Tuesday, 2 October 2012

Hoverfly abundance in 2012 - some early indications

In an earlier post on Volucella zonaria, I highlighted the difference of opinion between my interpretation of Volucella zonaria's  abundance in 2012 and the interpretation in Buglife's press release. I have now done a little bit of analysis which I think sheds light on what has happened this year.

The most compelling evidence for an unusual number of records is that of Helophilus trivittatus which has occurred in exceptionally large numbers. Meanwhile, lots of observers have noted that Episyrphus balteatus has been particularly scarce. Numbers of Volucella zonaria do not appear to me to have been exceptional and a basic analysis of the data from the photographic record seems to confirm this. In order to get a broader picture I also looked at Volucella inanis which has some similar characteristics and is equally noticeable and recordable.

In the past few years I have assembled a dataset of  over 9,800 records from photographs on the web and sent to me as casual inquiries or reports. This dataset is interesting because it largely reflects the species that are seen by the general public and as such is representative of the reports that might have stimulated Buglife's press release.

The data show some interesting trends for 2012.

Aphidophage and aquatics

Figure 1. relative frequencies of Episyrphus balteatus, Eristalis intricarius and Helophilus trivittatus 2004 - 2012

Episyrphus balteatus as a proportion of records has fared very badly with around half the long-term average number of records.

Eristalis intricarius appears to have done relatively well but its numbers do not exceed those of the better previous years

Helophilus trivittatus has had an exceptional year with three times more than the long-term average of records.

Social wasp and bee associates

 Figure 1. Relative frequencies of Volucella species from 2004 to 2012.

Volucella bombylans shows no obvious trend with numbers bouncing around within a consistent level of variation..

Volucella inanis seems not to have changed greatly in its frequency over the past three years and numbers lie below the long-term average which is skewed by data for 2004.

Volucella pellucens appears to have occurred in similar numbers to previous years, but shows a marked drop in frequency in 2011 and a recovery in 2012.

Volucella zonaria numbers are marginally up on the long-term average, but at such a low level that it is difficult to see any reason to assume that anything unusual has happened this year. Part of the reason that numbers are elevated is that additional data arrived via Buglife (about 10% of the sample) and consequently the figures are skewed. Removing these data from the sample would bring the proportion down to 4.89% and this equates to marginally above the long-term average (4.74%) which is not statistically significant.


The data presented above are extremely preliminary and will need to be compared against the bigger dataset in the Hoverfly Recording Scheme but they do seem to confirm some of the apparent trends for 2012.

The poor performance of some aphidophagous species is clearly illustrated by Episyrphus balteatus. I will return to this on another occasion as it seems that some greater analysis is needed for this guild.

By comparison, the Eristalines appear to have done exceptionally well.  This must also be a point of further investigation but the data for both Helophilus trivittatus and for Eristalis intricarius, both of which have not been particularly obvious in recent years, suggests a strong gain in numbers.

The data for Volucella zonaria are interesting, however, when you look at the numbers of records in 2011. This shows a quite clear dip in the numbers relative to other species and suggests that V. zonaria was not as frequent in 2011. The reason for this is most likely to be a response to the exceptionally cold winter of 2010/11. There are precedents as Stuart and I showed in our papers exploring the responses of Volucella zonaria and V. inanis over the past 60 years (see http://www.bacoastal.co.uk/Entomology/2004-Volucella-zonaria.pdf and http://www.bacoastal.co.uk/Entomology/2003-Volucella-inanis.pdf). Both species underwent substantial range contraction in response to the cold winter of 1963, but clearly that was considerably more dramatic than that of 2010/11. It is also very noteworthy that a similar dip can be detected in Volucella pellucens which I did not expect. Further analysis is needed for this species too.

Looking at the data, I suspect that any detailed analysis should omit the data for 2004 and 2005 because there were few photographic records exist for those years - see below. However, what is also becoming clear is that web-based data may prove to be a useful means of monitoring certain aspects of the country's biodiversity, even though it has significant limitations because only a proportion of the fauna can be identified from photographs.

Figure 2. Numbers of photographic records from internet sites from 22004 to 2012.

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