Just before Christmas, I reported on the first record of Achnanthidium catenatum for the UK, from a lake in Wales and pondered, in this post, what we meant by a “record” of a diatom. Simply finding a few empty valves (the silica cell walls) is not, by itself, evidence that there ever was a viable population of A. catenatum in Llyn Padern. What, indeed, should we use as criteria for regarding this as a new addition to the UK flora?
My instinct is that the more valves I find in a sample, the greater the probability that these indicate the existence of a viable population. Even so, it could still just indicate the presence of a population at one point on one occasion. If I were able to find A. catenatum at several locations around the lake, or at the same point over an extended period of time then, perhaps, we can conclude that it is an established part of the flora of this lake and, therefore, of the UK.
What if I cannot demonstrate this, even after visits to Llyn Padern? We know that it was present (albeit as dead cells) in the summer of 2013 but is this enough to add it to a list of UK records? Perhaps those of us who study algae need to adopt the practice of ornithologists of distinguishing between species that nest and breed in the UK (“British Residents”) and those that are “Summer/Winter Visitors” or “Passage Migrants”? (apologies to Northern Irish readers, but the website www.birdsofbritain.co.uk does not appear to make any distinction between UK and Great Britain).
In other words, we should aspire to a two-tiered checklist that accounts for the often erratic occurrences of microorganisms that are readily dispersed by a variety of physical and biological agents. Then again, a further complication is that populations may lay dormant for long periods and then proliferate for short periods, so our one-off record may actually have been present for much longer, but was missed.
And there’s a final problem: too often our records simply reflect the activities of the small number of people who collect and analyse diatoms and other microscopic organisms, making it very hard to understand how distribution patterns vary in space and time.
And, finally, you might well be asking whether these questions are relevant. In my opinion, yes they are, partly because it puts the diversity of the microbial world into context. According to www.rspb.org.uk there are 574 bird species recorded from the UK. There are at least four times as many species of freshwater diatoms, and about 10 times as many species, if we include all freshwater algae. There is a huge amount of diversity tucked away in a group of organisms that is largely overlooked or ignored.