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Adder Dispersal Distances |
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calumma
Senior Member Joined: 27 Jun 2003 Location: United Kingdom Status: Offline Points: 375 |
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Posted: 06 Aug 2009 at 9:56am |
My current take on available literature suggests that adder can disperse over
distances of ~ 2km (principally between hibernacula and summer foraging sites). In reality, the majority of such movements are likely to be much less than this - particularly give the fragmented nature of today's countryside. What experiences or opinions do other folks have? You can stop reading this rather long post now! For nerds interested in database stuff I have attempted to explain how I will be using this info below I'm attempting to pin down potential movement distances for all species as part of my attempts to better define favourable conservation status in Kent. This follows on from a meeting with NE and HCT that Steve (Vicar) and John Baker also attended. Still a way to go, but results to date are encouraging. KRAG have now been promised full access to the Kent habitat data - provided we can justify that access. Before making the request, I'm trying to put some of the background processes in place to demonstrate how the data will be analysed. One of those processes has been to calculate nearest neigbour distances for all records in the database. The database has been able to calculate distance between records for some time, but this information is calculated on the fly and not saved. I have now added a script that calculates nearest neighbour for each record and saves the result in a separate field. Unfortunately having more than 22,000 records on the system means that there are several thousand records for each species and this process takes a little while. Each of the species records has to calculate distances to every other of the records of that species to find out which one is the nearest One application of nearest neighbour is to illustrate metapopulation ranges. As an example of this, I have identified sites that have at least one other adder record within 0.1 - 2 km, and then plotted those sites on Google Earth with a circle of 2 km radius. Other validated records that do not have a neighbour within 2km are plotted with a circle of 500 m (just to put them on the map and differentiate them from the main metapopulation - these records could also be plotted with a 2 km radius). The results are useful for identifying areas where survey effort may be better directed. See the attached graphic: In trying to understand metapopulations (and defining favourable conservation status), I aim to look at the habitat that is available within each of these defined polygons. It may be better to consider range as 'potential range' and give distances that are defined by the time of year. For example, maximum dispersal distance is likely to be between hibernacula and foraging site. It may therefore only be relevant to plot distances (whether they are 2 km or another value) for records generated up to the end of April. Records collected during this period are likely to be for animals situated close to their hibernacula. Another possibility is defining potential dispersal distances by the number of sites that are situated within a predefined distance - the more sites located within the target range, the larger the potential maximum range plotted (up to a maximum limit that equates with the target range). The latter approach may help to control for populations where the factors limiting dispersal are unknown - sites with good dispersal potential are likely to have more records situated nearby. In fact I quite like the latter approach... Clearly there is more work to do and I'm likely to be a very busy boy over the next few months! |
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Vicar
Senior Member Joined: 02 Sep 2004 Location: United Kingdom Status: Offline Points: 1184 |
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Heh,
Yeah...this is going to be tricky at this level of abstraction I totally agree with your seasonal assessment. I've been using records at the beginning of the year (to mid April) as an indicator for hibernacula location. The trouble with locating dispersal data (IMO) is that you cannot tell from which hibernacula nodes the animals have originated. You could do some habitat analysis to identify potential barriers and make some assumptions...but are we even sure what a habitat barrier looks like? The only way I can see this providing definitive results is by mark-recapture (hugely intensive and difficult at the county level). We've had some success with Ca, as ALL sightings (almost) are photographed, databased and the unique animal identified. Trouble with Ca is that is takes years to build up significant stats and re-captures. We're only on year 2 now. Distribution is possible, dispersal...is going to have to be pretty wooly imo. As you say, there will be sites where the animals cannot disperse, due to habitat restrictions. How will you know that a 'summer' sighting is not from an undetected hibernaculum area? I think a useful goal would be to produce a very simple Vb HSI, using only data that is already available at county level. |
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calumma
Senior Member Joined: 27 Jun 2003 Location: United Kingdom Status: Offline Points: 375 |
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True, but setting the maximum distance can be problematic. For adder the most isolated population is currently 3.3 km and this figure varies for species (10.6 km for marsh frog). I do try to speed up the analysis using the most efficient algorithm that my brain can deal with and since I am interested in the distance to records that are not situated on the same spot, I also exclude duplicate 6 figure grid references (I chose 6 figure rather than 8 or 10 to maximise the number of included records). But I want my results now ! |
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Vicar
Senior Member Joined: 02 Sep 2004 Location: United Kingdom Status: Offline Points: 1184 |
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Just had another thought Lee....
