Wednesday 12 September 2012

Less common moths at garden trap sites

I thought it might be worth posting this short bit of data analysis for others to see.  I'm discussing a nearby planning/development issue with the Council and a developer, and am using my moth data as part of the evidence.  My initial hypothesis was that as I live on a coastal brownfield site, I should perhaps expect to see a greater proportion of less common species here.  What I have done is take data from some of the other garden sites and compare them to my record in terms of the number of:
  • Local distribution and rarer species
  • National B and rarer
These are the last two lines in the table below:
  Rhoose Point  AM Llandaff North GT Gorseinon BS Templeton Ave DS Cwmbach ME Sea View VS
  1 15 20 16   10
Common 276 328 489 410 392 412
Common(alien-foodplant) 1 1 1 1 1 1
Migrant 9 9 24 11 12 23
Local 46 62 134 84 75 128
Local(alien-foodplant) 1     1 12 1
Nationally Scarce A 1 2 1 1 1 2
Nationally Scarce B 2 4 10 6 2 8
Nb 4 1 8   4 15
RDBK 1          
Rare migrant           1
Vagrant/accidental           1
Total 342 422 687 530 499 602
% of local and rarer in total 16 16 22 17 19 26
% of National B and rarer in total 2 2 3 1 1 4

Whilst this has not supported my initial thoughts on the proportion of less common species at my trap site, it's interesting to see the disparity in the results.  Unsurprisingly the traps in rural sites do better (apart from mine!).  I wonder if there is a sampling bias here though, all sites apart from mine have long records and I have a suspicion that in the early years all the common species are picked up quite quickly and dominate the record, which may mean that as the years go by it becomes proportionately more likely that  new species encountered are in the Local distribution or rarer categories...?


  1. I'm sure you're right that the longer you trap at a site the greater the % of uncommon species you'll record.

    It stands to reason that uncommon species will generally occur in smaller numbers than common species. Also, rarer species tend to be more sedentary so will probably wander less often from their habitats into your garden. Both of these factors mean they're less likely to turn up in the trap, but over a long time period you might get the odd one.

  2. Another possible factor is that moths in general have become scarcer and are arriving at trap in smaller numbers. This probably affects scarce and local species as well as the common ones, so scarce and local species are getting numerically scarcer increasing the odds against them coming to a particular trap. Sites with a longer trapping history would have benefited from the comparative abundance of moths in the early days. There are species I used to get annually, a decade ago, that I now haven't seen for years.