How many individual living beings inhabit planet Earth at any given moment? Is it possible to make an estimate if you begin to eliminate certain life forms from the calculation process? For example discount all floras and include only animals large enough for a human to see with the naked eye.
I have only two pointless contributions to make:
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It is possible to make an estimate of just about anything. What that estimate eould be, I haven’t a clue.
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Flora is already plural.
No it isn’t. It’s a collective noun, but it’s not plural. Floras is an appropriate plural.
“Fishes” is a valid plural of fish, too, but you would only use it to mean “types of fish” rather than more than one fish.
Anyway, I don’t know how you would estimate this either but I believe that there are far more beetles in the world than people.
There is probably enough data in the world to be able to estimate this but it would be arduous to collect it all into one place to get that one number.
I think that’s how the OP intended to use “floras”, to mean “types of flora”.
I would like to help with this question, but I really don’t know anything about the science involved. I did find a rather elementary page on life, which has at least some rough numbers. The only species for which I know the population is humans, at around 6 billion. And there are over 4000 species of mammals, although I think that most of them are not as populous as Homo sapiens. I suspect that we outnumber horses but not mice. I don’t know if 1 billion per species would be a good average, but if so, then that’s 4 trillion mammals. Birds, reptiles, and fish all outnumber mammals. We could easily be looking at 20 trillion vertibrates, I’d imagine. And then invertibrates, especially bugs, vastly outnumber those. So based on my ridiculously inaccurate numbers, I’d say our final tally will be somewhere around 100 trillion. That’s 15 digits. I hope that someone more knowledgeable will say something, though.
Not a direct answer to the OP, but the estimated estimated total biomass of Earth is just under 2 trillion tons.
So all we need is the average mass of all organisms on Earth. Anyone wanna take a stab?
I have nothing to offer the OP, but I just wanted to say that no other board I know of has better grammar than the SDMB.
<solipsism on>
Just me.
<solipsism off>
Couldn’t resist.
This site lists the number of individual species at about 1.4 million, and offers a rough breakdown.
There are a not insignificant number of species of bacteria or insects, the numbers of which would be extremely difficult to guestimate.
So, I’m afraid this post isn’t much help either. Interested in hearing a possible answer though.
I think you’ve probably vastly underestimated the number of individual insects (and other macroscopic invertebrates) in question. Think of the number of ants in an ant colony, and then multiply that by the number of ant colonies in the world. Now multiply that by the probably more than a million species of insects. Now think of how small an individual krill is, and how many it takes to populate the worlds oceans. Now don’t forget earthworms, chaetegnaths, nematodes, copepods, and thousands of other taxa that must number in the hundreds of billions, or more. (Of course, the line between macroscopic and microscopic is a fuzzy one.) Add those all together and I think you’ll find that all vertebrates are no more than a rounding error. I think you need to revise your estimate upwards by at least a factor of a thousand. Probably more.
And, according to this page ants and termites make up 20% of the total biomass = 400 billion tons = 400 trillion kg. According to this page, an ant weighs 20 mg, so there’s 50,000 ants per kg. Multiplying that out, we get 20 quintillion ants and termites in the world. Wow.
I would suspect that if we stick to the animal kingdom, the total won’t be a whole lot more than that…factor of two or four maybe? Include plants and microscopic organism, and I don’t know. A whole bunch more, likely.
Okay, so I said 15 digits. bryanmcc thinks we should go up to 18 at least. And zut says 20. I’ve got no problem with this. Anyone want to go higher? 
I always wonder if individual cells or bacteria are counted as living beings. If they are, we may have several millions (?) in our body.
Actually, we have trillions of cells in our bodies. (That’s a million million, or 10[sup]12[/sup], or 1,000,000,000,000.)
And we have more bacteria (in numbers of cells) living in and on us than we have human cells. IIRC, some termite species have millions of protists living in their guts, which in turn contain hndreds of thousands (if not more) bacteria living in and on them. The numbers are mind boggling.
bryanmcc is right, in terms of mass and number, vertebrates are just a rounding error.