[Bruno]

Pitcher Plants generally have multiple pitchers, with nectar, but only one is an "active" trap at any given time. This means that the pitcher plant encourages ants to come to it; the colony as a whole benefits more than it loses from the calories it gains from the plant vs the individuals lost. The plant gains a small steady source of nutrients, and the ants guard it fiercely from herbivores as it is a ready source of nutrition for them.

The bait doesn't even need to be seeds. Farming a dungeon demon could set up the same relationship, particularly if clever humanoids figure out that feeding it livestock makes it less aggressive.