I think aliens would recognize more or less the same species we do. After all, various different human groups do. I suspect they’d group them into larger groups more or less like we do, initially making some of the non-phylogentic choices we do (e.g. whales among fish). They’d know about evolution so they’d use its patterns in their classification system, at least as they refined the system. The classification would be heirarchical or in some other way based on closeness/distance of relationships/similarity. We would probably disagree with the aliens about some things (we taxonomists disagree with everybody about some things) but have lots of agreement, too.
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This reminded me of how the Finnish have something like forty different classifications for snow. Maybe the aliens would hyperfocus on some aspect(s) which we have disregarded in our own systems.
(Imagine trying to convince such an alien that all the Canis familiaris were the same species under our system.)
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It could get really interesting however if the aliens were from a world where DNA/RNA is not the basis for replication/inheritance. Maybe they’re not even carbon-based. Perhaps their world’s evolutionary history was very different from ours.
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I disagree. I would explain my reasoning, but I think that should require another thread.
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RNA= virus aliens. 
Hey, what if the aliens are microscopic or small?
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For someone who apparently finds this topic silly or of no interest, you’ve posted more messages here than any other user. Some of the comments (a few of which have been flagged) have been disrespectful to the OP and those of us who find their topic thought-provoking. Not the way this forum works.
Have you considered the best response might be no response?
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I don’t want to seem rude to anyone, but I agree with you.
@tanker-aviator-naturalist a lot of your posts here have been only slightly related to the topic and a few were rather offensive.
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Let’s keep things on topic here, folks. If you think a post is inappropriate or off-topic, please flag it rather than let it derail the discussion.
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I delete all my posts. Sorry if I have performed idiotic behavior.
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My apologies
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I didn’t know these would be offensive. Sorry
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I don’t think that whether their inheritance is based on DNA/RNA or not would change the fact that organisms on earth at any one time occur in “kinds” that alien biologists would see. We see them because the variation is real and I can’t believe that aliens who had any interest in organisms would fail to perceive them.
There would be a lot of disagreement about rank, including whether to treat dog breeds species or not.
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True. Unless how they perceive relatedness is so different from us that they lump things in an alien (to us) way, perhaps based on utility or some philosophy. As others have noted, you can classify organisms in ways that are independent of their actual evolutionary relatedness. And as the OP added, what if they are advanced enough for space travel but behind us in understanding evolution? Maybe they have no interest in the subject. Anything is possible with hypothetical aliens!
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Please forgive me. 
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Aliens might classify lifeforms by the primary chemical composition of outer layers
As far as I know that’s only carbon and silicon, with most of life’s outermost layer being carbon based.
Dictyochales, Diatoms and Radiolaria would be lumped together as silica.
And everything else with chitin, calcium carbonate, cellulose, lignin, and keratin would be lumped with Carbon.
anything I missed?
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Not that I know of. Seems pretty spot on.
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Also, I was assuming the term aliens functions as a rhetorical device that symbolizes an external perspective, not a reference to a literal extraterrestrial.
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Given the way certain other threads are going, this looks more like intellectual arrogance on the part of taxonomists (“Well, of course OUR way is self-evidently the right way”). As a certain other member of the Forums likes to point out, “We see things not as they are, but as we are.”
Humans define invasive species as follows:
- non-native (or alien) to the ecosystem under consideration and,
- whose introduction causes or is likely to cause economic or environmental harm or harm to human [or alien] health.
So as long as those criteria are met by the incoming humans, the aliens would consider us invasive. There are many species on earth that fit criteria 1, but not 2 and are not considered invasive. We can’t always predict if criteria 2 will occur until after the fact.
Whether it’s within the rights of the aliens to try to stop us from visiting or settling their lands is a political question not an ecological one. It’s probably best that we not engage in political discussions in this forum. But to continue without hopefully not going too far, I think most humans are OK with the idea of us humans having the right to try to prevent an alien invasion of Earth–and I hope that humans can see this from the alien’s perspective and not attempt to invade their homeland.
Only if taxonomy does not reflect an underlying reality. Will aliens think the sky is blue? Is it arrogant to think that they would? No. We can measure the spectrum and see that it is 450–485 nm. This isn’t our perspective or biased opinion, it’s the reality. Likewise, if life evolved (primarily) as bifurcating lineages (as much of life has), then they’d almost certainly classify things based on this real pattern–rather than grouping dragon flies and birds together because they fly. Humans passed through a time when we didn’t know as much as we know now and did classify things differently (whales being fish not mammals for example). I’m assuming that these aliens know at least as much as we do. If they are unfamiliar with evolution, then I think they’d probably lump whales in with the fishes!).
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