How many species are there on Earth?

Okay, guess quickly: how many species do you think there are on Earth? (We’ll leave aside the problems with distinguishing species in largely asexual groups like bacteria.) A million? Ten million? A hundred million? Some estimates even go up to a trillion!

This is one of those questions that’s nearly impossible to even approximate an answer, for many species are cryptic, including those in the deep sea, bacteria, and the nematodes that burrow in the soil—not to mention all those species in the canopies of the rain forests.

This 5-minute video, from “It’s Okay to Be Smart,” essays an answer. but it doesn’t differ from mine above (i.e., “we have no damn idea!”.  It gives an estimate of 5 million as well, which, I suppose, is as good as anything else.

As lagniappe, watch this cool video about a HERMIT CATERPILLAR, which carries a leaf wrapped around it, and for the same reason that hermit crabs appropriate the shells of molluscs. I had no idea.  It appears that the video’s narrator actually discovered this species and its bizarre behavior.

The last half of the video, which isn’t as edifying, deals with the question, “What is a species, and how can we tell when species are different?” Its treatment is not so great, what with the inclusion of nebulous “genotypic species concept,” which is purely arbitrary, and the “mate recognition concept” which is just a subset of the Biological Species Concept (BSC). The video also completely overlooks the real “species problem”, which is this:  “Why does nature come in discrete packages instead of comprising an organic continuum?” That answer to that big question involves, as I think, the BSC (see Speciation by Coyne and Orr).


  1. ThyroidPlanet
    Posted January 29, 2017 at 2:57 pm | Permalink


  2. pck
    Posted January 29, 2017 at 3:03 pm | Permalink

    Thanks PCC! Speciation is to most fascinating subject in biology for me, and Speciation is a fantastic book – are there any plans on ever releasing a second edition?

  3. Brian Salkas
    Posted January 29, 2017 at 3:11 pm | Permalink

    So I have a question on the speciation of bass.
    Are the largemouth and the smallmouth bass different species even though they can (and often do) produce fertile offspring? There are some health problems that hybrids are predisposed to, but I don’t know if that would justify calling them different species. Is anyone claiming that they are the same species?

    • ploubere
      Posted January 29, 2017 at 3:22 pm | Permalink

      It seems that that is the very problem with our classification system that he is discussing, it’s not really possible to put everything into tidy boxes.

  4. ploubere
    Posted January 29, 2017 at 3:19 pm | Permalink

    Nice series, good information.

  5. jaxkayaker
    Posted January 29, 2017 at 3:20 pm | Permalink

    The last estimate I saw (about 10 years ago) was that there were 1 million named species of animals and a predicted 10 million species of animals. Another estimate I saw predicted that there were expected to be 10 million species of just insects alone.

  6. Gregory Kusnick
    Posted January 29, 2017 at 4:07 pm | Permalink

    “Why does nature come in discrete packages instead of comprising an organic continuum?”

    To clear up a possible point of confusion, I presume you’re not claiming here that the BSC somehow explains the existence of individual organisms (as opposed to, say, a continuous biofilm covering the entire planet).

    Rather, by “discrete packages” you mean something like distinct breeding populations, or clusters of similar phenotypes, and the “continuum” would then be free gene flow and/or a smooth gradation of phenotypes among organisms of all kinds.

    • Posted January 29, 2017 at 4:34 pm | Permalink

      Yes, sorry, I meant “species in one area” as when you look out your window at a bird, you know that it’s either a sparrow, a pigeon, a cardinal, a starling, and so on. Birds don’t form a continuum where such judgments are subjective. If they were, field guides would be of no use. Now of course there are difficult calls, but Orr and I lay out the whole issue of discreteness of GROUPS in chapters 1 and 2 of Speciation.

      • Posted January 30, 2017 at 12:40 pm | Permalink

        Also Scott Atran and Douglas Medin summarize some work here ( that shows there is also wide cultural agreement: non-scientific experts in a given area (at least where studied) largely agree with the species identified by scientific research. (This was astonishing to me at how much so it was – I think in one Mayan example it was 42 / 43.)

        I’m no expert on speciation or “folk concepts” of course, so if there’s work in cognitive anthropology I’m missing here, I’d love to hear about it.

  7. veroxitatis
    Posted January 29, 2017 at 4:18 pm | Permalink

    The “Hermit Caterpillar” is indeed an interesting creature. I wonder how many species use natural external materials to fashion protective coverings. I expect the trait goes back a very long way. There are, for example, the “brick building” rotifers, Floscularia ringens. These live in a self constructed tube which consist of pellets of compressed detritus and bacteria.

