Monday, 30 July 2018

How are hoverflies doing in 2018?

Initial impressions suggest that 2018 has been a very odd year for hoverflies. The season started very late, and there was precious little activity during the winter. Numerous observers have remarked upon the low numbers of species and the apparent absence of some of the most reliable species such as Eristalis pertinax and E. tenax. At this stage of the season it is quite difficult to be sure what is going on but I hope I have come up with a way of presenting the data in a way that tells its own story.

The following graphs are based on the following processes:
  • Records extracted from Facebook and other media such as Flickr have been used. The reason for this choice is that it is one block of data that I know has been assembled consistently. Also, it is the only block of data that I have ready access to (other data go on the HRS database that Stuart manages).
  • I created a baseline by generating a set of tables for each species in the years 2013 to 2017 inclusive and then used these data to create an average for each week for each species. This is the background data used for comparative analysis and shows up as a blue line on the charts.
  • I then overlaid weekly data for each species up to the end of week 30 in 2018; i.e. up to Saturday 28th July.
There are charts for nine species, all of which are relatively straightforward to identify from photographs and are regularly recorded by contributors. This choice is ideal because it comprises species for which there are good numbers of records and which therefore make the outputs as statistically robust as possible.

Interpreting graphs is always tricky and in this case we have to think about the shapes of the charts and not the absolute numbers, although there will be places where numbers of records are strikingly different.

The reason for concentrating on patterns is that the graph of average numbers is based on a five-year period in which we have seen recording activity grow dramatically but in that time there has also been a shift towards members maintaining their own spreadsheets (a big thank you to all who have). It means that no two years are made up of the same group of recorders, so, in addition to seasonal and yearly variation, there are also demographic changes too!

The critical consistencies are that the data have been checked/ extracted by a tight nucleus of specialists and extracted by just three of us (Geoff Wilkinson, Ian Andrews and me). In addition, we see a growing throughput of novices who gradually turn into more experienced recorders whose ability to recognise hoverflies increases over the years. Some may drop out, but there will also be new recruits.


These nine species tell a significant story.

Firstly, the numbers of hoverflies recorded were low during the winter lower than average) and the spring burst of activity was delayed for about two weeks. This is amply demonstrated by both Episyrphus balteatus (Figure 1) and Eristalis pertinax (Figure 4).

The impact of the drought in July is clearly demonstrated in Volucella pellucens (Figure 8) and Eupeodes corollae (Figure 2). There is possibly also a bigger impact on Rhingia campestris (Figure  6),. I expect the summer brood to fail in south-eastern England if experience in other droughts holds true; but the impact will probably only really become clear when the summer generation emerges or fails to emerge.

One of the more perplexing issues is whether we can detect the impact of the long wet winter? I think this may be apparent in Volucella zonaria (Figure 9). I think there is probably enough evidence to suggest that numbers are substantially down this year. We cannot discount the impact of drought because there was an uncharacteristic dip in the emergence at a point when it might have been expected that numbers would be rising rapidly; nevertheless, the overall numbers of this highly noticeable species appear to be heavily down on the average. I suspect there has been a winter impact and we might be able to detect this using occupancy models at the end of the year

I am also somewhat surprised by the degree to which the chart for Volucella inanis (Figure 7) seems to be mirroring previous years. In broad terms, the first generations of Eristalis tenax (Figure 5) and Rhingia campestris (Figure 6) are also  pretty close matches against the long-term average.

Finally, there always have to be winners in this sort of situation and I think this is amply demonstrated by the graph for Eupeodes latifasciatus (Figure 3), which has been far more abundant than in the last 5 years.

Figure 1. Episyrphus balteatus

Figure 2. Eupeodes corollae

Figure 3. Eupeodes latifasciatus

Figure 4. Eristalis pertinax

Figure 5. Eristalis tenax

Figure 6. Rhingia campestris

Figure 7. Volucella inanis

Figure 8. Vlucella pellucens

Figure 9. Volucella zonaria

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