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).