The start of Autum?

The data for hoverfly records extracted from the UK Hoverflies Facebook page seem to be showing that this week is the start of the autumn slowdown in hoverfly abundance. I suspect that at least some of this is weather-related with poor conditions in several areas as I gather from posts.

So, I wondered whether there were regional differences in the trends and sorted the data according to three basic regions - Sorth of a line between the Humber and the Mersey (appx), Midlands - roughly down to a line between the Severn and the Thames, and South. For this analysis I used a centred 5 day running mean to smooth the inevitable spikiness of the graphs.

What do the respective graphs tell us? Well, I have been a bit surprised by the data. In previous years, the smallest graph has always been the North where we normally have far fewer recorders. It looks as though there have been more records from the north this year (Figure 1), which is a welcome change. Northern England and Scotland are often much less well-represented in a wides spectrum of recording schemes.

Figure 1. Daily records for 3 regions in 2018 presented as a 5-day centred running mean

Southern England records normally far-outstrip those of either the North or the Midlands, mainly because the vast majority of recorders seem to be based in southern England. It is difficult to be sure what has happened this year but there seems not to have been the same level of activity in the south. Perhaps that is because a larger number of people moving to spreadsheets came from the South? I think that is unlikely because we would expect recruitment of newer recorders to mirror past recruitment to a very large extent. So, my suspicion is that the effects have been far more pronounced in the South and that this has had an impact on recorder activity too.

I therefore looked at the proportions of daily records from the three regions and was surprised to see a significant difference at or around the point where the heatwave struck.In southern England it hit around 20 June and records plummeted by 25 June. In the north, however, records stayed high until around 10 July before dropping quickly (Figure 2). It has to be borne in mind that Figure 2 represents the proportion of all records, so as records from one area drop, those in other areas will effectively rise. So, the drop in the proportion of northern records really reflects a rise in the numbers of records in the south as it recovers from the harshest impact of the drought and heatwave. There may, nevertheless also be a somewhat delayed effect on the northern fauna

 

Figure 2. Daily records for 2018 from the middle of April represented as proportions for each region on a 5-day centred running mean

I think that some of the more erratic peaks from late July onwards reflect differing weather patterns such that the three regions have presented rather different recording opportunities. So, there is a bit of interesting work to be done linking these patterns to the respective local weather variation.