Monday, 12 December 2016

Some trends from photographic records

During the course of this year there have been indications of some species being commoner or scarcer when compared with previous years. I therefore thought I ought to take a look at the data to see what has happened.

The following graphs are organised in sequence for the most positive change in declining order, and then comparable charts for the most significant declines.

To reach these results I too the counts for all species since 2009 and constructed a table of the proportion each year's records for each species ([total species x records for 2016 divided by total records for 2016] multiplied by 100). The species illustrated are those that showed the greatest gain or loss over the data for 2015. The results have not had a regression line superimposed, but many tell quite an interesting story.
Figure 1. Hoverflies showing a positive variance from 2015
It comes as no surprise that the species that shows the greatest variance from 2015 is Eupeodes corollae (Figure 1). The numbers of this species in 2016 were remarkable in July and early August and this shows in the graph. The data clearly show that its numbers fluctuate very broadly, so one cannot read too much into the spike for 2016 as it is quite likely to be followed by a dip in 2017. A similar explanation holds for Eupeodes latifasciatus (Figure 3)

I was surprised to find that both Melanostoma scalare and Syritta pipiens seem to have undergone a consistent upward trend. I suspect that this reflects the developing maturity of the UK Hoverflies Facebook group, which is now dominated by a group of between 50 and 70 people who regularly report everything that they see, and are always on the lookout for new species. Both M. scalare and S. pipiens are relatively small and would probably have been overlooked by many of the casual photographers whose records populate data for the early years of photographic recording. This change is seen in other species such as Melanostoma mellinum (Figure 3) and Platycheirus albimanus (Figure 4). A similar trend seems to follow in Meliscaeva auricollis (Figure 2), but I am less sure that this is the result in a change in recording patterns, although that cannot be discounted.

Figure 2. Hoverflies showing positive variance from 2015 (2)
The graphs for Eristalis nemorum and Eristalis arbustorum (Figure 2) also suggest a rise in representation in the dataset, but as numbers appear to fluctuate more markedly, it is possible that these species are showing natural fluctuations. The genus Eristalis is well represented in the data each year and therefore one might expect these species to have always been a major component of the dataset. There is a complication, however. It is now 3.5 years since the UK Hoverflies Facebook Page was established. In that time lots of members have become better acquainted with hoverflies and now know the angles they need to use to get a chance of a positive determination. It is likely that this is a partial factor behind any apparent rise an numbers.

Figure 3. Hoverflies showing positive variance from 2015 (3)

The slight downward trend for Myathropa florea (Figure 3) over the period since 2009 is possibly a further indication of the changes in the composition of the recording community: it is a species that is usually very obvious and is more likely to be recorded by the novice.

Figure 4. Hoverflies showing positive variance from 2015 (4)
The final two species chosen for positive variance over 2015 tell two very different stories. Helophilus trivittatus (Figure 4) is a migratory species that had an exceptional year in 2012 and has not been in evidence subsequently. The story for Helophilus pendulus (Figure 4) is potentially more complicated because there appears to have been a spike in abundance in 2013. Two immediate possibilities spring to mind: that this is a partial migrant or that 2013 was a good year for this wetland species because there was more rainfall in preceding months. This needs a bit of further investigation.

Moving on to species that showed a decline between 2015 and 2016, there are nine examples to consider. Here, the results are more difficult to interpret. Two species, Volucella inanis (Figure 6) and V. zonaria (Figure 5) show consistent declines since 2010 that I suspect are simply because they used to form a large part of the data because they are big and obvious. With the shift towards more comprehensive recording, their impact on the dataset has diminished. The same possibly obtains for Merodon equestris (Figure 6) and perhaps also Sericomyia silentis (Figure 7); although the latter may also have responded to lower rainfall in a similar manner to Helophilus pendulus. The shape of the graph for Episyrphus balteatus (Figure 7) is also likely to reflect the changes in the composition of photographic recorders and the trend towards comprehensive recording in place of ad-hoc recording by relative novices.

We are left with three aphidophagous species whose graphs involve significant spikes and dips. The shape of the graph for Scaeva pyrastri (Figure 5) is likely to be a combination of changing recorder composition and a natural fluctuation in the abundance of this migratory species whose numbers peak intermittently. It is noteworthy that the other two species in this group, Eupeodes luniger (Figure 5) and Syrphus ribesii (Figure 6) are also aphidophagous. Both are largely resident but may be bolstered by migration, and both are well represented by relative novices. I suspect these graphs primarily represent natural fluctuations but may also be influenced by a growing awareness amongst recorders of the characters that need to be depicted if a firm identification is to be achieved.

Figure 5. Hoverflies showing negative variance from 2015 (1)

Figure 6. Hoverflies showing negative variance from 2015 (2)
Figure 7. Hoverflies showing negative variance from 2015 (3)

Concluding remarks

These graphs are presented to illustrate the ways in which individual species' abundance varies within the photographic dataset. A fuller analysis using the entire HRS dataset is needed to be sure of some interpretations, but I think there are some important positive messages to be absorbed. For me, the critical issue is that photographic recording is maturing with the advent of the UK Hoverflies Facebook page and the mentoring that has been possible. The datset is definitely shifting away from the bold and showy species towards a much wider spectrum of species. This is to be expected as members gain experience and start to develop visual cues that take time to acquire.

Within the data, there are also indications of possible natural fluctuations that require a bit more investigation to make anything of possible causes. Nevertheless, the data also show that whilst it is probably unwise to place too much reliance upon data collected prior to 2013 as a monitoring tool, more recent data are potentially very powerful and will become increasingly important as group members develop their skills.

No comments:

Post a Comment