Monday, 15 October 2018

A poor year for Volucella pellucens?

As the summer progressed I became increasingly aware that we were not seeing many records of Volucella pellucens and whilst in the field it was noticeable by its absence. Was I imagining things or is there some justification for these feelings? Its season is now over unless we see a stray individual in the next couple of weeks, so perhaps a bit more sense can be made of the data?

I've tried a variety of ways of expressing the data in graph form and think the two following versions do help, but they are tricky to interpret!

The first thing that strikes me is that the height of the female peaks in 2015 and 2018 (Figure 1) are of similar height. There are two possible reasons: comparatively fewer males were recorded in these years; and the season was shorter in these years. In the case of 2018 I suspect both factors are at work.

Rather more significantly, I think Figure 2  indicates that numbers in 2018 were substantially lower than in previous seasons and hence the peak for females is markedly down on preceding years. The male peak seems to be broadly analagous to 2015 and 2016 but is substantially smaller than 2017. Some inter-annual variation is to be expected, I don't think we can make too much of this. What is possibly more significant is the substantial gap between the male and female peaks in 2018 that coincides with the heatwave between weeks 25 & 28 (i.e. 20 June to 10 July).

At this stage, it may be conjecture, but there is the possibility that the heatwave had a significant effect on the lifespan of the male generation and possibly also that of the females. If so, why might this be? It seems unlikely that the heat itself would have been a problem - the flies could simply have sought out shady places, especially as they tend to be woodland denizens.I think the more probable explanation is that neither males nor females could access sufficient nectar because normal sources had failed due to the heat and water stress. Is this a species that has suffered as a result of the heatwave?

Will the impact be expressed in numbers present in 2019? Only time will tell!

Figure 1. Volucella pellucens records extracted from social media covering the years 2015 to 2018 and represented as proportions of the records for this species in each year.

Figure 2. Volucella pellucens records extracted from social media covering the years 2015 to 2018 and represented as proportions of the records of all species in each week.

Sunday, 14 October 2018

The onset of an early Autumn?

I have just been looking at the data for the past 6 years to see whether it is possible to say how 2018 compares with previous Autumns. On the ground, it seems as though the numbers of records has dropped precipitously this year and that this has been matched by the numbers of species; but is this really the case?Moreover, do major storms completely knock down numbers?

I tried various formats for presenting the data before I hit upon the idea of superimposing 2018 over preceding years as a running process. So, Figures 1 and 2 present the numbers of records and numbers of species. I think what they show is that 2018 is substantially down on previous years in terms of the numbers of records, but that the numbers of species encountered is still holding up quite well against previous years.

Figure 1. Numbers of records extracted from social media between 2013 and 2017 with those for 2018 superimposed.

Figure 2. Numbers of species in data extracted from social media between 2013 and 2017 with 2018 superimposed.

Sunday, 7 October 2018

You've never had it so good!

A post on the UK Hoverflies Facebook page today raised the question of why Callicera spinolae took so long to recognise and be added to the British list? It was part of a wider question about the lack of both C. rufa and C. spinolae from Verrall’s 1901 monograph. The answer is perhaps worth setting out a blog post because it highlights the huge difference between 1901 and 2018, and what has happened in the intervening 117 years.

In today’s world, where we can jump in the car and travel to the far north of Scotland in less than 12 hours, we tend to forget (not realise) what life was like at the turn of the 20th Century. There was very limited public mobility – only the extremely wealthy could afford a car, roads were poor and travel long and arduous. So, most people recorded from a local area. People, when they did travel, went to the honeypots – hence the old collectors all went to the New Forest or to a few places in Speyside on the railway – you can read their accounts in the journals of the day and then find the specimens in the NHM collection (e.g. the Volucella specimens of FC Adams). There is something rather special about holding a specimen that was the subject of a short note in the EMM or Entomologist’s Record over 100 years ago; it provides such as poignant connection with the past.

We now have access to a vast literature too. My library contains the main works from northern Europe. What is more, quite a lot are in English or have keys in English. That was not the case even a generation ago!  The literature in 1900 was extremely limited – prior to Verrall’s work you would have had to have access to a substantial library of major foreign monographs and a big reference collection. This sort of facility would have been confined to the major museums of the day, or the extremely rich (or both). People tend to forget that today we have access to comprehensive (and cheap) literature plus the internet at the touch of a button. It was a rich man’s game then – hence being dominated by the likes of Verrall, who was course Clerk at Newbury Racecourse, or Yerbury (Lt. Colonel).

