Improving the Diversity of Hoverflies on iNatUK: Introducing "What to look out for"

One of the things that makes iNatUK's data less useful for the HRS than it might be is that there is a very strong bias towards a small number of charismatic, large and easily identified species. This is inevitable with citizen science to a degree, but doing something about it sounds like a fun challenge for us all!

To help us improve the diversity of what we observe, I have been sifting through the hoverfly records held by the NBN atlas and I plan to produce a post for each month entitled "What to Look Out For" in which I will highlight some of the seasonal species that are underrecorded in iNaturalist, but which should be identifiable from photos and are common enough to hope we could find. I intend that this will include information on how and where to find them, as well as how to identify them.

To flesh out the scale of the challenge a bit: by comparison with the more comprehensive NBN atlas database only 22 species are overrecorded on iNat! This leaves 251 species underrecorded, of which 86 have 0 observations on iNat.

One simple and general way we could improve our hoverfly diversity is by paying more attention to hoverflies in the first half of the year. Here's why I say this: The top 10 species in iNat (accounting for nearly 50% of all observations) include just one that peaks before July (and only four of the top 20) - but nearly 60% of UK species peak before July!!!

In each "What to look out for" I will list the species that are at peak abundance that month in a section called "MMM is the best month to see...", and I will also list species that are starting to increase significantly (first month at >10% of peak abundance) in a section called "MMM is the first month you might see...". I will choose a few species from each section to highlight with more information so that we can look for them intelligently - but you will also be able to look through the rest of the list for other suitable candidates. The choice of what to highlight will be based on a combination of 1) How underrecorded is the species on iNat? 2) How rare is the species? (i.e. is it realistic to think we might find some?) 3) How identifiable is it from photographs?

To each species I have attached a pair of numbers: e.g. Eristalis tenax (1,91%). The first number is the rarity expressed as a centile of abundance according to NBN: a low number suggests a common species, a number close to 100 indicates something vanishingly rare. The second number is the over/underrecordedness in iNat: A positive number indicates that the species forms a higher proportion of records in iNat than in NBN (i.e. overrecorded), and a negitive number implies that it forms a smaller proportion of iNat records than in NBN (i.e. underrecorded - a species with no iNat observations is '-100%').

To give you an idea of the rarity scale, here are a few other British species with increasing rarity values: Episyrphus balteatus (0,45%), Epistrophe eligans (10,45%), Eupeodes latifasciatus (19,-55%), Volucella inflata (31,-38%), Eriozona syrphoides (40,-64%), Pipiza luteitarsis (49,-86%), Chalcosyrphus eunotus (60,-95%), Blera fallax (77,-100%), Neocnemodon brevidens (90,-100%), Syrphus nitidifrons (100,-100%).

(Unfortunately I have not been able to find a particularly good source of information for the presence/distribution of species on the island of Ireland - so comments on distribution will only cover Great Britain. Note that fewer species are present in Ireland - obviously the commonest species in Britain are the most likely to also be present on the Emerald Isle. My apologies.)

You may want to look through the remainder of species I don't elaborate on for additional species with low rarity and significant underrecordedness, especially if you examine collected specimens - there are some extremely common species almost completely missing from iNat because they require microscopy - e.g. Neoascia podagrica (7,-98%), Syrphus vitripennis (6,-99%), Platycheirus clypeatus (8,-99%), Sphegina clunipes (20,-100%).

Hopefully this will help us improve hoverfly diversity on iNatUK, and be quite fun too!

[Boring bit: I compared iNatUK's hoverfly records for each species with the records for that species on the NBN atlas to determine whether they are under- or overreccorded on iNat. To do this I looked at the proportion of all hoverfly records that represented the species in question: if the species is a greater proportion of all hoverflies on iNat than it is on NBN I call it 'overrecorded', and 'underrecorded' if it is a smaller proportion of all hoverflies on iNat. If a species represents 1% of hoverfly records on NBN but only 0.5% on iNat it is 'underrecorded by 50%', if the same species represented 5% of hoverfly records on iNat it is 'overrecorded by 500%'. Crude, but hopefully that makes some sense.]

[Incidentally, the 22 overrecorded species are: Volucella zonaria (+1034%), Epistrophe melanostoma (+473%), Cheilosia caerulescens (+331%), Volucella inanis (+309%), Myathropa florea (174%), Merodon equestris (+169%), Eristalis similis (+155%), Eristalis tenax (+91%), Volucella pellucens (+78%), Chrysotoxum festivum (+63%), Meliscaeva auricollis (+63%), Eristalis pertinax (+57%), Dasysyrphus albostriatus (53%), Episyrphus balteatus (+45%), Epistrophe eligans (+45%), Eupeodes corollae (+39%), Epistrophella euchroma (+36%), Rhingia rostrata (+32%), Xanthogramma pedissequum (+26%), Sphaerophoria scripta (+25%), Chrysotoxum veralli (+23%) and Scaeva pyrastri (+17%). Most of these make sense as the sort of species citizen scientists like ourselves will most easily notice, others (such as Epistrophe melanostoma) seem to be there more as a quirk of small numbers. Rhingia rostrata and Cheilosia caerulescens may be there because of their recent explosion in numbers - NBN goes back to a time when they were much rarer but most of iNat's data is very recent.]

Posted on 20 January, 2024 22:33 by matthewvosper matthewvosper


Fantastic. Thank you for this post. I think it will be extremely useful and will certainly incentivize me to get out and record. I've just bought the new Hoverflies of Britain and Northern Europe which I hope will also help.

Posted by steve_orridge 6 months ago

Excellent idea Matthew, monthly guides will be really useful. Thanks for all your hard work!

Posted by john_baines 6 months ago

Thanks both. I've done as far as May so far, but I kind of want to release them all together so they're easy to switch between. I certainly have my eye on getting that book too!

Posted by matthewvosper 6 months ago

As a random thought - is there also a bit of bias in NBN where recorders there tend to focus on the harder to see species and not bother recording the widespread ones as much? I know I overrecord a number of species but that is mainly down to those are the most common and widespread and that I often search flowers for invertebrates coupled with getting records in for locations I have set up on my local patch (e.g. I am finding some places have loads of E. pertinax and no E. tenax). Species to see in the month and tips on finding them is a great idea and will be most useful, I know there a few early species that like birch catkin I keep meaning to go and find.
Will also be interesting to see how species distributions start to change over the years with climate change - am I correct in thinking Volucella inanis is spreading north at the moment - would also explain a bit of overrecording, although I am always a sucker for a shot when I see one!

Posted by a_emmerson 6 months ago

I known V zonaria's been spreading so possibly. Yes, I think there are biases in the NBN data too, but it is certainly much more representative - and it was very easy for me to access. The decline of the house sparrow drove home the importance of recording common species, but it would be cool if we could redress the balance in iNat a bit :). Birch catkins, yes - but I see Willow mentioned more. As I'm going through the year I'm coming to the conclusion that part of the reason for our imbalance is that we do tend to look for hoverflies on flowers. But that's not the best place to find some species - especially in Eristalinae. Some hardly visit flowers at all. I'm going to be paying more attention to logs, rot holes, tree trunks, sap runs, low vegetation and the bases of larval food plants this year :)

Posted by matthewvosper 6 months ago

'it would be cool if we could redress the balance in iNat a bit :)'
Mission accepted!

Excellent on the alternatives to flowers for looking for them - will be a great help, getting impatient for spring to start already.

Posted by a_emmerson 6 months ago

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