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New Estimate Suggests 5.5M Species On Earth, Not 30-100M
An anonymous reader writes "How many species share our planet? According to a recalculation by an international research team, the number is significantly lower than we thought — only around 5.5 million."
Please turn in your geek badge at the door. http://en.wikipedia.org/wiki/Gray_code
But what definition of species does this estimate use? It may seem odd, but there really isn't a scientific consensus of how to define a "species". That's not to say there aren't strong opinions out there, but it tends to vary from field to field depending on what questions a particular group of biologists is trying to answer. When you actually dig down and look carefully, there are shades of gray and blurring of lines all over the place (as would be expected for a world that is constantly evolving - there's no clear day on which one species becomes two).
(If you're trying to count species from the point of view of a billionaire with a Pokemon mindset, you're going to be disappointed because there will never have a perfect checklist for you to collect)
The study doesn't take into account bacteria, archaea nor unicellular eukaryotes. That's where by far most of biodiversity (species count and number of genes and metabolic pathways) and biomass (carbon and nutrients) lie. Typical macroworld arrogance :(
They did not "remove a whole group". The previous estimates of 30 to 100 million species also did not include bacteria.
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The only definition of a species is that two organisms that cannot mate are, by definition, different species.
To illustrate the subtleties in the actual definition(s) used by biologists, a prof in a class I was in wrote a definition very much like the above, and asked the class "What's wrong with this definition?" He was impressed when I spoke up and said "According to that definition, you and I are not the same species." We were (and probably still are ;-) both male, so he just grinned and said "Ya got it." Funny thing was that a good percentage of the class still had a puzzled looks on their faces, so he had to explain to them what I'd just said.
He later mentioned that there are other important problems with such definitions. One is that people generally want "the same X as" to be a transitive relation. But Ma Nature throws monkey wrenches into such things. Thus, the domestic dog Canis familiaris can interbreed with wild wolves and jackals, but wolves and jackals can't interbreed (or rather, they can, but the few offspring are sterile). So dogs are the same species as wolves and jackals, but wolves and jackals are different species. There are many examples like this.
A more subtle sort of example is what are sometimes called "range species", in which matings of critters not too far apart are fertile, but when the distance gets above some threshold, fertile hybrids are no longer possible. This happens in a lot of shoreline species.
We've had a couple of centuries to work out such ideas, and biologists have been fairly successful at dealing with this fairly important concept. But you need more carefully worded definitions than the above.
If you want to read about an especially difficult "species" distinction, google for the results of mating lions with tigers. That should convince anyone how tricky it is to get the definition right.
Those who do study history are doomed to stand helplessly by while everyone else repeats it.