    • Richard Bond
      Posted January 29, 2017 at 6:52 pm | Permalink

      The larvae of several species of caddisfly do this.

    • Posted January 29, 2017 at 7:09 pm | Permalink

      The larvae of the c. 7000 species of caddisflies (order Trichoptera)all live in tubes constructed of sand, twigs, or other detritus. They carry the tubes around with them, much like the featured caterpillar. At least one in the Sierra of California constructed its tube out of small gold nuggets. Rather heavy I’d think.

    • Mark Sturtevant
      Posted January 29, 2017 at 7:17 pm | Permalink

      There are also bagworms, moth larvae that make their own tubes out of bits of twigs and leaves and silk. They crawl around tree branches, hanging from their thoracic legs and not from their mouths that I know of. These are smaller than the one found here, though.

  8. Hempenstein
    Posted January 29, 2017 at 4:46 pm | Permalink

    “All species look the same after they’ve been thru a Waring Blender.” -WW Cleland (famous enzyme kineticist)

    • Posted January 29, 2017 at 7:22 pm | Permalink

      Because after that treatment, they’re not species any more.

    • W.Benson
      Posted January 29, 2017 at 8:54 pm | Permalink

      That is why enzyme kineticists have not contributed much to the question of why distinct species exist.

  9. Kevin
    Posted January 29, 2017 at 5:06 pm | Permalink

    I am not sure the aliens would need that answer or be interested in it.

    The machinery for life is what they would want. You could probably take a handful of sand, some DNA of about 100 or so animals and these aliens could, in millions of years create millions more species all different form the ones on this planet.

    Bottom line, aliens would be interested in the recipe, not the permutations.

    • Posted January 29, 2017 at 7:14 pm | Permalink

      Nah! Who cares about that sort of thing?

      General principles operate everywhere, including wherever the aliens came from. It’s the fantastic variety of products of evolution that would interest the aliens most I’m sure — based on my extensive experience with aliens and their interests.

    • Michael Fisher
      Posted January 30, 2017 at 1:08 pm | Permalink

      I’m with Achrachno

      The aliens will be highly engaged by the ‘music’ made by the interactions of all those “…forms most beautiful and most wonderful…” – it’s the orchestra [biosphere] playing as one rather than the brute facts re the construction of the instruments that is so beautiful.

      Also the gourmet type of aliens will be very happy with the variety available already on the hoof!

  10. Mark Sturtevant
    Posted January 29, 2017 at 7:15 pm | Permalink

    Very good. I use the meadowlarks as an example of the challenges in identifying species based on similarities or differences. I did not know they hybridized a bit, and that is totally cool.

  11. Dale Franzwa
    Posted January 29, 2017 at 11:56 pm | Permalink

    An awfully lot. Does that help?

  12. AdamF
    Posted January 30, 2017 at 1:54 am | Permalink

    This paper is well worth a look when trying to address this issue.

  13. John Ottaway
    Posted January 30, 2017 at 3:23 am | Permalink

    Although I could guess from the context, I did look up “lagniappe”. I don’t believe it is in common use here in the UK

    You know words, PCC. You have the best words

  14. Kevin
    Posted January 30, 2017 at 11:30 am | Permalink

    The video made me wonder if devout Christians believe that if aliens were to land that they would actually seek out the pastors and bishops of the lands as they are the most reverend of humanity. Such a thought is actually quite nauseating.

    • Michael Fisher
      Posted January 30, 2017 at 1:12 pm | Permalink

      There’s quite a few SciFi short stories & novels based on rational [or innocent] aliens getting to meet our religious loons – it never goes well

  15. Posted January 31, 2017 at 3:54 am | Permalink

    Interestingly It’s Okay to be Smart did a great video on the issue of species continuum when addressing when we became humans.

  16. gravelinspector-Aidan
    Posted January 31, 2017 at 7:27 pm | Permalink

    for many species are cryptic, including those in the deep sea, bacteria, and the nematodes that burrow in the soil

    Don’t forget the rocks. While it’s extremely hard to get good (in the sense of microbiologically uncontaminated) samples from thousands of metres from the surface (or even tens of metres, if you start in the bottom of a mine), most of the time when people have done it, they find … bacteria.
    There are fluxes of fluids, minerals which are in chemical disequilibrium with those fluids, and a temperature gradient. That seems to be enough for life to survive.
    That’s an environment of around the same volume as the oceans (average of about 5km depth ; area about one third greater than the oceans). Even if the microbe density and diversity are low, it’s still a lot of volume.

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