Tom Merkle wrote:

There's a balance principle in biology that makes it virtually
impossible for an organism that evolves on a world as small and devoid
of resources as Mars to develop something that is devastating to the
very robust earth's biosphere. The vector just moves in the wrong
direction. Evolution just doesn't work that way. It's remotely
possible that something that came from Mars would be mildly successful
in our environment--but that's it. Even the examples that NASA's
'caution' document have of successful 'species invasions', the vector
moves from the more robust, open environment, to the less open, less
robust environment. This is why the majority of species invasions that
you hear about take place in the New World. Australia has it worst,
America second worst, remote Pacific Islands the third... but you
rarely hear about species invasions in Africa or Asia. Why? Africa and
Asia both are bigger, more robust, have more species. African, Asian,
and European species are the ones doing the invading.

I think your generalization makes considerable sense, but in real
life things are always more complicated. I gather that several
African lakes have had their ecosystems severely compromised by the
introduction of European sport-fishing species, which are
out-competing many of the local cichlids, threatening to drive them
to extinction. By the same token extraterrestrial organisms might
succeed in the more isolated or 'hypotrophic' terrestrial
environments: e.g. one can imagine a Martian bacterium doing well in
our high arctic or antarctic regions, to the detriment of the local
bacteria, lichens, &c.

--
Odysseus