MCWagner summed it pretty well (though that won't stop me from doing it myself too!).
I don't mind a science-based story, but I would've liked more fully fleshed characters. It did feel very much like an Asimov story in that respect.
If the science conundrum had been baffling and mysterious then I'd probably give it higher marks, but when I heard about the two apparent species on the planet that both had odd characteristics, my first thought was "They're two distinct stages of a single species' life cycle, like the larval and pupal stages of an insect." It really surprised me that this crossed my mind a few minutes into listening but the professional biologist who is devoting all of his time to studying these lifeforms did not think of it. He seemed way too intent on classifying into Earth-based classification systems--that seems like a rookie mistake to go to another planet and assume that its life will fit neatly into our own classification systems. I mean, on our planet, isolated ecosystems can produce lifeforms that don't fit well into our ecosystem--like our good friend the platypus.
And yes they should've included in their plans that laborers would need more calory intake. Between this oversight and the biologists' lack of critical thinking, I really wonder how long this colony is going to last. What else did they mess up? Did they forget to bring plants to produce oxygen? Did they forget to bring antibiotics?
As others said, I too was wondering what the "grunts" were for way too long. I was picturing pig-men for quite a while, either genetically engineered workers or a captured alien race. One thing that SF writers have to focus on that mainstream writers don't is clarity between expressions and reality, especially in a short story and especially at the beginning of that story. If you describe someone as "walking on air" or "moving mechanically" at the beginning of an SF story, it could be either literal or figurative, so for full reader comprehension it should be a bit more explicit. Same here with the expression "grunts", could've been spelled out that they were humans right away.
I am glad that it had an actual ending though, it seems like quite of few of the EA stories recently are lacking those.