The question “how many species are currently living on earth?” is a particularly difficult one to answer. The simplest answer is that we haven’t a clue, and estimations of the true number vary wildly depending on the approach used.
A simpler question should be “how many animal phyla are there?” Again, we do not know. Thirty-three phyla (our phylum, vertebrates, is just one) have been described, but that number is increasing, largely due to scientists who look in unusual places. Recent additions include the Cycliophora, which was found living on the lips of lobsters. Of all places.
Putting aside the true number of species currently alive, the number of known species is not even known. There is no single catalogue of all the species that have been described by science. Even if there were, the list would be heavily beset by problem of ‘synonymy’ – that is, a single ‘true species’ having been named multiple times, and so showing up in the catalogue as a misleadingly high number. For this reason, estimates of species number sometimes decrease, as scientists spot these synonymous species.
There are a variety of different methods of estimating the true number of species, each giving a different answer.
One study by Terry Erwin involved the collection of beetle species. In the study, 19 tropical trees were gassed, and around 1200 beetle species were collected in nets as they fell from the canopy above. Assuming a ratio of specialist species to generalist ones, and assuming that beetles represent a specific proportion of all species, the publisher of the study estimated that there are in excess of 80 million species present worldwide.
A contrasting study by Peter Raven estimated that there are 3 to 5 million species globally, based on the fact that temperate species are well described (as that’s where most of the world’s scientists live), and the assumption that there are two tropical species for every one temperate species.
All estimations are fraught with error, of course, and not one of them amounts to more than an educated guess. Ideally, what is needed is an objective way of measuring organisms down to the species level with no overlap, no synonymy, and which will work on all species. No such tool exists, though ‘DNA barcoding’ is showing some promise. DNA barcoding is a technique that attempts to assign an honest and unique identifier to all species. It has thrown up some interesting results, such as the discovery to hidden ‘cryptic’ species, as well as merging others, and showing some species are confusingly mislabelled.
In the process of finding the answer to this question, many scientists have asked “exactly what is a species, anyway?” The problem of defining ‘species’ has troubled people for years; each definition fails in one way or another. For example, the most widely use definition, referring to ‘reproductively isolated organisms’ does not work with asexually reproducing bacteria. The tighter the definition of species, the harder it is to fit organisms into it. Darwin himself called species “an arbitrary term, one used for the sake of convenience.”
Members of the species of bacteria that causes legionnaire’s disease have been shown to be as genetically divergent as is a typical mammal to a fish. Clearly there is something wrong if this is the case. As ‘species’ may be impossible to define exactly, it is silly to focus any effort on precisely finding the number of species on Earth. I feel we could gain more understanding of the diversity of life on Earth by studying the molecular, morphological and behavioural diversity, rather than awkwardly shoe-horning organisms into the 18th century ideals of ‘species’.