Changing fortunes of Sargus soldierflies

Yellow-legged Centurion soldierfly (Sargus flavipes) – photo: Martin Harvey

Following the release of the new photo ID guide to soldierflies in genus Sargus, for the Soldierflies and Allies Recording Scheme, dipterist Ruud van der Weele got in touch via Twitter to say that he had recently seen a presentation showing that all species of Sargus were declining in the Netherlands, with the exception of Twin-spot Centurion S. bipunctatus. Ruud asked whether there was any evidence of similar declines in Sargus species in the UK.

Caveat to almost everything shown below: there are very few records in the recording scheme from before the 1980s, and even since then the numbers are still quite low and there are many potential biases to consider. The charts below are a simplistic visualisation of the available data and no statistical rigour is claimed!

If we look just at the raw numbers of records in the recording scheme database, S. bipunctatus is doing very well, S. cuprarius has almost vanished, while S. flavipes and S. iridatus seem to be declining from a 1980s peak.

Sargus soldierflies - number of records per decade

However, the amount of soldierfly recording has increased greatly from the 1980s onwards, making it hard to interpret the changes in the chart above.

Total number records for all soldierflies per decade

The variation in recording effort can partially be taken into account by looking at the number of records per decade for each Sargus species as a proportion of the total number of soldierfly (Stratiomyidae) records received for each decade.

Records of Sargus soldierflies as a percentage of all soldierfly records per decade

Remember that the above chart is based on very small amounts of data up to the 1980s. Since then, the story is much the same though: S. bipunctatus is perhaps on the increase (or perhaps lends itself to being photographed now that photo recording is more prevalent), S. cuprarius has almost vanished, and S. flavipes and S. iridatus seem to have declined since the 1980s, and maybe from a higher peak before then.

A much more rigorous interpretation was made by Charlie Outhwaite and colleagues as part of her work on trends in recording scheme data (Outhwaite, C.L., et al. 2019. Annual estimates of occupancy for bryophytes, lichens and invertebrates in the UK, 1970–2015. Scientific Data 6: 259.) The chart below is my simplistic plotting of the occupancy estimates for the three Sargus species for which there was sufficient data to analyse (for full data including 95% credible intervals see the published dataset: Outhwaite, C.L., et al. 2019. Annual estimates of occupancy for bryophytes, lichens and invertebrates in the UK (1970-2015). NERC Environmental Information Data Centre.)

Simple plot of occupancy estimates for three species of Sargus soldierfly

Occupancy models are a method of accounting for the biases within non-structured data such as that collected by recording schemes. This is a much more sophisticated way of analysing the scheme data than the charts shown earlier in this post, and it is reassuring to see that there is some similarity in the results, with S. bipunctatus once again holding its own while there are declines in S. flavipes and S. iridatus.

So it does look as if trends in these species in the UK are consistent with those reported for the Netherlands. What is causing this is another question of course.

Update at 11 February 2020: Thanks to Charlie Outhwaite for providing more information on the occupancy estimates referred to above: “There is a Shiny app that allows you to choose a species and explore the data from the Outhwaite et al. data publications. The app can be used to view occupancy and detection plots for individual species, although we emphasise that the models developed here may not be optimal for every individual species considered, and that the occupnacy estimate plots should not be used uncritically for single-species assessments.”