In the case of Callicera they were probably a lot less abundant. They are hard animals to find, even now, and although C. rufa is extremely widespread in Scotland I cannot remember when it was last recorded there as an adult. I work Speyside on an almost yearly basis at around the right time of year and have never seen it. Larvae are relatively straightforward to find. In the case of C. spinolae, I think it has genuinely become commoner and is expanding its range. It now makes a mockery of Schedule 41 – BAP Priority species that is popping up all over the place! I’m fairly certain the same obtains for C. aurata. I think the chances are that C. spinolae was an accidental vagrant that may only have set up transient populations until recently. I checked GBIF to see whether Callicera were any more abundant in Europe and am far from certain they are! There are precious few C. spinolae depicted, but I wonder if that is partly because there is taxonomic confusion with C. aurata and perhaps even C. aenea?

There have never been vast numbers of Dipterists; and especially so when it comes to taxonomically competent Dipterists. There is a strong British tendency to consider our fauna to be so well-known that we don’t expect other species and as we didn’t have access to European keys we never thought much about what else there might be. When Alan Stubbs wrote his book in 1983 the best he had was Seguy and van der Goot – and you know the British ability with foreign languages, especially Dutch! An indication of just how few competent Dipterists there are was shown by the data in the last atlas – of 750,000 records, half were supplied by just 20 people. Even today, I would reckon there are probably fewer than 20 who might be considered anything approaching Alpha taxonomists! And there are now no Syrphid specialists in the major museums.

We can always reflect and wonder what we might have found, had we been able to investigate the Britain of 1900, but to do so we would have had to be able to travel and to access comprehensive literature. This is the price we pay for progress: on the one hand we have books, fast travel, the internet etc. On the downside we have destroyed a lot of the British countryside in a quest for better living standards. It is especially sobering to think that the Great Man himself died of Dropsy at the age of just 64 – I cannot imagine that happening today with our wonderful health service.

Saturday, 6 October 2018

Drought & the Diptera Crash

In the most recent Bulletin of Dipterists Forum (No 86: page 18), Alan Stubbs writes on the impacts of drought upon Diptera. He also highlights the problem of developing long-term datasets that are compiled in a consistent manner. He draws attention to his own long-term garden monitoring effort and that of Jenny Own in Leicester and reflects on the absence of any other similar efforts. His concluding comments focus on Recording Schemes and suggest that they should be developing ‘practical ideas on how some aspects of monitoring might be achieved’.

I’m afraid my immediate response is that we have been trying to achieve a long-term approach for several years. We tried to get a garden monitoring scheme up and running some years ago, but it proved impossible to get enough traction to make the results meaningful. More recently, we launched our ‘carrot flower challenge’ that generated a certain amount of interest but also highlighted the numerous problems of getting something practical to work. My carrots were eaten by slugs and I think several others met the same sorry end! Back to the drawing board! The other big problem that these efforts highlighted was the need for a much bigger pool of 'shakers and movers' who would act as motivators and administrators. Sadly, I am woefully lacking in any ability to run a multitude of initiatives!

So, are there any other practical ideas? I don’t have any! Formal monitoring programmes need a big pool of interested and capable people. Amongst Diptera schemes, I think the only one where there might be enough recorders is the HRS. Even there, we are still relatively skills light, with a very small nucleus of experienced specialists. We are, however, in the fortunate position of having a growing nucleus of active recorders.

I think that we therefore need to think about this in a different way. There are quite a few members who regularly record from their garden or favoured walk. The current data run is relatively short, but it will get stronger over time. To pick up any sort of trend, one needs a data run of at least 5 years and a run of ten years or more is needed to generate anything meaningful. The sooner we have a big pool of people regularly recording from a favoured site, the greater the likelihood of having a reliable dataset to test theories in new situations. Let’s face it, the drought and heatwave of 2018 will not be the last such event; indeed, we must expect more in the not too distant future.

In the meantime, we do have a well-tested system for using opportunistic data to look at trends using occupancy models. These models are still evolving, and we are starting to have to tackle the problem of changing recorder methods having an impact on trends. I have written on this subject in the past (Data requirements for occupancy modelling – 23 May 2018). My feeling is that we have got to work with modelling teams to refine existing occupancy models and to develop new ones that take account of the range of species that can be tackled by a wider pool of enthusiasts. Beyond this, we do need to start to look quite carefully at existing data to try to understand the events that they portray.

I think we probably have enough data to start to interpret the effects of the 2018 heatwave and drought, but the big issue is context and whether any interpretation can rule out other compounding factors. There will inevitably be a variety of signals within the data, some of which will complicate our interpretations. Trying to make sense of these signals is one of the impending challenges.