You should either remove or alter the plants present to see if the effect of rodents on ants is removed. For example, you could artificially alter the number of small-seeded plants to try to disrupt this effect.
Work Step by Step
Since the plants serve as intermediaries, they are like gears between pedals and wheels in a bicycle. Remove a gear, and you see that the pedals no longer affect the wheels. This is analogous with altering numbers of one or the other kind of plant.