Wednesday, 15 November 2017

Interpreting Sphaerophoria - or can we?

Today, I went through the species accounts for the genus Sphaerophoria, which is one of the more challenging genera for the student of hoverflies. Most records have to be based on males, because there remains quite a lot of uncertainty about the identity of females.

In my experience, most Sphaerophoria fall into two groups. One is largely associated with ericacious heaths and is substantially northern and western in distribution, or is confined to the major heathlands of Dorset, Hampshire and Surrey. It is chacaterised by S. batava, S. fatarum, S. philanthus and S. virgata. The main exceptions are S. scripta, S. interrupta, S. loewi, S. rueppellii and S. taeniata which are mainly grassland and wetland species. Some have clear distributions (e.g. S. tarniata and S. ruppellii) and others such as S. interrupta and S. scripta are widespread and more difficult to fit to a habitat.

So, what is happening to this genus? Quite a few appear to be declining, especially those that are associated with ericaceous habitats. Can we be sure that there have been declines? Anecdotally, I certainly see far fewer S. fatarum and S. philanthus than I used to, and I don't see the numbers of S. batava and S. taeniata that I used to. Part of the reason for my experience is that I no longer work the Surrey heaths where so many of these species occur. BUT I do spend a lot of time in Scotland and I am always pleased to even find a Sphaerophoria. Perhaps I go too early in the year? or perhaps something is happening?

So here are a few maps. Each is as yet un-edited for doubtful records so there may be changes, especially in the square spots which are NBN records which are often very doubtful:

Each map presents a few problems, but the main one is whether the decline in coverage is real or follows a general drift away from difficult taxa. The surprise, therefore is S. taeniata, which seems to be holding its own, at least in terms of coverage. Meanwhile coverage for the other four is clearly declining. There might be a range of reasons, however:

In the case of S. batava, S. fatarum and S. philanthus, these are species that generally occur in ericaceus communities, which in southern England are most widespread in Dorset, Hampshire and Surrey. Both Dorset and Surrey no longer get the attention they once did from recorders who looked seriously at Sphaerophoria. Dave and Ted Levy did a huge amount of work for their Dorset and Somerset atlases, Whilst Graham Collins and I did a great deal in Surrey at the same time. This year, we learned that Ted Levy was retiring from recording, having been unwell for several years, whilst I left Surrey many years ago. So, it is likely that these combined factors may have affected records from southern England - lots of records in the 1980s, but very few recently. BUT, what about Scotland? I make an annual pilgrimage and do a fair bit of recording there. I see very few Sphaerophoria but then perhaps I am going too early or too late in the year?

Then, what about S. rueppellii? It is a classic 'Thames Estuary' species but is not turning up in the numbers it used to. Unlike many of this genus, it can be identified from good photographs but is not often reported (we do see mis-identifications quite frequently). Perhaps it is declining? I suspect something is happening because the map for S. interrupta seems to indicate quite a significant reduction in records from south-east England. As this is the commonest species after S. scripta, and it does seem to have a significant northern and western distribution, I wonder whether SE England is becoming unsuitable for it. If so, my instincts are that this is a result of climate change and in particular periods of intense hot weather and drought.

So, do the maps and trends tell us something about how reliant we are on a small cohort of specialist recorders, or is something more insidious happening? I really don't know!

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