The Dune ecosystem is much less thought out than it appears to be. Herbert drops bits and pieces about the Dune ecology here and there so getting an overall picture is difficult, so we don't realize how silly it is.
* We don't know where sand plankton comes from.
* Sand plankton lives in the top layers of desert sand, eats spice.
* Some of the tiny sand plankton individuals grow larger, migrate deeper down, and become sand trout.
* Sand trout excrement combines with water pockets deep below, is biologically active, grows, releases gases and explodes, transporting it back to surface, becomes spice.
* We don't know what sand trout eats, but it should eat something in order to produce excrement.
* Some sand trouts grow larger, become sand worms.
* Sand worms eat sand plankton.
So the thole ecosystem consists only one species, with 3 life stages: (1) sand plankton, (2) sand trout, (3) sand worm. And stage 1 lives by eating the excrement of stage 2, and stage 3 lives by eating stage 1.
Ecologically and energetically this is silly. The species just eats itself and its own excrement, and there appears to be no energy input to the system.