Interpreting phenology histograms

Since the great changes brought about by the UK Hoverflies Facebook page I have had growing concerns about a small part of the HRS dataset. Most of it is probably fine, but as time goes by it is becoming increasingly clear that many more species than we thought hitherto have relatively short emergence periods. There can be regional variation which means that for species that occur widely, but not at high densities, the aggregated phenology histograms are misleading. In many cases they show a very elongate tail into the Autumn. I don’t think this interpretation is correct, although there is always the possibility of occasional aberrations.

I’ll use four species as examples. Two are widespread and regularly recorded both by traditional net and microscope recorders and by photographers; the third is a recent arrival but is almost entirely recorded by photographers and the final one is a very early spring species. The important separation is that since we have had the Facebook group I have done the bulk of the determinations that have found their way onto the dataset (until this year). In that time, all three species have been found to have very tight emergence patterns.

Epistrophe nitidicollis generally occurs in May and June, perhaps into July. It is quite infrequently seen and I suspect is regularly reported as a misidentification of a Syrphus. I am pretty sure the tail on the graph (Figure 1) arises from such misidentifications.

Figure 1. Phenology of Epistrophe nitidicollis

Epistrophe melanostoma arrived in the UK in 1986 or thereabouts and has since become established in south-east England. It is very similar to E. nitidicollis and this is one reason why it may not have been recognised prior to 1990 when a Dutch specialist (Paul Beuk) was living in the UK and found it on my local patch. Remarkably, we get most records from photographs and, as can be seen from Figure 2, there is no extended tail of records. More importantly, whilst I think there is growing evidence that E. nitidicollis is declining in abundance, E. melanostoma is becoming far commoner.

Figure 2. Phenology of Epistrophe melanostoma

Platycheirus tarsalis is another spring species. It is quite difficult to separate from P. manicatus and I suspect that there have been numerous mis-identifications in the past. Unlike P. tarsalis, P. manicatus has at least two broods and continues well into the Autumn. I think this probably explains the tail in the graph for P. tarsalis (Figure 3), although there may also be confusion with female P. peltatus amongst recorders who don’t use keys.

Figure 3. Phenology of Platycheirus tarsalis

Melangyna quadrimaculata flies from late February to April, perhaps into May in Scotland but I have never seen it there despite many trips in late May and early June. Nevertheless, there is a tail in the graph (Figure 4), which I think can be explained by misidentification of very dark male Leucozona laternaria; I have seen this mistake made on several occasions and suspect that in my novice days I too might have made this mistake!

Figure 4. Phenology of Melangyna quadrimaculata

How do we resolve this issue? I am at a loss, because there will probably be some genuine records in the tails. However, there are almost certainly a lot of misidentifications. I suspect the best we can do is to provide a written interpretation to accompany graphs but there remains the danger that the graphs will be mis-interpreted, and the data glitches will be reinforced by further misidentifications. Alternatively, should we clean up the graphs and adopt a cut-off date after which we don’t accept records? That runs another set of risks because we might then be accused of massaging the data to fit our interpretation.

We are faced with a conundrum! I think we need to develop a far more comprehensive analysis of the phenology of those species that are regularly recorded. I suspect it will prove to be illuminating!