Wednesday 14 November 2012

Another Trend Test

Since the first article was recieved with interest, I thought I'd quickly try a slightly different tack on a different species. Dusky Thorn was the one hailed in the State of Britain's Larger Moths as showing the greatest decline of all species, so I thought I'd see if my rudimentary stats would show something up. So below we have two graphs. The first is in line with what I'd done for the Heart and Dart & Large Yellow Underwing, in that it is the number of individuals weighted against 'Trap Nights'.

It is now know that this is a species with large peaks and troughs in it's abundance, and it certainly had a few good years as soon as the report was published. It did occur to me that my method would come unstuck if a lot of recording additional effort was put in at times of year when the given species was not on the wing - so more people taking part in the Winter Garden Moth Scheme would cause a drop in the averages of summer species, so I tried this:

And this shows the average catch across the various years - so recording effort is taken into account, but only when the moth is trapped.Interesting that the huge peak of 2009 in the previous chart has been levelled out. Essentially it was well recorded in 2009 compared to total recording effort, but the average catches were, well, average I suppose! 2011 looks like it was a poor year for this common garden species, but I've not yet got all the 2012 data in so can't say whether that is a trend or a blip.


  1. This species looks like many others, affected by the severe winter in 2010/11. As you say lets wait for the 2012 results to see if there is any recovery. It was certainly down in my records. None in my garden or Whitchurch for 2011, when I usually get 1-2 at both sites and just 2 records from Whitchurch in 2012 - the only ones on the GMRG dataset for this year! Really bad if that's the case.

  2. None in my garden this year, but I did see one sitting on one of the fuel pumps at Coryton filling station.


  3. Hmm, this is where the average trap count falls down - the minimum value is always going to be 1, whereas clearly if it's only seen at a couple of sites on a couple of nights then there is a bigger problem! I guess to properly work I'd need to work out trapping effort for each known site at the appropriate time of year. I'll have to think about this one!