Saturday, 26 March 2016

The benefits of total recording

Many members of the UK Hoverflies Facebook page record everything they see on a daily basis; others are confined to weekends, but are really very active. The combined efforts of this increasing band of recorders are showing what can be done with less well-known groups of invertebrates.

What I particularly like is the fact that there is no discrimination between records of common or rare species. All records get logged, and as a result we are starting to build a more realistic picture of the relative abundance of hoverflies across the country and at particular times of year. This approach is essential if we are to truly understand what is happening to wildlife.

Which dates matter?

It is all very well recording first and last dates, but what happens in the intervening days/weeks/months is really crucial. First/last dates are rather serendipitous whereas the median date for all records is far more reliable and meaningful. That means that we need all data and an army of diligent recorders. I suspect that data is pretty robust within moth recording where lots of people make daily counts. The same probably holds for butterflies and maybe dragonflies. But, can we say the same for other invertebrate groups? Probably not.

So, it would be nice to think that the advent of the UK Hoverflies Facebook group heralds a major forward step for hoverflies. Feedback I have received from some specialists is not always positive – the question 'why bother with common species' has been posed, and it has been argued that 'this does nothing to advance our knowledge of Britain's fauna'. It remains to be seen whether they are right, but I would like to think there is developing evidence to the contrary.

What can be done with the data?

Two small analyses that I undertook today illustrate the benefits of mobilising an army of recorders with varying levels of experience. To my mind, enthusiasm matters as much as experience. People with enthusiasm will go out when conditions are less than perfect; conversely, those of us who have yaers of experience will look out of the window, note that there are clouds and it is slightly windy, and settle back to the computer. microscope or whatever we were doing.

The data largely show this. Hitherto, hoverflies have been the preserve of a relatively small body of recorders. For forty years, about 20 people supplied 50% of the data assembled by the Hoverfly Recording Sscheme (HRS), very few of whom would have made much effort until the spring had fully arrived. The dynamics have changed, and although we still have a nucleus of about 25-30 highly productive specialist recorders (above 500 records a year), we have many more whose combined efforts are really helping to develop a valuable picture (several of whom now contribute many hundreds, or in one case thousands, of records). The new recruits start from a very different perspective and want to get out.To illustrate this, I offer two brief analyses relating to patterns of spring emergence.

Composition of the early spring fauna

If one starts with the question 'how does spring 2016 compare with previous years?' We can break the data into the numbers of records of individual species, but this does not take account of variations in the numbers of recorders. So, presenting data as a proportion of all records received for a given timescale is one option.
Figure 1. Records of hoverfly species in March 2014 to 2016 presented as the proportion of all photographic records covering the period 01-25 March.
Figure 1 illustrates this quite nicely: it shows how, in the period 1-25 March, relative composition of records changes from 2014 to 2016. This figure is based solely on photographic records, which is really useful because relatively few entomologists are stirring from their winter hibernation. The photographic team, however, is raring to go and every time there is a bit of sunshine they are out. I seriously doubt one could manage to secure this sort of picture from data prior to 2013, although it would be possible later in the year when insect activity increases and traditional entomologists stir from their torpor!

What is very clear is that the composition of the fauna in different years can vary considerably amongst the more abundant species. As might be expected about a quarter of the total species list makes up the bulk of the records, but within this there are two obvious trends. In 2014 and 2016, Eristalis pertinax is far more prevalent in the data than in 2015. There is an obvious reason when you look at the composition of 2015 data: In 2015, Episyrphus balteatus, Eristalis tenax and Meliscaeva auricollis make up far more of the dataset. These three species are typical of winter months and E. balteatus and E. tenax predominantly over-winter. Thus it seems that 2015 was a later year than either 2014 or 2016 where Eristalis pertinax was much more abundant. E. pertinax, in contrast does not generally over-winter and thus the dominance of this species in the dataset tends to suggest that spring has arrived a little earlier.

And at species level?

We have known for a long while that species' phenology changes from year to year, and that in some emergence times have advanced considerably. These changes are nicely illustrated by Epistrophe eligans, which is highly responsive to spring temperatures; its spring emergence is now several weeks earlier than it was 40 years ago. We can now pick this up because it is easily photographed and recorded. In the past, many recorders might have logged the first record but all too often I have been told 'I'll give first and last dates but I cannot be bothered with the rest'. Now we have the whole lot! And it beautifully illustrates differences in both season and latitude.

Figures 2 and 3 present the data for Epistrophe eligans in 2014 and 2015 for three zones: south (to the 200km division across the UK, midlands (200-400km) and north – above 400km i.e. pretty well above the line between the Humber and the Mersey. What a difference it shows. Emergence times are separated both between the years and across the latitudes. To do this, I have separated the data into weekly blocks from 1st March onwards. The data were then turned into a weekly percentage of total records, after which they were turned into a centred three week running mean. This smoothing process helps to refine the picture and it is pretty powerful.
Figure 2. Phenology of Epistrophe eligans in 2014 based on photographic records.

Figure 3. Phenology of Epistrophe eligans in 2015 based on photographic records.
These graphs suggest that spring in 2015 came a little earlier than in 2014. It remains to be seen how 2016 fares, but the records of E. eligans to date point to a situation more akin to 2015 than to 2014, albeit potentially marred by bad weather forecast for the coming week.

And the moral of the story?

Commoner species matter! One gets too few records of rarer species to develop strong pictures of change; but at least in some commoner species one can detect change very nicely. That does not mean there is no place for recording rarer species or taxonomically difficult species – it is essential, but it may only ever be the preserve of a small sub-section of recorders.

And, for that splendid band of photographic recorders? I hope these brief examples show just how important your contributions are. This level of activity starts to elevate the hoverflies from the margins to the mainstream of insect recording.

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