  Given the existence of the M_BH-sigma relation, models of self-regulated
black hole (BH) growth require both a fuel supply and growth of the host bulge
to deepen the potential, or else the system will either starve or self-regulate
without sustained activity. This suggests that bright quasars must be triggered
in major mergers: a large fraction of the galaxy must be converted to new bulge
mass in a dynamical time or less. Low-luminosity AGN, in contrast, require
little bulge growth and small gas supplies, and could be triggered in more
common non-merger events. This predicts a transition to merger-induced fueling
around the traditional quasar-Seyfert luminosity divide (growth of BH masses
above/below 10^7 M_sun). We compile observations to test several predictions of
such a division, including: (1) A transition to bulge-dominated hosts. (2) A
transition between 'pseudobulges' and 'classical' bulges hosting the remnant
BHs: pseudobulges are formed in secular processes and minor mergers, whereas
classical bulges are relics of major mergers. (3) An increase in the amplitude
of small-scale clustering where mergers are more efficient. (4) Different
redshift evolution, with gas-rich merger rates rising to redshifts z>2 while
secular processes are relatively constant in time. (5) An increasing prominence
of post-starburst features in more luminous systems. Our compilation of
observations provides tentative evidence for the predicted division around the
Seyfert-quasar threshold. We discuss how future observations can improve these
constraints and break degeneracies between different fueling models.
