From where
do we obtain the cells of our test subject

Although i don't know what is the simulated system, starfish are predators that eat mostly  microalgae,  snails, sponges, bivalves, mussels and other small marine animals. removing starfish from any system would definitely increase this animals that they prey.

Your answer is water because there is combined mass of water found on, under, and over the surface of a planet. your answer is water.