Monday, 13 November 2017

Interpreting data: the problem of Platycheirus

Platycheirus  is one of the largest and more perplexing genera of hoverflies in Britain. There are often good and relatively straightforward male characters (on the front and middle legs) provided you know where and how to look (at high magnification). Females are often much more difficult, with characters that are somewhat subjective and open to misinterpretation: I know they give me problems, so I suspect they give others problems too! (The same applies to Melanostoma).

I have already highlighted the problem with female immarginatus/perpallidus but there are others such as the split between females within the clypeatus complex (clypeatus, europaeus, occultus, and  ramsarensis) and in the peltatus complex (nielseni/peltatus especially). These problems mean that one has to be careful interpreting the data. So, as an example I shall describe the problems with Platycheirus ransarensis.

A bit of history

P. ramsarensis was split from P. clypeatus by Speight & Goeldlin (1990) and Goeldlin et al. (1990). It occurs in oligotrophic (acid, base-poor) situations, most frequently beside moorland streams and lakes, and usually where there are small flushes with abundant sedges or rushes.

When the split was first announced, there was considerable interest amongst the active Dipterists of the day and many went back through their collections to see what they contained. As P. clypeatus was known to be a problem species I suspect many of had a better collection of specimens than we had for easier species. We also took a great interest in looking for this species and our summer field meetings happened to be substantially northern at the time; so lots of records were assembled. There was also considerable activity in the Sorby Natural History Society at the time and they too generated lots of records. Look in the uplands, and it seems that P. ramsarensis is almost a standard species to be expected! More recently, there has been a lot of activity amongst the Devon Fly Group and Dartmoor turns out to be a hot spot. I expect with similar activity both Exmoor and Bodmin Moor will turn out to be other south-western hotspots.

Figure 1. Trend for Platycheirus ramsarensis
Figure 2. Distribution of Platycheirus ransarensis

Interpreting the trend

The overall trend from 1980 says an increase, but I think that would be a misinterpretation. Equally, the trend from 2000 onwards is down, but that too is probably misleading. I think the steep rise prior to 2000 is increased awareness and interest in the Platycheirus splits, whilst in subsequent years there has been a decline in interest in these difficult species that gives an impression that they are occurring less frequently.

In reality we simply don't know what is going on with this one! My field experience suggests that where one looks for it there is a pretty strong chance of finding P. ramsarensis. It is likely to be substantially under-recorded and will probably be found to occur much more widely on the Pennines, in the Lake District, upland Wales, and on the Southern Uplands of Scotland. Further north, I would expect it to be quite widespread across the Western Isles and in the Highlands but probably absent from much of the lower ground of north-east Scotland.

So, will we define a reliable trend? I suspect not unless we start to see an absence from places such as Dartmoor. Whilst the Devon Fly group has many active specialists corroboration of any apparent trend should not be an issue. But, were we to see a decline in specialist activity we might see a negative trend that is a reflection of changing recorder activity rather than a loss of this species.

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