I'd put down good odds that the life they were going to sacrifice was the kind that has the best aetheric capacity barring Ancients. See there is one thing that neither Emet or anyone else mentions. How long they waited to put the sacrifice plan into action. It very well may have taken a very long time to cultivate enough aether to offer up to Zodiark. Long enough for the races we call the spoken to develop. I highly doubt that the 14th and their followers were over concerned about bees and trees being feed to Zodiark.

They most likely got the idea from Zodiark himself. We know that primals can speak and reason within a limited sphere of relevancy to their goals. He was created to restore the world and bringing back the Ancients lost during the End of Days would be well within his sphere.