Recorder activity - a possible proxy for looking at the impact of weather on datasets?

I have had the feeling that we had lost a lot of recorders from the UK Hoverflies Facebook Group and that activity on this page was down on 2017. However, upon closer inspection I think activity on this page rather more accurately reflects the prevailing conditions and we might even be able to use past years' activity as an indication of the conditions that prevailed then.


I attach two graphs. The first comprises the numbers of contributors in each week in 2017 and 2018. The second is the numbers of records generated. The results come with a health warning because in 2017 we started to encourage members to keep their own spreadsheets. This trend has continued into 2018 and we now have about 95 people maintaining their own spreadsheet or putting records onto iRecord. So, the graphs are not entirely comparable. Nevertheless, I think there are sufficient consistencies to make something of the results.

Figure 1. Numbers of contributors in each week

Numbers of records in each week

What is immediately apparent is that we had a much later winter this year, and that is clearly shown in both graphs. A clear drop in the numbers of records since the start of June (week 23) is not matched with recorder activity, whose decline is far more marked about 3 weeks later. So it looks like the numbers of records is not completely related to the numbers of recorders. It is, of course, possible that a small number of  very active recorders have switched to spreadsheets, but I am unconvinced that this is the case (I can think of one). To test this theory I split the recorder data (500 recorders) into a series of classes. The most active recorders this year range from one who cas contributed nearly 500 records to a longer tail of between 200 and 50 records. These I have placed in a single class because they form a suitably sized group (37). The gradated scale is then shown in Figure 3 with increasing numbers in each class.

Figure 3. Recorder activity in 2018 split into 5 groups depending upon the volume of records submitted.

We can see that the same broad pattern emerges in all of the groupings of recorder activity, which suggests that the fall in the numbers submitted is not simply a change in recording method but that it actually reflects a change in recorder activity. In reality, it looks as though recorder activity was probably greater in 2018 until the start of the drought! We will have a far clearer picture at the end of the year, but I think the trends are sufficiently clear from this sample of data.

What is also clear, is that the numbers of contributors has dropped during the drought. In other words, recorder behaviour may be a useful proxy for hoverfly abundance. We should, of course, also recognise that it may be that the hovers are about but the weather is discouraging recording! My standard ground-truth of graphs is to ask whether I have detected any changes during the course of my own field work? Have I changed my recording behaviour and why? Answer, yes I have and this is because it is darned hard work and unproductive on many days! I suspect a lot of datasets will show similar trends, but it will be interesting to see whether this is the case for all invertebrates? If press reports are true, then the trends for butterflies may be different.