When do we need voucher specimens?

Yay, in the last few days this blog passed 100k views! And just when I had a whole week of not being able to find the time to add anything...

Anyway, on Friday I have been considering voucher specimens. First, in case somebody from outside the field reads this, what are they?

Imagine somebody did a study of essential oils found in the South American mint genus Minthostachys, and they published a paper reporting a pulegone-dominated oil for Minthostachys glabrescens, a menthone-rich oil for M. verticillata, and a carvone-rich oil for M. mollis. Twenty years later, a taxonomist revises the genus and finds, for example, that the name M. glabrescens had for decades been misapplied to a completely wrong species (as per the type), and that the circumscription of M. mollis needed to be changed.

A new taxonomic treatment of the genus is published, and you might now, if you were interested in its ethnobotany, biochemistry or commercial exploitation, be interested in knowing how the old oil data relates to species as currently circumscribed. What those guys who did the oil study called glabrescens definitely wasn't true glabrescens, but what was it instead? Which currently accepted name applies to the sample that had the pulegone-rich oil?

If all there was in the oil paper were names and biochemical data you're stuck. This is where voucher specimens come in. For good scientific practice, the authors of that study should have deposited a herbarium specimen of each sample they analysed in an officially recognised and accessible research herbarium (the kind of institution serious enough to be listed in the Index Herbariorum), so that we can examine them even fifty years later and figure out what exactly it was that they had in their study.

So, in short: a voucher specimen is a herbarium / museum / biodiversity collection specimen that is cited in a publication to allow later scientists to verify the taxonomic affiliation of a sample used in a scientific study. It could be a dried and pressed plant, a needled insect, a fish skeleton or a stuffed bird; it could be connected to a morphological data set, a DNA sequence, a biochemical profile or a new species name. (In the latter case it would not be a mere voucher but a type. Although types are even more valuable, the principle is the same.)

Among biodiversity researchers the importance of vouchers is well understood. It is, or should be, virtually impossible to publish a study in a good botanical journal without citing a list of voucher specimens underlying your data either in a table, in an appendix, or as part of the paper's online supplement. And we are often rather exasperated that not all colleagues in related fields have the same approach. The biochemical example from above was chosen deliberately, as I have run into many essential oil or ethnobotanical studies that neglected to cite vouchers, meaning that their results are pretty much unreproducible and scientifically near worthless.

That being said, however, I have started to wonder whether some colleagues don't go a bit overboard with this. The occasion is discussing a seed reference collection, about which several people have asked me "is it vouchered"? So the idea is, we can only use the seed samples that have a herbarium specimen as a reference somewhere.

But we are not talking here about biochemical data, a DNA sequence uploaded to GenBank, or a morphological description. The seeds themselves are biological specimens, aren't they? Can't they be their own voucher?

Granted, there may be many cases where only having the seeds is not good enough to narrow taxonomic affiliation down to species level. But is that really different in principle from a perfectly acceptable herbarium specimen of a flowering plant ... in a genus where you need fruits to identify to species?

So to me the point of a voucher is that it is a lasting, biological reference specimen for a piece of data. But a lasting biological specimen should not necessarily need another lasting biological specimen as its reference. In some cases it may be good enough to be a specimen in its own right. Or to look at it another way, there can be botanical specimens that are not dried and pressed whole plants.