Adder Dispersal Distances

And here it is.



What this graphic shows is an estimate of what could be considered minimum
(darker circles) to maximum (lighter circles) distribution.

The minimum distribution is based on a value of 0.64 km, whilst the
maximum distribution is based on a value of 3.32 km (the maximum
confirmed, non-historical, nearest neighbour value).

Of course, at the moment these figures and calculations are just for fun -
and to see if in principle this type of analysis is worthwhile. I need to review
some aspects of the calculations script. In particular I need to decide what
records are included in the nearest neighbour calculation.

P0 = percentage probability of a sighting being from 0 to 1 km to its nearest neighbour.
P1 = 1 to 2 Km etc

heh,

What you see is the difference between real data and a mathematical function. I have tried curve-fitting, with some success.

Yes, the right hand columns of that table (which was too wide to post) has percentile distances such as 95% etc.

I can't see any way of building a distribution profile without using range bins...however; the range bins can be any size (just a number in the code).

I think I derived the 95%iles etc using 100m bins.

I'll dig out the numbers.

Vicar wrote: How and where you survey affects these figures significantly. There needs to be a metric of county survey completeness, which will have to include negative records (where we looked but didn't find animals).

Yes I quite agree. I have attempted to investigate this by looking at the number of
occupied survey units and comparing this with the number of survey units for which data
has been collected. There are still some issues that I need to overcome with this analysis
but in Kent, adder has been recorded in 229 km squares. This represents an occupancy of
just 5.2 %. However, when you control for survey effort the occupancy increases to 14.2%.
Defining effort becomes problematic when targeted survey results are combined with
anecdotal records.

You probably have this information already Steve, but if anybody else would like to quickly
work out how many 10 km or 1 km squares there are in a vice county
this BRC website is a very useful resource.

What I think I am now settling on is using the upper quartile nearest
neighbour value to define where core range is likely to be. This
represents 75% of recorded observations. To this I apply a 'buffer' that is
defined by the 95 percentile. I have chosen 95% rather than 100% to eliminate
outliers. The figures I currently have are 0.64 km for core range and 1.2
km for the buffer (measured from the edge of core range). Since what I
aim to describe is the predicted distribution of each species:

Predicted distribution = core range + buffer.

For adder this is 1.84 km.

Reading from your graph, this would represent a distance at which there
is a ~25% chance of encountering adder in Surrey.

It is also annoyingly close to the 2 km that I started this thread with

Of course not all areas within the predicted distribution will be suitable and this
is where the HSI assessment will come in.

Now the big question Does anybody have similar data for areas
outside of Kent and Surrey?

Edit: note that I sometimes use the terms 'distribution' and 'range' to mean the same
thing. I've cleaned up the text to make it a little more consistent.

Edited by calumma