That's a great question. I don't think I have seen it addressed. Well, I think I read a couple scientist say like there is no way there would be dinosaur size creatures on Tatooine, (Star Wars) because there is not enough biomass (plants?) to sustain something of that size...
The bugs in starship trooper were planet hopping, which was kind of crazy... but let's give them that, if there was no bio mass, they would have died or moved on? If there is biomass and they consume it all, they're going to die or move on? Do they get to a certain mass, catabolize and make queens and the queens go the next planet? Or, terraform the land to grow crops or bacteria colonies... some ants farm... Yeah, the bug thing was more difficult, but also but from the book perspective- it was meant to be a ludicrous metaphor as anti war statement.
Aliens... OMG, everything after Aliens, James Cameron is rubbish. And I keep seeing them and swearing them off... aliens, per the newer stuff, are bioweapon, engineered, not something that evolved. Regardless of which movie we use, you're right, there is not enough biomass to explain the numbers or the growth- unless, it doesn't need biomaterial, food to grow. some of the artificial metal life, Lee Cronin, I don't know if they have turn off point. They keep making cells as long as there is material to consume, and then just sit there indefinitely? that little 'first alien' critter grew on that ship from chest bursting size to full adult in mere hours? What was it eating? It seemed liked it was killing crew, not consuming.
Clearly, it requires a host for reproduction? Does reproduction cease when it runs out of viable hosts? Those tardigrades can last a petty long time, can the face hugger egg sac last indefinitely, waiting for another space fairing host to come along?
Humans, intentionally or through ignorance, has been out of balance of balance with nature for a moment? Will our short sightedness lead to human extinction? Will we survive, but be seriously impoverished? I think your question argues that we should all be striving for greater diversity, in biology and psychology and sociology...
