Monday, 1 May 2017

Dealing with species complexes

One of the big problems we have when identifying specimens from photographs is the level of resolution needed to make a firm identification. In many species the critical characters are subtle or simply cannot be depicted from anything but the most awkward and unlikely angles for live animal shots. Therefore, if we cannot make a firm identification we will often go only as far as genus (occasionally Tribe); or we will go as far as the point where a particular species split occurred.

The main reference point for species splits (in Britain) is the first edition of Stubbs and Falk (1983), which established the foundations of the modern species list. At that time some 250 species were known, with various ideas presented about possible splits (species A, B, C etc). Over time, some of those ideas have been confirmed as reliable new species and splits in what was then regarded as a single species have been made. One or two have been lumped (e.g. Baccha elongata and B. obscuripennis).

The problem in recording terms is what to do with pre- and post- split data? Junking all the pre-split data is not wise – it will find its way back in again and may contaminate the data for the original species. So, unless original voucher specimens have been re-examined and a new diagnsis has been made, we allocate these records as sensu lato (in the broad sense), often notated as sl. or agg.

There then comes the problem of what to do when species complexes are presented as photographic records? In many cases we simply cannot make the relevant split from the photograph so one option would be to log at generic level. This is not very satisfactory, however, because we can be reasonably sure about its identity if basing the identification on the earliest edition of Stubbs & Falk. So, I log such specimens as sl.

The main splits that are relevant here are:

Cheilosia albitarsis to C. albitarsis and C. ranunculi
Platycheirus clypeatus to P. clypeatus, P. europaeus, P. occultus and P. ramsarensis
Platycheirus peltatus to P. peltatus and P. nielseni
Platycheirus scutatus to P. aurolateralis, P. scutatus and P. splendidus
Xanthogramma pedissequum to X. pedissequum and X. stackelbergi (and X. dives in Europe)

Those apart, we think that at some point Dasysyrphus venustus will get split into at least two species – hence we are careful here. There has also been a lot of uncertainty about the status of D. hilaris which is almost identical to D. venustus – hence if there is no face shot we lump these together and say venustus agg. – but for data purposes I log as Dasysyrphus sp. When this one splits we won't be able to do it from photos because the main useful characters are on the sternites - there will be at least three species - D. venustus, D. hilaris and at least one additional species but potentially two or more.

Then there comes Melanostoma – we just cannot be sure what will be doable once this is sorted out! We had thought that a recent review using DNA had eliminated the developing theme of at least five species within M. mellinum and two or more in M. scalare, but M. mellarium has been added to the mixture and I (or others) have yet to do a careful analysis of specimens to be sure what we have.

Finally, we have the problem of groups of species that Alan Stubbs recognised as groups based around a central name. This is mostly used in Cheilosia where there are a number of species that share a common obvious feature such as projecting hairs on the face, or hairy/bare eyes etc. In those Alan has erected 'groups' and a similar approach has been adopted by van Veen (see my post on identification guides). In our diagnoses, we may well say that the depicted specimen is likely to be a member of the grossa, pagana, variabilis etc. groups but we don't give an aggregated name because there is no certainty of which couplet (split between two species sharing most but not all of the same characters) one can get to.

Much of this more complicated taxonomy is well-known to those Dipterists who have used the various editions of Stubbs & Falk, but newcomers using the WILDGuide will not. Unfortunately, there is only so much space in such a volume and it was only ever designed as an introductory guide rather than a comprehensive replacement for Stubbs & Falk. If you progress to the microscope then you will need this volume.

Stubbs & Falk is arguably obsolescent because there have been a dozen or so new additions to the British fauna since 2002 when it was last updated. We have said that we will provide a supplement, but as yet that has not happened – it is one of several major jobs on our list – maybe next winter!

No comments:

Post a Comment