounts of drift fencing in, others very little, some put them in parallel lines or "X's" across sites, whereas others target specific habitat areas.
 
Time of year is also important, the spring migration to the pond is probably much stronger and shorter than the late summer/autumn migration to hibernation sites and of course, the habitat quality as always is important.
 
As I'm a bit of a geek, I love wading through trapping data to try and figure out what is going on at a site. But using that data or its interpretation on another site is pretty much a waste of time due to the number of variables.
 
A case I'm working on at the moment has caught around 50 individuals, all of them in the areas you would expect, but all of them were female. What are the chances of that?


Posted By: Noodles
Date Posted: 08 Nov 2012 at 4:37pm
I believe all those variables were taken into account and investigated during data analysis in the Cresswell report. Other similar projects nowadays would also take that approach to analysis. Actually i think the Cresswell study is extremely valid and useful and one that we should be building upon with continued research. As Jon C suggests the original distances quoted now seem very arbitrary.


Posted By: GemmaJF
Date Posted: 08 Nov 2012 at 4:39pm

Quote It is difficult to compare trapping data between sites for a host of reasons. There is a minimum trap density required based on the level of population at the site (small/medium/large), and in some cases ecologists deliberately lay more traps around and close to ponds because they believe there will be more newts there. I've been on sites where they are a 2 metres apart.

On the flip side, you could argue that trapping density doesn't really matter. If a newt has to walk 5 metres along a fence line before it hits a trap, over the course of that evening the likelihood is that it would walk an extra 5 metres and fall in the next one so it would be captured anyway.
 
Drift fencing is also a problem when comparing sites. Some people put huge amounts of drift fencing in, others very little, some put them in parallel lines or "X's" across sites, whereas others target specific habitat areas.
 
Time of year is also important, the spring migration to the pond is probably much stronger and shorter than the late summer/autumn migration to hibernation sites and of course, the habitat quality as always is important.
 
As I'm a bit of a geek, I love wading through trapping data to try and figure out what is going on at a site. But using that data or its interpretation on another site is pretty much a waste of time due to the number of variables.
 
A case I'm working on at the moment has caught around 50 individuals, all of them in the areas you would expect, but all of them were female. What are the chances of that?




Absolutely, I totally agree with all the points above.

I would say the chances of an all female 'population' were pretty slim, one would assume the males left earlier in the season. Why? Perhaps they know another pond full of females!

I would have thought the seasonal dynamic would be the main issue when collecting data. No doubt it would have been a case that mitigations would have been targeted when newts were migrating. So how far out the majority of captures were is not really indicative of distance they may travel to terrestrial habitat in a season. Simply indicative of where the migrating animals were intercepted. 



Posted By: GemmaJF
Date Posted: 08 Nov 2012 at 4:45pm
Originally posted by Noodles Noodles wrote:

I believe all those variables were taken into account and investigated during data analysis in the Cresswell report. Other similar projects nowadays would also take that approach to analysis. Actually i think the Cresswell study is extremely valid and useful and one that we should be building upon with continued research. As Jon C suggests the original distances quoted now seem very arbitrary.

Trying to take all the variables into account when dealing with ecology is simply impossible. 

We have trouble mathematically taking all the variables into account for an apparently 'simple' natural phenomenon such as flight. To think this is achievable during a field study in the real world involving the distribution of wild animals with variables we don't even know about can't be valid?

There are so many published scientific reports in ecology with flaws in basic reasoning. At best they relate to the dynamics of the study site, providing a 'snap-shot' of what might be happening. Often they do not even give a true or valid representation even of that!

Perhaps these people should actually study maths before writing the papers!

You state above that the report is valid, but what about the glaring flaw in the methodology let alone all the other variables?

Here is analogy of what they actually did:

A shop keeper opens a shop in the middle of town. After a couple of months he decides to do a survey to see if people know about his shop. He asks five random people in the street that the shop is in if they know about his shop. 4 of them say yes.

He then travels 1km out into the suburbs and asks five random people if they know about his shop. They all reply NO.

He concludes that his customer base in mainly in the street where the shop is.

He totally misses the point that 1km out 5000 people know about his shop, he's just far less likely to meet one of them while taking his random sample.... 




Posted By: Caleb
Date Posted: 09 Nov 2012 at 9:32am
Originally posted by GemmaJF GemmaJF wrote:

He asks five random people in the street that the shop is in if they know about his shop. 4 of them say yes.

He then travels 1km out into the suburbs and asks five random people if they know about his shop. They all reply NO.

Surely if the trap density is uniform, it would be more like going 1km out and asking 500 people? Or am I missing something?


Posted By: GemmaJF
Date Posted: 09 Nov 2012 at 5:06pm
I see your point Caleb a better analogy would ask more people further out, but my point remains that detectability has dropped by a factor of 4 each time the radius doubles. 

Lets say the trapping was a ring fence with pitfall traps around a pond just after the breeding season. At 50m out one can expect a lot of animals to intercept the fence and fall in the traps. At 100m out they have 4 times the area (3 times if you subtract the first 50m circle) to wander about in, so would be less likely to intercept the ring fence however many traps are in place. One just can't make the assumption that they walk away from the pond in straight lines until they hit the fence and then hope to extrapolate from that population distribution.

As the data comes from mitigation work, I doubt they released animals caught at 50m to the other side of the fence either to see if they would have gone further, so the results just end up very skewed towards large numbers nearer the pond.

The main point I'm really making is that we should all be careful when interpreting results of studies. They can 'look' very conclusive at times but even a gentle analysis often reveals flaws. I would not for example let the findings of this report influence any assessment  I made about the impacts of a potential development on a GCN population for example. Others will take the findings and interpret it as proof, this is how we often end up with things working down to the lowest common denominator.



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