Monday, 15 August 2016

A bit of analysis

Yesterday, in a post on the UK Hoverflies Facebook Page I tried to make some sense of what is happening to hoverfly populations this year. It generated several useful comments that got me thinking further. In that post, some basic graphs were presented in which the general differences between the datasets for the period 1 July to 13 August in 2015 and 2016 were illustrated (Figures 1-4). My initial thoughts (anecdotally) were that recorder effort had changed, with much more data coming from a smaller core of very active recorders and that whilst recorder effort had risen, the numbers of species per recorder had dropped. My interpretation of this was that the data lent some credence to the belief that hoverfly numbers were substantially down in 2016.
Figure 1. Basic data for 1 July to 13 August 2015 showing total numbers of daily records, recorders and species recorded.

Figure 2. Basic data for 1 July to 13 August 2016 showing total numbers of daily records, recorders and species recorded.

Figure 3. Comparisons of numbers of records per recorder in the periods 1 July to 13 August in 2015

Figure 4. Comparisons of numbers of records per recorder in the periods 1 July to 13 August in 2016
It was pointed out in comments that this might not be the case and that the data could simply be skewed by the numbers of new recruits who only posted shots of the commonest species. This morning, I therefore took a further look at data for the same period (plus one day). The results were illuminating (Table 1)!

Total records
Numbers of recorders each year
Top 10 recorder contributions each year
809 = 27.2%
1311 = 27.07%
2049 = 34.78%
Top 20 recorder contributions each year
1053 = 35.4%
1927 = 39.79%
2899 = 49.21%
  Table 1. Breakdown of photographic data for the period 01 July to 14 August in successive years 2014 to 2016

The data suggest that there has actually been a drop in recorder activity, but that those that are active have contributed vastly more records. To some extent I think the latter is true. There is a very nice group of regular contributors who now record almost daily from their garden or wildlife patch. This is the essence of the Garden Hoverfly Recording Scheme that Alan Stubbs pioneered 25 years ago. It is precisely what Stuart and I have been hoping to generate. The data are certainly skewed by some changes in recorder activity. For example, one major recorder in 2014 changed to a slightly different technique and although we have his data on a separate spreadsheet they don't appear in this analysis. Were they to be included, the trend would be greatly more pronounced. In addition, several other very active recorders in 2015 have switched to maintaining their own spreadsheets for submission monthly or at the end of the year. Again, their data would accentuate the change that has happened.

So, what has happened to recorder activity? This is best summed up by looking at the composition of the top 30 recorders within the photographic dataset that I have used. The recruitment/loss process is shown in Table 2. It is greatly encouraging and bodes well for the future of hoverfly recording!

Number of recorders
Contributing all three years
Contributing in 2015 & 2016
Contributing in 2014 only
Contributing in 2016 only
  Table 2. History of the top 30 recorder contributions to the photographic dataset between 2014 and 2014

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