How many species are there?
Tough question. Nobody really knows. A new review in Trends in Ecology & Evolution takes a look at the myriad of answers and how we can try to narrow down the options.
Even if we discard the incredibly complex microbial world, estimates range roughly from 2 to 50 million. And getting an accurate count of how many species have been identified so far isn’t easy either. the main problem here is synonymy, which means exactly what you think it does. Some species have been described more than once. For example, different live stages have often been described as separate species, even though it was found later that they are actually part of the life cycle of a single organism.
- Fungi: about 99 000 catalogued species, estimated total number (including unknown ones) somewhere between roughly 600 000 and over 1.6 million.
- Plants: over 310 000 known, estimated total around 400 000.
- Terrestrial animals: over 1.23 million known, estimated total around 8.75 million.
- Marine animals: over 190 000 known, estimated total over 2.2 million.
Lots of exploring left to do, it seems.
Where do these estimates come from?
The authors distinguish three main categories of methods that are used to do this:
- Extrapolations from fractions: using unknown fractions in samples. Say you sample insects somewhere, and you find that a third of your sample consists out of species unknown to science. Using some correction factors then allows you to extrapolate this to a global level. Of course, this method faces the often flawed assumption that known species form an unbiased subset.
Extrapolations through scaling: here species number is linked to higher taxonomic units, such as genera, families and orders. The idea here is that there is a certain, fairly stable, number of lower-level units in a higher one for each large organism group, such as, for example, marine animals or seed plants. So, a certain number of species in a genus, a certain number of genera in a family, and so on. This, of course, is an assumption in itself, for which there is no particular theoretical reason.
- Mechanistic models: taking into account the species discovery process is key here. The taxonomists (people who describe new species) are viewed as metaphorical predators, and the species waiting to be discovered as metaphorical ‘prey’. Based on this idea, a model is constructed, that allows estimates with confidence intervals (basically allowing people to say: “with this percentage of certainty, the total number of species is somewhere between this and that.”). Of course, there are only so many taxonomists, which additionally, might work methodically, genus per genus or something like that.
So, where, and how, to look for all these missing species?
A promising approach seems to be to focus on so-called biodiversity hotspots, regions with high numbers of endemic species (or, species that occur only at a specific location). Many of these hotspots are already focal points for conservation efforts, but nevertheless, many of them suffer from habitat loss. Also, several of them are found in regions that are remote or subject to political instability.
But, even if we can settle on likely places to look, should we think about how we are looking? In other words, are some species harder to find than others? And indeed, there are some fairly obvious matters to take into account here:
- Size: larger species are usually easier to find than small ones. Speaks for itself, really. However, larger species are still being discovered.
- Colors (or, the “ooh shiny syndrome”): conspicuous organisms are noted with more ease. Brightly colored species are better ‘targets’ than those without eye-catching markings.
- Behavior: some species are just very shy, hiding at the first sign of danger. Others are more confident or curious and less likely to run from the human intruders.
- Location: organisms bound to a specific site of limited size are harder to find than species that occur over large areas of land or water. Also, some places, say the deep-sea or deep, narrow caves, are just harder to explore.
- Rarity: obviously rare, endangered species are harder to find than those that are very abundant and doing well.
By the way, here’s a composite image of the Top 10 of newly discovered species from 2012 (more information? Click.)
Where does this leave us? How are the efforts going? Some estimate that it might take another 500 years to describe most of the species on earth, others say it can take 1 000 years just to make headway in the jungle of fungus species. The authors, however, identify some bright spots:
- Museums might already possess many unknown specimens.
- DNA barcoding can aid in distinguishing very similar species and in describing species in general.
- Taxonomists are increasingly to the internet, making it easier to share their work and prevent the issue of synonymy. This is complemented by online, global attempts to catalogue all species.
- Species discovery and mapping is finding its way to smartphones and crowd-sourcing, greatly increasing (wo)manpower (no sexism here), so to speak.
In general, there are still a lot of species that are waiting to be discovered. Maybe even in your backyard…
Scheffers, B.R., Joppa, L.N., Pimm, S.L., & Laurance, W.F. (2012). What we know and don’t know about Earth’s missing biodiversity. Trends in Ecology & Evolution DOI: 10.1016/j.tree.2012.05.008