Saturday, 27 September 2014

Reflections on 2014 - a good year for hoverfly recording?

As autumn draws in and the numbers of hoverflies seen rapidly declines, it is perhaps worth reflecting on what has happened in 2014. Obviously only a proportion of data are available for analysis, but I think the photographic dataset is pretty robust, so I have taken a quick look to see what story it conveys.

If one looks at absolute numbers of records, it looks as though hoverfly recording has gained speed and the numbers of records have grown exponentially (Figure 1). Part of this is undoubtedly the establishment of the UK Hoverflies Facebook group, which has provided a very active forum to promote recording and where occasional casual records can be submitted. There will be big blocks of data from others coming in during the Autumn and Winter, so we will not really know for some time how well recorded 2014 will be. Even so, the data are encouraging. There are lots more casual recorders and a fair number of people who now regularly record hovers. Two photographers have each contributed in excess of 450 records each and there are seven others who have contributed between 100 and 250 records.  Big blocks of data are a great boost to the scheme, especially when they paint a picture for a single site.

Figure 1. Growth in the photographic dataset between 2011 and 2014.

The second aspect of photographic recording in 2014 relates to the abundance of hoverflies represented. We saw lots of activity throughout the winter and a really productive April, followed by a cold wet May. The graph of photographic records for 2014 (as a percentage of all records) shows how hoverfly abundance came to a shuddering halt! (Figure 2) If compared against a similar graph for the period 2002 to 2014 covering all of the photographic records I hold (Figure 3), it is clear just how different May was to the general pattern What is less clear is the degree to which this had a knock-on effect into June and July. Membership of the Facebook group grew very rapidly at this time and a growth in recorder numbers may have masked some of these effects.

Figure 2. Monthly totals of photographic records extracted for 2014.

Figure 3. Monthly totals for photographic records covering the years 2002 to 2014.

The graph for 2014 illustrates one of the developing problems for many invertebrates - they are definitely emerging earlier in the year (see earlier posts), but just when there is a need for warm sunny weather there is a cold wet spell that affects both adults and developing larvae. This may be one reason why some species appear not to be doing well. Obviously, this is just a single year's data and there are limitations because the membership of the UK Hoverflies Facebook Group group has been growing throughout the summer. Inevitably, the relative numbers of records may be different, but the overall picture would probably be more stark if this was to be taken into account.

I think it is a great endorsement of what is possible when an interactive medium such as Facebook is used to encourage biological recording, but there may be issues to deal with in due course. Readers will know from previous posts that relatively few representatives of some genera are photographed, and that there are some genera that really cannot be identified from photos. This means that there will be an inevitable need to look at analytical techniques to make sure that interpretations of trends are not influenced by the recording technique. Stuart has started to look at this and has concluded that there is a skew developing (as I have previously predicted).

Data skews are not necessarily a problem provided they are recogised and taken into account when undertaking analysis. The big question now is how to make sure that some forms of recording are promoted to generate comprehensive data sets. Stuart Ball has looked at the data that he, me and a couple of other very active recorders were generating and clearly there is a difference in what occurs. My own analysis (unpublished) showed that there was just over a 50% overlap between the 30 commonest represented species recorded using photography and those recorded by my own sampling - which comprehensively covers all species encountered. Data held by the HRS lies somewhere midway, which indicates that there is undoubtedly a skewing effect. BUT, does this matter? - Maybe not.

My instincts are that we will have to split data for certain analytical processes and that the next target for me to pay attention to is developing a set of replacements for the people such as Stuart, me, Alan Stubbs etc. I guess we need at least 20 such recorders to form the nucleus for the next 30 years (and must hopefully encourage a few to take on the role of growing a further generation).

What I think is greatly encouraging is that there is so much more interest in hoverflies and that so many new people are making active contributions that really are useful. The Hoverfly Recording Scheme is about so much more than mapping. Provided we recruit a broad constituency of new recorders we will hopefully be able to continue to keep hoverflies high on the agenda of research into the effects of landscape and climate change on Britain's wonderful wildlife.