Thursday, 26 June 2014

Shifts in phenology

Epistrophe eligans is one of the commoner spring hoverflies. It's larvae are often predacious upon aphids on fruit trees and therefore it can be quite common in gardens. When I first started recording hovers in the 1980s I saw it most frequently in May. Bu 2000 its earliest dates were in the third week of March This year is was 9 March! It is clearly very responsive to temperature and could be a really useful model for following climate change.

This year there have been good numbers of photographic posts of this species. The majority of records are from the midlands and southern England, with far fewer records from northern England and Scotland (the latter is at the extreme of its range). I therefore wondered if I could show differences in emergence at different latitudes? So, the following graph is split along three lines - south of a line between the Severn and the Thames set by the grid squares SSS, ST, SU and TQ; midlands between the Severn-Thames line and a line between the Dee and the Humber set below the grid squares SD, SE and TA; and a final area north of the Dee-Humber line.

Even using very limited data for one year, the differences are clear when the data are cleaned by creating a three week rolling mean (Figure 1.).

Figure 1. Phenology of Epistrophe eligans in 2014 using photographic data.
I tried the same technique with my own data over 4 epochs, 1980s, 1990s, 2000-2009 and 2010 to date. This looked as though it would work too, but I found that the change in my main recording area has affected the data - my 1980s and 90s data were assembled in southern England; now it is mainly from the midlands, so the graph does not really work.

These two  simple observations help to show how a network of recorders might generate data that could be used on a yearly basis to follow the effects of inter-yearly variation in numbers. Some similar results emerge from data for Episyrphus balteatus (Figure 2). This graph is based on the data from photographers too - and again it shows how numbers can vary hugely from year to year.

Figure 2. Inter-yearly phenology of Episyrphus balteatus based on photographic records.