Tuesday, 23 December 2014

Phenology of Episyrphus balteatus in 2014 and 2013

In a few moments of inactivity I took a look at the phenology of a few species seen this year. One that I thought worth looking at was Episyrphus balteatus as it is reported in very large numbers. The attached histograms are strictly for photographic records - I will look in more detail once the garden records have arrived.

Data were plotted in four blocks:

  •  All records for 2014 and 2013 plotted separately
  • South of a line created by the northern limits of ST/SU/TQ plotted as separate years 2014 and 2013.
  • Midlands - up from the northern limits of ST/SU/TQ to the southern limits of SE/SD plotted as separate years 2014 and 2013.
  • North - north of the limits of the southern limits of SE-SD plotted as separate years 2014 and 2013.
The results were a bit of a surprise:

  1. It looks as though there might be as many as four generations in southern England - I expected 3 but there is a bit of a March generation too.
  2. In 2014 I found no migration peak at the end of July but there is quite a spike by late June - and a pronounced spike in the Midlands.
  3. The number of generations in the midlands is a bit messy - probably 3, but I cannot be sure. It begs the question - can one come up with a series of thermoclines to come up with differences in phenology?
  4. The northern population seems to have a distinct single generation.

What I think it starts to tell us is that every year is very different, and that this species probably has slightly different emergence strategies even between southern England and the Midlands. In 2014, in which we had an exceptionally mild spring and autumn it looks as though there were probably at least 4 generations in southern England, whereas the further north one gets it seems to be broadly univoltine.What I find particularly puzzling is that the late July migration spike was not picked up this year. I have a feeling that this may only be detected by absolute counts.
Figure 1. Total 2014 records of Episyrphus balteatus against week number.
Figure 2. 2014 records of Episyrphus balteatus from southern England.

Figure 3. 2014 records of Episyrphus balteatus from the English midlands and Wales.

Figure 4. 2014 records of Episyphus balteatus from northern England and Scotland.

Figure 5. Total 2013 records of Episyrphus balteatus against week number

Figure 6. 2013 Episyrphus balteatus records from southern England
Figure 7. 2013 Episyrphus balteatus records from the English Midlands and Wales.

Figure 8. 2013 Episyrphus balteatus records from northern England and Scotland

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.

Sunday, 31 August 2014

Identifying hoverflies from photographs

Identifying hoverflies from photographs is not straightforward. Photographs depict a single plane and do not allow rotation to look at particular characters. By comparison, preserved specimens can be viewed from many angles under the microscope. In addition, most keys have been developed from preserved specimens, which can differ markedly from live animals. The art of identification from photographs is therefore in its infancy. As photographic techniques improve, identification techniques may also improve; but there will always be some hoverflies that cannot be identified at all from photographs. That said, it is realistic to assume that around 50% of the British fauna can be identified from at least some photographs. There are a number of obvious ways in which photographers can be more assured of a positive identification:

  • The higher the resolution of the photo, the more chance of actually seeing key characters. For example, really nice sharp and well-filled frames can expose hairs on eyes, leg hairs and occasionally the pilosity of the arista.

  • Views from several angles top, front face and side view often combine to provide enough information to give a positive ID.

  • It is worth developing a knowledge of the family so that you have a rough understanding of the genus you are photographing. Each genus depends upon a slightly different range of characters and once you have a feel for the genus it should be easier to make sure that key characters are depicted.

There are some genera that cause particular problems - the most frequently illustrated genera that don't get identified are within Cheilosia, Eristalis, Platycheirus and Syrphus. That is not to say that they are the most difficult to ID in other circumstances, but they are the most frequently depicted genera from awkward angles. In addition, there are some tribes and families that are most unlikely to get identified because they rely on characters that are difficult to show in photographs. These include the Platycheirus where pits on the underside of male tarsi can never be seen in live specimens, and Eumerus, Pipizella and Sphaerophoria where it is not possible to examine the male genitalia. The tribe Pipizini is altogether difficult, even under the microscope and is unlikely ever to be readily identified from photographs; so too are many Cheilosia.

But, if one ignores the problems (a good idea) IDs can probably be given in 60% of cases to around 150 species. The others fall into the too difficult group or will only be ID's from an exceptional photo.

The following is a tabulation of the commonest ID problems that I encounter:

My ID (not necessarily right!)
ID posted
Eristalis intricarius
Criorhina berberina
Volucella bombylans
Eristalis pertinax
Eristalis tenax
Eristalis sp - various often not possible to go further
Eristalis tenax
Eristalis pertinax
Eristalis sp - various often not possible to go further
Eristalis sp.
All sorts of views, often at angles that show few characters or are well out of focus.
Eristalis rupium (quite regularly on iRecord)
Syrphus sp.
Epistrophe diaphana
Eupeodes latifasciatus
Megasyrphus annulipes - several on iRecord
Syrphus ribesii - the chosen name for about 90% of posts, suggesting that little attention is paid to text in the main keys or that Chinnery is being used.
Parasyrphus sp.
Eupeodes sp.
Xanthogramma pedissequum (agg)
Eupeodes corollae
Eupeodes luniger
Eupeodes latifasciatus
Parasyrphus punctulatus
Eupeodes luniger
Eupeodes corollae
Unidentifiable Eupeodes
Eupeodes luniger
Eupeodes corollae
Eupeodes latifasciatus
Leucozona lucorum
Volucella pellucens
Cheilosia illiustrata
Merodon equestris
Volucella bombylans
Platycheirus albimanus
Platycheirus scutatus
Platycheirus scutatus (agg)
Platycheirus albimanus - a problem I think resulting from the WILDGuide that I hope we will rectify in edition 2.
Scaeva pyrastri
Eupeodes luniger
Volucella pellucens
Leucozona lucorum
Xanthogramma pedissequum (agg)
Eupeodes nitens - a Chinnery mistake