Monday, 17 April 2017

An early spring?



Each spring, observers often remark upon whether plants and animals are emerging earlier or later than previous years. In the last 30 years, the general impression has been that springs are getting earlier and this impression is reinforced by the data. Amongst the hoverflies, there are several whose emergence has shifted by as much as two or three weeks, with some emerging at crazily early dates. But, does the first reported emergence actually tell us very much?
In reality, a one-off event is inconsequential; it is far more important to look at the overall phenology of a species or a group of organisms. And, when one looks at phenology, it is not the first and last dates that are important, it is the degree to which peak numbers shift that tells the full story. Thus, analysts get very frustrated when recorders say ‘I’ll give you first and last dates but I cannot be bothered with anything else. Without the supporting context, first and last dates are utterly meaningless.

Making sense of the data

Monitoring photographic data compiled by recorders who are relatively unselective is a great way of developing data on readily recognisable and useful indicators of seasonal change. The Hoverfly Recording Scheme has been doing this for around ten years, but it is only since the advent of the UK Hoverflies Facebook group that the volume of data has reached a level where the data are sufficiently robust to look at differences early in the year. In the past, one would have had to wait for a year or more for relevant data to arrive. Now, we have the data almost immediately to hand and can start to interpret the impressions of observers almost on a ‘real-time’ basis.
This year, the overall impression has been that spring got going very early. Was that really the case? I thought it was worth looking at a suite of indicator species to find out. Initially I compiled a long list of species that looked to have emerged earlier than usual. This was rapidly whittled down to just three species because many of the potentially early species are reported in relatively low numbers. They are not really very useful because the reports depend entirely upon chance. Records of widespread and abundant species provide a much more solid basis for analysis because many more people will see and report them.

Approach

For this analysis I took three species that fit my criteria of being abundant, easily recognisable and widely reported. They are: Epistrophe eligans, Leucozona lucorum and Dasysyrphus albostriatus.
As 2017 has only just started, the median date for these species cannot be calculated. The median date for early emergence can, however. So, I compiled a table of the first three dates for each species in 2014 to 2017. From this, I calculated the median date for each year for each species and then ranked them according to date of median emergence (Figure 1). This initial analysis shows that the early emergence dates for 2017 are indeed earlier than in previous years, with two out of three species ranking first and the other lying second in the rankings.
Figure 1. Tabulation of first two third dates for three early hoverflies

Can one get any other ideas on the degree to which this year is early when compared with previous years? My answer to this was to look at the spread of dates for each year, taking the median dates for the three species and creating a second median. This is probably statistically wrong, but as a crude analysis it helps to paint a picture (Figure 2). Taking these dates, a further median can be created between the earliest and latest medians of the combination of three species. This date is 31 March. One can look at the degree to which each combined median varies from the central date (Figure 3) which suggests that 2017 is possibly as much as 7 days earlier than the median for the previous 3 years.
Figure 2. Median dates for the combination of species over the period 2014 to 2017

Figure 3. Variation in median dates from the median for the period 2014 to 2017

Conclusions

Is this believable? Time will tell, but my general impression is that the species lists for 2017 contain animals that would not have been seen for at least two weeks further into the season even within the last ten years. If one compares with 20 years ago, the evidence is very strong that hoverfly emergence has advanced by several weeks and that the field season is getting longer. In some cases, it is likely that unless you get out early, some short-lived species will have come and gone before you have mobilised!