Using neutral theory, species become common or rare as
a consequence of random processes: chance variation in who a predator happens to eat, or whose dispersing offspring happen to land on a vacant bit of real estate on the seafloor.
Just another few more dumb questions: is noise
random because it is the
consequence of a physical
process that is causing it, or is it
random because you defined it that way; on what basis can it be said that ENSO is noise; on what basis can you say that any trend is constant (no discontinuities); and taking all that into account on what basis can you say that noise averages out the more data you have, leaving behind a pristine trend?