I have had two conversations about rare algae over the past two weeks. The first was an invitation to contribute to an exercise to develop a list of diatoms that might form the basis of a “red list” of endangered algae. The second was a retort from a colleague that such an act would be meaningless as algae don’t have the same biogeographical restrictions on their distributions as higher organisms, and that all algae will grow anywhere so long as the environment is suitable. The argument that algae don’t have biogeographical restrictions is an old one (summarised as “everything is everywhere, the environment selects”) but several recent papers have shown this to be wrong. Some species do appear to be cosmopolitan, as my previous post shows, but others do seem to be restricted to particular regions. Even if the local environment does play a large role in determining the algae that are found at a location, that does not seem to obviate the need for a list of endangered algae. On the contrary, it might even help focus attention on locations where efforts to restore a site might make a real difference.
The problem, in my opinion, is more basic: phycologists working in freshwaters do not have a strong tradition of systematic recording of the distribution of organisms. You only need to look at the Freshwater Algal Flora of the British Isles, and to see how many species are described as “probably cosmopolitan”, to realise the scale of the problem.
Because of their widespread use in ecological assessment, the diatoms are one group of freshwater algae where there may be enough data to start making some sensible judgements about the rarity, or otherwise, of individual species. I had a look at a dataset compiled for a project that I was involved with a few years ago in order to see what might be possible. This dataset comprises 6500 samples from 3305 sites spread across Great Britain and Ireland, most of which also have location information. The basis for conservation assessments is the distribution in 10 km squares, termed “hectads”, of which 1111 were represented in my dataset, out of a total of about 3000 in Britain. The two criteria I am using are “nationally rare” for species that occur in 15 hectads or fewer, and “nationally scarce”, for those which are only found in 16 to 100 hectads. Using these criteria, I produced a “long list” of 150 diatom taxa that are “nationally scarce” and a further 226 which may be “nationally rare”. This, however, is where the real work starts.
Scanning down this list, I see several problems that need to be addressed before we can make serious judgements about the rarity, or otherwise, of particular taxa. However, I do also see a number of taxa on this list that I do believe to be genuinely rare or scarce and which are, at least, worthy of more study. The problems are many and will spill into the next post but let’s make a start:
- The dataset I’m using is for rivers, and I will need to merge it with some additional datasets to get good coverage of lakes and also of soft water and acid habitats. I would not trust this provisional analysis to give an accurate overview of the distribution of acid-loving Eunotia species, for example, nor of planktonic diatoms such as Asterionella formosa;
- I also noticed some species typical of brackish taxa which have been recorded occasionally in freshwaters (e.g. Bacillaria paxillifer). More comprehensive coverage of coastal and estuarine environments would probably show many of these to be quite common. The same reasoning applies to those diatom species associated with terrestrial habitats (e.g. Hantzschia amphioxys).
- Most of the samples in our database come from rock or plant surfaces and it is likely that diatoms that prefer other substrata have been under-recorded, which will complicate interpretation of their distribution. Absence of evidence is not evidence of absence.
- Many of the taxa that are rarely recorded belong to taxa that have been subject to taxonomic uncertainty over the past few decades, leading to variations in how they have been recorded. Some of the rare diatoms are “varieties” of common species but as these often are (or were) poorly described in the literature, many analysts have not tried to distinguish them.
- Finally, we have to be sure that the records actually represent living populations. Because diatomists usually work from the empty silica frustules, we cannot tell whether a cell was alive at the time it was collected. If you find a number of frustules of the same species, then it is reasonable to assume that some of these were alive, but if a species is represented by a single frustule, we have to consider the possibility that this was washed into the site from elsewhere, and never actually grew there.
The positives from this process are that I think we can start to make some judgements about the rarity (or otherwise) of diatoms that are reasonably well circumscribed in the literature (i.e. a low chance of misidentification) and where the underlying taxonomy has been relatively stable. A further criterion at this stage is that the candidate taxa must be common in streams and rivers and, ideally, associated with hard surfaces rather than soft sediments. That’s quite a lot of caveats, but in the next post I’ll start to sort through the list and see if we have some genuine candidates for “scarce” or “rare” diatoms.
The dataset referred to was developed for:
Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, J., Rippey, B, Hirst, H. and Yallop, M (2008). Assessment of ecological status in U.K. rivers using diatoms. Freshwater Biology 53: 403-422.