At some point, you may want to quantify the results (Area FRVs etc). It may be beneficial using grids rather than radii, else you'll end up having to calculate the dispersal overlaps. Now that will take a long time to process! (<-- T-shirt wearer). Also, you can make the precision of a sighting a variable, which is useful for obfuscation (or aggregation). 6-figure OSGBLGR = 1 hectare etc. You're right that this needs to be done, else that one tin which has been checked 1000 times will bias some of the stats. Excellent point re square distances Caleb! - must check I've done that. |
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Vicar
Senior Member Joined: 02 Sep 2004 Location: United Kingdom Status: Offline Points: 1184 |
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This I had not heard! Question... I've often seen neo adders hunting lizards at Thursley on the wet bog areas. Presumably the hibernaculum isn't very close by, as I would have thought that flooding would be an issue. My conclusion was that neos may also migrate to summer grounds (following scent trails). Maybe there is a hibernaculum close by? or maybe animals behave differently in different environments? I guess it's hard to say without definitive mark-recapture. |
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calumma
Senior Member Joined: 27 Jun 2003 Location: United Kingdom Status: Offline Points: 375 |
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Yes, I agree this is useful and have already set up precision as a variable (actually for other reasons, but can see you point here as well). The comparison between area and available habitat is a little ways off so I'm not too concerned about that aspect of the analysis just yet (so long as I cover my bases and ensure the data now being generated is being stored in the most appropriate format). Edited by calumma |
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calumma
Senior Member Joined: 27 Jun 2003 Location: United Kingdom Status: Offline Points: 375 |
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I've just exchanged emails with Gareth discussing this very issue. A problem is the number of available records for different lifestages. For example the db holds 1139 adder records, but only 227 of them are for immatures. Whilst I accept that different lifestages will have different dispersal distances, ultimately what I am hoping to illustrate is the maximum potential dispersal from each site (as defined by a 6 fig grid reference). It *may* therefore be appropriate to use the overall maximum dispersal distance for the initial analysis. Of course there is no reason why the db cannot hold the additional dispersal data and the analysis updated once sufficient records become available. |
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Suzi
Senior Member Joined: 06 Apr 2005 Location: United Kingdom Status: Offline Points: 1025 |
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I would just like to ask what depth of a suitable habitat do adders need/like? A route that I walk every year to spot adders (stoney track with coarse grass firebreak backed by heather which adders lie along the edge of) has not produced one so far. Sure I might not be there at optimum times but nevertheless not a one so far this year over a dozen visits at least. Around this route the cattle grazing has intensified and the cattle now have access to the heather itself. Horses have been introduced close by as well. A large area of molina was burned out adjacent to the route probably during winter. Add to this the scraping in the vicinity of bare patches and I just wonder how much adders will take. I guess this group of adders might have pushed off onto another part of the heath but again cattle, scraping etc. Sorry to have hijacked this thread! |
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Suz
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calumma
Senior Member Joined: 27 Jun 2003 Location: United Kingdom Status: Offline Points: 375 |
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I am at this very moment trying to get to grips with this issue myself Suzi.
In a nutshell, the larger and more connected the site the better - but what we really need is a HSI that is directed towards reptiles. To date my efforts on development of a reptile HSI have been thwarted by the issue of site definition. For amphibians, HSI's are usually applied to a pond with each pond representing a discrete 'site' that is relatively easy to define (so much so that I have recently created a new table in my db to hold waterbody as site data, but I digress...). Defining a reptile 'site' can be very problematic and the habitat factors that need to be quantified are likely to vary quite considerably across large sites. Rather than attempting to define HSI based on 'site', it may therefore be more useful to define HSI based on recording unit. Of course, this approach may result in area becoming a fixed variable (e.g. hectare blocks of land). I'm not sure if this is useful and it certainly wouldn't help answer your question. It may therefore be acceptable to attempt to define the recording unit as a clearly demarcated block of habitat (e.g. a grassland field or even a patch of scrubby grassland in the middle of a large expanse of arable). The larger the 'site', the more of these habitat units there are likely to be. An alternative approach is to define reptile HSIs in broader terms at the landscape level - but individual site area could be lost in such an approach. In effect this is what I am trying to achieve with the favourable conservation assessment of adder in Kent - available suitable habitat vs occupancy of said areas etc. It may be worthwhile spinning off your question into a new thread - what is the smallest discrete block of land in which adder has been recorded? |
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Vicar
Senior Member Joined: 02 Sep 2004 Location: United Kingdom Status: Offline Points: 1184 |
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ROFL!...I have literally JUST sent you an email lee about exactly this issue!
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