There can be few words as poignant as ‘endling’, the name given to the last surviving individual of a species. Tell me you don’t find this image of Benjamin, the last Thylacine, heartbreaking? Or that you weren’t moved by the plight of Lonesome George? And what about Martha, the passenger pigeon? Doesn't her story make you weep at our limitless environmental profligacy? But what links all of these endlings is that we know they were once members of a thriving population - in Martha’s case, one of the most abundant vertebrate species on the planet. There are other species which are known only from a single individual. These species, perhaps lacking the poignancy of endlings but arguably more significant to students of biodiversity, are known as 'singletons'.
Singletons of course are to be expected when your survey area is small, or if your look for only a short period of time. If I counted the birds in my garden for an hour tomorrow morning, I’d expect to see multiple individuals of a few species - magpies, wood pigeons, house sparrows - but I’d be very surprised to see more than one sparrowhawk or wren. However, if I extended my search to my whole street, or to the whole of Sheffield, the number of singletons would drop off precipitously. And at the scale of the UK, over the course of an entire summer, any breeding bird species by definition must be represented by at least two individuals, so there should be no singletons at all in our core avifauna. Any that remain can be dismissed as shivering vagrants blown across the Atlantic, ornithological curiosities but ecologically insignificant.
Enter the sea, though, and the number of singletons remains stubbornly high, even when we expand enormously our study region. For instance, in an analysis of European benthic invertebrates I did a few years ago, I found that about 10% of the species in our very large database (2,300 species sampled from >15,000 locations throughout European seas) were singletons. Similar patterns appear when interrogating the Ocean Biogeographic Information System database I blogged about recently. For instance, OBIS contains records for >11,500 species occurring in the seas around Britain, yet over 45% of these are represented by a single record. At the global scale, 20% of the almost 80,000 marine animal species which occur in OBIS are singletons.
What's happening here? Are these singletons simply very rare species? We expect most species to be rare, but do our surveys of marine habitats really cover so small an area that we never pick up their conspecifics? And if this is so, what does this mean for marine ecosystem functioning? Do these rare species play a role? Individually, maybe not - Kevin Gaston has written on the dominance of common species in terms of numbers, biomass, and probably ecosystem functioning, in most communities. But collectively the singletons in a sample can be abundant, and if there were particular biological characteristics associated with being a singleton, then this could be significant. Unfortunately, about the one generalisation we can make about rarely observed marine species is that we know little about their biology, so we’re not yet in a position to answer this question.
An alternative explanation is that many singletons are mistakes. When analysing diversity surveys, we can take steps to ensure that taxonomic names are consistent, for instance by using the World Register of Marine Species to ensure that we use the accepted name for each species and not one of its (often many) synonyms (I've done that in the cases mentioned above). But what if the person who sorted the sample simply got their identification wrong? There’s not much we can do about that kind of mistake, although one would hope that errors of identification are not so frequent as to explain the very high prevalence of singletons.
Probably we won’t know the answers to such questions until sampling of a few large marine ecosystems reaches a sufficient intensity that we can have confidence that surveys accurately reflect the composition and relative abundance of the species present. For now, we can at least use the presence of singletons to tell us something about how far away from such complete knowledge we are. As I suggested in my last post, in certain marine systems the answer to this is: a very long way indeed. In the meantime, we are becoming more and more aware of the threats facing many marine species. We must hope that the singletons we find in our surveys are only statistical loners, the first observed rather than the last remaining individual. If they do in fact represent the Benjamins, Marthas and Lonesome Georges of their kind, then marine biodiversity is in more trouble than we thought.