I actually find this conversation kinda fun. Perhaps the producers should be made aware that we at candlepowerforums are going to try to find explanations for their apparent complete lack of understanding of nature
One is that there may be other subterranean life forms, just as competitive, but which don't come out above ground.
<font size="2" face="Verdana, Arial">Well, following a reasonable set of rules on life, we know that the top of the food chain will be vastly outnumbered by the bottom of the food chain. At the very bottom, there needs to be a huge amount of lifeforms that convert heat or light to biomass. Hard to imagine any other way. The birds can't be surviving on plants that use light, since the birds can't be in the light. On earth, the only creatures that survive just on heat are below the ocean. Hard to imagine an ecosystem under the earth, based on heat, that's at all realistic. In short, we have a huge amount of top-of-the-food-chain predators, we have the movie clearly trying to show us that everything else is dead (except a few glow worms), and we have no sign of even bottom-of-the-food-chain lifeforms (e.g.,plants) much less the intermediate lifeforms required to keep a balanced ecosystem.
Finally, if the creatures have wiped out all other life (which they haven't, exactly - they did find some small glowing worms or something..), then they may be gradually dying out.
<font size="2" face="Verdana, Arial">But again, have we ever seen anything like that in nature, where a species can just wipe out its food sources but continue to thrive itself, until there are many predators and no prey?
I think there are a few possible things that can save us here. One is, as you suggest, that we didn't see the entire planet, so maybe there are lifeforms elsewhere. Another plausible storyline might be that people who came to the planet in the past created the birds through genetic engineering, creating thousands then losing control. Now maybe we do have a situation where the planet can be relatively wiped out.
Now don't get me started one the plausibility of a three-star system with earth-like planets in a stable orbit
Or how that orbit can be such that there's never a night time on any part of the planet.