by Francesco Lami
Slightly later than most other biology blogs (and while waiting for the other motherfucker on this site to start being productive), here’s the new study that estimates that the number of currently living species of Eukarya (organisms with cell nucleus and organelles – animals, plants, algae, fungi, protozoa…) is around 8.74 millions, which is quite impressive, especially considering that the number doesn’t include Bacteria and Archaea (prokaryotes). Mora and the others who ran the study noticed a quantitative relationship between the number of taxa (groups of organisms) and the number of lower taxa contained in each of them and so on, and thought that maybe there was the same relationship between these taxa and the number of species contained in them. It might be a false assumption, as the authors themselves pointed out, but from what we know now there’s no reason to think so (yet), and anyway it’s a pretty clever method. It couldn’t be applied to prokaryotes since the higher taxon approach is invalid for those little fuckers (they’re too variable, they evolve too fast, and there’s probably too many of them).
To the surprise of no one, of these 8.74 millions, 7.77 millions are animals, and that’s simply because a vast majority of animals are insects, which are one of the most successful and diverse groups of organisms to have ever existed. Plants have 298.000 species, fungi 611.000, protozoa 36.400, chromists (algae & co.) 27.500. It’s interesting to note that not only the animals, but the fungi too (which are, in fact, part of the same clade) are much more numerous than plants. An explanation I can think of is that autotrophs only have to fill the role of “producers”, while heterotrophs are “consumers”, and they can fill different levels and feed on different organisms, and thus they adapted for very different lifestyles.
Interesting is also the fact that the model predicts that “only” 2.2 millions of species out of these 8.74 millions live in the ocean. It might seem strange, since oceans cover 71% of Earth’s surface. I’ve read a very interesting and reasonable explanation of this fact in the twenty-years-old-but-still-extremely-valid book The Diversity of Life by legendary myrmecologist (yeah, it sounds weird) Edward O. Wilson. The fact is that on land and, to a slightly lesser degree, in freshwater, travelling is much more difficult, and there are a lot of natural obstacles and barriers. This contributes to the isolation of groups of organisms, and isolation (and subsequent interruption of sexual exchange among populations) is one of the keys to speciation and diversity. In the ocean,there are far less natural barriers, and travelling is much easier thanks to currents, so it’s more difficult for populations to lose contact with each other and diverge. (I’d also like to add that while ocean is enormous, most of the marine biodiversity is localized near coastal regions, which cover a much smaller area).
Previous estimates of the number of species included 3 millions, 30 millions and 100 millions. Today, roughly 2 millions of species (Eukarya, Bacteria and Archaea) have been described. I’m not sure if we’ll ever be able to describe all of them and determinate if this new study was correct. The challenge seems very though, especially since nowadays, because of us, many species are lost forever even before their discovery.