Saturday, 2 May 2015

Hoverflies in April

As we pass into May 2015 the weather has followed a bit of a modern pattern: a warm sunny April followed by a cold snap in early May, just when the insects are getting going. Hopefully the change will not extend over a long period but we do need some rain!

As part of my ongoing efforts to provide feedback to contributors I produced a set of graphs this morning for the UK Hoverflies Facebook group. They tell quite an interesting story for developing hoverfly activity in April. Real activity seems to have started a few days earlier than 2014 and to have continued to gain pace for much of the month (Figure 1). This contrasts with 2014 where there was a bit of a dip mid-month. In both years, however, there was a strong dip towards the end on the month.

Figure 1. Centred 3-day running means for the number of hoverfly records from photographs in April 2014 and 2015.
Analysis of the composition of the lists created shows how a relatively small number of species dominate the data (nearly 1600 records in 2015). Six species make up 63.5% of the records and 90.2% of the records are derived from just 20 species. The six most abundant species recorded were (in Alphabetic order): Epistrophe eligans, Eristalis pertinax, Eupeodes luniger, Melanostoma scalare, Platycheirus albimanus and Syrphus torvus. Details of the overall species list can be seen in Figure 2.

Figure 2. log2 representation of records of hoverflies recorded from photographs in 2015. Species represented by single records are not represented by a column owing to the use of log2
As in previous analyses, it can be seen that the dataset is dominated by a small number of species but that the overall list is substantial with large numbers of species represented by occasional records (Figure 3). This situation is primarily a function of the recording technique. My active field sampling highlights much wider species representation and far more species within genera such as Cheilosia and Pipiza.

Figure 3. The relative abundance of species within the photographic dataset for hoverflies in April 2015: over 60% of the species are represented by 9 or fewer records.
In addition to maintaining a log of firm identifications (or as near firm as possible), I have maintained a log of partially identified photographs (502 records). This log indicates that around 23% of photographs posted cannot be reliably identified. It is a rate that is not inconsistent with previous analysis (see The rate at which partial records accumulates differs little from the main dataset, but does fluctuate (Figure 4), partially depending upon the popularity of photographing the genus Syrphus (Figure 5).

Figure 4. absolute numbers of records of photographs that cannot be identified to species with a 3-day centred running mean super-imposed.

Figure 5. Total numbers of representatives within Tribes (red) with numbers of individual genera (blue) reported at log2 scale. Single records are not represented on the scale, hence some gaps.
These fairly simple analyses help to show the potential power of photographic records in some aspects of invertebrate monitoring. The development of a network of photographers is therefore a definite enhancement of data collection and has the potential to help to provide a real-time impression of how invertebrates are responding to seasonal fluctuations. Problems with identification remain an issue, however, and it is therefore important to think about the ways in which photographic recording can be used to maximum effect, without compromising the continuing need for data collected by traditional methods.

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