By that logic, how do scientists claim there were ever mass extinctions of species in earth's history?
Yes, that is what I would like to know.
I mean, it's pretty obvious how one first comes to suspect a mass extinction: an interesting species is observed in strata from A to B, but no longer past B into C, and then
another interesting species is also noted to disappear at the B/C boundary. All it takes is those two species, after which new evidence will be gathered to support the hypothesis of something sudden and untoward happening at that juncture.
But this only tells something about the fate of "stable" species that had been present for several dozen million years before the event. If younger species die out at the boundary, or elsewhere, we can't tell whether they are part of a mass extinction effected by "outside forces", or simply unfit to live on this planet to begin with. Alternately, they may be established species whose time is past.
If something as new as recent species of mammals or birds cease to exist here and there across the planet, we can't really tell that such things haven't been happening continually for the past billion years. The fossil record would tell us little or nothing about the comings and goings of such brief candles.
If something as fundamental as frogs or sharks start dying out, then something is afoot. But it could be that the time of amphibians has finally come to an end, now that the climate is fluctuating madly in a cycle of ice ages and hot interglacials. Who are we to interfere?
As for what is natural... Apart from major sterilizing phenomena like supernovae, this planet has undergone many disasters that are selective killers. Volcanic activity results in ends very similar to man-made pollution. Tectonic movements do what air travel does, only infinitely more slowly; changes in currents act faster, comparably to sea travel, in propagating alien species to faraway shores.
In light of this, I have relatively strong faith in Earth's ability to adapt to technological species. I have less faith in said species' ability to manage a viable ecosystem if they work from the basis of trying to maintain status quo.
Timo Saloniemi