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BUT once you have said that, they are solid reasons to exclude the results of human culture from result of other natural phenomenon: the timescale at witch theses phenomenons occur (e.g. isolation of an island for millions of years vs extinction of its endemics in decades) and their magnitude.

Well, this was actually my point. I argue that they are solid evidences that anthropogenic effects on the biosphere are order of magnitude higher than other normally occurring phenomenons. If you represent the rate of extinction and dispersion (especially intercontinental or long distance dispersion) caused by human activity VS caused by natural phenomenon you will surely see a very significant difference which would I think clearly justify the distinction of Natural VS Anthropogenic. I think that these two notions should absolutely not be merged because they translate a completely different story on a completely different timescale.

In my personal opinion "timescale" and "magnitude" should not be factors in defining whether something is "natural" or not. Going down the scientific road how do you quantify at what time and size something becomes 'unnatural'? Is there a threshold? Any publications with evidence for this?

Following your logic an earthquake or a meteor hitting Earth would then be 'unnatural' because of the huge magnitude of the extinction it could cost in a very short timescale...?

The point here is not to abandon the book definitions of terms like 'natural'. Yes, we need to use them sometimes to set some borders relevant to a particular case. Yes, you need that if you pitch a conservation project to a potential sponsor and so on, but we were talking more about what it really means in the bigger picture of the Universe and expressing opinion on what we really think at the end of the day when we get home, not in front of the black board.

Ilian Velikov wrote:Following your logic an earthquake or a meteor hitting Earth would then be 'unnatural' because
of the huge magnitude of the extinction it could cost in a very short timescale...?

Quite obviously, no definition condensed in one sentence could ever cover all the possible real-life
cases/exceptions of anything... There will always be some "loose threads" sticking out from it...

An asteroid hitting the Earth - a brief (= short-time) and most dramatic occurence - is most certainly
natural by all definitions. But what about detonating a thermonuclear bomb, which is equally brief
and would produce more or less the same effects - is it a "natural" disaster, or an "unnatural" one?
If natural (?) beings like Homo sapiens had produced it and detonated it, is it less natural than an
impact of an asteroid? Ilian would probably say no. I haven't yet reached a decision. There has been
introduced a concept of "extended phenotype" when it comes to the particular species of Homo sapiens...
It would include your clothes, probably your car as well, maybe even more than that, maybe even a
thermonuclear bomb... not clear for the time being.
In short, is the "technosphere" included in the "biosphere" or not? NATURALLY included or UNNATURALLY?
Or NOT AT ALL? As I said, difficult, tricky questions. I wouldn't tackle them lightly, and certainly not here.
As a scientist, when writing a paper, I would always prefer to use (much) more than one sentence
to describe the actual circumstances of what's really going on. Then everyone should get the right
message, irrespective of the terse definitions of this or that. I think it's the right way, and have been
using it all the time, for many years, not to specify how many... But I have the feeling that the readers
of our scientific (or any other) papers are always very much grateful if we do so. I certainly am, reading
other people's papers.
Peace on you, my friends. And may the Force be with you.

BTW, I strongly recommend reading the book
John Nicholas Gray (2002): Straw Dogs: Thoughts on Humans and Other Animals.
Not that I would readily subscribe to everything written there, but it's a revealing read indeed.

Please do not misrepresent the arguments or definitions I was using :

Following your logic an earthquake or a meteor hitting Earth would then be 'unnatural' because of the huge magnitude of the extinction it could cost in a very short timescale...?

The magnitude or time scale of an event does absolutely not define if something is natural or not. What define if something is natural or not is whether it is man made or not. I used the time scale and magnitude of extinctions and introductions to justify that this distinction of Natural VS Anthropogenic is of huge scientific importance. Because they translate completely different stories on a completely different timescale.

Furthermore, an earthquake does not results in mass extinctions and neither earthquakes nor meteors hitting the Earth displaces species.

In short, is the "technosphere" included in the "biosphere" or not?

Absolutely not because objects are not living organisms.

Karim Chouchane wrote:Please do not misrepresent the arguments or definitions I was using...

No, no, we do understand them perfectly, just maybe we don't endorse them. But, actually, who is this "WE"?
Ilian and myself are not "a team fighting together" at all, we just happen to share a certain liking for critical
thought, that's all. (It's also called "philosophy", in some other circles, but I wouldn't provoke with aspirations
like that.)

Karim Chouchane wrote: What define if something is natural or not is whether it is man made or not.

Well, if that's really your final word on that subject, this particular discussion (with you, at least) should be closed.
Nothing left to discuss. Ilian has raised some interesting ideas/opinions (not so terribly new in the philosophy of
science, but still unresolved - and "his own"), but if you just say NO, and noone else jumps in, then that should be
the end of it.